Old fashioned phone receiver for cell

Old fashioned phone receiver for cell

The laws regarding the use of the radiospectrum are actually fairly uniform, and established byinternational treaty, for the obvious reason that radio signals do notrespect international borders. The laws and regulations condinate the use of radio frequencies (frequencies, transmitting power etc.) and define the needed permission to use transmitters/receivers.Intentionally interfering with legitimate radiocommunications is illegal in virtually all countries. Generally a license is needed to operate a radio transmitter, unless you use a special radio system that is defined not specifically to need any special permit.Electromagnetic radiation is a wave that combines electric and magnetic fields, moving out from its source as an expanding sphere and having waves as the feilds alternate in value. This kind of radiation has different properties as its wavelength changes. We call this radio waves.Waves of a very long wavelength (low frequency), such as thousands of meters, tend to travel along the surface of the earth and even penetrate into the water. These are useful for communication with submarines. Radio, television, cellular telephones, walky-talkies, 2-way police radios, and other such communication/broadcast systems use electromagnetic radiation, or "Radio Frequency Waves". Each communication service uses a part of the spectrum that is suitable for its needs.A radio wave used to transmit audio signals is a complex signal that contains the carrier frequency of the broadcast station and the audio signal to transmit (usually from the microphone or audio amplifier source). The function of the radio receiver is to recover the audio signal that was modulated onto the RF carrier at the radio station, and apply it to the speaker, reproducing the sounds of the announcer. There are various ways to combine the carrier frequency and the audio signal together. This process is called modulation. The most commonly used modulation methods are amplitude modulation (AM), frequency modulation (FM), single sideband modulation (SSB) and phase modulation (PM).Also digital signals can be modulated to radio frequency carrier.When the signal is transmitted, there are many impairments on the way until the signal gets to the receiver: Typical impairments are:

  • The absorption of radio signal power (air and surrounds absorb signal)
  • Signal refelctions caused by the ground and obstacles (signal detected in the receiver is a sum of direct and reflected waves which can cause an effect known as fading)
  • Co-channel interference (distant radio transmitters on the same frequency will disturb the reception)
  • Intermodulation distortion (transmitters on different frequencies can disturb each other)
  • Background noise in receiver (thermal noise generated by the receiver electronics itself)
  • Atmospheric noise (bursty noises from thunder storms and similar)
  • Industrial noise (RF noise from electronics, sparks)
Familiar forms of radio communications include such as AM/FM, short-wave, police/fire, radio, television, and so forth. These narrowband services, which avoid interfering with one another by staying within the confines of their allocated frequency bands, use what is called a carrier wave. Audio signals and data messages are impressed on the underlying carrier signal by modulating its amplitude, frequency or phase in some way and then are extracted upon reception. In receiving side the narrowband radio receivers are fitted with a front-end filter that prevents transmitters operating outside their reception bands from causing trouble.
  • http://www.rfic.co.uk/ - RF and microwave design is a complex and large subject. This site gives you information and theory to understand it.    Rate this link
  • AC6V's Amateur Radio and DX Reference Guide    Rate this link
  • A very brief introduction to microwaves - By "microwaves" we mean the range of radio frequencies between about 1 GHz 300 GHz. For comparision.    Rate this link
  • Counterintelligence and Counterterrorism WWW Page - information about surveillance and countersurveillance    Rate this link
  • Doug Lung's R.F. Technology Page - Focusing on TV RF Broadcast Technology    Rate this link
  • Exposure Limits / Conversion Tables - information on limits for electric and magnetic fields    Rate this link
  • FCC Interference Handbook - electronic version of FCC Interference to Home Electronic Entertainment Equipment Handbook    Rate this link
  • How Radio Scanners Work - This article will look at the basics of scanner operation, examine radio scanning as a hobby, and show you how to get started listening to public airwaves you may not have known existed!    Rate this link
  • How Radio Works - "Radio waves" transmit music, conversations, pictures and data invisibly through the air, often over millions of miles. There are many everyday technologies that depend on radio waves. This document will explore the technology of radio so that you can completely understand how invisible radio waves make so many things possible.    Rate this link
  • How the Radio Spectrum Works    Rate this link
  • Introduction to Scanning    Rate this link
  • Mobile commucation and cellular phone link    Rate this link
  • New Scanner Users FAQ    Rate this link
  • RF Design articles online    Rate this link
  • RF Energy Safety - Reprint from QST and the ARRL Handbook    Rate this link
  • RF Integrated Circuits - list of some commonly used types    Rate this link
  • Safety of Microwave Energy    Rate this link
  • Signals and Measurements for Wireless Communications Testing    Rate this link
  • The quest for the ideal rf amplifier    Rate this link
  • The RC Transmission! - text explains what kind of signals are sent from the TX to the RX of an "FM" radio control set, how they are decoded, and ideas about producing it yourself    Rate this link
  • The Radio Equipment and Telecommunications Terminal Equipment Directive (1999/5/EC) - On 9 March 1999, the Council and the European Parliament adopted a Directive, defining new rules for the placing on the market and putting into service of Radio Equipment and Telecommunications Terminal Equipment.    Rate this link
  • The Smith Chart - an RF engineering aid by Phillip Smith of RCA many years ago    Rate this link
  • Useful radio terms: A tutorial - This is a tutorial to decibels and other radio terms you might encounter in various radio applications.    Rate this link
  • Why my radio bought from USA does not work well in Europe ?    Rate this link
  • System-Level Design Tackles Tough Soft Radio Framework Challenges - Recent design trends tends to add layers and virtualization even to high-rate and hard real-time systems such as wireless receivers. System-level design tools and high-level frameworks, such as the software communication architecture (SCA) developed within the Software Defined Radio (SDR) Forum, are serious threats to the "good old days" of "classical" C and VHDL programming approaches for DSP and/or FPGA/ASIC wireless systems.    Rate this link
  • ERO Frequency Information System - General Search for Radio Spectrum Utilisations    Rate this link
  • www.rfic.co.uk - RFIC and Microwave design tutorials    Rate this link

Frequency allocation

Continuous set of logically related frequencies based on the FCC allocation services :

25 - 28 HF Hi & CB 28 - 30 Amateur Radio 30 - 50 VHF Lo 50 - 54 Amateur Radio 54 - 72 TV Audio 72 - 76 Land Mobile 76 - 88 TV Audio 88 - 108 FM Broadcast 108 - 137 Commercial Aircraft 137 - 144 Government 144 - 148 Amateur Radio 148 - 174 VHF Hi 174 - 216 TV Audio 216 - 222 VHF Hi 222 - 225 Amateur Radio 225 - 400 Military Aircraft 406 - 420 Government 420 - 450 Amateur Radio 450 - 470 UHF Standard 470 - 520 UHF "T" 520 - 760 TV Audio 760 - 806 UHF "T" 806 - 824 UHF Public Service 849 - 869 UHF Hi 894 - 956 UHF Hi 956 - 1240 Land Mobile/Private/Paging 1240 - 1300 Amateur Radio

The origins of radio communications are in the 19th century.

  • 1864 James Clerk Maxwell presented the Maxwell Equations for electromagnetic radiation
  • 1876 Alexander Graham Bell invented the telephone.
  • 1887 Heinrich Hertz discovered "hertzian waves" which are now called as radio waves.
  • 1896 Guglielmo Marconi carried out the world?s first radio transmission.
At the beginning of our century, e.g. the police forces in Europe and in the US were using radio telephony equipment. During the 50?s and 60?s, first radio telephone networks were introduced for public customers in the US.As the radio telephony services became more popular, the insufficient availability of radio frequencies became obvious. At the 60?s and 70?s, new technologies like dynamic channel allocation and cell-based networks were developed in order to decrease the congestion in the radio frequencies. In the 80?s, several analogue cellular radio networks entered to service around the world. Each country has proceeded in its own way in adopting standards for these networks. These standards are not mutually incompatible.Later international standards, like GSM, were introduced.
  • United States Early Radio History - An assortment of highlights -- plus a few lowlifes -- about early U.S. radio history. Articles and extracts about early radio and related technologies, concentrating on the United States in the period from 1897 to 1927.    Rate this link
  • The Broadcast Archive - We hope this will become one of your favorite links to broadcast history. The goal is to continue adding historical materials on both pioneer and current broadcast radio stations, as well as links and references to other locations containing accurate materials on broadcasting. While the emphasis is on professional broadcasting, especially radio, certainly there are some important links to early amateur broadcasting, as well as various companies where the value of radio was exploited.    Rate this link

Modulation is necessary to allow radio wave carriers to carry information. The simplest modulation is CW (continuous wave) modulation used in early morse tranmissions: when the radio user presses the key, the transmitter start transmitting and when key is not pressed thereis no transmission. This a simplest form of digital modulation.A radio wave used to transmit audio signals is a complex signal that contains the carrier frequency of the broadcast station and the audio signal to transmit (usually from the microphone or audio amplifier source). There are various ways to combine the carrier frequency and the audio signal together. This process is called modulation. The most commonly used modulation methods are amplitude modulation (AM), frequency modulation (FM), single sideband modulation (SSB) and phase modulation (PM).Common abreviations for different modulation methods used for radio communications:

  • AM (amplitude modulation): The amplitude of carrier is chaged according the modulating signal. The amplitude of the output is a function of the input signal (usually audio or video signal). In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating data appears in the form of signal components at frequencies slightly higher and lower than that of the carrier. These components are called sidebands. The lower sideband (LSB) appears at frequencies below the carrier frequency; the upper sideband (USB) appears at frequencies above the carrier frequency. The actual information is transmitted in the sidebands, rather than the carrier; both sidebands carry the same information.
  • CW (continuous wave): The carrier frequency is constantly on when transmitter is activated. Continuous wave transmission is used primarily for radiotelegraphy. This is the transmission of short or long pulses of RF energy to form dots and dashes that will correspond to some code such as the Morse Code, sometimes referred to as interrupted continuous wave (ICW).
  • DSB (dual sideband): This is basically an AM modulation where the main carrier freuquency is suppressed (only sidebands are left).
  • FM (frequency modulation): The frequency of carrier is chaged according the modulating signal. It means that the RF-frequency will change acording to the input audio signal. A FM demodulator produces an output voltage that is proportional to the instantaneous frequency of the input. In frequency modulation (FM), the frequency of the carrier wave is varied in such a way that the change in frequency at any instant is proportional to another signal that varies with time. FM offers increased noise immunity and decreased distortion over the AM transmissions at the expense of greatly increased bandwidth. Frequency modulation uses the instantaneous frequency of a modulating signal (voice, music, data, etc.) to directly vary the frequency of a carrier signal. Modulation index, b, is used to describe the ratio of maximum frequency deviation of the carrier to the maximum frequency deviation of the modulating signal.
  • ICF (interrupted contunuous wave): This is the transmission of short or long pulses of RF energy to form dots and dashes as used in Morse code.
  • WFM (wide-FM): This modulation used in normal FM radio broadcasts. The FM band has become the choice of music listeners because of its low-noise, wide-bandwidth qualities; it is also used for the audio portion of a television broadcast. Normal FM radio uses +- 75 kHz deviation. TV sound used +- 25 kHz bandwidth. This FM system offers increased noise immunity and decreased distortion over the AM transmissions at the expense of greatly increased bandwidth.
  • NFM (narrow-FM): A FM modulation with only few kHz of modulation deviation. Narrowband FM is defined as the condition where modulation index is small enough to make all terms after the first two in the series expansion of the FM equation negligible. In narrowband FM, commonly used in two-way wireless communications, the instantaneous carrier frequency varies by up to 5 kilohertz above and below the frequency of the carrier with no modulation.
  • NBFM (narrow-band-FM): A FM modulation with only few kHz of modulation deviation. Narrowband FM is defined as the condition where modulation index is small enough to make all terms after the first two in the series expansion of the FM equation negligible. In narrowband FM, commonly used in two-way wireless communications, the instantaneous carrier frequency varies by up to 5 kilohertz above and below the frequency of the carrier with no modulation.
  • PM (phase modulatio): In phase modulation charried signal phase is chaged according the modulating signal. Phase modulation, like frequency modulation, is a form of angle modulation (so called because the angle of the sinewave carrier is changed by the modulating wave). The two methods are very similar in the sense that any attempt to shift the frequency or phase is accomplished by a change in the other. The converse also holds: When the instantaneous phase is varied, the instantaneous frequency changes. But FM and PM are not exactly equivalent, especially in analog applications. When an FM receiver is used to demodulate a PM signal, or when an FM signal is intercepted by a receiver designed for PM, the audio is distorted. This is because the relationship between frequency and phase variations is not linear; that is, frequency and phase do not vary in direct proportion.
  • USB (upper sideband): Single side band transmission which uses upper side band from AM modulation. This means that the signal is above reference carrier frequency. Because LSB and USB are essentially mirror images of each other, one can be discarded.
  • LSB (lower sideband): Single side band transmission which uses lower side band from AM modulation. This means that the signal is below reference carrier frequency. Because LSB and USB are essentially mirror images of each other, one can be discarded.
  • SSB (single sideband): Single sideband is an AM signal where one everythign else than one of the sidebands (upper or lower) is removed. In this transmission there is only one sideband AM modulation products, no base carrier or other sideband.
  • VSB (vestigial sideband): Vestigial sideband is an AM signal with most of one (redundant) sideband filtered out to save bandwidth. This is used in analogu TV broadcasting. VSB transmission is similar to single-sideband (SSB) transmission, in which one of the sidebands is completely removed. In VSB transmission, however, the second sideband is not completely removed, but is filtered to remove all but the desired range of frequencies.
The simplest form of Amplitude Modulation is MCW (Modulated Carrier Wave). This consists of keying the modulator with a fixed AF tone, say 400 Hertz, with for instance Morse Code. This is known as class A2 transmission. Modulating a transmitter with voice or other frequencies, in amplitude, is known as Class A3 transmission. The channel spacing for wideband FM broadcast stations is typically 0.2 MHz (200 KHz), such as between station A and B above. For NBFM - narrow band FM - the channel spacing may be 20 KHz, or even 12 1/2 KHz, or less.Also digital signals can be modulated to radio frequency carrier.Some simple digitial signal modulation methods:
  • MCW (Modulated Carrier Wave) consists of keying the modulator with a fixed AF tone. Active tone described one state and no tone the other. This can be applied to for example to an AM transmitter for radio transmission.
  • FSK (frequency shift keying) may be applied to an AM transmitter so that, for instance, binary 0 = 1 KHz and binary 1 = 2 KHz modulation. Anything from Morse to RS 232 serial computer data may be sent by this means.
  • FM modulation: The radio carrier itself may be "frequency shifted" based on the modulating signal.FM offers increased noise immunity and decreased distortion over the AM transmissions at the expense of greatly increased bandwidth. In digital FM, the carrier frequency shifts abruptly, rather than varying continuously. The number of possible carrier frequency states is usually a power of 2. If there are only two possible frequency states, the mode is called frequency-shift keying (FSK). In more complex modes, there can be four, eight, or more different frequency states. Each specific carrier frequency represents a specific digital input data state.
  • ASK (Amplitude Shift Key): The carrier amplitude is changed based on incoming data. Generally logic 1 indicated higher transmitting level and logic 0 means lower transmitter transmitting level. ASK modulation allows for the carrier to be "on" for both the transmission of a "0" and a "1". The carrier, during the transmission of a "0", is reduced in amplitude.
  • OOK (on-foo keying): OOK modulation (On/Off Key) is the special case of ASK (Amplitude Shift Key) modulation where no carrier is present during the transmission of a zero. OOK modulation is a very popular modulation used in control applications. This is in part due to its simplicity and low implementation costs. costs. OOK modulation has the advantage of allowing the transmitter to idle during the transmission of a "zero", therefore conserving power.
  • PSK/PM (phase shift keying / phase modulation): The phase of the radio signal is changed based on the modulating signal
  • OFDM (orthogonal frequency-division multiplexing): Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies. In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM). The difference lies in the way in which the signals are modulated and demodulated. Priority is given to minimizing the interference, or crosstalk, among the channels and symbols comprising the data stream. Less importance is placed on perfecting individual channels. OFDM is used in European digital audio and TV broadcast services.
  • MCM (multi-carrier modulation): Multi-carrier modulation (MCM) is a method of transmitting data by splitting it into several components, and sending each of these components over separate carrier signals. The individual carriers have narrow bandwidth, but the composite signal can have broad bandwidth. The advantages of MCM include relative immunity to fading caused by transmission over more than one path at a time (multipath fading), less susceptibility than single-carrier systems to interference caused by impulse noise, and enhanced immunity to inter-symbol interference. Limitations include difficulty in synchronizing the carriers under marginal conditions, and a relatively strict requirement that amplification be linear. The technology lends itself to digital television, and is used as a method of obtaining high data speeds in asymmetric digital subscriber line (ADSL) systems. MCM is also used in wireless local area networks (WLANs).
  • COFDM (Coded Orthogonal Frequency Division Multiplexing): Coded Orthogonal Frequency Division Multiplexing (COFDM) [1, 2] has been specified for digital broadcasting systems for both audio (Digital Audio Broadcasting (DAB)) and (terrestrial) television (Digital Video Broadcasting (DVB-T)). COFDM is particularly well matched to these applications, since it is very tolerant of the effects of multipath (provided a suitable guard interval is used). Indeed, it is not limited to 'natural' multipath as it can also be used in so-called Single-Frequency Networks (SFNs) in which all transmitters radiate the same signal on the same frequency. A receiver may thus receive signals from several transmitters, normally with different delays and thus forming a kind of 'unnatural' additional multipath. Provided the range of delays of the multipath (natural or 'unnatural') does not exceed the designed tolerance of the system (slightly greater than the guard interval) all the received-signal components contribute usefully. COFDM is a modulation scheme which is especially tailored to work well with selective channels and isolated CW (or analogue TV) interferers. COFDM is an OFDM system where signal is split into several narrowband channels at different frequencies. With other rectangular-constellation modulation systems, such as 16-QAM or 64-QAM, each axis carries more than one bit, often with Gray coding. At the receiver, a soft decision can be made separately for each received bit.
  • 8-VSB (Eight-level VSB): Eight-level VSB (8-VSB) was developed by Zenith for inclusion in the Advanced Television Systems Committee (ATSC set of digital television (DTV) standards used in USA.

