An analysis of corona-generated interference in aircraft

Triboelectric charging, occurring when an aircraft is operated in precipitation, raises the aircraft potential until corona discharges occur from points of high dc field on the aircraft. These corona discharges generate noise which is coupled into receiving systems. The magnitude and spectral distribution of this radio interference, called precipitation static, depends upon three factors: 1) the strength and spectral characteristics of the source discharges, 2) the manner in which the disturbances produced by the discharges couple into the antennas, and 3) the magnitude of the discharge current and its distribution among the discharging extremities. The coupling between the antenna and the noise source is discussed using a reciprocity relationship. Because the geometry of an aircraft is complicated, and a purely theoretical approach to the determination of coupling factors is not possible, a technique developed for measuring absolute values of coupling factor as a function of frequency and position on the aircraft is described. The spectral character of the corona-noise source is studied, including a study of how the source spectrum is affected by altitude. To test the validity of the theory and the results of the laboratory work, calculations are made to predict the noise currents induced in the two test antennas employed in a flight-test program conducted on the Boeing 367-80 aircraft (prototype of the KC-135 and 707). The results of these predictions are compared with the noise spectra measured in flight.