Text 24. Radio Transmitters

General Considerations. A radio transmitter is known to be es­sentially a device for producing radio-frequency energy that is con­trolled by the intelligence to be transmitted. A transmitter accord­ingly represents a combination of oscillator, power amplifiers, har­monic generators, modulator, power-supply systems, etc., which will best achieve the desired result.

Commercial transmitting equipment is ordinarily mounted on a framework of structural-steel members fronted by a vertical metal panel containing the controls and meters necessary for adjusting and monitoring the transmitter. All equipment appearing on the panel is at ground potential, instruments which must be observed during ad­justment or operation and which are not at ground potential being located behind the panel and viewed through windows. The steel frame is normally enclosed with wire mesh of some sort and is provided with doors that cut off the transmitter power when opened. This type of construction requires a minimum of floor space in proportion to the amount of apparatus involved, makes the transmitter accessible for inspection and repairing, and eliminates all hazard to persons.

The design of most radio transmitters, particularly those intended for broadcast and short-wave transmission, is dominated by the need of maintaining the transmitted frequency as nearly constant as pos­sible over long periods of time. In broadcast work two or more trans­mitters are commonly assigned the same carrier frequency, and in order to minimize the resulting interference it is essential that the carrier frequencies be as nearly as possible the same.

The Microphone Transmitter. The microphone transmitter may be one of the ordinary carbon granule type. Without going into details, it will suffice to state here that such a microphone consists simply of an elastic diaphragm bearing against a mass of carbon granules enclosed in a suitable chamber, the carbon granules forming part of an electri­cal circuit. When the microphone is not being spoken into, the dia­phragm remains stationary and exerts a constant pressure upon the carbon granules, the resistance of which remains, therefore, constant. On the other hand, when the diaphragm is set vibrating, as it is done by speaking into the microphone or through a noise or sound reaching it, the pressure exerted by the diaphragm against the carbon granules changes, and this change of pressure causes the resistance of the car­bon granules to increase or decrease in accordance with the displace­ment of the diaphragm from its position of rest.

When the microphone is not being spoken into, the alternator produces a high-frequency current of constant amplitude, i. e., an undamped current; the amplitude of this current is adjusted to the maximum by adjusting the inductance so as to make the natural frequency of the circuit equal to the frequency of the alternator.

Now, assume, for the sake of simplicity, a vibrating tuning fork to be placed in front of the microphone. The harmonic vibrations of the tuning fork will bring about harmonic vibrations of the micro­phone diaphragm, and these will produce variations in the resistance of the microphone. Since no other part of the circuit is undergoing any change, it is plain that a variation of the microphone resistance will produce a corresponding variation in the amplitude of the high- frequency antenna current. Thus, when the diaphragm is displaced inwardly the resistance of the microphone and, therefore, of the en­tire alternator circuit, decreases, and the amplitude of the current supplied by the alternator must necessarily increase, the reverse tak­ing place when the diaphragm is displaced outwardly.

Notes

consideration [ksn^ids'rei/n]	міркування
equipment [ik'wipmant]	устаткування
accessible [aek'sesabl]	зручний
to eliminate ['ilimineit]	усувати
hazard ['haezad]	небезпека
carbon ['ka:ban]	вуглець
granule ['graenju:l]	зернятко, зерно
to suffice [sa'fais]	вистачати
it will suffice	буде достатньо
to exert [ig'zart]	напружувати
chamber ft/eimba]	камера
to decrease [di:'kri:s]	зменшувати
inwardly ['inwsdli]	всередині