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radio communication[′rād·ē·ō kə‚myü·nə′kā·shən]
telecommunication by means of radio waves. Radio communication requires the use of both transmitting and receiving equipment. The transmitting equipment, which includes a radio transmitter and a transmitting antenna, is installed at the point from which messages are transmitted. The receiving equipment, which includes a radio receiver and a receiving antenna, is installed at the point at which messages are received.
In the transmitter, sinusoidal oscillations are generated at a carrier frequency belonging to some range of radio frequencies. These oscillations are modulated in accordance with the information being transmitted. The modulated radio-frequency oscillations constitute the radio signal. The signal passes from the transmitter to the transmitting antenna, which excites correspondingly modulated electromagnetic waves in the surrounding space.
The radio waves travel to the receiving antenna, in which they excite electrical oscillations. These oscillations are passed on to the receiver. The signal thus received is very weak, since only a very small fraction of the radiated energy reaches the receiving antenna. For this reason, in the receiver the signal first is fed into an amplifier and is then demodulated, or detected. As a result, there is obtained a signal analogous to the signal used to modulate the carrier-frequency oscillations in the transmitter. Usually after an additional amplification, the signal is converted by an appropriate reproduction device into a message equivalent to the original message.
At the reception point electromagnetic oscillations from extraneous sources of radiation may be superposed on the signal. These superposed oscillations can interfere with the correct reproduction of a message and are therefore called radio interference. There are two other types of radio interference. First, the quality of radio communication can be adversely affected by variations with time in the attenuation of radio waves on the path from the transmitting antenna to the receiving antenna. Second, distortion can result when radio waves propagate simultaneously along two or more paths of different lengths. In this case, the electromagnetic field at the reception point is the sum of radio waves that are displaced in time. The interference causes distortion of the radio signal; the influence on the reception of radio signals is particularly great in long-distance communications. The growth of radio communications and the use of radio waves in, for example, radar and radio navigation are based on the principle of electromagnetic compatibility—that is, the requirement that different systems and equipment using radio waves function simultaneously without undesired mutual interference.
The transmission of radio signals through open space in the form of radio waves means that theoretically the signals can be received by persons other than those for whom they are intended. In other words, radio signals are subject to interception or monitoring. This disadvantage of radio communication is not shared by telecommunication through closed lines, such as cables or radio wave guides. The privacy of telephone conversations and telegraph messages is provided for by the Communications Rules of the USSR, by the corresponding regulations of other countries, and by international agreements. When necessary, secrecy is ensured by automatic equipment for disguising radio signals. Coding is one method of providing secrecy.
As early as the 1890’s, T. A. Edison made an attempt at radio communication and was granted a patent for his invention. Edison’s work was undertaken before H. Hertz discovered electromagnetic waves in 1888. Although Edison’s efforts did not result in a practical solution, they provided a stimulus for other work on the realization of wireless communication. Hertz invented the spark transmitter, which subsequently underwent various improvements and remained for several decades the type of transmitter most widely used in radio communications. The possibility and basic principles of radio communication were discussed in detail by W. Crookes in 1892, but it was not expected at that time that these principles would soon be put into practice. The development of radio communications began after A. S. Popov, in 1895, and G. Marconi, in 1896, constructed sensitive receivers that were fully suitable for achieving wireless signaling—that is, for radio communication. On May 7, 1895, Popov gave the first public demonstration of the radio apparatus he had designed and of the wireless transmission of signals by it. Radio communications can thus be regarded as having originated on this date.
Not only was Popov’s receiver suitable for radio communication, but he also used it, with some added components, for the automatic recording of lightning discharges. This achievement, which also occurred in 1895, marked the beginning of radio meteorology. Active work on the commercial use of radio communication was undertaken in Western Europe and the USA. In 1897, Marconi established the Wireless Telegraph and Signal Company, Ltd., in England. He founded the American Marconi Company in 1899 and the Marconi International Marine Communication Company, Ltd., in 1900. In December 1901 he sent a radiotelegraph transmission across the Atlantic Ocean. A. Slaby, G. von Arco, and K. F. Braun organized the manufacture of radio-communications equipment in Germany in 1902.
The development of radio communications throughout the world was stimulated by the obvious great value of radio communication for navies and marine transport and by its beneficial role in rescuing shipwrecked persons. The representatives of 29 countries took part in the first International Radiotelegraph Conference, which was held in Berlin in 1906. The conference adopted the Radio Regulations and an international convention that went into effect on July 1, 1908. The regulations fixed the allocation of radio frequencies among the various radio services. The International Frequency Registration Board was established, and the international distress signal SOS was adopted. At the international conference in London in 1912, the allocation of frequencies was somewhat modified, the regulations were made more precise, and there were established new services for radio beacons, the broadcast of weather reports, and the broadcast of exact time signals. The radio conference of 1927 prohibited the use of spark transmitters. Since such transmitters generate radiation over a broad spectrum of frequencies, they prevent the efficient use of radio frequencies. Spark transmitters were permitted only for the transmission of distress signals, since their broad radiation spectrum increases the probability of reception of the signal. The development of radio equipment was based primarily on the use of electron tubes from 1915 until the 1950’s, when the introduction of transistors and other semiconductor devices was begun.
