radio communications conducted with the aid of a network of radio transmitter-receivers that are usually spaced apart at intervals defined by the line of sight of their antennas. Each relay station receives a signal from a neighboring station and amplifies and retransmits the signal to the next station.
Radio-relay systems are used for multichannel transmission of telephone, telegraph, and television signals at decimeter and centimeter wavelengths. The use of decimeter and centimeter wavelengths makes it possible to operate simultaneously a large number of radio transmitters using signal bandwidths up to several tens of megahertz; furthermore, at these wavelengths the level of atmospheric and industrial radio interference is low, and it is possible to use highly directional antennas. Since a stable propagation of decimeter and centimeter waves occurs only within line-of-sight limits, communications over long distances make it necessary to construct a considerable number of relay stations. In order to increase the distance between stations as much as possible, antennas are mounted on masts and towers 70–100 m high and, whenever possible, in elevated locations. In flat regions the distance between stations is usually 40–50 km; the use of tropospheric radio communications in individual links of the network makes it possible to increase this distance to 250–300 km.
Several transmitter-receiver units are usually installed in each station. These units are housed in a common equipment building and use a common power supply, common antenna supports, and common antennas. This provides the line with several communications trunks and increases the transmission capacity. In order to allow for the simultaneous transmission of signals on many telephone channels, frequency-division and time-division of channels is used. Frequency division provides a greater number of channels per trunk than does time division, for example, up to 2,700 as opposed to 100; however, the apparatus for time division is simpler and more compact.
Radio-relay communications lines are subdivided into high-capacity lines (main lines), medium-capacity lines (regional lines), and lines with a small number of channels, which are used for communications along railroad routes, gas and oil pipelines, and power lines. There are also lines with a small number of channels used with mobile relay stations for military purposes.
The first radio-relay communications line, built in 1935 between New York and Philadelphia, featured five channels. As a result of advances made in the area of microwave engineering, the construction of radio-relay lines has proceeded at an accelerated pace since the 1950’s. By the early 1970’s, all developed countries had constructed complex radio-relay networks with several thousand telephone channels in each line. By the mid-1970’s, a set of standardized radio-relay units was developed in the USSR for operation on lines over total distances up to 10,000 km; such units made it possible to provide up to eight trunks for each line, with each trunk having a capacity of 1,800 telephone channels.
REFERENCESBorodich, S. V., V. P. Minashin, and A. V. Sokolov. Radioreleinaia sviaz’. Moscow, 1960.
Gusiatinskii, I. A., E. V. Ryzhkov, and A. S. Nemirovskii. Radioreleinye liniisviazi. Moscow. 1965.
Gusiatinskii, I. A., and A. A. Pirogov. Radiosviaz’ i radioveshchanie. Moscow, 1974.
I. A. GUSIATINSKII