Generation and Control of Radio-Frequency Power

The radio-frequency power required by a transmitter is practically always obtained from a vacuum-tube oscillator. Vacuum-tube oscillator is capable of converting direct current1 power into alternating-current2 energy of any desirable frequency up to 300,000,000 cycles or higher. Over the range of frequencies used in long-distance radio communication, i.e., 12 to 30,000 kilocycles, the power that can be obtained from vacuum-tube oscillators is in the order of tens to hundreds of kilowatts, the efficiency with which the direct-current power is transformed into alternating-current energy being in the neighborhood of 50 per cent or higher.

A number of other methods of obtaining radio-frequency energy have been used at one time or another during the history of radio. Among those are the high frequency alternator, the frequency multiplier, and the oscillatory spark discharge. The high frequency alternator is a special high-speed inductor-type alternator with many poles. Such alternators are capable of generating several hundreds of kilowatts with reasonable efficiency when operating at frequencies of 50,000 cycles of less. The frequency multiplier utilizes a moderately high frequency alternator from which the desirable radio frequency is obtained by the use of magnetic-type harmonic generators. In this way it is possible with an alternator giving a frequency of 5000 cycles to produce considerable quantities of power at frequencies of from 20,000 to 40,000 cycles. This type of arrangement at one time had a very prominent place in radio but now, has practically disappeared.

The oscillatory spark discharge was the earliest and for many years the only method known for the generating of radio-frequency power. In this type of transmitter a condenser is charged to a high potential, which then breaks down a spark gap, permitting an oscillatory discharge through an inductance, this process being repeated about one thousand times each second. The spark transmitter thus radiates a series of wave trains, each of which being a damped sinusoidal oscillation. This method is capable of generating large quantities of radio-frequency energy with good efficiency but is in disfavor because of the radiated waves being not simple sine waves but rather waves of a number of frequencies superimposed on each other. The result of excessive interference with radio signals being transmitted on slightly different frequencies.

Modulation.

The transmission of information by radio waves requires that some means be provided to control the radio waves by the desired intelligence. In radio telegraphy this control is obtained by turning the transmitter on and off in accordance with the dots and dashes of the telegraph code. In the radio telephony the transmission is effected by varying the amplitude of the radio-frequency wave in accordance with the pressure of the sound wave being transmitted. Thus the sound wave would be transmitted from a radio-telephone station by causing the amplitude of the radiated wave to vary. In the transmission of pictures by radio a similar method is employed, in which the amplitude of the wave radiated at any time is made proportional to the light intensity of the part of the picture that is being transmitted at that instant.

When the amplitude of the alternating - current wave is varied from time to time, the wave is said to be modulated. Thus the wave radiated from a radio-telephone station is modulated by the voice or sound wave, while during the transmission of a picture the modulation is in conformity with the light intensities of different portion of the picture, and in the case of radio-telegraphy the modulation is by the telegraph code. Except in the case of telegraphy, the modulation of the radio-frequency wave is usually performed by means of vacuum tubes that control the amplitude of the generated of radiated high-frequency energy in accordance with the intelligence to be transmitted. ~ 3338

 

 

Comments:

a direct current (DC) – 1) постоянный ток 2) прямой (однонаправленный) ток

an alternating current (AC) – переменный ток, электрический ток, меняющий своё направление с частотой 50 Гц - в России и 60 Гц - в США.

an alternator – генератор переменного тока, синхронный генератор

a spark discharge – искровой разряд

a magnetic type harmonic generator – генератор гармоник магнитного типа, генератор гармонических колебаний магнитного типа

a spark gap – искровой промежуток, искровой разрядник

a wave train – цуг волн, последовательность волн

a damped sinusoidal oscillation – убывающее синусоидальное колебание

a sine wave – гармоническая волна, синусоидальная волна

to be in disfavor – быть в немилости

superimposed waves – наложившиеся волны

a conformity – соответствие, сходство

light intensities – значения силы света