Radio employs electromagnetic waves to send information over long distances without wires. A broadcast radio station uses such waves to convey a continuous programme (or programmes) of sound. Unlike modern multimedia systems, this kind of transmission isn’t usually interactive, obliging us to listen to the current programme or switch to another station. This is ideal when want to listen a programme but need to do something else at the same time. In addition, since it lacks any visual elements, radio can stimulate the imagination in ways that are totally impossible with television or the Internet.
So, despite other technologies, radio remains popular, attracting even more listeners in recent years. It’s even more surprising to find that many radio stations still employ the same technology that was used in the earliest broadcasts.
The mechanics of radio were first demonstrated in the early 1800s by Michael Faraday and Joseph Henry, who both found that a current in one wire could induce a current in another unconnected conductor. In 1820, Hans Christian Oersted found that an alternating current (AC) flowing in the first wire produced a changing magnetic field, resulting in a similar current in the second wire, an effect known as electromagnetic induction. Today, this phenomena is used directly in power transformers, usually running at a frequency 50 to 60 cycles per second (also known as hertz or Hz). However, radio frequency (RF) waves operate at much higher frequencies, usually between 30,000 Hz (30 kHz) and 300 GHz, where 1 GHz is 109 Hz.
In 1864, James Clerk Maxwell proposed that electromagnetic induction was caused by a form electromagnetic radiation, the latter of which includes radio waves, X rays, gamma rays and light. All of these are at different frequencies, together forming what is known as the electromagnetic spectrum. Heinrich Hertz managed to produce radio waves in the late 1880s, employing oscillating circuits, made up of capacitors and inductors, to transmit and receive the signals.
The idea of using radio for communication came from Guglielmo Marconi, who, in 1899 sent the first wireless telegraph or radio telegraph message across the English Channel and, in 1901, sent a message from England to Newfoundland. The wireless telegraph employed Morse code, involving the use a telegraph key that switched the transmitter’s oscillator on and off in a sequence of ‘dots’ and ‘dashes’. This was known as interrupted continuous wave (ICW) or continuous wave (CW) operation.
Morse code wasn’t ideal for sending or receiving information. Fortunately, the radio telephone, introduced in 1906, could convey the spoken word by means of amplitude modulation (AM), in which the power of the transmitter was varied in proportion to the volume of sound produced by the speaker.
A wartime ban on non-military broadcasting in the USA during World War I postponed any further developments, although many amateur stations sprung up in 1920, the first true radio station being KDKA, Pittsburgh, Pa. By 1922, more than 500 stations were licensed by the American government, each being given a three-letter code or, later on, a four-letter code, by the US Department of Commerce. Most operated on a common wavelength of 360 metres (m), although more powerful stations were permitted to broadcast live music on 400 m, both of these frequencies being in what is now known as the Medium Wave (MW) or Medium Frequency (MF) band. By 1924 these stations had accrued a total of 20 million listeners.
The private British Broadcasting Company was established in 1922, but in 1927 was transformed by Parliament into the British Broadcasting Corporation (BBC), becoming the sole provider of radio in the UK. The new organisation had its own board of governors and a director general (DG), the first of whom was Lord John Reith, who committed himself to “to educate, to entertain, and to inform”. The BBC’s first programmes were broadcast using the call sign 2LO, the ‘LO’ standing for London, and operated on a wavelength of 1500 m (200 kHz), located within the Long Wave (LW) or Low Frequency (LF) band.
In 1926, RCA set up the National Broadcasting Company (NBC) in the USA, whilst the rival Columbia Broadcasting System (CBS) was established in 1927. The influence of radio perhaps reached its zenith with a broadcast in 1938 of an adaption of H G Wells’s science-fiction story The War of the Worlds, whose account of an alien landing caused widespread panic.
A simple receiver consisted of a aerial connected to a tunable circuit, usually comprising of a coil and a variable capacitor, the latter containing fixed and movable vanes for tuning, although some older receivers used a fixed capacitor and coil with an adjustable slug of iron. The signal was applied directly a detector, also known as a rectifier, in which the current could flow in one direction only. In a crystal receiver, the detector consisted of a crystal of galena or carborundum, onto which a movable pointed wire, the cat’s whisker, was placed. This in turn was connected to a capacitor in order to remove the RF carrier signal, the resultant audio signal being connected to a pair of headphones. Using a long aerial and a good ground connection it was possible to receive signals from distances of up to 1,600 km (1,000 mi).
