Excitation of Higher-Order Antenna Modes by Multipath Propagation

Deep fading on microwave line-of-sight radio relay links is known to be caused by two or more rays arriving at the receiving antenna at slightly different angles in the vertical plane. Destructive interference between these rays can produce deep fades (30 dB or more) of the received signal. In the experiment described, it was found that deep fading is often accompanied by strong higher-order mode excitation of the receiving horn-reflector antenna. The particular higher-order mode measured in this experiment was the , which is excited by horizontally polarized rays arriving off the antenna axis in the vertical plane. The mode as well as the dominant mode of the receiving horn-reflector antenna were continuously monitored for the summer of 1966. The experiment was conducted on a 22.8 mile path in New Jersey at a frequency of 6 GHz. Statistical distributions for the received signal level in the dominant mode, as well as statistical distributions for optimum selection of the and signals, are presented.