Both OOK and ASK receivers genrally require an adaptable threshold or an automatic gain control (AGC) in order toensure an optimal threshold setting. The FSK modulation does not usuallyrequire this because it incorporates a limiter that keeps the signal envelopeamplitude constant over the useful dynamic range.

Radio modulation related link

  • Explore inside of a Radio - This page investigates the inside of a cheap beach radio.I will show you different compontents and explain what they do. There are lot of stuff you can re-use from such radio.    Rate this link
  • OOK, ASK and FSK Modulation in the Presence of an Interfering signal - This paper discusses three popular modulation schemes in the presence of an interfering signal. This paper will review the three modulation types and develop a mathematical model for the prediction of error due to interference.    Rate this link
  • Quadrature FM Detectors - FM stands for Frequency Modulation. It means that the RF-frequency will change acording to the input audio signal. A FM demodulator produces an output voltage that is proportional to the instantaneous frequency of the input. There are three general categories of FM demodulator circuit: Phase-locked loop (PLL) demodulator, Slope detection/FM discriminator, Quadrature detector    Rate this link

Walkie-talkies provide a cost-effective alternative for cellular phoneuse in business or family conmmunications at short distance. With a set of small and robust walkie-talkie radios, its easy for your group to remain in contact. Simply press the Push-to-Talk buttonto instantly speak to your group. Most ypical control in walkie-takie radio is CHANNEL, which is used to select the desired channel.Many walkie-talkie systems involve some form of SQUELCH system. If the walkie-talkie picks up unwanted, partial, or very weaktransmissions, turn SQUELCH clockwise to decrease the walkietalkie's sensitivity to these signals. Turn SQUELCH counterclockwise if you want to listen to a weak or distant station. General operation advice for using walkie-talkie radio is to hold the walkie-talkie 2 or 3 inches from your mouth. Press and hold down the transmit speak into the microphone in a normal voice. In most systems the walkie-talkie's automatic modulation circuit adjusts themicrophone's sensitivity to allow a wide variety of voice levels. Do not speak too loudly when transmitting. It does not makeyour signal any stronger, and might distort your transmission.

General information

  • Directive 1999/5/EC - Directive of 9 March 1999 of the European Parliament and of the Council on Radio Equipment and Telecommunications Terminal Equipment and the mutual recognition of their conformity (1999-04-07 OJ No L 91/10).    Rate this link
  • Handheld Radio Equipment Page - This page attempts to keep track of the array of low power systems available to the public (excluding telephones) such as CB, FRS, GMRS, MURS, SRBR etc.    Rate this link
  • Personal Radio Services - Services issued in USA by FCC. This is official FCC page on those services.    Rate this link
  • R&TTE Directive - As of 2000 april 8th within the European Economic Area (EEA) Radio and Telecommunications Terminal Equipment (R&TTE) is brought under the CE Marking scheme. So far a type approval was required for equipment like telephones, mobiles e.g. DECT, GSM and DCS1800, transmitters like remote controls and the like. Now, like most other equipment, the type approval is replaced by a self certification scheme in accordance to the R&TTE Directive (99/5/EC).    Rate this link
  • Two Way Radio Directory - A comprehensive directory of Two Way Radio resources with over 1500 links.    Rate this link


  • CTCSS (Continuous Tone Coded Squelch System) - CTCSS (Continuous Tone Controlled Squelch System) a sub audible tone in the range of 67 to 254 Hz.    Rate this link
  • CTCSS, PL, Tone Squelch, and other Necessary Evils    Rate this link
  • CTCSS? DCS? Sub-channels? - With "tone squelch" the radio receiver can be set to only open its squelch if this tone is received. Any interference would not have the correct tone present, and so the radio would remain quiet. Continuous Tone Coded Squelch System was developed into a system with numerous standard tones to choose from, often 37. Some manufacturers call these "sub-channels". DCS is similar to CTCSS in practice, but uses a digital code instead of continuous analogue tones.    Rate this link

Citizens Band (CB) in USA

CB is one of the Citizens Band Radio Services. It is a two-way voice communication service for use in your personal and business activities. Expect a communication range of one to five miles. 27MHz CB was the first system that the public were free to use for business purposes, with a license of course, and that anyone else with a CB could legally listen in. Nowadays in USA license documents are neither needed or issued, when you use an unmodified FCC certificated CB unit.

  • Citizens Band Radio Service - Citizens Band (CB) Radio Service is a private two-way voice communication service for use in personal and business activities of the general public. Its communications range is from one to five miles.    Rate this link

Citizens Band (CB) in Europe

The Citizens Band (CB) operating at 27 MHz has been used in many European countries. Those devices has been called with names like LA, CB and PR27. The LA version is amplitude modulated and PR27 version is FM modulated. In many countries operating CB radio needs a license. Here is short introduction to different versions (based on sitation in Finland, some specfications can vary in other countries):

  • LA: Channels 1-22 and 11A, AM or FM modulation, 5W power
  • PR: Channels 1-40, FM modulation, power 4W
  • CB: Channels 1-40, AM/FM/SSB modulation, power levels: 4W FM, 1W AM, 4W SSB
Nowadays there is european wide directives for this kind of deivces. This kind of devices are marked with mark R....PR27 and covered by telecommunications terminal directive 1999/5/EY. This kind of devices should also have CE mark in them. European "EU" Band is 26.965 MHz - 27.405 MHz (ETS 300 135/MPT 1333 "CEPT/EU Channels", total 40 channles) and 26.965 MHz - 27.405 MHz (MPT 1382 December 1997, sometimes referred as CEPT or "EU" channels, total 40 channels). Allowed frequency band can vary somewhat from country to country (some countries have more channels, so there cna be specific models only to be used on some specific countries). The maximum transmitter RF carrier power output allowed is 4 Watts and the antenna is restricted. CB radio is is voice only service. Generally no data transmission is allowed.


PMR446 stands for Personal Mobile Radio. PMR446 is a European standard licence-free radio service. PMR-446 is a licence free communication band in Europe that anyone can use for two-way radio communications.

The PMR 446 specification is largely based around the American Family Radio Service known as FRS that has been in existence for a few years and have proved extremely popular as an alternative to CB Radio. PMR446 was introduced in spring 1999 to supersede some other short range radio systems.

PMR446 is Europe-wide licence free standard for hand-portable two-way radios, anyone, individual or business, can make use of affordable and useful walkie-talkie radios. PMR 446 is a Europe-wide standard for radios that can be bought and used by anybody for business or leisure purposes. This means that in most European Union countries PMR 446 walkie talkie radios can be used with no special permission or license needed. PMR446 standard allows for license-free legal use of same walkie-talkies throughout the European Union.

PMR446 walkie-talkie radios are simple to operate. The system has 8 channels on UHF frequencies (around 446 MHz, 12.5 kHz channel spacing). The allowed transmitting transmission power is 500mW max, which gives a working range of up to two or three kilometers in good conditions. PMR446 radios use FM modulation (F3E) for audio. PMR446 radios have 0.5W ERP transmitting power and a fixed antenna on equipment (no external antenna allowed).

PMR446 radios are recommended (but not obligatory) to use CTCSS selective squelch system (sometimes called sub-channels). Most radios use CTCSS system with up to 38 channels (the number of supported tones and tone numbering can vary from manufacturer to manufacturer).

PMR446 related specifications are ETS 300 296 (RF) and ETS 300 297 (EMC). The channels for PMR446 are as follows:

  • Channel 1 - 446.00625 MHz
  • Channel 2 - 446.01875 MHz
  • Channel 3 - 446.03125 MHz
  • Channel 4 - 446.04375 MHz
  • Channel 5 - 446.05625 MHz
  • Channel 6 - 446.06875 MHz
  • Channel 7 - 446.08125 MHz
  • Channel 8 - 446.09375 MHz

PMR446 walkie-talkie radios are made by a variety of companies, including Motorola, Maxon, Kenwood, Goodmans, Icom, Maycom, Multicom, Cobra, Yeasu, Panasonic and others. Radios for use on this service are less expensive than conventional licenced equipment. The very cheapest are suitable for leisure use whilst the more expensive are ideal for professional business applications.

In most European countries you do not need a license, or pay any type of "user fees" or subscriptions. You simply purchase a radio, and batteries, and then you may immediately use the radio. Most PMR446 sets use either normal AA size cells, or the smaller (half the weight) AAA cells. There are also radios that feature a rechargeable battery pack.

When using this type of radios please note that PMR446 is not a cellular system or secure communications channel. All transmissions may be listened to by other PMR446 users, or those people with scanners. Please also note that PMR446 radios are only allowed to be used for voice communications. The typical coverage range of PHR446 system with 500 mW transmissionpower is around half kilometer to one kilometer. In very goodconditions (for example on open sea), the coverage of few kilometers is possible.

  • CTCSS? DCS? Sub-channels?    Rate this link
  • Delboy's PMR446 Radio Forum    Rate this link
  • Euro446 - This page gives you general information PMR446 radio system.    Rate this link
  • European Radiocommunications Committee on Exemption from Individual Licensing of PMR 446 equipment (ERC/DEC/(98)26)    Rate this link
  • Information on PMR446 Walkie Talkies - PMR446 is a European standard licence-free radio service.    Rate this link
  • Licence-Free PMR446 Radio - User perspective introduction to PMR446 radios.    Rate this link
  • PMR446 Personal Mobile Radio - This PMR446 website is an information guide dedicated to the subject of personal mobile radio. This page has also informationon using PMR446 server network for eQSO, forum engine from the people who brought you internet gateway linking for pmr446    Rate this link
  • PMR446 Personal Mobile Radio Reviews    Rate this link
  • PMR446 radio service - This document is an introduction to PMR446 system.    Rate this link
  • PMR446 Walkie Talkie Radio System - This is a general introduction document.    Rate this link
  • PMR446 Radio Modifications 1 - Telcom 100 Modulation and Squelch Adjustment, Telcom TE200 Squelch Modification, Binatone 100/Goodmans Tracker Mk1 external antenna connection, Alinco DJ446 Power Adjustment    Rate this link
  • PMR446 Radio Modifications 2 - TWINTALKER 3000 moduleation adn squelch adjustments, Alan 441 Squelch Adjustment, TEAM TECOM 446 modification for standard UHF handheld band (420 to 470 Mhz), Entel Euro-Wave squelch and modulation adjustment, AUDIOVOX PMR modulation adjustment    Rate this link
  • Simple Inductive Coupling - This document shows a simple inductive coupler used to improve reception on the PMR446 band.    Rate this link
  • The "naughty stuff" page... - PMR446 is ideal for UHF radio experiments, without needing an amateur radio license. This page has technical information on PMR446 radio system. This pages has also information on PMR446 antennas.    Rate this link

Family Radio Service (FRS)

Family Radio Service (FRS) is a very low power short range two-way radio service in the 460 MHz band in use in USA. FRS was created specifically for the use of families and small groups. This service allows the group to use a small, easy to use, and relatively inexpensive two-way radio for the purposes of voice communication between members of the group. FRS standard license-free radios are for sale to the general public.The Family Radio Service is a service developed for use by the general public at large. This service is not intended as a "hobby" service; and currently, usage reports indicate this is a typical trend. Users of FRS typically wish only to communicate with others of their own group. You do not need a license, or pay any type of "user fees" or subscriptions. You simply purchase a radio, and batteries, and then you may immediately use the radio. Family Radio Service walkie-talkies have 14 channels, use UHF frequencies and have a legally-limited transmission power of 500mW. FRS radios are legal to use only in the USA. The Federal Communications Commission (FCC) authorized Family Radio Service in 1996 as a short distance, unlicensed, two-way voice service for general purpose use. Family Radio Service is meant to be used for direct, personal voice communications among two or more people. FRS radios are personal two-way (send/receive) radios which conform to the FCC FRS specifications. In brief, they're an inexpensive and easy way to communicate with family and friends over short distances (under 2 miles). FRS radios offer 14 separate communications channels, and each channel can handle up to 38 separate conversations or "talk groups." Channel and talk groups are shared by FRS radio users on a "take turn" basis, and they cannot be assigned exclusively to any specific individual or organization. FRS Channel Frequency Assignments:

Channel 1: 462.5625mhzChannel 2: 462.5875mhzChannel 3: 462.6125mhzChannel 4: 462.6375mhzChannel 5: 462.6625mhzChannel 6: 462.6875mhzChannel 7: 462.7125mhzChannel 8: 467.5625mhzChannel 9: 467.5875mhzChannel 10: 467.6125mhzChannel 11: 467.6375mhzChannel 12: 467.6625mhzChannel 13: 467.6875mhzChannel 14: 467.7125mhz Notes: You may ONLY use FRS radios in the United States and Canada! All FRS units are compatible with one another in basic operation. Radios with CTCSS tones all use essentially the same tones. These tones are just in a different order. Many manufacturers advertise "privacy codes" on their radios. Wording "privacy code" is misleading, because all transmissions may be listened to by other FRS users (in channel monitor mode), or those people with scanners. Many radio models are manufactured for both European PMR446 and FRS in USA, and there are very few differences apart from cosmetic ones and channel frequencies.

Analogue Trunking Radio Systems

Use of analogue trunked radio systems began back in the mid 80's. In trunked radio system every radio on the system 'listens' on a control channel, that is a data transmission giving the radios all their instructions.When a call is received, or made, the controlling data transmission tells the radios who wish to speak to each other which channel they need to switch to. When speaking on their voice channel a normal 'talktrough' repeater is used to allow the sets to talk to each. This system allows efficient use of radio channels. A system of 12 repeaters and controller could support several hundred if not a thousand or more customers (not all of them need to be allocated their own frequencies). Simply put, trunking permits a large number of users to share a relatively small number of communication paths - or trunks. Commercial telephone communication is a wireline version of trunking. Equipment is available from many manufacturers as MPT 1343/1352 is a open standard. The analogue trunking system band is spilt into two parts, so that receiving and transmitting has differnt frequencies (usually 8 MHz difference).