Unfortunately, the crystal detector wasn’t entirely reliable, often requiring an adjustment of the pointed wire. However, in 1905, Sir Ambrose Fleming invented the diode valve, which was far more satisfactory, even though it required an external source of power. This was followed by Lee De Forest’s audion or triode valve, which could be used to amplify radio signals.
Early receivers containing triode valves used tuned radio frequency (TRF) technology, which meant that each stage of amplification had to be tuned to the required station, which was often unsatisfactory when used across a wide range of frequencies. The superheterodyne receiver, devised by Edwin Armstrong in 1918, overcame this by mixing the incoming frequency with a signal from a local oscillator, so as to produce a fixed frequency signal, whatever the receiving frequency. The subsequent stages of amplification were all tuned to this intermediate frequency (IF), which was usually set at 455 kHz in an AM broadcast receiver. Virtually all domestic receivers made after World War II were built using this technology.
Although valves worked well, they weren’t always convenient, since they normally needed a high tension (HT) supply, usually 90 volts (V) or more, and a low tension (LT) supply, the latter for the valve heaters. This meant that radios had to be powered from the mains or needed two sets of batteries; an HT battery, in the form of a primary cell, and a LT battery, normally in lead-acid rechargeable form. The arrival of the transistor, which replaced the valve during the 1960s, eliminated such problems, allowing truly portable radios to be built with a single battery pack.
Despite the arrival of FM, as described below, the AM system remained popular, mainly because it gave reliable reception in almost all conditions. However, the maximum audio bandwidth available to transmitters in the MF broadcasting band (535 to 1,605 kHz) was around 5 kHz, which was totally inadequate for high-quality sound reproduction. In the USA, various systems were introduced to add stereo to AM broadcasts, using the two sidebands of the AM signal in different ways to convey the stereo information. In many other countries, however, this wasn’t considered worthwhile.
The AM system suffers from electrical interference, as caused by lightning. In 1933, Edwin Armstrong invented frequency modulation (FM), in which a constant RF carrier is transmitted, with its frequency varied according to the volume of the sound. Subsequent standards specified the maximum frequency deviation as 75 kHz above or below the carrier frequency, whilst the maximum modulating frequency was set at 15 kHz, sufficient to give reasonable sound quality.
It wasn’t until the 1950’s that FM became established. Transmitters were assigned to part of the Very High Frequency (VHF) spectrum known as Band II, between 88 and 103 MHz, although later extended in the UK to 108 MHz. Each BBC network had a block of frequencies 2.2 MHz wide, with the Light service on 88.0-90.2 MHz, the Third on 90.2-92.4 MHz and the Home on 92.4-94.6 MHz. Most transmitters generated 100 to 100,000 watts (W), giving a range of 24-105 km (15-65 mi).
FM receivers included automatic frequency control (AFC), in which a direct current (DC) signal was wired back from the FM detector to the local oscillator, correcting any drift in the latter that might otherwise cause the signal to fade. Most AM and FM receivers also incorporated an automatic gain control (AGC) system, sometimes known as automatic volume control (AVC), in which another DC signal from the detector was used to adjust the amplification of the IF amplifiers, ensuring that the listener wasn’t deafened by a strong signal after tuning in to a weak station.
By the 1960s, most FM stations were broadcasting in stereo. Although various systems based on a high-frequency subcarrier were proposed, the Zenith system was generally adopted throughout the world. Although this gave good results, if suffered from a problem with increased background noise, particularly when receiving weak signals. Some countries adopted Dolby B noise reduction to minimise the problem, although this was never considered acceptable in the UK.
The arrival of television in the 1950s didn’t influence radio in the USA, which saw an increase to over 4,900 FM stations and 4,200 AM stations by 1990, although the national commercial networks had long since disappeared. However, National Public Radio and American Public Radio continued to supply programming to public stations across the country.
