Radio Transmission Center

a complex of buildings and technical equipment for the radio transmission of telegraph and telephone messages, music, images, and the like. The first radio transmission centers were built near the cities of Nauen (Germany, 1908), Rugby and Caernarvon (Great Britain, 1908 and 1913), Bordeaux (France, 1910), and St. Petersburg and Moscow (1914). The basic technical equipment of a radio transmission center consists of radio transmitters, antenna systems that are connected to the transmitters by feeder lines, and, where needed, a grounding system. The technical buildings are located near the antennas. They contain the radio transmitters (as many as several dozen in large centers) and transmitter-operating equipment, such as electric power supply, water-cooling, evaporative-cooling, and air-cooling systems for high-power electron tubes. They also contain equipment for switching and remote control of the antennas and for blocking off areas that may constitute a danger to personnel. In addition, the technical buildings house equipment for signaling and control to maintain normal operations, traffic-control and telephone communication facilities, and an electric clock system that provides a correct time signal for all technical service areas.

A transformer substation is located within the radio transmission center. It supplies electric power to the center’s equipment, either from an AC network or from its own sources, such as a diesel power plant. The transmission center’s equipment must ensure the reliable operation of the radio transmitters and maintain their technical performance within the required limits, as defined by such criteria as power output, frequency stability, and coefficient of nonlinear distortion. Most of the equipment is automated.

The radio transmitters in a transmission center are classified according to their function as radio broadcast transmitters, television transmitters, and special-purpose transmitters. The last group includes transmitters used in radio navigation, radio direction-finding, space communications, and ionospheric research. The operating wavelength of a transmitter is chosen according to the transmitter’s function and in accordance with radio communications regulations. The main type of transmitter is a shortwave transmitter that operates at wavelengths of 10–100 m at power ratings of 1, 5, 20, 50, and 80–100 kilowatts (kW). Radio transmitters for long-distance broadcasting in the short and medium wavelength bands have power ratings of 500 and 1,000 kW, and transmitters for regional broadcasting are rated at 150 kW for medium wavelengths and up to 100 kW for short wavelengths. Broadcasting transmitters in television transmission centers (telecenters) have power ratings of 5–70 kW and wavelengths in the meter or decimeter range. Meter range frequency modulated transmitters with power ratings of 1–20 kW are used for high-quality local radio broadcasting.

High-power transmitters for medium-wave radio broadcasting consist of several modules. Power outputs of such modules are combined in a common, intermediate oscillatory circuit or, if there is an even number of modules, in a special device called an additive bridge. (This device is also used for shortwave and meter wavelength transmissions.) Such modular arrangement makes it possible to continue the transmission—although at a somewhat lower power—if one of the modules breaks down. Radio transmission centers are also equipped with reserve transmitters that are capable of a smooth frequency changeover within a given range of wavelengths and that can be hooked up to an operating antenna. In case of breakdowns, such transmitters temporarily replace the radio transmitters that went out of order.

In present-day (1975) radio transmission centers, the transmitters are usually operated by remote control from a central console that initiates start-up, shutdown, retuning to a different wavelength, and other operations. The controls themselves may also be automated and may be operated by a traffic-control station located several dozen km from the center. Trunk cables and radio relay lines are used to transmit electric signals containing the message from the point of communication (telegraph, telephone central, radio office or station) to the transmission center. Since the operating of a center’s transmitters causes considerable interference with radio reception, radio transmission centers are built at distances of 50–80 km from radio reception centers and densely populated areas. However, television transmission centers are usually built within city limits.