. Radio, miracle of the 20th century ... a story of human achievement that stands unrivalled in the history of humanity, together with a colorful portrayal, giving a broad, general view of the whole subject of wireless telegraph and telephone and its marvelous development . alradio frequency oscillation, but follows the change in theamplitude of these oscillations. This change is repre-sented by one of the envelop lines as stated. When a radio wave has its amplitude varied so that itsenvelope is made to assume any desired curve the waveis said to be modulated. The instrument or apparatusthat a
. Radio, miracle of the 20th century ... a story of human achievement that stands unrivalled in the history of humanity, together with a colorful portrayal, giving a broad, general view of the whole subject of wireless telegraph and telephone and its marvelous development . alradio frequency oscillation, but follows the change in theamplitude of these oscillations. This change is repre-sented by one of the envelop lines as stated. When a radio wave has its amplitude varied so that itsenvelope is made to assume any desired curve the waveis said to be modulated. The instrument or apparatusthat accomplishes this object is called a modulator. The simplest way of modulating a radio wave is bychanging the resistance of the antenna. This change inresistance changes the intensity of high-frequency cur- 308 RADIOTELEPHONY rent in the antenna. This changes the amplitude of theoscillations as the amplitude varies with the intensity ofcurrent in the oscillation. Figure 48-A shows this simplearrangement where T is a microphone transmitter. Speech in the mouthpiece of the microphone varies theresistance of the aerial circuit, thus causing a variationin the current. A modification of this method is to havethe microphone shunt a condenser in the antenna circuit 30O Volts. 200 vo/+f} 7T Oscillations with200 Volts on P/afe ^- Oscillation? with300 Volts on Plate or a part of the inductance in the circuit. By this methodnot only is the resistance varied but the antenna has itsnatural period of oscillation varied by the action of themicrophone. This change in natural period throws theantenna circuit out of tune with its primary and hencechanges the amount of current in it. This change inresonance can be made to add its effect to the change inresistance caused by the microphone. The methods ofmodulating described in this paragraph are not usedvery much because both of them waste a great deal ofenergy. Another method of modulation is called the absorptionmethod. Figure 48-B is a
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