The BBC continued to operate its Home service, providing regional broadcasting via several frequencies on the MF band, the Light service on the BBC’s original LW frequency and the Third programme on a limited number of MF transmitters. These services were duplicated on the VHF/FM system, although there were some variations in programming between FM and AM transmitters. This arrangement was slightly modified in 1967 with the arrival of the Radio 1 pop channel, which was accommodated on 247 m, whilst the Light service became Radio 2, the Third became Radio 3 and Home became Radio 4.
The expansion of both BBC and commercial local radio in the seventies and eighties led the BBC to abandon the regional nature of Radio 4, moving it onto its LF transmitter and putting Radio 2 on the MF band. Although FM frequencies were also reorganised, the BBC networks kept their original slots, with local and commercial stations going higher up the band. Radio 1 was also assigned a 2.2 MHz area in the region of 97.6-99.8 MHz, although out of tuning sequence with other BBC services, whilst Classic FM, a national commercial station, was given a narrower 2 MHz slot at 100-102 MHz.
Unfortunately, there wasn’t space on FM for the new Radio 5 Live station, which had to go on MF at 693 and 909 kHz. Other country-wide stations also appeared on MF, including TalkSport, which was assigned 1053 and 1089 kHz, and Virgin, which was given 1215 kHz. Meanwhile, the BBC’s World Service, which provided global coverage via its higher frequency Short Wave (SW) transmitters, continued to broadcast on MF at 648 kHz.
Many local radio stations now operated entirely separate services on their FM and AM frequencies, which meant that most people needed equipment that covered all the bands, even though the BBC had previously said that a VHF-only tuner would accommodate their services.
Although simple superhet receivers had always been popular, the digital advances of the 1980s made such devices seem rather old-fashioned. It was now possible to digitally generate a highly-accurate frequency within what was known as a synthesiser receiver. The result could be combined with an incoming AM signal at the same frequency so as to reproduce the original audio information, a process known as direct conversion. More importantly, the synthesised frequency never drifted, so the receiver never went off tune. However, for this to work, all transmitters had to use frequencies that were multiples of a base frequency, which was set at 9 kHz. In the case of the BBC’s LF transmitter, for example, this meant moving down to 198 kHz. Having made these changes, it was possible for anyone with this kind of receiver to dial in a frequency or channel number and receive the required programme immediately.
FM broadcasting, although adequate for the 1950s, was considered rather ‘mid-fi’ by the 1990s. This brought about experiments with Digital Audio Broadcasting (DAB), a new system that operated in VHF Band III. The earliest receivers only worked on the DAB wavebands, although later models also accepted standard FM signals. However, things became rather more complicated with the introduction of Digital Video Broadcasting (DVB), which also provided radio stations such as BBC7 and OneWorld Radio over its Freeview service.
The whole situation is now horribly confused, with various services using one or more of the AM, FM, DAB and DVB systems, whilst some stations are also available via the Internet.
Radio programmes can be heard using one or more of the following methods:-
DAB employs terrestrial transmitters operating on Band III, in the range of 217.5 to 230 MHz, as formerly used by ITV for its 405-line television programmes and by a few military organisations. Some other countries also use L Band for DAB transmissions, although not all receivers can accommodate these higher frequencies. The actual audio signal is encoded using MPEG and Coded Orthogonal Frequency Division Multiplexing (COFDM) technologies. The signals can be received by means of a DAB receiver or a special tuner card in your computer.
FM operates over terrestrial transmitters using Band II, in the range of 88 to 108 MHz. The audio frequency response is restricted to 15 kHz, which is rather poor by modern standards. In addition, the level of background noise can be excessive, especially when listening to stereo material from a station that can only provide a weak signal.
AM is sent over terrestrial transmitters using the Medium Frequency (MF) band, also known as Medium Wave, in the range of 535 to 1,605 kHz (560 m to 187 m) and the Low Frequency (LF) band, also known as Long Wave (LW), in the range of around 150 to 250 kHz (1900 to 1170 m). The audio frequency response is limited to a meagre 5 kHz, which results in a poor sound quality, whilst interference due to lighting and electrical appliances can be irritating.
