VHF-UHF Precipitation-Static Interference Effects on Aircraft

Abstract

In early researches very occasional precipitation static was reported at VHF frequencies. With increasing IFR traffic density, even occasional interference can be serious and for military operations, interference on communications channels can present a serious hazard during ground controlled instrument approaches as well as jeopardizing possible mission success. Experimental investigations of military and commercial aircraft have disclosed that the severe interference is caused by electrically isolated metallic sections on the aircraft exterior which become charged by frictional contact with atmospheric particles and then sparkover to the airframe. This results in a severe broadband interference spectrum centered at a wavelength corresponding to the dimensions of the section which is sparking. Although this type of interference has been previously observed, the investigations again illustrate the severity of the interference and the difficulty in identifying it. Also discovered in the investigations which had not previously been found was receiver desensitization resulting from corona discharges from commercial grounded type VHFUHF antennas. This can occur without the flight crew being aware of it. This investigation indicates that all VHF-UHF antennas having corona discharges from exposed metallic extremities may suffer this desensitization while in flight without the flight crew being aware of it as the interference does not break the squelch. The magnitude of the desensitization may reach 30 db. The impact of this phenomena on commercial airline and military operations, where ground controlled approach may be the only type of instrument approach available for some aircraft, suggests a serious potential problem which needs to be identified and corrected.