TETRA is digital mobile radio technology that has been accepted throughout Europe. It is a standard defined by ETSI (European Telecommunications Standards Institute), and brings new features to mobile communications. It combines the features of mobile cellular telephones with fast data communications and the workgroup capabilities of PAMR and PMR.This system offers small handsets, up to 28.8kbit/s data rates, almost instantaneous call set up times, "press to talk" (PTT) capability, broadcast facilities and hand over between cells. TETRA uses TDMA (Time Divisional Multiple Access) technology at 410 - 430 MHz frequency range.TETRA offers fast call set-up time, addressing the critical needs of many user segments, excellent group communication support, Direct mode operation between radios, packet data and circuit data transfer services, frequency economy and excellent security features. TETRA uses Time Division Multiple Access (TDMA) technology with 4 user channels on one radio carrier and 25 kHz spacing between carriers. This makes it inherently efficient in the way that it uses the frequency spectrum. TETRA trunking facility provides a pooling of all radio channels which are then allocated on demand to individual users, in both voice and data modes.The new all digital civil Tetra (Trans European Trunked Radio) system operates in the band 410-415 MHz Portable Transmit and 420-425 MHz Base Transmit (it might be expanded in the future).For civil systems in Europe the frequency bands 410-430 MHz, 870-876 MHz / 915-921 MHz, 450-470 MHz, 385-390 MHz / 395-399,9 MHz, have been allocated for TETRA by the ERC Decision (96)04.For emergency systems in Europe the frequency bands 380-383 MHz and 390-393 MHz have been allocated for use by a single harmonized digital land mobile systems by the ERC Decision (96)01. Additionally, whole or appropriate parts of the bands 383-395 MHz and 393-395 MHz can be utilized should the bandwidth be required.

  • TETRA MoU - TErrestrial Trunked RAdio (TETRA) is an open digital standard defined by the European Telecommunications Standards Institute (ETSI). The TETRA Memorandum of Understanding (MoU) represents 85 organisations from 29 countries working with TETRA.    Rate this link

Walkie-talkie circuits

Many people constantly ask for walkie-talkie schematics, so here is some links on this topic. Building this kind of circuit need expertise in high frequency circuits and special equipment. The most probable outcome of your attempts is that you get tired of trying to make it working reliably or to work at all. Well-working walkie-talkie circuits are carefully designed radio circuits, even though some of them seem to be quite simple in construction. Best ones are complex circuits. I suggest that you choose the easy way and buy a ready-made walkie-talkie radios if you need this kind of device. It will be easier to make to work, works more reliably and is approved to use in your country. Home constructed ones will most propably work much poorer than commercial ones and are illegal to operate.

Amateur radio is a friendly, high-tech hobby that's got something fun for everyone! When you pass an exam, you an become an Amateur Radio operator--no matter what age, gender or physical ability. People from all walks of life pass their entry-level exam and earn their Amateur (ham) Radio license. They all share the diverse world of activities you can explore with ham radio. The rules for earning an Amateur Radio license vary depending on which country you live in.Many hams enjoy learning about radio technology, often though building their own equipment or antennas,Amateur Radio is a valuable, licensed radio Service that provides important communication in times of emergency. For example it has been proven that even during the power outages ham radio operators, using battery backup power, were able to help coordinate emergency workers while the cell phone networks were overloaded.

  • An amateur radio design tutorial    Rate this link
  • ARRLWeb - the American Radio Relay League's home on the World Wide Web!    Rate this link
  • AX.25 Amateur Packet-Radio Link-Layer Protocol    Rate this link
  • Baycom Modem - A BAYCOM Packet Radio modem is the cheapest way to get on the air, here is the circuit diagram of it    Rate this link
  • British Amateur Television Club (BATC) - non profit making video engineering club    Rate this link
  • eHam.net - Amateur Radio Community Site    Rate this link
  • Electronic projects and radio design links for the amateur radio and electronics hobby enthusiasts    Rate this link
  • Electronic projects by Roberto Barrios - radio programming cable for Kenwood TM-V7 and TM-G707, programming cable for Yaesu FT50, VX1 and VX5 , Luxman I Bus Interface, radio interface box for the Motorola Radius GP300    Rate this link
  • Finnish Made PMR, Trunking and Cellular (NMT) Phone Amateur Radio Conversion Pages - modification examples for Nokia, Mobira, Televa, Benefon etc.    Rate this link
  • Grounding Systems for Amateur Radio Stations - you need good grounding for both safety and good operation of radio equipments    Rate this link
  • HAM Intelligent Calculator (HAMIC) - a program designed to simplify a number of calculations commonly used by HAMs, it is designed for the HAM radio hobbyist, but may be useful to others as well. This software is available for a 30-day free trial.    Rate this link
  • Hamquest - an internet auction site created solely for the ham community    Rate this link
  • Ham Radio Online - articles and projects    Rate this link
  • Headset and PTT circuits for popular handheld transcievers    Rate this link
  • ICOM Microphone Notes - how to interface other than original microphones and headsets to it    Rate this link
  • Interface for SSTV using a Soundcard    Rate this link
  • Introduction to Packet Radio - presentation slide set    Rate this link
  • Linux HAM HOWTO    Rate this link
  • Meteor & Colorgramme - Radioastronomy Meteor scatter detection software, uses a small electronic interface    Rate this link
  • Mini-Circuits Application Notes - collection of RF component and circuit related application notes    Rate this link
  • Monitor Internet Amateur Radio Links    Rate this link
  • Moppeakatemia - This page has information how to modify old Nokia, Mobira, Benefon, AP, Ericsson/GE, Icom, Motorola, Philips, Storno etc. radios for amateur radio use. The information in this page is available in Finnish and partly in English.    Rate this link
  • Morse Code Beacon Keyer - This circuit sends morse code messages that are stored in an EPROM chip. The keyer can output either a one-shot message such as "CQ DX DE XXXX" if the start button is pushed, or it can run continuously by switching the free run switch on. The continuous mode is useful for making low power (QRP) beacons.    Rate this link
  • Morse Code on The Internet - With Morse code slowly disappearing off the air, there seems to be a growing number of people who carry out conversations in Morse over the internet. Several Windows and Linux clients using VoIP or special protocols, such as EchoLink, EchoLinux, MorseMail, CW Communicator or CWirc exist for Morse lovers worldwide to pound brass and make contact with one another.    Rate this link
  • Morse Code Practice Oscillator    Rate this link
  • Morse Code Buddy Model O-2 Code Practice Oscillator - The Code Buddy was featured as a construction article in the July 1996 issue of Nuts & Volts Magazine.    Rate this link
  • Narrow Bandwidth TeleVision - Ths document illustrates a simple but facinating electromechanical technique for transmitting a small image over amateur radio bands.    Rate this link
  • Online Radio & Electronics Cource - free on-line course orgnaized by Austrailian Amateur Radio Club    Rate this link
  • Packet Radio Linkpage    Rate this link
  • Peter Parker's Project Page - A collection of homebrew projects for the amateur radio enthusiast. A few projects include full constructional articles, but most have just schematic diagrams and a few notes.    Rate this link
  • PSK31 Home Page    Rate this link
  • Radio Interface Schematics    Rate this link
  • Repeater World HQ - Links, links, and more links - 10,000 links to ham radio sites!    Rate this link
  • Sound Card Interface with Tone Keyer - This is an improved version of the audio interface commonly used to connect a PC's soundcard to a transceiver's receive and transmit audio circuits for PSK31, SSTV, . The usual version of this type of interface (including the commercial "RigBlaster") requires the use of a serial port to provide PTT (push-to-talk) control for the radio's transmitter. This version includes an audio tone detector that keys the transmitter whenever transmit audio is generated by the application running on the PC.    Rate this link
  • Sound Card to Radio Interface Home Page    Rate this link
  • SSTV - Slow Scan Television - articles and web links    Rate this link
  • Suomen Radioamat??riliitto ry - The Finnish Amateur Radio League - SRAL    Rate this link
  • S??satelliittiharrastajan k?sikirja - weather satellite reception handlbook in Finnish    Rate this link
  • TAPR - an international organization dedicated to amateur digital communications    Rate this link
  • The Design Of The Brookdale ATV Repeater System Exciter - The ATV exciter described here uses a pair of Hamtronics model TA451 2-watt narrow-band FM voice transmitters to develop video and audio carriers on 439.250 MHz and 443.750 MHz.    Rate this link
  • The DXZone - over 4000 amateur radio links collection, with forums, free classified ads and product reviews    Rate this link
  • The Q code    Rate this link
  • Track and Solve Electrical Interference    Rate this link
  • Understanding Power Splitters - how they work, what parameters are critical, and how to select the best value for your application    Rate this link
  • Understanding Soundcard Interfacing - This page is an attempt to correlate the interfacing schemes for various Radio models, and Sound Card configurations.    Rate this link
  • Wireless Network Link Analysis - This set of tools analyze the link length available with different antennas and transmitters on different frequency bands. It calculates approximate received power levels and fade margins for wireless links.    Rate this link
  • VK2TIP Ian Purdie's radio design tutorial pages - for amateur, ham radio, electronic project enthusiasts as well as the electronic hobbyist    Rate this link
  • Welcome to Amateur Radio!    Rate this link
  • www.mods.com - www.mods.com is the biggest WWW sites on internet for tips, tricks, and modifications of HAM rigs, HAM modems, etc. Here you will find modifications of all types of HAM radios and HAM modems etc.    Rate this link
  • The G8GTZ Amateur radio and TV reference page - This page has lots of HAM radio and TV links.    Rate this link

An antenna is an RF component used to transform an RF signal, traveling on a conductor, into an airbourne wave and vice versa. Antennas are passive devices that radiate and pick up radio frequency energy (RF). Antennas are typically designed so that they work with the desired operation frequency, have a wanted radiation pattern and are matched to the cable connected to them (most often 50 ohm coaxial cable, can also be 75 ohm coax or 240-300 ohm flatline).

Antennas do not create RF energy. In transmitting applications antennas focus the energy in a pecific area or direction, which increases the signal strength in that direction or area. This is specified as Gain in units of dBi. An antenna with 0dBi gain is one which radiates in all directions equally. An antenna with 12dBi gain, has a direction in which the signal is 12db stronger than in another direction. In reception the antenna gain helps to the antenna to pick up signals from one direction stronger than from other directions. This directivity is very important if you need to receive weak signals in noisy environment.

Every antenna and every antenna feed-line have a characteristic impedance, or opposition to electrical current. In an ideal situation, the impedances of line and antenna match perfectly, and 100 percent of the electrical energy sent to the antenna is converted to radio energy and radiated into the atmosphere. In a less than ideal case, when the impedances aren't perfectly matched, some of the electrical energy sent to the antenna won't be converted to radio energy, but will be reflected back down the feed-line. The energy reflecting back from the antenna causes standing waves of electrical energy in the feed-line. The ratio of highest voltage on the line to lowest is the standing wave ratio. In the perfectly matched system, the SWR is 1:1. Typical radio equipment (transitters and receivers) are designed for 50 ohm impedance (many consumer radio receivers and TVs are designed for 75 ohms impedance). An ideal antenna solution has an impedance of 50 ohm all the way from the transceiver to the antenna, to get the best possible impedance match between transceiver, transmission line and antenna. Since ideal conditions do not exist in reality, the impedance in the antenna interface often must be compensated by means of a matching network, i.e. a net built with inductive and/or capacitive components. Antenna matching is essential in transmitting circuits. A poorly matched antenna connected to a transmitter means that some part of transmitting power does not get to the antenna, but is lost somewhere else, for example on radio equipment output stage (poor matching or missing antenna can lead to transmitter damages on high power transmitting systems). In receiving antennas poor impedance matching causes signal attenuation, meaning poorer radio reception.

To radiate efficiently, a transmitting antenna has to be resonant. If the antenna is not suitable for the transmitted frequencyand transmitter impedance, the result is very much reducedperformance and even a transmitter damage (usully with highpower transmitters). At first sight the radiation resistance of an antenna has no influence on the radiated power, as long as you match your transmitter to this resistance. But unfortunately the radiation resistance is not the only resistance that is consuming the transmitter power, there are also the loss resistances. These losses occur within the antenna (+ the antenna matching system) and in the environment of the antenna (ground, objects near the antenna). In receiving the antenna quality is not so critical if maximumperformance is not needed. If the antenna is not optimal, thereceived signal is just weaker than with optimal antenna. Antenna operation and coverage are the same whether the antenna is transmitting or receiving.

The oldest antenna structure is the dipole, or Hertz, which is usually fed by a transmission line at the antenna's center point. It is self-resonant at a length of one-half the operating wavelength, with an impedance of 72(ohm). Ideally, an antenna should be one-half the wavelength of the transmitting frequency. Maximum current flow at the center of the half wave, maximum voltage at the ends. The impedance at the center happens to work out at about 72 ohms, which matches standard 75 ohm coaxial cable very nicely. Thus, the half wave antenna is most usually broken into two equal quarter waves and fed by coaxial cable at the center. This type of antenna is known as a half wave dipole, and is the fundamental type by which the performance of other types of antenna are judged. Half wave dipole antenna is a single band antenna that offers 2dB of gain in a relatively narrow frequency range.

Slightly younger than the dipole is the monopole, also called the "whip", "quarter wave ground plane" or Marconi, antenna. It is constructed as a system where one leg of a half wave dipole is replaced by a sheet of metal at right angles (this acts like a reflector). The monopole is a vertical dipole; however, the phantom reflection of a conductive ground plane underneath the antenna replaces one leg of the dipole. This antenna is one-fourth-wavelength long, and its impedance is 36(ohm), one-half that of a dipole. The roof or trunk of a car, or body of a walkie talkie acts as a good reflector. The feed impedance of a quarter wave ground plane is around 40 ohms, sufficiently close to 50 ohm coaxial cable to form a potential match. This antenna has theoretically circular azimuth radiation pattern. Unfortunately, the ideal full conductive plane under the antenna usually is nonexistent or erratic. The actual azimuth pattern, thus, depends heavily on installation and use, in contrast to its theoretical circular pattern. The elevation radiation angle is also a function of the ground-plane situation and antenna's height above ground.

The third type of antenna, the loop, can be rectangular or circular and resonates at a perimeter length of one wavelength; it is fed by simply breaking anywhere into the loop. Although loops are often mechanically difficult to support at long wavelengths, they are practical when frequencies get up to hundreds of Mhz.

Discone Antenna is a relative of the 1/4 wave ground plane antenna optimized for wide frequency bandwidth reception. It typically offers 0dB of gain, on frequencies from about 120-1300 MHz, and with a vertical element on top, it is usable down to about 30 MHz. Gain is achieved by compressing the radiation pattern into a donut shape with little of the signal radiating upwards or downwards, concentrating the pattern perpendicular to the vertical axis of the antenna. This antenna type is called a discone because it is comprised of two parts, the disc, a group of elements parallel to the ground around the top, and the cone, the diagonal radial elements around the bottom. These could be made from a solid metal disc and a cone shaped sheet metal radial.

There are also many other antenna constructions. Many of the more complicated antennas are antennas that have more controlled directivity than those simple basic antennas. Directional antennas are used for example for point-to-point communications applications, cellular base stations and TV signal reception. Those antenna consist typically of a large number of antenna segments placed at suitable distance from each other. Quater wave length segments are very common and useful. The most well known antennas of this kind of are Yagi and Log Periodic antennas. The most useful feature of this kind of beam antenna in reception, is that the can be rotated to null out a signal you do not want or maximizing the one you do want. In transmitting applications you can point your signal to where you want to send it.