The following further options use what are essentially non-radio technologies:-
The services provided by Freeview on DVB in the UK also include some radio stations, all with digital sound. To receive these programmes you’ll need a FreeView set-top box or a digital television with an integral decoder.
Most satellite and cable TV services also offer selected programmes from radio sources. These are often digital and of very good quality, but to hear them you must have the appropriate hardware.
Many radio stations use real-time audio streaming (RTAS) to send their output over the Internet, allowing them to be heard across the world. For practical purposes you should have a non-metered Internet connection, and, for best results, a broadband connection, as well as the appropriate software for the stations that you want to hear. Unfortunately, audio streaming provides what can only be described as ‘mid-fi’ quality, although at least you can hear the station. Further information about the Internet and audio streaming can be found elsewhere in these guides.
Signals at higher frequencies can only travel over short distances. For VHF, this usually requires you to be in ‘line of sight’ of a transmitter, although sometimes you can receive a signal reflected off another structure or building. Unfortunately, VHF can suffer badly from the consequences of changes in a layer of the atmosphere known as the ionosphere, which is influenced by weather conditions and sunspot activity. The ionosphere then reflects VHF signals, allowing them to travel over hundreds of miles. The worst problem is co-channel interference, caused by foreign programmes on the same channel, although adjacent channel interference can also be a nuisance. With FM, both kinds of interference can be minimised by using a directional aerial, although this isn’t appropriate for DAB, where such interference is normally less of a problem.
Signals in the MF band normally travel over very long distances, which means that they’re usually easy to receive. However, atmospheric conditions can also effect this band, resulting in whistling and other unpleasant effects. Signals in the LF band are able to travel over much greater distances, allowing virtually all of the UK to be covered by the BBC’s Radio 4 transmitter. and are less affected by the ionosphere. However, both wavebands suffer equally from impulsive interference, as produced by lightning, unsuppressed motor engines or switches and other devices in electrical equipment. Problems of the latter kind can only be corrected by fitting appropriate suppressors at the source of the interference.
Radio transmitters and receivers are connected to a wire or metal conductor known as an aerial or antenna. The radio frequency (RF) signal from such an aerial can be as low as 0.1 microvolt (µV), although in most domestic AM receivers it’s around 50 µV. Modern receivers often don’t need to be connected to an external aerial since they have a built-in internal aerial. However, where an aerial socket is provided you can assume that better results can be obtained by using one.
An aerial must be insulated from the ground and can be horizontal, for radio waves that have horizontal polarisation, or vertical, for vertical polarisation. DAB and AM broadcast aerials are vertical, requiring a vertical receiver aerial, whilst traditional FM aerials are horizontal. The situation with FM is slightly complicated, since although horizontal polarisation was originally adopted, virtually all stations now use either slant polarisation or circular polarisation, both of which give better results with a vertical car aerial, whilst, in theory at least, you may get a better performance with the aerial at 45°.
The internal aerial for DAB or FM is usually in the form of a metal rod, either built into the body of the receiver or in the form of an extendible whip aerial. Better results can be obtained with FM when the aerial is tilted down to 45°.
The internal aerials for the LF or MF bands are often in the form of a single ferrite rod, onto which are wound the tuning coils for each band. This kind of aerial usually works very well and little improvement can be obtained using an external aerial, except perhaps where you’re trying to receive signals over very long distances.
For optimum transfer of energy, both the transmitting and receiving aerials must have a length equal to half the signal’s wavelength or a multiple thereof. For example, the wavelength of an FM station at 100 MHz is 3 metres (m), requiring a 1.5 m (4 ft 11 in) aerial. To receive an AM station at 1,000 kHz would, in theory, need a 150 m (492 ft) vertical aerial. Fortunately, you can use a quarter-wavelength Marconi antenna, the ground itself effectively doubling the length.
External aerials specifically designed for either DAB or FM are available. Note that DAB signals are vertically polarised, whilst those for FM normally require a horizontal aerial. Since DAB uses Band III, formerly employed for ITV’s 405-line television service, you can also use an old Band III TV aerial, although a directional aerial of an ‘H’ or multi-element form isn’t an advantage, since DAB receivers are meant to accept signals from multiple transmitters. You could remove the reflector elements to make such an aerial less directional, although this may upset its performance. Most users of FM only need to receive material from a single transmitter, allowing the use of a directional aerial, which also reduces the background noise.
