Abstract :
RADIO communication makes use of a medium common to the whole world for the transmission of many signals simultaneously. In order to accomplish this a high-frequency electromagnetic wave has one of its characteristics varied in accordance with the instantaneous variations of the signal to be transmitted. The control of these variations is called modulation. The various simultaneous messages can then be separated by: (a). Differences in the frequency band used. (b). Differences in signal strength. (c). Differences in direction of the source.