Yagi antenna is named after it's inventors Mr Yagi and Mr Uda. Yagi antenna is a single band antenna that offers typically 10-20dB of gain and 10-30dB of front-to-back isolation in a relatively narrow frequency range. A yagi antenna is built out of a group of dipoles all the same length, connected to a boom, to hold them a specific distance apart. It offers excellent gain, and front-to-back isolation, and a narrow beam width that it will receive from. The gain is determined by how many elements are used as directors, and is achieved by limiting how many directions a signal can be received from. The down side is, it will only have gain in a narrow frequency range of about +/-1% of the center frequency. Yagi antenna is most commonly used by commercial and amateur operators, since it is an inexpensive and very efficient type of antenna for single band.

Log Periodic Antennas are remarkable antennas that exhibite relatively uniform input impedances and radiation characteristics over a wide range of frequencies. Log-periodic (LP) antenna is a broadband, multielement, unidirectional, narrow-beam antenna that has impedance and radiation characteristics that are regularly repetitive as a logarithmic function of the excitation frequency. The length and spacing of the elements of a log-periodic antenna increase logarithmically from one end to the other. The Logarithmicly Periodic Dipole Array (LPDA) is a beam antenna optimized for wide frequency bandwidth. It offers 5-15dB of gain with a moderate 10-15dB of front-to back ratio; the beam width is fairly wide when compared to a Yagi. It is a group of dipoles of decreasing size (with the longest in back and the smallest in front), connected to a boom, to hold them a specific distance apart. The tapering of the elements is what gives it the wide frequency range, by always providing an element that resonates near the frequency that your operating on. It is most commonly used in TV antennas, where operation on many frequencies is required.

Thare are also antenna types that can be integrated easily to circuit board. The patch antenna is a conducting surface separated from an underlying ground plane by a dielectric; a double-sided circuit board often works as a dielectric. Each edge is one-half wavelength at resonance, or you can use a circular patch with a radius of 0.3[lambda]. You feed the antenna through a small hole in the ground plane.

Antennas in mobile applications are often smaller than the free-space or ideal-ground self-resonant dimensions indicate. In addition, the antenna is near other electronic circuitry, a user's body, an enclosure, power circuitry, and structures. Fortunately, antennas that are smaller than resonant size can still be effective radiators or energy receivers. Pagers, for example, use loop antennas that are about (1/10)[lambda]. However, the impedance-matching circuitry between the antenna and the power amplifier or front end causes losses and, thus, wasted power, reduced coverage, or weaker received-signal strength.

TV antennas are antennas that are optimized for the TV bands reception. If you look closely at a TV antenna you will notice that the taper of the elements is not uniform. There will be several long ones (Chan 2-6 at 54-88MHz) then several medium long ones, usually interspersed with the long ones (Chan 7-13 at 175-216MHz), and then a bunch of short ones, all the same length (UHF 470-812MHz). UHF elements on a TV antenna are almost alwasys a Yagi design, and the reception range that they advertise is only on one channel or few TV channels. There are also antennas with wider response. A typical 4-bay bow tie, it has about 6dB of gain, a 15dB front-to-back ratio and resonates across a wide frequency range. Nowadays there are also quite good wideband TV antennas that use Logarithmicly Periodic Dipole Array (LPDA) design. Broad band LPDA TV antennas are always optimized only TV frequencies, and do not typically receive other frequencies well.

It's relatively easy to build an antenna that covers one specific frequency. It's a lot harder to make one that covers a wide range of frequencies well. There are also special antenna constructions for special applications. When you need to flood a wide but defined area with RF energy, such as for perimeter security systems, tunnels, and cellular- or 802.11-system interior zones, one approach is to use an RF-leaky feeder cable to provide controlled radiation.

Ideal free-space antennas have a purely resistive impedance. Smaller antennas usually have a lower resistive component to their impedance, and most part capacitance and/or inductance. For example, an antenna with several ohms of resistance, fed by matching circuitry with a comparable resistance, wastes half the transmitted or received power in the matching circuitry. The lower antenna resistance causes higher antenna currents and ohmic losses through matching components. Short dipoles and monopoles have a capacitive impedance. Therefore, the matching circuitry that transforms the antenna's complex impedance into an apparent resistance must introduce inductance to compensate. You implement this inductive loading in monopoles as a discrete wire coil at the antenna base, a coiling of the antenna whip at its base, or a continuously wound helix around a flexible core?the common, rugged, bendable "rubber-duck" antenna. Most pagers and wireless wands use loop antennas. Unlike the dipole and monopole, the smaller-than-resonant loop antenna is inductive and needs capacitive compensation to yield the resistive result.

Impedance matching is necessary to keep the VSWR low enough for your application. Relatively low-power mobile units can often accept VSWR values as high as 1.5 or 2, although higher power base-station transmitters usually need VSWRs lower than 1.5 to prevent output-stage damage. You should also filter the transmitted RF signal to minimize interference and intermodulation.

A good general rule for antennas is as big as practical, as high aspractical, as clear of obstructions as practical, and watch out forpower lines.There's really no substitute for a decent rooftop antenna on TV and radio reception. When installing and using antennas that are outside, please pay attention to a proper lightning protection. At basic this is a good grounding of all metal parts in the antenna with a grounding system that can survive lightning strike. In addition ther could be need to have some overvoltage protectors on the antenna lines (if you need those or not can vary depending on the enviroment and value of equipment connected to antenna).

The cabling between antenna and the transmitting/receving equipment cause also losses. Those need to be taken into consideration when designing the antenna positioning. It doesn't matter how good your antenna is, if you are feeding it with lossy coaxial cable. The loss that a coax has, is determined by many factors, most having to do with the density and effectiveness of the shield and the dielectric, and the length of the cable. Frequency is the other major contributing factor in determining your losses. The higher the frequency, the higher the loss. Here is a chart of some common 50 ohm coax and their loss at different frequencies for comparison:

Losses in dB per 100 feet (30m)
50MHz 100MHz 500MHz 900MHz
---------- ----------- ----------- -----------
RG-58A/U 3.3 4.9 13.3 20.0
RG-8/U 1.2 1.8 4.7 6.7
Belden 9913 0.9 1.4 2.9 4.2
1/2" Heliax 0.56 0.83 2.0 2.8
The losses scale proportionally with length. Half as long, half the loss in dB. Double the length causes double the loss.

An antenna system needs to be correctly constructed to work well. If you have an antenna system that once worked well, but is not working well anymore, here are few tips to find and fix receiving antenna problems (most tips apply also to transmitting antennas as well). First visually inspect every inch of you antenna system. Look for loose connections, corrosion, cut or burnt cable, cracked insulation, foreign metallic objects or birds nests on the antenna, bent antenna elements, antenna aiming, and problems with splitters in line. Next unhook the cable at the antenna and place a short across it. The measured resistance should be "low" depending on cable length. Then remove the sort and measure again. Now the resistance should be high. If you are using a coaxial line (as opposed to twinlead) check the balun at the antenna... or just replace it. they don't cost much. Look for a "blob" inline near the antenna that might be an inline amplifier, check that is is working correctly and getting the operating power it needs (could be separate powerinc cable or powered through the antenna coax cable). If the antennas you have are many years old, consider replacing the antenna, because many cheap typical consumer antennas just don't seem to hold many years in hard environment.


  • FCC Interference Handbook - This electronic version of our "Interference to Home Electronic Entertainment Equipment Handbook" is provided as a service to our customers on the World Wide Web by the Compliance and Information Bureau of the FCC.    Rate this link
  • It was a stormy night... The importance of grounding - This article tells about the importance of proper grounding of antennas.    Rate this link
  • About TV and FM Antennas - TV and FM Antenna Tips, FAQ, Reception Help, Interference and Amplifier Guide, etc.    Rate this link
  • A new look at the Ufer ground system - Mr. Ufer developed the concept of concrete encased grounding electrodes which are suitable for radio antennas    Rate this link
  • Antenna Info - basics of antenas and most commonly used antenna types    Rate this link
  • Antenna information - information on dipole and GP transmitting antennas    Rate this link
  • Antennas - some basic information and resource links    Rate this link
  • Antennas for Ham Transmitters - This document describes how to construct various type of antenna for Ham Radio Transmitters.    Rate this link
  • Antennas, some rules of thumb for beginners - Every now and then somebody asks for antenna suggestions. Quite often these people asking about antennas are beginners who are afraid of making the wrong choice - a contibution on antennas by James R. Duffey KK6MC/5 aka Dr. Megacycle.    Rate this link
  • Antennas: critical links in the wireless signal chain - right antennas can strengthen the chain by yielding better signal coverage, increased S/N ratio, reduced bit error rate, and lower power consumption all at very low cost    Rate this link
  • Basics of Dual-Polarized Antennas Tutorial - information on special antennas used in celluar networks    Rate this link
  • Erilaiset antennit - Basic information on different antennas in Finnish.    Rate this link
  • Exploring the secrets of the Smith chart - an indispensable tool    Rate this link
  • FM Antenna Configuration vs Performance - White paper from Harris Broadcast Communications Division    Rate this link
  • Good ground, great signals - how to do geounding of antennas well    Rate this link
  • Guide to Scanner Antennas - There have been many questions regarding how to select an antenna for scanning. This is a Readers Digest version of antennas, meant to give new users some idea of the different antennas and their good and bad points. At the end, there are some specific recommendations on how to build a general purpose VHF/UHF antenna system.    Rate this link
  • HAM antenna documents and programs    Rate this link
  • How does a CB radio antenna work?    Rate this link
  • Smith Chart Basics - The Smith chart appeared in 1939 (Ref. 1) as a graph-based method of simplifying the complex math (that is, calculations involving variables of the form x + jy) needed to describe the characteristics of microwave components. Although calculators and computers can now make short work of the problems the Smith chart was designed to solve, the Smith chart, like other graphical calculation aids (Ref. 2), remains a valuable tool.    Rate this link
  • Install your antenna properly    Rate this link
  • Jim's Notebook - all sorts of interesting and useful antenna data, techniques, hints and such    Rate this link
  • Near field or far field? - How do we define the far field of an antenna system, and what criteria define the boundary between it and the near field? The answer depends on your perspective and your design's tolerances.    Rate this link
  • Pasternack Connector Identifier - pictures to help to identify different coaxial cable connector types and impedances    Rate this link
  • Radiation impedances of wire and rod antennas    Rate this link
  • Smart Antenna Design: Antenna Diversity    Rate this link
  • Smart Antenna Systems Tutorial - A smart antenna system combines multiple antenna elements with a signal-processing capability to optimize its radiation and/or reception pattern automatically in response to the signal environment    Rate this link
  • Up On The Roooof - tips for installing FM and TV antennas to roof    Rate this link
  • Why an Antenna Radiates - You don't have to know how an antenna works to use one, but getting a handle on this subject can deepen your understanding of radio. Here's a searching look at the mysterious process by which our antennas hurl energy from Here to There.    Rate this link
  • Which HAM Radio Antenna is the Best Choice for Point to Point Communication?    Rate this link
  • Antenna SWR Tuning - The SWR, or Standing Wave Ratio, of an antenna is a measure of how efficiently your radio is radiating the energy it produces when you transmit.    Rate this link

Antenna cabling issues

Even the best antenna and the most expensive receiver will not produce an acceptable output (audio or picture) if the transmission line has not been carefully selected and correctly installed. The transmission line from antenna to receiver is more important than most people realize. Proper transmission line from transmitter to receiver is also important. There are three basic types of transmission line used for antenna connection: 300 ohm twinlead, 75 ohm coaxial cable and 50 ohm coaxial cable. 300 ohm twinlead and 75 ohm coaxial cable are typically used for antenna connections in consumer TV and radio reception application. 300 ohm twin-lead has a characteristic impedence which allows the signal to be best transfered from the 300ohm antenna to the 300 ohm input connections on the TV (on those TVs that has those). Using a different cable could reduce the signal level but it may not be a factor if you have a high signal strength. 75 ohm coaxial cables are typically very low loss coaxial cables that are used to transport signals from antenna to TV in applications where shielded cable is needed and the signal input is matched to 75 ohms (usually the antenna itself has different than 75 ohm impedance, and it is matched to 75 ohm cable impedance with suitable matchign network/balun built into antenna). Common antenna network wiring is typically built using 75 ohm coaxial cable and coaxial antenna signal inputs on TVs and FM radio receivers are matched to 75 ohms. The 50 ohm coaxial cable is the type used on on radio transmission applications, and the most often used coaxial cable type in professional radio applications. You will see 50 ohm coaxial cable in almost all radio transmitters, cellular phone antenna cabling, radiophone antenna wiring, etc.

  • Microwave connector reference - This document has pictures and some basic information on BNC, TNC, N, UHF, C. SMA, SMB, SMC and APC-7.    Rate this link
  • Most Often Asked Questions About Power Splitter / Combiners    Rate this link
  • Putting a Balun and a Tuner Together - A balun is a transformer that converts an AC signal from balanced to unbalanced, or vice versa. This document has information on antenna baluns and their use.    Rate this link
  • Quarter-Wave Power Divider - use this to phase two antennas together for increased gain    Rate this link
  • RF Directional Couplers - The equations that describe the performance of transformer based directional couplers are derived. The best theoretical performance available from a directional coupler, using ideal transformers, is a function of the turns ratio, and the terminating impedances. At VHF and UHF frequencies, wire gauge and core material can be chosen to closely approximate the response based on the solution of these equations.    Rate this link
  • Signal Cabling - quite often the humble feed line is to blame if the communication does not work    Rate this link
  • The purpose of a balun - information on benefits of balusn in eadio reception    Rate this link
  • Weatherproofing Andrews Connectors - Andrew connectors are designed to be pretty weatherproof by using tight tolerances and O-ring gaskets in a couple of places. It is ALWAYS a good idea to weatherproof any connector joint; in fact, it is done practically 100% of the time by professional installers.    Rate this link
  • Coaxial Cable -- The Neglected Link - Is a better grade of coaxial cable worth the price difference? This analysis of the importance of shielding in coax lines explains why the answer is "Yes!"    Rate this link
  • Selecting and Installing Transmission Line - Transmission line or downlead, is the wire that carries the signal from the antenna output terminals to the receiver input terminals. Even the best antenna and the most expensive receiver will not produce an acceptable picture if the transmission line has not been carefully selected and correctly installed. The transmission line is more important than most people realize. Color television reception is sensitive and highly susceptible to interference from many different sources. Transmission line that is carefully chosen and neatly run by an installer who knows what he is doing will reward the customer with clear, distortion free color TV reception.    Rate this link
  • Transmission Lines    Rate this link

