Control of phase-fading in long-distance radio communication

The paper describes a continuation of fading-control experiments by methods which rely on transmissions being made at adjacent frequencies. Measurements have been made at distances of 700 and 3 300 km. from the transmitter, and it is found that phase-fading is due to interference between two main sky-rays of comparable intensity. The path-differences between these rays have been measured with the help of the Appleton-Barnett frequency-change technique and the corresponding control modulations applied. The most satisfactory type of controlled signal is obtained by imposing two modulations having frequencies in a 3 : 1 ratio and by suppressing the carrier. The optimum control frequencies were found to be 200 and 600 cycles when the distance between sender and receiver was 700 km., whereas at 3 300 km. the best frequencies were 50 and 150 cycles. Reception of the doubly-modulated signals shows that, when the control frequencies have been adjusted to correspond to the sky-ray path-differences, the phase-fading is eliminated. This removes all of the deep troughs of fading, and the residual intensity variations are then well within the capabilities of simple automatic-volume-control apparatus at the receiver.