Index

Radio Electronics Pages

    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 (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.