Antenna building projects

  • 2304 Tech List - Many radio atenna building project documents on-line. Pages have also many radio equipment plans on-line.    Rate this link
  • Antenna extension provides open-door policy - garage door opener receiving antenna for 315 MHz    Rate this link
  • Antennas for 136kHz - The main subject will be transmitting antennas for 136kHz as this often is the most important part of a longwave amateur radio station.    Rate this link
  • Antenni ns. kettujahtiin ja radiosuunnistukseen - This is plan for 27 MHz directional loop antenna. The text of this document is in Finnish.    Rate this link
  • A UHF Discone Antenna for scanners - The availability of scanners and wideband receivers covering the upper UHF spectrum above 800 Mhz necessitates reasonably priced, wide bandwidth, effective antennas. The most commonly used arrangements for wideband reception on scanners using an outdoor antenna system are multiband trap antennas, a discone, or a simple ground plane antenna.    Rate this link
  • Balloon or kite antenna? Why not    Rate this link
  • Build a Yaki-Uda Antenna - instructions for building and designing 3 and 6 element yagi antennas    Rate this link
  • Build a UHF Discone Antenna    Rate this link
  • Cheap Yagi Antennas for VHF/UHF - The antennas described in this article are relatively small, easily constructed from common materials/tools and have surprising performance. No baluns or gamma matches are used in this design. This simplified feed uses the structure of the antenna itself for impedance matching. You can build these antennas for ! There are plans for 144, 222, 421.25, 432, 435, 450, 902, 903 and 1296 MHz.    Rate this link
  • DIY GPS Antenna - antenna for 1.5 GHz band for GPS receiver    Rate this link
  • Down To Earth Antennas - reception antennas which work close to earth    Rate this link
  • Easy-Up Beverages - A Beverage antenna is a very giid antenna type for mediumwave DX use.    Rate this link
  • Go ATV Portable With This Simple Beam Antenna! - a simple broadband yagi for 70-cm band    Rate this link
  • How to build a Corner Reflector Antenna: Corner reflector antennas for UHF reception and transmission    Rate this link
  • How To Build A Tin Can Waveguide Antenna for 802.11b Wireless Networks or other 2.4GHz Applications - Got no dough for a commercial antenna? Looking for an inexpensive way to increase the range of your wireless network? A tin can waveguide antenna, or Cantenna, may be just the ticket. This design can be build for under U.S. and reuses a food, juice, or other tin can.    Rate this link
  • How To Make A Simple Ground Plane Antenna - A ground plane antenna is a very simple antenna. It is a quarter wave vertical radiator and four radials that simulate a ground for the radiator. It directly matches to 50 ohm coax and should show a VSWR of 1.5:1 or better at its center frequency. It is useful from HF through UHF, although it tends to be large below 30 MHz.    Rate this link
  • Hybrid quad reflector array with approx. 6 dBd gain    Rate this link
  • Long Loopstick Antenna - to improve AM radio reception without using a long wire or ground    Rate this link
  • Practical indoor antennas for everybody - HF foil antennas, VHF/UHF discones, wire beams, VHF/UHF rotables, tape antennas and telescoping VHF/UHF indoor beams    Rate this link
  • Radial Wire Antennas - The Radial Wire aerial is simply a length of wire laid on the ground in a straight line. Radial Wire aerials are a very simple and easy way of obtaining amazingly good radio reception on longwave, mediumwave and the lower shortwave bands.    Rate this link
  • Satellite Antennas Hints and Kinks: Low Cost Satellite Antennas    Rate this link
  • Simple dipole antenna for 88-108 MHz FM Reception    Rate this link
  • Small, 915-MHz antenna beats monopole - This antenna is effective and compact (smaller than 3-in monopole).    Rate this link
  • Low Noise Antenna Using the Universal Magnetic Balun and the Antenna Feeder Isolator - In most receiver installations the feeder is connected to mains earth at the receiver. This can cause mains borne noise to enter into the receiver because either the feeder/mains earth is the return path for the antenna or simply by noise being coupled to the antenna by the feeder. Using the Isolator will ensure that mains borne noise cannot enter the feeder and couple noise to the antenna either directly or by the feeder radiation. The Isolator can reduce noise at low frequencies by up to 40dB.    Rate this link
  • Easy Homemade 2.4 Ghz Omni Antenna - An easy step-by-step guide go making a homemade wireless antenna, for a fraction of the cost of commercial antenna. Uses readily available parts, and requires no specialist tools or knowledge. Or in geek speak - a diy homebrew omnidirectional colinear dipole design suitable for 802.11 wifi compatible hardware with external antenna connector.    Rate this link
  • Musings on the Miracle Whip design - The whip described here will tune up to a respectable SWR on 40 metres and up.    Rate this link

Active antennas

  • Active Antenna II - This circuit is designed to be used on receivers that use untuned wire antennas, such as inexpensive units and car radios.    Rate this link
  • Active Antenna III - This circuit is designed to be used on receivers that use untuned wire antennas, such as inexpensive units and car radios.    Rate this link
  • AM/FM/SW active antenna - an active antenna that can be used for AM, FM, and shortwave, on the shortwave band this active antenna is comparable to a 20 to 30 foot wire antenna    Rate this link

Antenna amplifiers

Antenna amplifies can work in helping weak radio signal reception - within their limits. Most TV/FM boosters are simple, broadband VHF amplifiers. They provide an extra amplification stage for the tuner. This kind of amplifiers amplify anything entering to them that is within their operation frequency range. This means that they amplify the signal, but they will also amplify the noise. Most designs have pretty good noise characteristics, but they may belacking in other areas. In particular, some are easily overloaded by stronglocal signals (e.g. TV stations and public service band stations caninterfere). When this happens, the FM signals can become badly distorted.If you are subject to multipath problems, the booster make make them worse.The first step to improving radio reception is always to check, and possiblyimprove, the antenna. Make sure that your antenna is properly connected toyour tuner and that the feedline impedance matches the tuner antenna imputimpedance. Make sure the feedline is properly connected to the antenna. Ifyou do not have a good outdoor antenna, get one. I have never seen abooster help a simple, indoor antenna.Radio receivers and tuners vary a lot in sensitivity. Some work better with weak signals than some other. For some less sesitive radios, extra amplifier can be very helpful. If you have a very snsitive radio, you might not benefit at all from the antenna amplifier. Besides pure signal amplification need, the antenna amplifiers are often used to compensate the cable losses from the antenna signal source to receiver that long cables can cause. In this kind of applications, it is the best idea to put the amplifier as near the signal source as possible to get best results.

  • FM-Band Preamplifier - Here is a high performance RF amplifier for the FM band which can be successfully built without any special test equipment. The grounded-gate configuration is inherently stable without any neutralization if reasonably good layout techniques are employed. The output transformer is designed to resonate with the FET's drain capacitance at about 92 MHz giving the amplifier the highest gain at the low end of the band where the weaker stations operate. No tuning capacitor is needed as long as the transformer is built precisely as described. The performance of the amplifier is quite good. The noise figure is below 2 dB and the gain is over 12dB.    Rate this link
  • Hacking The Original 915 MHz WaveLAN (NCR 915 MHz WaveLAN 2 Mbps DSSS) - amplifier circuit, datasheets, antenna designs, etc.    Rate this link
  • UHF Preamplifier - This circuit is designed to work at UHF frequencies in the range 450-800MHz. It has a gain of around 10dB and is suitable for boosting weak TV signals.    Rate this link

Antenna adapters and couplers

An RF isolator keeps reflected power from returning to the transmitter output and keeps other signals from getting into the transmitter. A typical RF isolator is a three-port device with an input port, an output port and a load port (can be treated as black box).

The normal direction of RF flow is into the input port, through the isolator, and out of the output port. Any RF signal entering the output port is directed through the isolator to the load port where it is dissipated as heat in the dummy load. When used like this an isolator is sort of an RF lobster trap. Unidirectional, it isolates a source and load so that any reflected energy at the load is trapped or dissipated. Isolators typically provide around 30 dB or so isolation. Isolators can be connected in cascade to achieve a higher degree of isolation.

Heat is an enemy of the isolator. Don't exceed the power input rating of the isolator. It is important that the power rating of the dummy load be sufficient to handle a significant level of reflected power. As a rule of thumb, the power rating of the dummy load is chosen to be about 50% of the transmitter power output rating. You will see dummy loads that are attached directly to the isolator and those that are mounted away from the isolator. Dummy loads that are mounted away from the isolator tend to reduce the amount of heat transferred to the isolator in cases of high reflected power.

Normally, the isolator is used immediately at the output of a transmitter. By using an isolator on the output of a transmitter, many interference problems can be avoided, especially at densely populated transmitter sites. An isolator's primary function is to prevent signals from nearby transmitters from entering its transmitter's final stage where they might mix with the transmitter signal or harmonics to produce strong intermodulation products. Such intermod products might cause interference to collocated receivers. A secondary function of the isolator is to provide the transmitter output with a constant 50 ohm load impedance. Because the isolator dumps off any reflected power to the dummy load, the transmitter sees a near-perfect load impedance.

A large percentage of communications facilities use shielded cable grounded at both ends or coax cable to interconnect voice and data circuits to a radio or multiplex channels. This cable often carries current derived from the AM broadcast station's signal--current that will modulate your signal and that represents a form of interference. There are times when the signal from the local AM broadcast station gets into your station equipment. You don't want it for reasons other than not liking the program content. Communications towers at a height nearly equal to the quarter wavelength of the AM station frequency not only influence the radiation pattern of the broadcast station, but also provide considerable energy in the ground return of the tower. Multipoint grounding, or the "ground everything" concept, will not eliminate this problem, it will merely reduce it to some lesser level. One of the simplest methods of interference reduction is to isolate the affected equipment from these ground currents. Suitable dc/RF isolator consists of two tightly coupled tuned circuits or special RF transformers. They will pass the RF signal, but will block DC and other RF frequencies on the cable shield. The object of this procedure is to dc/RF-isolate all transmission lines from the equipment by not allowing current supplied by the tower to return to ground through the baseband cable, audio lines, data ports and the like.

Antenna tuner is a device designed to transform the impedance at the feed line input to a value that your transceiver can handle (typically 50 Ohm). In practical terms, all a tuner does is act as a kind of adjustable impedance transformer between the antenna system and the radio. It takes whatever impedance the antenna system presents and attempts to convert it to 50 Ohm (or something reasonably close to that value) for the transceiver. When the transceiver "sees" a 50 Ohm impedance, it is able to load its maximum RF output into the system. That power is transferred through the antenna tuner, to the feed line and, ultimately, to the antenna (minus any losses incurred along the way). Most antenna tuners have an inductance rotary switch and two capacitors. The capacitors are often labeled ANTENNA and TRANSMITTER. In some antenna tuners the inductance switch is replaced with a continuously variable inductance, popularly known as a roller inductor.

  • The Care and Feeding of the R.F. Isolator - The application of suitable R.F. isolators with complementary filtering devices now becomes more and more important as the number of receivers and transmitters on one site increase all the time. The purpose of this bulletin is to review the characteristics of R.F. isolators and their operation and discuss the ways in which these devices are applied to control interference due to intermodulation products and to provide other benefits. A better understanding of R.F. Isolators, their benefits, limitations and short comings and the best ways to employ them will be covered in this bulletin.    Rate this link
  • A Happy Tune: Tiny ATUs for the Trail - A tiny homemade QRP antenna tuning unit - ATU or transmatch - is inexpensive to build and is easily customized for special needs.    Rate this link
  • Simple Inductive Coupling - This document shows a simple inductive coupler used to improve reception on the PMR446 band. A short length of coax cable, some coated copper wire, a PL connector (Male or female), solder and a soldering iron.    Rate this link
  • RF isolation, cheap and easy Isolation may be best for eliminating AM broadcast signals and induced electrical hum in some communications base station - There are times when the signal from the local AM broadcast station gets into your station equipment. You don't want it for reasons other than not liking the program content. Communications towers at a height nearly equal to the quarter wavelength of the AM station frequency not only influence the radiation pattern of the broadcast station, but also provide considerable energy in the ground return of the tower.    Rate this link
  • Practical notes on the ferrite isolator - An isolator is sort of an RF lobster trap. Unidirectional, it isolates a source and load so that any reflected energy at the load is trapped or dissipated.    Rate this link
  • The isolator: An RF traffic director - An RF isolator keeps reflected power from returning to the transmitter output and keeps other signals from getting into the transmitter output. It also provides a good impedance match to the transmitter output. This article describes the basic RF isolator and how it is used.    Rate this link
  • Antenna Tuner Operation - In practical terms, all a tuner does is act as a kind of adjustable impedance transformer between the antenna system and the radio.    Rate this link
  • What is an antenna tuner?    Rate this link

Antenna baluns

Baluns are sonverters which convert the unblanced antenna signals from coaxial cable to a blaanced format suitable for antenna types which need balanced signals (for example dipole antennas). Besides this conversion the baluns will quite often do some form of impedance conversion in the process. Different blauns are needed in different applications. In some applications only balanced-unbalanced conversion is needed, while in some oother applications also impedance conversion is needed. From a technology viewpoint, alot of baluns are also based upon 1:1transformations (in differing configurations.) Another popular balun with antennas is 4:1 balun. A typical 4 to 1 balun acts as a center tapped auto transformer. Theunbalanced signal uses one end of the winding and the grounded centertap, while the balanced signal uses the ends of the winding. Usingcoax or twisted pair transmission line for the windings is a way toget very good coupling and wide bandwidth. If the transmission linewinding is cut to the right length, it can make a narrower band balunwith no magnetic core. Sometimes 4:1 balun configuration is built witgh with a natural 1:1 impedance transformer wired in the right way.

Impedance matching

Antenna filters

Signal attenuators

Signal splitters and combiners

Antenna related measuring equipments

Other antenna related circuits

In the United States, the FCC (Federal Communications Commission) decides who is able to use which frequencies for which purposes, and it issues licenses to stations for specific frequencies. Other countries has their own regulating organizations. It is the operator's responsibility to run the transmitter in accordance with the frequency regulating authority of their country.

General broadcasting links

FM radio broadcasting

The most commonly used radio broadcasts are the FM radio broadcasts operating at around 88-108 MHz frequency band. FM broadcasting offers around 50 Hz to 15 kHz bandwidth with stereo sound. The deviation used in FM broadcast is 75 kHz (peak deviation). Typical transmitter power for FM station can be from 10W up to 100 kW.Those broadcasts were originally mono broadcasts, but were later converted to stereo. The stereo broacasts use special technique to be compatible with mono receivers. The reason for this is that in order to avoid the wrath of all the ownersof monaural FM sets, the FCC in its wisdom decreed that a "compatible"system would be necessary before they would approve FM stereo. Theengineers quickly noted that the A+B signal from two microphones gives apassable monaural signal (especially if "one-point" miking is used).Now the problem was how to get A and B out of A+B. To do this the (A-B) signal is also needed. The way to do the stereo to send (A-B) in some clever wayand the receiver could reconstruct A and B by "matrixing". The methodadopted was to "multiplex" this (A-B) signal onto the main carrier byusing it to modulate a SUBcarrier located at 38 KHz. Double-SidebandSuppressed Carrier (DSBSC) modulation was chosen. This gave a "lowersideband" extending downward from 38KHz (less 20Hz or so) to 23KHz(because the highs were cut off at 15 KHz.) A "pilot carrier" was putat 19 KHz which allows the receiver to recover the precise frequencyphase of the 38 KHz carrier so that recovery of the (A-B) signal couldproceed. The compatiblity was good, because pilot at 19KHz and A-B from 23 to 38 kHz are so high in frequency that most people wouldn't hear them, andmost older monaural FM sets and loudspeakers won't reproduce themaudibly anyway. In addition to the stereo broadcasts, some radio stations also transmit extra SCA audio (for example to transmit MUZAK) with their broadcasts. This audio is transmitted usign FM broascast at 67 kHz carrier (uses 53 to 81 kHz frequency range). The ratings of FM transmitter, antenna, radio-frequency cable, and tower depend upon the desired coverage and its geography. Ordinary FM receivers can catch signals on limited coverage ares. Sophisticated ones can also receive the broadcasts from longer distance.Several test instruments are essential in an FM station. Most critical are spectrum analyser, audio analyser, FM demodulator, field strength meter, RF power meter, oscilloscope, multimeter and programme amplifier. A backup power system (usually generator) is necessary to keep the station working when normal supply fails. Modern FM radio stations usually nowadays heavily process the audio before they transmit it. Typical processing is the FM broadcasting following: Audio Amplifier adjusts level of input signals from left and right channels torequired intensity. Usually this stage includes nowadays some some form of automated signal level control, compression and/or limiting. Stereo Coder converts the left and the right channel signals into .L+R. and .L-R. elements. It multiplexes them with a synchronising pilot-signal of 19 KHz. It can also combine signals of traffic radio, radio data system, or Subsidiary Communications Authorisation channels. Modulator superimposes the signal on Carrier Frequency. Synthesiser can set transmitter's Rated Frequency in steps (usually 10 KHz steps over the entire range 88-108 MHz). It synchronises the signal with Reference Frequency (from stabilized cystal source). Audio Amplifier, Stereo Coder, Modulator and Synthesiser together make up the Exciter portion of an FM transmitter. Power Amplifier intakes a weak signal from exciter and amplify it to the high power that is sent to the transmitting antenna. A normal FM station transmits at power of hundreds or thousands of watts. There are strong incentives for music to be processed in such a waythat it's "louder" and more attention-getting, when played over FMradio. There are a bunch of reasons for this, having to do withcompetition between stations (the station with the loudest-soundingsignal is believed to have a better chance of "grabbing" a listenerthan one with a quieter-sounding signal) and the conditions underwhich popular music is often played back (in cars, boomboxes, Walkman-and MP3-players with cheap headphones, etc. in conditions with highambient noise levels). Most popular-music FM station directors feelthat it's to their advantage to squash the &^%$ out of the music thatthey broadcast.Usually the music dynamics are squashed downinto a couple of dB of actual dynamics when they are broadcasted to FM radio stations. Automatic dynamics controlling and compressing equipment also help the people working at the radio studio: you don't have to be very careful how loud you play back something, everythign comes out at around same volume (in earily history there was a separate people for controlling the audio level and quality that gets transmitted out).FM adio broadcasting have differences between USA and Europe. North American FM broadcast channels are on the odd 200KHz frequencies: 99.5MHz, 100.1MHz, etc. In Europe, channels can be on any multiple of 100KHz, even or odd. This means that some digital tuner from the USA will not tune European stations properly. FM stations in Europe use a different pre-emphasis than those in North America, 75 and 50 microseconds respectively. Using wrong pre-emphasis has effect on the frequency response in reception.