Until 1920 radio communications employed, for the most part, wavelengths ranging from hundreds of meters to tens of kilometers. In 1922 radio amateurs discovered that decameter, or short, waves could propagate over any distance: they are refracted in the upper layers of the atmosphere and are reflected from these layers. Decameter waves soon became the principal means of long-distance radio communication. Signals transmitted in this way over long distances are received by sensitive receivers and large comparatively beamed antennas. Such an antenna structure occupies a large piece of land, which is called the antenna field. Similar structures are used for radiating decameter waves. To reduce interference, the receiving equipment is housed in a special radio reception center located outside the city and far away from radio transmitters. The transmitting equipment is housed in a radio transmission center. Both the reception and the transmission centers are connected with a central telegraph office located in the city. Signals are sent from the central office for transmission, and received signals are relayed to the central office.
During the 1930’s the use of meter waves was introduced. The 1940’s saw the introduction of decimeter and centimeter waves. These three types of waves propagate essentially along a straight line and do not bend around earth’s curvature. This limitation to line-of-sight transmission means that direct communication by means of such wavelengths is restricted to distances of 40–50 km. The frequency ranges corresponding to these wavelengths extend from 30 megahertz (MHz) to 30 gigahertz, a width 1,000 times greater than the width of the frequency ranges below 30 MHz, which corresponds to a wavelength of 10 m. Because of this great width, the frequency ranges above 30 MHz permit huge amounts of information to be transmitted by the methods of multichannel communication. At the same time, the limited transmission range and the possibility of producing a sharp beam with a comparatively simple antenna mean that the same wavelengths can be used at a number of locations without mutual interference. Transmission over considerable distances can be achieved by relaying through ground repeater stations or communications satellites, which are located at very high altitudes—approximately 40,000 km—above the earth’s surface. Communications systems using satellites or ground repeater chains can simultaneously carry over great distances tens of thousands of telephone conversations or tens of television programs. Such systems are thus incomparably more efficient than conventional long-distance radio communications based on decameter waves, whose importance is consequently declining. Decameter waves, however, remain of value as a useful standby and as a means of communication where traffic is low.
High-power (tens of kilowatts) transmitters permit radio communication by means of meter waves over distances of ~ 1,000 km within a narrow range of frequencies (several kilo-hertz). This is a result of the scattering of the waves in the ionosphere. Use is also made of the reflection of radio waves from the ionized trails of meteors burning in the upper layers of the atmosphere. In this case, however, the transmission of information is intermittent. Consequently, meteor-trail systems cannot be used for radiotelephony.
A small fraction of the energy of decimeter- and centimeter-wavelength radiation can propagate beyond the line of sight to distances of hundreds of kilometers because of the electrical in-homogeneity of the troposphere. When transmitters of comparatively high power—several kW—are used, this effect permits the construction of radio-relay systems with repeater stations of intervals of 200–300 km. Larger intervals are possible, but then the usable frequency spectrum is narrowed—that is, the amount of information that can be transmitted decreases.
Radio links are used to transmit telephone messages, telegraph messages, digital data, facsimile images, and television programs. Waves in the very high frequency range or higher are generally used for television programs. Depending on their purpose and length, radio links are classified as international or domestic. Domestic links are subdivided into trunk lines and regional lines. Trunk lines run between the capital of the USSR, the capitals of the Union republics, and the krai and oblast centers. Regional lines connect points within an oblast or a raion. The planning of radio-link development takes into account the inclusion of radio communications in the integrated automatic communications system.
Radio services are the organizational-technical measures and means for establishing and maintaining the systematic functioning of radio communications. They can be classified according to, for example, their purpose, their range, and their structure. Examples of services are the earth-space service and the space service, which encompass all types of radio communications involving the use of one or more satellites or other space vehicles. Other examples are the fixed service, where communication is between permanently located stations; the mobile service, where communication is between a mobile station and a land station or between mobile stations; radio broadcasting; and television. To meet industrial and official needs special services exist in some ministries and organizations. Thus, there are services for civil aviation, the railroads, river and marine transport, and such municipal agencies as the fire department, militia, and health department. Another example is the use of radio for internal communication in industrial and agricultural enterprises and in some institutions. Radio communications are of great importance in the armed forces.
REFERENCESReglament radiosviazi. Moscow, 1975.
Izobretenie radio: A. S. Popov: Dokumenty i materialy. Edited by A. I. Berg. Moscow, 1966.
Razvitie sviazi v SSSR, 1917–1967. Edited by N. D. Psurtsev. Moscow, 1967.
Chistiakov, N. I., S. M. Khlytchiev, and O. M. Malochinskii. Radiosviaz’i veshchanie. Moscow, 1968.
Gusiatinskii, I. I., and A. A. Pirogov. Radiosviaz’ i radioveshchanie, Moscow, 1974.
N. I. CHISTIAKOV