Index


Radio Electronics Pages

    General information

    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.

    History

    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

    Radio signal modulation

    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

    Radio phone information (walkie-talkie)

    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

      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

      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.

      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

      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.

    Antennas

    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.

      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.

      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.

      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.

    Broadcasting

    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.

      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.

    Digital Audio Boradcast (DAB)

    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.

    Receivers

    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.

    Receiver accessories

    • Add a signal-strength display to an FM-receiver IC - The Philips TDA7000 integrates a monaural FM-radio receiver from the antenna connection to the audio output. External components include one tunable LC circuit for the local oscillator, a few capacitors, two resistors, and a potentiometer to control the variable-capacitance-diode tuning. The IC has an FLL (frequency-locked-loop) structure. You can obtain the information related to the intensity of the received signal at the output of the IF filter.    Rate this link

    Transmitters

    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

      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

    Radio transceivers

    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

    Radio remote controlling

    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.

    Radio modems

    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).

      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

    Radio frequency interference

    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.

      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.

    Other unsorted links

    • FM-only radios - Very complete radio collector's site of FM only radios. Site include historical information, theory, photographs, and a few unique projects.    Rate this link
    • MEIsearch - mobile electronic industry search    Rate this link
    • The Wireless Set No. 19 website - The Wireless Set No. 19 website celebrated this vintage military WW II "tank" radio, now collected, restored and carefully used by some Amateurs all over the world. This site includes technical information, operating procedures (from war-time manuals), photos, first-hand experiences, weekly and monthly on-air nets and much, much more.    Rate this link

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