FM backscatter sounding as a means for monitoring propagation conditions during short-wave broadcasts

Abstract

Employment of megawatt power levels in shortwave broadcasting, the declining sunspot cycle, and the growing number of broadcasters competing for spectrum space make it increasingly desirable for transmitter personnel to know when unusually low MUF's prevent a given broadcast from reaching its destination. Affected transmitters can be either shut off, thus saving running costs, or retuned to propagating frequencies. Backscatter sounding is known to be a satisfactory means for determining propagation conditions along the transmission path, but owing to the practical problems of integrating such sounding into broadcasting operations, has not been appreciably used as yet. It is shown that application of the FM ranging principle to backscatter sounding is feasible, permitting the broadcasting carrier itself to be used for sounding purposes without disturbance to transmitter operating conditions or appreciable annoyance to the distant listener. It is necessary that an associated receiving site within the ground-wave range of a given transmitting plant be available. Useful sounding measurements can be made with the aid of equipment normally found in any receiving station. In fact, an operator can gauge skip distance by estimating the pitch of an audible tone. On balance, this technique appears to have significant practical advantages in comparison with the pulse or modulation-correlation alternatives.