AM Broadcasting

Am broadcasting is the first voice broadcasting system in use and it is still with us. In amny thigns AM is worse than FM on technical perspective. From a technical perspective, there should be no contest between AM and FM. The 9 kHz RF channels in the LF and MF bands set the maximum audio bandwidth for AM at 4.5 kHz. In practice, the frequency response of most AM radios is typically -3 dB at 3.5 kHz, whereas FM offers 15 kHz bandwidth as well as stereo. AM reception is relatively stable even on moving car, this is why some people prefer it on car and on voice programs. At the end of the 1990s, AM listening is showing signs of decline. AM remains viable for news and sports services, but is less likely to be successful for music formats.

Local broadcast stations in USA use an amplitude-modulated (AM) transmitter connected to a vertical antenna with a minimum of 120 buried radials for a ground plane. The radiated field has a strong ground wave extending as far as several miles from the antenna. This ground wave can cause noise problems to other systems. Communications towers at a height nearly equal to the quarter wavelength of the AM station frequency not only influence the radiation pattern of the broadcast station, but also provide considerable energy in the ground return of the tower. If your tower is within the influence of a broadcast tower or electrical substation, you should expect some of this energy to appear in the ground system of your station. A tower grounded at its base without other connections will not bother anyone other than to produce pattern disturbance. However, a closed current loop is provided when conductive appendages such as microwave dishes, VHF, UHF and 800MHz/900MHz antennas are attached. Cables leaving the tower at some elevation usually are attached to the electronic equipment, thus providing another current path separate from the tower ground. Often the shunt current is of sufficient magnitude that it interferes with the station ground.

Sky-wave propagation is generally regarded as a disadvantage in the MF bands used for AM broadcasting. However, it also offers the possibility of covering large areas with a single transmitter, especially in the MF and HF bands. Where there are low levels of both co-channel interference and man-made noise, sky-wave coverage is very attractive for international broadcasting. A major problem for sky-wave services is that multipath propagation through the ionosphere causes time-varying selective fading.

AM broadcasting bands are nowadays adapting also to new digital technologies. The combination of advanced digital modulation schemes with new algorithms for the digital compression of audio signals offers tremendous potential - even within 9 kHz or 10 kHz RF channels. Digital systems can offer enhanced performance - probably giving performance equivalent to monophonic FM services - whilst being much less fragile than AM in terms of immunity to interference and selective fading. Digital Radio Mondiale (DRM) is investigating such systems, with the objective of agreeing a singlestandard for digital radio in the AM bands. This could be used as the long-term replacement of AMbroadcasting in the HF bands, as well as in the LF and MF bands. Ideally, the DRM solution will be applicable to existing AM transmitters with only minor modifications. Unfortunately, the real cost of switching from AM to digital services is in thepurchase of millions of new radios that listeners need to buy to get those new services. AM broadcast stations un USA have frequencies is 10 kHz steps. In Europe AM station frequencies have generally 9 kHz steps.

  • AM broadcasting - is the end in sight? - The introduction in the 1950s and 1960s of FM broadcasting sounded the death-knell for AM broadcasting. As FM was so much better than AM, it seemed obvious that AM would disappear within 10 years - or 20 years at the most. In fact, AM is still with us. Why has it refused to lay down and die?    Rate this link
  • AM Broadcasting: The New Cellular Design Factor - As cellular systems have sprouted towers across the landscape, a seemingly unlikely conflict has arisen between cellular operators and old-fashioned AM broadcast stations. Awareness of this important, but obscure, situation has just been heightened by the Federal Communications Commission (FCC). Typical cellular towers are just the right height to reradiate signals transmitted from AM broadcast stations.    Rate this link
  • A Chronology of AM Radio Broadcasting    Rate this link
  • Using Synchronized Transmitters for Extended Coverage in AM Broadcasting - This paper provides an overview of "simulcasting" the technique of extending radio broadcast coverage areas by using multiple transmitters operating on the same frequency. The paper discusses how simulcasting works and new solutions that address specific technical obstacles that have historically limited use of this technology. This paper is primarily directed at chief engineers, directors of engineering, professional engineers and consulting engineers who desire a technical introduction to simulcasting and transmitter synchronization.    Rate this link
  • DRM (Digital Radio Mondiale) - DRM is a full stereo fully digital broadcast system. The quality of the broadcasts are close to that of FM radio but transmitted on normal AM bands.    Rate this link
  • Technical Standards and Requirements for AM Broadcasting Transmitters (BETS-5) - This document contains the technical standards and requirements for the issuance of a Technical Acceptance Certificate (TAC) for AM broadcasting transmitters. The information in this document is specific to Canada.    Rate this link
  • Low Power Broadcasting FAQ (Frequently Asked Questions)    Rate this link

Broadcasting accessories

A system to add text information to FM radio broadcasts whichRDS radios can receive.

The Terrestrial Digital Audio Broadcasting (DAB) is a new form of radio broadcasting. DAB is a multiplexed system that can be used in either centralised or distributed architecture. DAB is an agreed universal standard that has been implemented from the UK to Taiwan. The BAD system was developed in Eureka 147 project.It transports digital audio (MPEG compressed audio) over radio waves using COFDM modulation system.

In a receiver processes modulated signals are induced into antenna and the receiver electronics delivers a reproduction of the original modulating tone, audio or video. The signal can then be amplified to drive a reproducing device such as a loudspeaker, earphone, tape recorder or video monitorGenrally reception is the induction of EM waves into the antenna to produce a voltage in that antenna, and amplifying it. Selection is tuning of one particular frequency from all the signals induced into the antenna. This is called selectivity. The better the receiver is at differentiating between the desired and undesired frequencies, the better the selectivity rating. Sensitivity of a receiver is based on its noise figure, the minimum required S/N ratio for detection of the modulation, and the thermal noise of the system. The equation for the minimum input signal is noted below:

S = NF + n0 + S/N where S is the minimum input signal required (dBm), NF is the noise figure of the receiver, S/N is the required output signal to noise ratio (for adequate detection, usually based on the acceptable bit error rate), and n0 is the thermal noise power of the receiver (dBm).For sake of simplicity, we will estimate the required output S/N ratio (Manchester data) to be 5dB. To calculate S, we still need n0. n0 is defined as: n0 = 10log10 (k T B / 1E-3) in dBm where k is the Boltzmann's constant (1.38 E-23), T is temperature in Kelvin, and B is the noise bandwidth of the system. At room temperature (T = 290?K) in a 1Hz bandwidth, n0 = -174dBm (commonly expressed as = -174dBm/Hz).For a 300kHz IF bandwidth, n0 is calculated to be .119dBm.Detection is the action of separating the low frequency audio or video signals from the higher frequency carrier. This is also called a demodulator. Perhaps the simplest way to study receiver technology is to keep this in mind: whatever is done to change the modulating signal at the transmitter, it must be undone at the receiver. Propably the simplest form of receiver is rystal radio set with one tuned circuit and one rectifier as the demodulator for AM signal.The first radio receivers were Tuned Radio Frequency (TRF) receivers. In TRF these receivers, all the RF amplification is carried out at the incoming received frequency. This kind of receivers ver bulky to use, because in order to be able to tune to different signals, all of these RF stages must be tunable in step with each other.A dramatic improvement was made in receiving efficiency with the discovery and introduction of the superhetrodyne receiver. Basically, the output from a variable "local" oscillator in the receiver is mixed or hetrodyned with the signals from incoming radio transmissions. In mixing an incoming radio signal with the local oscillator signal, there will be present at the output the original two signals plus the sum and the difference signals of the two, plus harmonics of these sum and difference signals. For instance, in receiving an FM station (Station "A") on 99.7 MHz, the local oscillator could be tuned to 89 MHz. The only modulation product we are interested in is the difference frequency, 10.7 MHz, which is called the I.F. or intermediate frequency. 10.7 MHz is the normal FM receiver intermedia frequency (some other radio types use different frequencies). The advantege of fixed I.F. is that this tuned amplifier stage can be made to operate at fixed frequency, so it is much easier tomake high perfomance than variably tuned stages. Some receivers are "double-superhetrodyne", and this means that the output from the first intermediate frequency is hetrodyned with a second (fixed) local oscillator to produce a second I.F. for further amplification. It is possible to produce substantially greater selectivity by this means. For example 455 KHz is a common second I.F. for VHF radiotelephones operating on NBFM, and requiring the extra selectivity required for the narrower channel spacings. 455 KHz is also the normal (single) I.F. for AM receivers.In most superhet receivers, especially those in the commercial broadcast range, the I.F. is constant. Commercial AM I.F. is 455 KHz and FM I.F. is 10.7 MHz. For brodcast television signals an IF frequency of around 39 MHz is quite common.The detector, or demodulator, allows the extraction of the original modulating signal (audio). It essentially pulls the intelligence from the I.F., leaving a usable audio signal by filtering out the I.F. carrier. Different types of demodulators are needed for different modulations (for example AM and FM need a different kind of modulator).Sensitivity is the ability to receive weak signals and amplify them to a usable level. Most quality receivers will be able to amplify signals (lower) than 5 microvolts. That is, the smallest discernible signal is 5 uv in amplitude. Increasing the sensitivity in a receiver can be accomplished by adding more stages of amplification prior to demodulation (but there are physical limitations how much amplification can be done before component noise levels kick in). The signal-to-noise ratio is a comparison of the signal power to the noise power. This result should be high as possible. Noise should be kept to a minimum as it tends to cover up the weaker signals.Selectivity is the ability of a receiver to tune to a particular station without any other signal interfering with the reception. Selection of a proper I.F. frequency is important to image rejection. In most cases, the larger the I.F., the better the rejection. In some receivers, the oscillator frequency is higher than the received signal; in some cases, lower. Image frequencies are always taken into account in the design of all receivers.One of the most important aspect of a superhet receiver is the constant intermediate frequency. Signal demodulation methods:
  • Probably the simplest of all methods of demodulation is AM. It usually consists of a single diode and filter.
  • For DSB and SSB demodulation, the carrier frequency from an internal oscillator must first be introduced to the signal to replace the suppressed carrier in the received signal. The signal will then appear as a standard AM signal which can be easily detected by a diode-filter arrangement. This oscillator is sometimes known as a beat-frequency oscillator (BFO), and has to be within a few Hertz of the original carrier, or carrier as hetrodyned down to I.F.
  • FM demodulation is considerably more difficult than the demodulations described earlier. A FM demodulator produces an output voltage that is proportional to the instantaneous frequency of the input. There are three general categories of FM demodulator circuit: Phase-locked loop (PLL) demodulator, Slope detection/FM discriminator, Quadrature detector.
Practically all radio tuners are actually analog. There are some radios with digital control of tunign, but their actual tuning and radio processing is anolog electronics. Most radio receivers use a local oscillator for the tuning. There is an RF input amplifier and then a mixer stage. The idea is that when the mixer mixes the local oscillator frequency and the incomign signal, the result is that at ixer output the rest of the receiver electronics gets the wanted signal at suitable intermediate frequency. The frequency of what the RF amp and thelocal oscillator are tuned to is controlled by varactor devices. This is aspecial type of diode that changes it capacitive reactance with theinput voltage. There are some complex circuits around this device tomake it work. The net result is to imitate a variable capacitor foreach of RF stages, and for the local oscillator. This device is whatactually makes the tuned frequency change.Older radios used to have analogue tuning control (by adjusting a variable capacitor or varactor tunign voltage). Many modern radios with digital frequency control are controlled by a circuit called a PLL, that isalso working with a pre-scalar. When you tune to a frequency, you areseeing a pre-scaled display of the frequency that is to be tuned. ThePLL ( Phase Lock Loop ) is commanded to send a command to the varactorcontrol circuits to tune the station to the selected station. Thefrequency tuned is governed by a referenced, pre-scaled reference. Therate, linearity, and span of the tuning is controlled by a combinationof software, and adjustments of tuned circuits and bias references.When the frequency is close, there is an AFC ( Automatic FrequencyControl ) circuit that samples the output of the IF stages via an AFCdetector to make a feedback control to the PLL. This is to force it tosearch for the peak or centre of the carrier of the station being tuned.Peak signal is used for AM, and centre of span or bandwidth is used forthe FM reference.
  • 50 MHz Receiver based on MC3372 - The purpose of this project is to build a simple receiver for 50MHz. The Receiver is built around the circuit MC3372, wich is a narrow band FM receiver. The receiving frequency can be set with a LC tank or with a crystal.    Rate this link
  • A Cameo - This is an FM receiver circuit which can receive signals between 88 and 108 Mhz. With the TDA 7000 you can build an FM radio with a minimum of components; most of them so easy to manage. Nevertheless, this IC in itself, is a complete superheterodyne receiver with all the problems of alignment previously solved by the constructing engineers.    Rate this link
  • A high-performance FM receiver for audio and digital applicatons - This receiver design offers high sensitivity and low distortion for today's demanding high-signal environments.    Rate this link
  • Aircraft Receiver - A passive receiver for listening to the pilot, pdf file    Rate this link
  • AM-Receiver for Aircraft communication (118.250MHz) - This is a tunable AM-receiver for 118.250 MHz frequency. This receiver is manually tunable with some 100kHz around the 118MHz.    Rate this link
  • A Short Wave Regenerative Receiver Project - A regenerative radio receiver is unsurpassed in comparable simplicity, weak signal reception, inherent noise-limiting and agc action and, freedom from overloading and spurious responses. In the comprehensive electronic project presented here, Charles Kitchin, N1TEV has provided us with a three stage receiver project which overcomes some of the limitations of this type of receiver, principally the provision of an rf amplifier ahead of the detector.    Rate this link
  • Building a Three-Penny Radio - A crystal radio is nice because it needs no power, and the materials can all be home-made or at least found around the house. This is a little bit more advanced device. The heart of the radio is a special 10 transistor integrated circuit in a tiny three-legged bit of plastic. This circuit comes ready-made with several amplifiers, the detector, and an Automatic Gain Control circuit that boosts the level of faint stations to match the strong ones, so no volume control is needed. This is called a "Three Penny" radio because it uses three shiny pennies as anchors for the various parts the radio needs. This makes the construction very easy.    Rate this link
  • Build the 'Moorabbin': A regenerative receiver for the AM broadcast band - This circuit first appeared in Amateur Radio, November 1999. It doesn't need an antenna, gives speaker reception of local AM broadcast stations and also receives amateurs talking on the 160 metre band.    Rate this link
  • Build your own superheterodyne receiver - simple three-IC superheterodyne radio to receive stations in the 4.5- to 10-MHz range    Rate this link
  • Classic detector enables low-cost receiver - super-regenerative receiver's sensitivity is better than 1 mV, and it operates over a wide range of VHF    Rate this link
  • Compact 20: A 14 MHz Direct Conversion Receiver - This is a no-frills homebrew direct conversion receiver that is capable of receiving local and overseas stations on the popular 14 MHz band. The well-known NE-602 is used as the product detector. A MPF102 RF amplifier assures adequate sensitivity. The audio stage uses the common 741/386 combination. Component values provide high and low frequency roll-off suitable for SSB (can be mofied for CW also). This set is ideal for portable use because of its low power consumption.    Rate this link
  • Crystal Radio - very simple circuit    Rate this link
  • Deluxe Direct Conversion Transceiver (Super Sprint Rig) - This direct conversion receiver uses 74HC4053 as a mixer and is virtually immune to AM SWBCI. Unique 74HC86 XOR gate based heterodyned VFO, crystal oscillator and mixer.    Rate this link
  • Doppler D/F Instruments - a doppler type VHF radio direction finder for HAMs and experimenters    Rate this link
  • FM Band Preamplifier - This low noise amplifier will bring in those low-power stations    Rate this link
  • FM crystal receiver with slope demodulation construction project - This is a passive FM receiver for 87-108 MHz band. It works only near transmitter sites. Reception range is approx. 4-7 miles using a 3 Ele. Yagi antenna.    Rate this link
  • FM Radio Spectrum Analyzer - a device which will display the strength of all FM Radio stations simultaneously    Rate this link
  • FM-ULA-vastaanotin - This circuit is a receiver for FM radio band 88-108 MHz. It operates from 8-12V DC. This circuit is based on TDA7000 IC. The documentation for this circuit is in Finnish.    Rate this link
  • Improving Receiver Sensitivity with External LNA - This application note presents the system level trade-offs involved in adding a low-noise amplifier (LNA) to a remote keyless entry (RKE) receiver. The system's sensitivity improves 3.77dB, but the third order intercept degrades by 15dB.    Rate this link
  • Improving the performance of an a.m. radio receiver - Here are some suggestions for improving the performance of the simple radio receiver circuit. Both simple AM receiver and improvements are shown in this article.    Rate this link
  • Old Time Crystal Radio - plans of one electronics kit    Rate this link
  • One Transistor FM Radio Project - description of low-cost FM-receiver kit    Rate this link
  • Op Amp Radio - very simple AM radio    Rate this link
  • Poor Man's Spectrum Analyzer - information about an electronics kit    Rate this link
  • Simple AM Radio Receiver & Amplifier - This is an extremely simple AM radio receiver and amplifier circuit which is capable of driving a small, 8 ohm speaker. Construction is simple and could be performed on a breadboard with decent results.    Rate this link
  • Simple Op-Amp Radio - This is basically a crystal radio with an audio amplifier which is fairly sensitive and receives several strong stations in the Los Angeles area with a minimal 15 foot antenna.    Rate this link
  • Single chip builds tiny aircraft receiver - This is a simple AM receiver for 108 to 135 MHz using single-chip NE605.    Rate this link
  • Spectrum analyser kid - simple spectrum analyzer for frequencies between 0 and 33 MHz    Rate this link
  • The R2 Single Sideband Direct Conversion Receiver, Revisited: The R2a    Rate this link
  • receiver has microvolt sensitivity - three-transistor circuit receives signals in the 5- to 15-MHz short-wave band and operated from 9V battery    Rate this link
  • Super Scanner 45-860MHz with 0.01Hz stepsize. - This receiver is based on a TV tuner, a DDS circuit and a radio circuit. This receiver will work from 45 to 860 MHz and the step size can be down to 0.01Hz Why not use this receiver as a Spectrum Analyzer or a NOAA satelit receiver? How about that!    Rate this link
  • 45-860MHz Radio receiver based on UV916-tuner - This receiver use a TV-tuner, a simple radio-circuit and a interface to a computer. The computer control (set) the receiving frequency from 45-860MHz. The purpose of this project is to learn about tuners. The UV916 or UV918 tuner is easy to find in broke TV or VCR:s because it is a common tuner. This circuit can receive FM transmissions.    Rate this link
  • Crystal Radio Set Systems: Design, Measurement and Improvement - here are some early Blonder-Tongue Products to look at Electronic design by co-founder (now retired) Ben H. Tongue    Rate this link

