Control of wireless signal variations

The paper describes an experimental investigation, for simple conditions of short-distance propagation, of fading control of radio signals by methods which rely on transmission being made on adjacent frequencies. It has been found that the frequency separation of the signals required for optimum fading control is determined by the path-difference between the ground and sky rays and is independent of the mean frequency of the wave. In general it is necessary to read messages on such circuits by observing the change in direct current in the detector at the receiver, since the audio-frequency components are uncontrolled. Applications of the system of fading control are suggested in connection with ionospheric research, direction-finding, and broadcast telegraphy and telephony. The most satisfactory type of controlled transmission involves suppressed-carrier technique. The use of this system in short-distance propagation ensures that the night-time signal never falls below that due to the ground ray alone. In long-distance transmissions, to be described later, the effect of the control is to remove the deep troughs of fading.