The appropriate aerial or aerials can be mounted on an aerial pole, normally secured to a chimney or other solid part of a building. This is a specialist job, involving the safety of yourself and others, so it’s best left to the experts. However, before doing this, and assuming you have a non-metallic roof, it’s worth trying an aerial inside the loft, as the attenuation caused by roofing isn’t too serious on the VHF band, although any problems that do occur can be exacerbated by wet weather.
Aerials sold in the UK are designed for connection via a coaxial aerial cable, which should be of the low loss variety and have an impedance of 75 Ω: the use of inferior cable is a false economy. Some receivers, however, as well as some aerials, are designed to operate in conjunction with a 300 Ω ‘twin-wire’ feeder cable and matching hardware. Although most receivers of the 300 Ω type will be found to work perfectly well with a 75 Ω coaxial cable and aerial, better results may be obtained by fitting a balun, a small transformer that converts a 75 Ω circuit to 300 Ω, or vice versa.
Some aerial installers use a diplexer to combine the signals from a UHF television aerial and DAB or FM aerials onto a single cable. This device is normally fitted adjacent to the aerials, a similar diplexer being used at the destination to provide separate sockets for each service. Although this saves on cable, it results in a significant reduction in the signal level and may cause other complications. If possible, separate cables should be used from each aerial to the required destination. Note that problems can also occur when a splitter box is used to feed more than one receiver. An aerial amplifier can help in this situation, although it may introduce even more problems. If in doubt, use a separate aerial for each receiver.
A long wire aerial is often adequate for LF and MF reception. As the name implies, this is a long piece of wire, of almost of any type and length, strung out but insulated in the style of a washing line. Most aerial sockets for LF and MF also accommodate a ground connection: if possible, this should be wired to a metal stake inserted in the ground in a moist part of the garden. Failing all else, it can be wired to your mains earth, but not to a water or gas pipe, as this can have safety implications.
Normal AM signals require a total bandwidth of 10 kHz to accommodate an audio bandwidth of 5 kHz, since the transmitted signal consists of the carrier frequency as well as a 5 kHz upper sideband and a 5 kHz lower sideband. However, the carrier doesn’t contain any useful information, whilst the audio material can actually be extracted from one sideband on its own.
The single sideband suppressed carrier (SSBC or SSB) system reduces the bandwidth and transmitter power requirements by sending only one sideband. This allows twice as many stations to fit in a specified waveband and gives a 110-watt SSB transmitter a similar range to a 1,000-watt AM transmitter. However, receivers of this type, as used for ham radio, commercial radio telephones, marine-band radios and US citizens band radios, are more complex, requiring the carrier to be reinserted prior to mixing the signal with the output of a local oscillator.
A modified form of CW, known frequency shift keying (FSK) is used in teletype, facsimile, missile-guidance telemetry and satellite systems. This mechanism sends coded information by shifting the carrier frequency, typically in steps of between 400 and 2,000 Hz, producing sounds at the receiver that corresponded to the information being conveyed.
The technology for cellular telephones, originally developed at the beginning of the 1980s, uses a system that divides a given area into clusters, each containing several cells. In early versions, each cell’s transmitter covered a radius of about 13 to 19 km (8 to 12 mi) and conveyed around 120 two-way speech channels. To prevent interference, the neighbouring cells use different radio frequencies, although the frequencies in one cluster are usually repeated in adjacent clusters. As a phone user moves away from one transmitter, the signal is switched automatically to the transmitter in the next cell.
The European standard for digital Terrestrial Trunked Radio (TETRA), also known as Airwave, has been developed for use by the police and other public safety organisations. This offers secure digital communications, and, in common with cellular telephones, operates at microwave frequencies. Unfortunately, this has led to concerns over its safety, particularly as it uses a pulsed transmission system.
1997 Grolier Multimedia Encyclopedia, © 1997, Grolier Inc.
©Ray White 2004.