A Single-Sideband Musa Receiving System for Commercial Operation on Transatlantic Radiotelephone Circuits

In the operation of short-wave radiotelephone circuits selective fading is observed which is a result of the combination at the receiving antenna of waves which have arrived from the transmitter over paths of different lengths. The poor quality resulting from this fading may be mitigated by increasing the directivity of the receiving antenna in the vertical plane so as to favor the waves arriving at one angle to the exclusion of others. Friis and Feldman have described an experimental system designed to accomplish this end which they call a "musa" receiving system. This system was found under certain transmission conditions to give an improvement in the grade of circuit which could be obtained. A commercial installation of this type has now been constructed for use on the single-sideband circuits of the American Telephone and Telegraph Company from England. Two receivers have been provided for the operation of four radiotelephone circuits. The antenna system consists of a row of 16 rhombic antennas two miles long, each antenna connected by a separate transmission line to receivers located near the center of the row of antennas. In each receiver the signals from the antennas are combined in the proper phase to permit simultaneous reception from three adjustable vertical angles. The three signals are then added through delay-equalizing circuits or discretely selected on the basis of amplitude to obtain diversity reception.