Aircraft Precipitation-Static Radio Interference

Radio reception at normal frequencies on board high-speed, all-metal aircraft suffers severe interference under certain precipitation conditions. Flight research has shown that under these same conditions, an airplane will accumulate a relatively heavy electric charge. The resulting potential difference between this charged object and its immediate surroundings creates voltage gradients of sufficient magnitude on pointed and sharp-edged extremities of the craft to produce corona discharges. Local radio disturbances of varying character and severe intensity accompany these discharges. Several different mechanisms were found to be involved in the production of the discharges, and currents of magnitudes greater than one milliampere were observed under severe conditions from a single point. Numerous graphic records were taken in flight showing the correlation between the charge and discharge currents at various points on the airplane. These records also illustrated the rapidly fluctuating magnitude of the discharges and indicated that their directions reversed rapidly under certain space-gradient conditions. Ground tests involving the research airplane mounted on insulating supports and charged by means of a high-voltage rectifier, indicated that the radio interference and discharge currents observed in flight could be duplicated under controlled conditions on the ground. Further ground and laboratory tests verified and explained the recognized efficiency of the shielded radio loop in improving to a marked degree the signal-to- noise ratio in the presence of corona discharge interference. The mechanism of the interference was studied intensively in the laboratory.