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radio transmitter[′rād·ē·ō ′tranz‚mid·ər]
a device or assembly of devices used to obtain modulated electric oscillations in the radio-frequency range and radiate them through an antenna in the form of electromagnetic waves. A radio transmitter is the most important part of systems and equipment for the transmission of information by radio waves, including those used in radio communications, television, radio broadcasting, radar, radio navigation, and other areas of engineering, as well as in scientific experiments.
Radio transmitters are classified according to their operating wavelength and according to the oscillation power fed to the antenna as low-power (up to 100 watts [W]), medium-power (from 100 W to 10 kilowatts [kW]), high-power (from 10 k W to 1 megawatt [MW]), and very-high-power transmitters (above 1 MW). Transmitters are also classified according to the type of operation (telegraph, telephone, and others), according to the method of modulation (amplitude, frequency, phase, or other modulation), according to the type of electronic device used as an oscillator (electron tube, transistor, magnetron, klystron, and other types), according to their purpose (communications, broadcasting, radar, television, and others), and according to their mobility (stationary or mobile).
The simplest (single-stage) type of radio transmitter contains a self-excited oscillator, which converts the energy of a direct current (less frequently, of an alternating current) to the energy of radio-frequency oscillations, a modulator, and a source of electric power. However, transmitters operating at decimeter or longer wavelengths usually consist of several stages that have different functions; this is particularly true of medium-power and high-power transmitters. The multistage arrangement in radio transmitters is mainly the result of the need to obtain oscillations of sufficient power with a highly stable carrier frequency; permissible frequency drift is usually between 10–6 and 10–9. The use of various methods of frequency stabilization usually makes it possible to obtain sufficiently stable oscillations only in low-power, self-excited oscillators (driving oscillators) operating at frequencies that are usually lower than the operating frequency of the transmitter. In this case, the frequency is multiplied in the succeeding stages of the transmitter (frequency multipliers).
If the requirements for frequency stability are particularly stringent, a buffer stage directly follows the driving oscillator; the buffer stage protects the driving oscillator from feedback from succeeding high-power stages in the transmitter. In order to increase the oscillation power, a preamplifier stage (or stages) is used to amplify the voltage and power; the preamplifier excites the power output stage of the transmitter, a separately excited oscillator. The modulation of radio-frequency oscillations is achieved by changing one of the parameters of the radio transmitter. Modulated oscillations are fed through coupling circuits to an antenna or to a cable or wire transmission line.
REFERENCESDrobov S. A., and S. I. Bychkov. Radioperedaiushchie ustroistva, 4th ed. Moscow, 1969.
Rodionov V. M. Istoriia radioperedaiushchikh ustroistv. Moscow, 1969.
Model’, Z. 1. Radioperedaiushchie ustroistva. Moscow, 1971.
V. M. TIMOFEEV