How it works : AM Radio

AM (amplitude modulation) RADIO

Above: how an AM signal is formed. The signal to be transmitted, typically a sound wave,is an irregular mixture of different frequencies (A). The carrier wave generated by the transmitter is a sinusoidal wave of a constant, much higher,frequencies and when no signal wave is superimposed on it, of constant amplitude (B). The complete AM signal is produced by combining the two waves so that the amplitude of the carrier is constantly modified by the amplitude of the signal (C). A radio receiver separates the two signals to recover the original sound wave.

AM radio is the accepted name for the system of sound RADIO broadcasting used in the medium wave band in all parts of the world and in the long wave band in certain areas, notably Europe. The letters AM stand for amplitude modulation, the technique by which speech and music are impressed on the radio waves. The sound waves from the studio are converted into electrical signals by the microphones, and these electrical signals are used to vary the amplitude- the distance from 'peak' to 'trough' of the radio carrier wave generated at a constant frequency in the transmitting station. As a result, the signal radiated by the station varies in amplitude in step with the variations in the loudness and pitch of the sound in the studio. In the receiver, the amplitude variations of the radio signal are separated from the carrier wave by a process known as demodulation, and after further amplification are fed to the loudspeaker.

Amplitude modulation is also used by international broadcasting stations operating in the short wave band, and for some professional communications such as air traffic control, ship-to-shore traffic, and some mobile radio services.

AM radio stations in the medium wave band can provide a service over an area the size of a town or of a whole continent, depending upon the power of the transmitter which can vary from less than a hundred watts to hundreds of kilowatts. Long wave AM radio stations cover even larger areas. Normally, all radio waves travel only in straight lines, which would limit a radio station to an area of only about 40 miles (65 km) in diameter-as far as the horizon. Medium and long wave stations can actually cover much larger areas because their signals are bent round the curvature of the Earth by the ionosphere (an electrically charged layer in the upper atmosphere which can reflect radio waves).

In the US, where AM radio stations are operated on a commercial basis, coverage is generally and deliberately limited to one city and its environs. Major cities such as New York are served by large numbers of stations, each radiating a different programme.

In the UK, where until recently sound broadcasting was run entirely by the BBC, which is a public corporation, the emphasis has always been on nationwide coverage. This is achieved by distributing four programmes throughout the country. over a cable network and broadcasting them from a large number of strategically placed transmitters each radiating the same programme or programmes. This pattern has not been altered by the establishment of medium wave local stations operated by both the BBC and the newly formed Independent Broadcasting Authority.

Use of AM
Amplitude modulation was the first technique to be used when broadcasting began in the US and Europe in the early 1910s. It proved highly successful but by the 1940s improvements in components and techniques were creating interest in the possibility of a very much higher sound quality in broadcasting.

The quality of reproduction on AM radio is not particularly good because of limitations in the amplitude modulation technique itself and because of overcrowding and subsequent interference in the medium wave band. Interference is particularly serious at night because the nature of the ionosphere alters after sunset, so that it can then reflect medium wave signals back to Earth a long way beyond their normal service area. Here, they interfere with other stations, producing whistles and other unwanted noises.

The AM method is still used for most popular broadcasting. Although the alternative FM RADIO system gives better quality, it is more difficult to receive, and is less suitable than AM for use in cheap transistor portables and car radios.

Recently, the rapidly falling cost of microcircuits (whole circuits on a single tiny chip of silicon) has made it possible to use in cheap transistor radios the highly complex circuits and techniques which previously could be employed only in expensive commercial receivers. One such technique, known as single sideband, will greatly reduce the interference effects that have long plagued AM reception. Single sideband will also provide better sound quality, particularly in areas near the limits of the area covered by a station, and will take up less channel space in the overcrowded medium wave band. Successful trials in the US have shown that a modification of this technique can provide a stereophonic AM radio service which will be particularly useful for reception in cars.


Reproduced from HOW IT WORKS p103