A transmitter is a device which generates high frequency power, which, by means of a suitable antenna, is radiated (or transmitted) through space (or other suitable media). Transmitter signal may be modulated with information, (noise, voice, pictures (TV)). This modulation or information may be received and demodulated in a receiver, where the combination of a transmitter and receiver is known as a radio system.All carriers send over an RF signal are realy analog. The informationon this carrier can be digital if it is a digital broadcast that isbeing listened to or viewed. The RF carrier is a sinusoidal type wavewith very low distortion to avoid generating harmonic interference toother frequencies used by other stations. This carrier may be frequencymodulated or amplitude modulated, depending on the desired standard.Transmitters are usually controlled by a low level oscillator, the oscillators themselves being controlled by an LC (Inductor-capacitor) circuit resonant at the oscillator frequency, or by a quartz crystal. FM modulation is normally achieved by the use of a varactor diode, one which varies its capacity as a function of the applied voltage. If an audio voltage is applied, its capacity will follow the AF variations. Thus, such a capacitor is used to form part of an LC oscillator, or a quartz crystal oscillatorAM may be achieved by feeding the output from an audio amplifier to a transformer whose secondary is in series with the supply line feeding the RF oscillator (low level modulation) or Power Amplifier (high level modulation) of the transmitter. This adds to, or subtracts from, the DC power feeding the transmitter, thus varying the amplitude of the output as a function of the input to the audio amplifier.Remeber that radio transmitters need a permit to have and operateat the give frequency. In some countries (USA) you are allowed to dovery small power broadcasts if you do not cause interferenceand stay within the FCC limits. In other countries it might be illegalto have this type of radio transmitter with you (exceptions arecircuits for measurement purposes which are packed in metal boxes anddo not have antenna so can't be used for illegal broadcasting).Remeber that if you operate a radio transmitter without a permityou can get quite high fines or go to jail in many countries. Evenowning a radio transmitter without a permit is illegal in manycountries. Check the local legistlation before startingto build or use any radio transmitter. You shouldcheck the law before buying any transmitting equipment as a fine, confiscation of equipment and/or imprisonment can result from illegal use or ownership. The laws vary from country to country, therefore check local laws. General warning on radio transmitter operation: Do not transmit until you have matched the antenna connected to the radio transmitter. If you have not connected the antenns, you can burn out parts in the transmitter. Some transmitters are better protected against antenna problems, so if you try to transmit without antenna your luck may vary if yoiu damage something or not.

General information

  • CB-Radio Component Database    Rate this link
  • Estimating Transmitter Distance - Here you can find a simplified equation for analysing low power radio transmitters, for line of sight. It does not take into account probagation conditions or other limiting factors, but does include a variable for the losses in the antenna and tank circuit of a transmitter.    Rate this link
  • Hum Reduction in FM Stereo Transmitters - It is convienient and easy to use a wall transformer with FM stereo or AM transmitters, but often unwanted hum appears on the transmitted signal. This problem can be sometimes frustrating and difficult to correct.    Rate this link
  • UHF Construction Precautions - If you are building a kit or circuit that operates at VHF (over 30 MHz) or UHF (over 300 MHz) you should be made aware of proper wiring and construction practices. This is largely experience combined with a good theoretical understanding.    Rate this link
  • VHF Pirate Radio Electronics - This document explores the concepts involved in radio broadcasting relevant to the pirate radio operator on VHF FM. This document give a step by step tour of a typical VHF FM transmitter system starting with the output from the tape recorder or mixer, and finishing with a brief discussion of aerials.    Rate this link

FM transmitters and related circuits

  • Coilless FM transmitter - The RF oscillator using the inverter N2 and 10.7Mhz ceramic filter. Output driver inverters are in parallel the output impedance will be low so that it can directly drive an aerial of 1/4th wavelength. Since the output is square wave there will be a lot of harmonics in it. The 9th harmonics of 10.7Mhz (96.3Mhz) will hence be at the center of the FM band.    Rate this link
  • 1W PLL transmitter - Suitable for study purposes, you can use the RF part easily for your projects. However, unless you know how to build a serial data link and connect this to a PC or a microcontroller, you won't be able to use the PLL.    Rate this link
  • 2W transmitter - This is claimed to give mile of range or so on normal FM band (around 100 MHz). Don't forget that there is no such thing as free lunch, stability and spectral purity suck.    Rate this link
  • 3W FM Transmitter - operates between 90 and 110 MHz    Rate this link
  • 4 Watt FM Transmitter - This is a small but quite powerful FM transmitter having three RF stages incorporating an audio preamplifier for better modulation. t has an output power of 4 Watts and works off 12-18 VDC which makes it easily portable.    Rate this link
  • Coil-less FM Transmitter - Very crude transmitter which uses the 9th harmonics of 10.7Mhz (96.3Mhz).    Rate this link
  • FM Band Monaural Transmitter - Varactor diode gives high quality modulation, pdf file    Rate this link
  • FM Beacon Broadcast Transmitter (88-108 MHz) - This circuit will transmit a continuous audio tone on the FM broadcast band (88-108 MHz) which could used for remote control or security purposes. Circuit draws about 30 mA from a 6-9 volt battery and can be received to about 100 yards. A 555 timer is used to produce the tone (about 600 Hz) which frequency modulates a Hartley oscillator. A second JFET transistor buffer stage is used to isolate the oscillator from the antenna so that the antenna position and length has less effect on the frequency.    Rate this link
  • FM Broadcast VCO and Buffer - for 88-108 MHz    Rate this link
  • FM Band Monoaural Transmitter - high quality monoaural FM transmitter, FET based oscillator    Rate this link
  • FM Oscillator With BF900 Dual Gate Mosfet - This is a 100 MHz frequency FM transmitter circuit designed aroun BG900. Instead of the BF900, a BF905 or BF907 can be used without any problems. This circuit uses 15 V DC power (50 mA) and outputs RF output of around 55 mVpp to 50 ohm load. This circuit has a low frequency deviation.    Rate this link
  • FM Stereo Transmiter    Rate this link
  • FM Transmitter - low powered FM transmitter, includer circuit board and parts placement picture    Rate this link
  • High Fidelity FM stereo matrix generator circuit - circuit is for use with mono transmitters to give out clear stereo output    Rate this link
  • Low Power FM Transmitter - This circuit operates from 9V battery and transmits FM signal at frequency range around 90-108 MHz. This circuit can take microphone or line level signals for modulation.    Rate this link
  • Micro Power FM Broadcasting Circuits - accessories to transmitter circuits: audio limiter, audio mixer, antenna and essential modifications for Ramsey FM10    Rate this link
  • Mighty's Pirate Radio - many radio circuits, including mono and stereo FM transmitters    Rate this link
  • Phone Broadcaster - telephone call FM transmitter    Rate this link
  • Small Radio Transmitter - information about building a small radio transmitter, which has a PCB 1.75" x 2.5" (45mm x 68 mm) and has a range of about 30 yards or so at 85-108 MHz frequency range. This plan is based on Velleman kit circuit which is slightly modified (this article has a new circuit board design).    Rate this link
  • Um excelente gerador de FM para a faixa de 88 a 108 MHz - PLL syntetized generator of FM for the band of 88 the 108 MHz, text in portugese, you can use    Rate this link
  • We-man's radio circuits - FM transmitters, fax interface and antenna design    Rate this link
  • World's simplest transmitter - Range and stability is not premium, but it works! Its a demonstration of what can be done with little to none resources. This circuit is based on 74LS13 logic IC!    Rate this link
  • Stereo encoder I A simple FM stereo signal encoder circuit design that has been has circulating around for ages. Adding this circuit to a mono FM transmitter turns it to a stereo FM transmitter,    Rate this link
  • Stereo encoder II This is a stereo modulator circuit that can turn a mono FM transmitter to a stereo FM transmitter. This circuit have been mentioned to have a bit of noise, filtering is not sufficient, but otherwise it can be made to work extremely nicely.    Rate this link
  • World's simplest FM transmitter schematic If you cannot assemble this transmitter, then perhaps you should give up on assembling anything at all. Range and stability is not premium, might not meet all signal purity specifications, but it works! This circuit is a demonstration of what can be done with little to none resources. This circuit is based on 74LS13 TTL IC!    Rate this link
  • Simple small FM transmitter schematic This FM transmitter gives 200-500 meters of range.    Rate this link
  • FM Band Monaural Transmitter A high quality monaura FM transmitter is quite useful in a variety of applications. The audio from a TV or entertainment system may be transmitted for remote or portable listening and for wireless earphone listening. This is a very simple FM transmitter with FET based FM oscillator modulated with varactor diode.    Rate this link
  • FM Jammer - Here's a quick and simple project. The Amazing One Transistor FM Transmitter. It is a redesign of a circuit sold many years ago by PPG electronics called the TV Jammer. Though it transmits only a MONO FM carrier, it can still quite effectively "Wipe Out" most stations on a STEREO receiver from about 30 feet away (with the optional antenna).    Rate this link

FM wireless mics

Mini FM transmitters take place as one of the standard circuit types in an amateur electronics fan's beginning steps. When done right, they provide very clear wireless sound transmission through an ordinary FM radio over a remarkable distance Miniature FM transmitters with built-in microphone are sometimes called bugs. Bugs are generally used for room monitoring, baby listening and nature research.Please note that "BUGS" are illegal in many countries.

  • FM Transmitter Kits for Beginners    Rate this link
  • FM-ULA Transmitter - This circuit is a small transmitter that can be tuned from 0 MHz to 108 MHz, operates from 9V battery, takes 7 mA current    Rate this link
  • How to make an FM wireless BUG - This handy little device can be used for two purposes. The first is a FM bug, which transmits on the FM frequency, thus making it extremely easy to pick up. The second is a FM station blocker.    Rate this link
  • Resonant Frequency of a Colpitts Oscillator - gives formulas how to analyze the operation of the example 96.8 MHz wireless microphone circuit    Rate this link
  • Simple FM transmitter with a single transistor - This simple FM trasmitter works on 1.1-3 V supply (1.8 mA current @ 1.5 volts). This circuit gives around 30 meters range and has a built-in elecret microphone.    Rate this link
  • Small Radio Transmitter - information about building a small radio transmitter, which has a PCB 1.75" x 2.5" (45mm x 68 mm) and has a range of about 30 yards or so at 85-108 MHz frequency range    Rate this link
  • Wireless Microphone by Harry Lythall - SM0VPO - This project is a miniature, VHF FM (wideband) Wireless Microphone transmitter of the type that are commonly refered to as BUG's.    Rate this link
  • Wireless Microphone 2 by Harry Lunthall - This wireless microphone transmitter is claimed to have coverage range up to two kilometers.    Rate this link

AM transmitters

  • 136kHz 1kW transmitter    Rate this link
  • Building a very simple AM voice transmitter - If a crystal radio is the distilled essence of a radio, this transmitter is the matching distilled essence of transmitters. The transmitter goes together in about 10 minutes, and is small enough to fit in the palm of your hand. Depending on the antenna, the transmitter can send voice and music across the room, or across the street. The transmitting frequency of the design is 1 MHz.    Rate this link
  • Micropower AM band radio station - simple transmitter offers surprisingly good signal quality, pdf file    Rate this link
  • Micro Power AM Broadcast Transmitter - 74HC14 hex Schmitt trigger inverter is used as a square wave oscillator to drive a small signal transistor in a class C amplifier configuration    Rate this link
  • Class E AM Transmitter Descriptions, Circuits, Etc. - Class E amplifiers are very efficient amps and are generally built with MOSFET transistors. The principle is to drive the MOSFET's gate input with square waves to quickly put the device into it's low ohmic region and to do this when the voltage across the drain of the MOSFET is at or near zero volts. This greatly reduces the heat dissipated by the MOSFET and increases efficiency. A choke value for the drain is chosen so that it resonates at the operating frequency, in combination with the parasitic capacitance of the drain and the output filter. The "fly wheel" effect of the resonant tank causes the drain voltage to drop to zero before the MOSFET is switched back on. Efficiencies of 70% or more can be achieved this way. This article describes a simple Class E transmitter and is shown built for 40 meters. It uses a 74HC02 NOR gate as a crystal oscillator. The amplifier delivers about 2 watts output with a 9 volt supply and about 4 watts with a 12 volt supply.    Rate this link
  • The Grenade Transmitter - This is shortwave transmitter based on The Animal's infamous "Grenade" design. It operared at 12-14V power and outputs up to 10W AM transmission at 40 meter frequency area (6000-8000 Kilohertz). This design uses crystal to define the transmitting frequency and has limiter/compressor in the audio input.    Rate this link

Microwave transmitters

  • 2400MHz Signal Source - This unit is an attempt to make the simplest possible signal source for 13cm without the usual grief of not finishing up on the right frequency. The oscillator starts with a readily available 96MHz crystal and multiplying this by 25 goes up to 2400MHz. The use of filters takes the guesswork out of finding the correct frequencies. The output is -10dBm (100 microwatts).    Rate this link
  • 2.4 GHz FM ATV - a project concept - This article describes how to modify consumer wireless video link devices for ATV use.    Rate this link


  • 900Mhz 9600bps Radio Link - This is a reference design from the AVNET electronics page for a wireless 9600bps transceiver.    Rate this link
  • K8IQY's "Manhattan Madness" Project Page - This web site is a repository for information developed during the design, construction, and testing of ham radio oriented projects. Some are complete rigs, others are pieces of useful test equipment or test methods that are useful, and occasionally there will be a tid bit of information that stands by itself.    Rate this link
  • N5FC's 2N2222 40 Meter CW/DSB Transceiver - This is a project using no ICs and 22 2N2222s the active semiconductor devices. This circut operates from 12 Volts power source.    Rate this link

Radio transceivers combine the radio transmitter and radio receiver in one case. Walkie-talkies are one example of radio transceiver used as a communication device.

