Radio

B

Transmitters

Essential components of a radio transmitter include an oscillation generator for converting commercial electric power into oscillations of a predetermined radio frequency; amplifiers for increasing the intensity of these oscillations while retaining the desired frequency; and a transducer for converting the information to be transmitted into a varying electrical voltage proportional to each successive instantaneous intensity. For sound transmission a microphone is the transducer; for picture transmission the transducer is a photoelectric device.

Other important components of the radio transmitter are the modulator, which uses these proportionate voltages to control the variations in the oscillation intensity or the instantaneous frequency of the carrier, and the antenna, which radiates a similarly modulated carrier wave. Every antenna has some directional properties, that is, it radiates more energy in some directions than in others, but the antenna can be modified so that the radiation pattern varies from a comparatively narrow beam to a comparatively even distribution in all directions; the latter type of radiation is employed in broadcasting.

The particular method of designing and arranging the various components depends on the effects desired. The principal criteria of a radio in a commercial or military aircraft, for example, are lightness of weight and intelligibility; cost is a secondary consideration, and fidelity of reproduction is entirely unimportant. In a commercial broadcasting station, on the other hand, size and weight are of comparatively little importance; cost is of some importance; and fidelity is of the utmost importance, particularly for FM stations; rigid control of frequency is an absolute necessity. In the United States, for example, a typical commercial station broadcasting on 1,000 kHz is assigned a bandwidth of 10 kHz, but this width may be used only for modulation; the carrier frequency itself must be kept precisely at 1,000 kHz, for a deviation of one-hundredth of 1 per cent would cause serious interference with even distant stations on the same frequency.

B

1

Antennas

The antenna of a transmitter need not be close to the transmitter itself. Commercial broadcasting at medium frequencies generally requires a very large antenna, which is best located at an isolated point far from cities, whereas the broadcasting studio is usually in the heart of the city. FM, television, and other very-high-frequency broadcasts must have very high antennas if appreciably long range is to be achieved, and it may not be convenient to locate such a high antenna near the broadcasting studio. In all such cases, the signals may be transmitted by wires. Ordinary telephone lines are satisfactory for most commercial-radio broadcasts; if high fidelity or very high frequencies are required, coaxial cables are used.

C

Allocation of Wavelengths

What people actually listened to was, and still is, crucially dependent, not just on what programme-makers construct but on the allocation of wavelengths and the distribution of transmitters. The discovery that electromagnetic waves could carry radio signals over the horizon had raised the prospect of broadcasting on an international scale, but governments and broadcasting organizations rapidly realized the inherent problems of the growth in the medium: if radio stations operated on the same, or very similar wavelengths, listeners would suffer severe interference in reception.

In 1925 an international agreement over the allocation of wavelengths was reached in Europe in 1925 through the so-called Geneva Plan of the Union Internationale de Radiophonie, and in the United States, Congress passed the Radio Act in 1927 to create the Federal Radio Commission. Regulation of the world’s electromagnetic spectrum has since been enacted largely by national governments through the International Telecommunication Union based in Geneva; however, from these earlier dates onward, transmission technology has been concerned primarily with the range and frequency of the signal being broadcast.

D

Short-Wave Radio

Short-wave radio uses higher frequencies (3 to 30 MHz) and shares the ability to travel long distances. In this case, however, transmitters can switch their precise frequency several times throughout the 24-hour period to take continuous advantage of the reflective properties of the ionosphere. The first short-wave transmitters of the 1930s opened up the prospect of much more controlled long-distance radio broadcasting, and the International Telecommunication Union has since allocated much of the short-wave spectrum for just such use.

Most remaining parts of the short-wave spectrum are used for amateur (“ham”) radio, and various marine, air, and mobile land services. The very shortest radio waves—designated as very high, ultra-high, and super-high frequencies (VHF, UHF, and SHF)—are not reflected by the Earth’s ionosphere, and their use is restricted to television, satellite transmissions by microwaves, or VHF radio stations, the last now more popularly described by the term “FM” radio.

E

FM and AM Transmission

“FM” stands for “frequency modulation”, as opposed to “AM”, or “amplitude modulation”. Both terms apply to techniques for imposing a meaningful pattern of variations on an otherwise unvaried stream of energy during transmission, but they have also come to be applied to whole categories of broadcast radio.

AM modulates the carrier radio wave by varying the amplitude (strength of the wave) in accordance with the variations of frequency and intensity of a sound signal, such as a musical note. Such modulation is vulnerable to electrical interference, and the sound quality is variable. Throughout the first half of the century, most standard radio broadcasting was achieved using this technique, and today some music and a great deal of speech radio, which does not necessarily demand high-quality reception, is still found on the AM dial.

FM works by varying the frequency of the carrier wave within a narrowly fixed range at a rate corresponding to the frequency of a sound signal. It is used within the VHF band, so that the terms “VHF” and “FM” have become synonymous for most radio listeners. FM reaches only to the horizon, so a transmitter’s remit is local rather than national in scale. This geographical restriction has the advantage of reducing interference, and coverage is therefore more stable, day or night. The signal itself is inherently static-free, unlike that for AM, and a suitable receiving-set can take advantage of its more generous frequency range and dynamic range to reproduce high-fidelity sound.

FM’s quality advantage over AM, exaggerated further with the development of stereo, has proved particularly suitable for the broadcasting of music and explains the rapid growth in the number of FM stations—often associated with rock and pop—in the 1960s, 1970s, and 1980s (see Sound Recording and Reproduction).