A mode-averaging diversity combiner

A diversity-combining technique is described which reduces frequency distortion caused by multipath propagation in high-frequency single- or double-sideband voice radio transmission. By repetitively sweeping a null through the vertical polar pattern of the receiving antenna, the relative amplitude of signal components arriving by the various propagation modes is varied at a rate which is high compared with the highest audio modulating frequency. A running average of the time-varying resultant of the various mode voltages is diode-rectified to extract the audio intelligence. Transmission nulls caused by mode interference at particular frequencies in the radio spectrum are filled in by this averaging process. The technique requires no more equipment than conventional combining and would appear to offer superior performance. A test of a three-channel version of a combiner using this principle has now been made. To demonstrate the most distinctive feature of the combiner, its ability to fill in mode-interference nulls or minima in a radio channel, special frequency-modulated transmissions were used. It was found that substantial null filling occurred and that, as expected, the filling in is more effective when a pair of antennas is spaced along the line to the distant station than when the same pair is spaced transversely with respect to that line. It is concluded that the theory of this combining operation, as described in this paper, is correct to a first order of accuracy.