  • 7MHz SSB Transceiver VU3PRX - The transceiver described here is remarkably simple based on popular communication building block MC1496. It is fairly simple to build because most of the functions are performed by MC1496. The emphasis during the design of the project was on repeatability, minimum number of switching and ability to modify for multi band operation.    Rate this link
  • 900Mhz 9600bps Radio Link - This is a reference design from the AVNET electronics page for a wireless 9600bps transceiver.    Rate this link
  • A 70cm Wide-band Transceiver Concept - This article presents a description of a 70cm wideband transceiver for duplex operation. The modules are designed for operation in a 200 kHz wide duplex channel, freely selectable from 430-440 MHz with a 100 kHz channel spacing. First prototypes were introduced on the international Packet Radio Symposium in April 1997. Some minor changes resulting from further experience were added to the text since then.    Rate this link
  • N5FC's 2N2222 40 Meter CW/DSB Transceiver - This is a project using no ICs and 22 2N2222s the active semiconductor devices. This circut operates from 12 Volts power source.    Rate this link

You might have seen people at the park flying a model airplane or blimp, or controlling a miniature boat sailing serenely across a pond. You might wonder how this works. This kind of devices use radio controlling.The operation principle of the radio control is thatthe transmitter sends a control signal to the receiver using radio waves. The control signals are sent in some suitable coded form (many different codes for different uses exist). Besides remote controlled toys radio controlling can be used in very many other applications when some remote device needs to be controlled without wires.RC toys typically have a small handheld device that includes some type of controls and the radio transmitter. The transmitter sends a signal over a frequency (most RC toys operate at either 27 MHz or 49 MHz) to the receiver in the toy. The majority of RC toys are labeled with the frequency range they operate in. Most full-function controllers form simple have six on/off-type controls. Controllers for more advanced RC systems often use dual joysticks with several levels of response for precise control of RC servo motors in the controlled device.

General information

Remote controlling circuits

  • 433.92 MHz vastaanotinmoduuli - receiver circuit for 433.92 MHz data transmissions, uses ASK modulation, based on T7544 IC, text in Finnish    Rate this link
  • SAW-resonaattorilla toteutettu oskillaattori RF-out 433.9 MHz - This is a RF transmitter oscillator for 433.9 MHz operation, can be modulated (AM) with external signal    Rate this link
  • Four-way remote control uses series transmission - four channel remote controller system which uses 433 MHz band    Rate this link
  • RC Electronics Projects - lots of projects published in the UK magazine Radio Control Models and Electronics    Rate this link
  • 433.92 MHz RF Remote Control - schematics & projects using the TWS/RWS RF modules    Rate this link
  • Remote Control Schematics - Schematics for turning your IR transmitter into an RF transmitter, schematics for 433.92MHz RF transmitter & receiver modules with 4-bit & 8-bit remote control encoder/decoder circuits, 5-button, 4-Bit transmitter schematic for TWS-434 RF transmitter, 4-button, 4-Bit transmitter schematic for the TWS-434 433MHz RF transmitter module with Holtek HT-12E encoder IC, 4-Bit receiver schematic using the RWS-434 433MHz receiver module with Holtek HT-12D decoder IC    Rate this link

Radio modems allow digital data communications through radio waves. Conventional narrowband radio techniques rely on a base "carrier" wave that is altered in a systematic manner (modulated) to embody a coded bit stream. Carrier waves can be modified to incorporate digital data by varying their amplitude, frequency or phase. The radio-modem, although usually much slower than its telephone counterpart, has the in-built capacity of being a self-correcting data carrier. The deterioration of the speed of transfer in bad conditions can become annoying to the impatient operator.Radio modems are available usually in two different formats.Some modems are devices which contains the modem partand radio transceiver as a single device or module.The other kind of radio modems are modem devices which aredesigned to to interface to a normal "voice communication" radio or similar device (usually through headphone,microphone and PTT connections).The standard way to convert the serial data signal to a suitable format which can be transmitted through the audio channel provided by the radio. Radio amateur transceivers and other transceivers designed for voice communications can not be connected directly to terminals or computers to perform digital data transmissions. The binary signal exiting from the serial port of a computer has a rectangular shape, has a strong DC component and a great amount of harmonic frequencies. Transceivers are generally planned for voice (or telegraphy) transmission and admit particularly frequency modulation (F3A), thus they can not transmit DC (frequency leading towards zero) nor frequencies that exceed human voice ran ge (passband being almost 3 KHz wide). To overcome these and other obstacles you don't have to re-plan or rebuild a transceiver. You can instead interpose between the PC and the transceiver a particular modulator/demodulator which transforms digital signals of the PCinto audio signals between 300 and 3,000 KHz (passband of the transmitter modulator); these signals are thus sent to the transceiver as if coming from a microphone. The device, normally called a modem. It typically generates a continuous sinusoid signal centered in the passband of the transmitter input, whose parameters (amplitude, frequency, phase) are varied by the serial digital signal coming from the PC. Simplest approach is amplitude modulation:You generate an audio tone when there is 1 on line and notone when there is 0 on line. This is the transmitter.The decoder is just a tone decoder (just listens iftone exists).The techniques of modulation are basically the following:

  • AM or amplitude modulation (used for very slow speed) changes the transmitted signal amplitude to each of the two logic statuses 0 and 1, the most extreme version of this turns the carrier completely on and off
  • FM or frequency modulation (for speeds up to 1,200 bytes/sec) pre-sets a given frequency value in the generated sinusoid signal to each of the two logic statuses 0 and 1
  • PSK or phase modulation (for speeds up to 4,800 bytes/sec) pre-sets two different phases of the carrier wave to the two logic statuses 0 and 1
  • combined phase and amplitude modulation (for higher speeds) combines the modification of
The most common modem modulation method for radio communications use is some form of FSK (Frequency Shift Keying). This type of modulationpasses nicely through many kinds of radio radio based "voice channels".Simple FSK modulation through few kHw woice band channel can usuallygive transfer speeds up to 1200 bps or 2400 bps. For more speed some more advanced modulations are needed.Those fster communication modulation methods need to beoptimized for radio communication (modulationsused in fast telephone modems are optimized only fortelephone line and usually perform very poorly on radio channel).Many commercial radio modems provide a simple wireless link with the ability to send any message protocol at any data rate up to the maximum quoted. They provide a simple means of passing RS232 data streams with no message overhead in the form of extra addressing or error-checking codes.Generally with this kind of modem when there is enough signal strenght and not much interference, data supplied to the transmitter input is reproduced at the distant receiver's output.When operating near the limit of maximum range will the received output be prone to data corruptions. The transmitter is usually switched on by asserting the transmitter control line. A short delay (varies depending on device type used) is required to allow the transmitter and receiver to establish the link before any data can be sent.When the transmitter is switched off at the end of a message, a short burst of noise may occur before the receiver's mute operates (this noise should be ignored by the system connected to the receiver).In radio communications the user may need to provide error checking and/or addressing to each mesage if the application requires guaranteed data integrity.When using radio communication some for of error checks,error correction, and re-transmission methods are very often needed tobe able to get reliable communication over noisy radio channels.This noise on radio channel causes transmission errors which needsto be corrected by mode or handled in some way by the communicationprotocol itself. The needs vary depending on the transmission distance,available signal power, radio band used and radio modem type.The free space propagation signal propagation rule for radio communications is that a 6dB change in transmitter power is required to double / halve the range. An imperial 'rule of thumb' for in-building operation requires a 15dB change in path loss capability to change the range by a factor of two (hhis is a very cruel law).There are international and national bodies that allocate frequency bands and issue authorization to transmit signals. In some countries, there are bands that are allocated for public use without the need for any special authorization. This is an important factor to consider when selecting a radio modem, since getting authorization to broadcast information is often not an easy task. The bands that are allocated for public use are of particular interest. The 900 MHz band in the United States and 2.4 GHz in most European countries are allowed for spread spectrum communication without any special authorization (but there are limitations on the amount of power that one can use to transmit signals). In Europe there are also frequencies 418MHz, 433.92MHz and 868-870 Mhz for low power short distance licence exempt communications (there are ready made modules available for this, check your country regulations if their use is allowed in your country if you live in Europe).

General information

  • Introduction to Packet Radio by Larry Kenney, WB9LOZ - This series of eighteen articles was originally written in 1988 to appear in Nuts & Volts, the newsletter of the San Francisco Amateur Radio Club. The series has been widely distributed since then, with revisions issued in 1991, 1993, and 1995. Occasional revisions were made to this version on the web thereafter, in the late 1990s.    Rate this link
  • Data Radio Standard Test Methods Project - A project to establish Standard Test Methods for the evaluation of radios used in data communications especially aimed at Amateur Packet Radio.    Rate this link
  • Federal Standard 1052 - This standard is one of a series of standards pertaining to automatic high frequency (HF) radio equipment and operation. The basic standard is Federal Standard (FED-STD) 1045A, Telecommunications: HF Radio Automatic Link Establishment.    Rate this link
  • Introduction to Packet Radio - Packet has three great advantages over other digital modes: transparency, error correction, and automatic control. The operation of a packet station is transparent to the end user; connect to the other station, type in your message, and it is sent automatically. The data on acket radio system is usualy sent using radio modems. AX.25 (Amateur X.25) is the communications protocol used for packet radio.    Rate this link
  • Introduction to Packet Radio    Rate this link
  • Internet Gateways to Amateur Radio tcp/ip Servers    Rate this link
  • Selecting a Radio Modem Technology forIndustrial Automation Applications - Article from Instrumentation &Control Systems Magazine (July 1997).    Rate this link
  • The Protocol AX.25 - The name AX25 originates from the recommendation X.25 of CCITT, adding letter A that stands for Amateur; AX25 is therefore Amateur packet radio link layer protocol. The purpose of this protocol is to define the frame structure and to set the requirements of the station that sends or receives that frame or packet.    Rate this link
  • Using Radiometrix modules operating on 418 or 433.92MHz in North America under FCC Regulation Part 15, Section 231, Paragraph (e)    Rate this link
  • RFC 1226 - Internet protocol encapsulation of AX.25 frames This memo describes a method for the encapsulation of AX.25 (the Amateur Packet-Radio Link-Layer Protocol) frames within IP packets. This technique is an Experimental Protocol for the Internet community.    Rate this link

Radio modem project

Radio modem interfacing

Soundcard to radio interfacing

Nowadays there exists radio modem software for PC which uses PC soundcard as the signal input and output device. Here you can find information how to interface PC soundcard to your radio.

  • Sound Card Interface with Tone Keyer - This is an improved version of the audio interface commonly used to connect a PC's soundcard to a transceiver's receive and transmit audio circuits for PSK31, SSTV, . The usual version of this type of interface (including the commercial "RigBlaster") requires the use of a serial port to provide PTT (push-to-talk) control for the radio's transmitter. This version includes an audio tone detector that keys the transmitter whenever transmit audio is generated by the application running on the PC.    Rate this link
  • Sound Card to Radio Interface Home Page    Rate this link

Telephone modem type equipment to radio interfacing

Interference is any unwanted signal which precludes reception of the best possible signal from the source that you want to receive. Interference may prevent reception altogether, may cause only a temporary loss of the desired signal, or may affect the quality of the sound or picture produced by your equipment.Interference to home electronic equipment is a frustrating problem; but, fortunately, there are several ways to deal with it.

  • Computer EMI    Rate this link
  • Dealing with Computer generated RFI/EMI - One of the most frustrating problems about using computers with radios, whether it be for controlling purposes or for decoding, is the amount of RFI generated by these machines. Most of the time, the RFI generated is enough to render certain bands useless and on other bands, it may drown out any weak signals and distort or interfere with signals that you want. The bad news is that, there is no way that I know of to completely remove the computer generated RFI in most situations. The good news is that there are definite steps that we can take to reduce the RFI to a very acceptable level and in some cases, it will almost disappear altogether. This document is a compilation of suggestions from various persons and some of the things I have tried with my own system when dealing with this problem. Many of the documents I have seen relate to situations involving transmitters and how not to generate them (RFI). This document is written from a receiving point of view.    Rate this link
  • EMI from RF-Driven Lamps    Rate this link
  • General Information about EMI/RFI    Rate this link
  • How to avoid radio interference to audio equipment and telephone - written in Finnish    Rate this link
  • QST October 1992 Lab Notes on Telephone EMI - amateur radio telephone interference    Rate this link
  • Radio Frequency Interference (RFI) Information Page    Rate this link
  • Radio Frequency Interference To/From Touch Lamps and Dimmers    Rate this link
  • The ARRL Technical Information Service - lots of documents on radio interference    Rate this link
  • Using ferrites for interference suppression - One of the most used yet least understood techniques for reducing both incoming and outgoing RF interference is the application of ferrite sleeves to cables and at interfaces. This tutorial is meant to shed some light on the use of ferrites, and also presents some comparative frequency domain measurements both to illustrate some of the points, and to give designers an idea of what they might expect in using and specifying a ferrite component.    Rate this link

Jamminc and blocking radio signals

If you want that certain radio signal does not get to a certain place you have generally two options: shielding and jamming.There is no device that can simply "block" an RF transmission,other than by transmitting another RF transmission on the samefrequency that is overwhelmingly strong compared to the onebeing "blocked". This is called "jamming", and it is in generalillegal worldwide, because intentionally interfering with legitimate radiocommunications is illegal in virtually all countries. There are a couple of companies that manufacture cell phone blockers, but the use of those is very much limited to only few countries where use of them is allowed. The only other option is to shield the receiving device(s) from the transmission. A grounded electrically sealed metal box will keep the radio signal not from getting into it. In some cases not completely shielded or wire mesh box can attenuate the radio signals quite much (the shielding effects of such not completely closed structure depends very much on the signal frequency).For instance, cell phones in a theater could be rendered inoperative by making the theater a completed closed conductive chamber; i.e., asealed (electrically) metal box.

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