High-Voltage D-C Point Discharges

D-c corona discharges from pointed and sharp-edged conductors have been found to give rise to severe radio interference on aircraft under certain precipitation conditions. Laboratory studies, employing a point-to-plane arrangement, have shown that these discharges are greatly influenced not only by voltage and polarity but also by the geometry of the points. Three distinct types of positive and two of negative corona have been found. One of these types, that occurring on sharp, slender points, is essentially common to both polarities and is free from radio interference at all frequencies up to at least several megacycles. Those discharges common to rather bluntly-backed points extend out some distance from the electrodes and are made up, in general, of a steady ion-migration component plus a series, orderly or otherwise, of ionic bursts. The latter phenomena, through an impulse excitation mechanism, give rise to the radio interference. A number of oscillograms were taken with the Duddell-type oscillograph showing the simultaneous current and radio-interference characteristics of the various types of point discharges. These oscillograms, taken with carefully constructed amplifiers, show the steady component of the discharge to be free from radio interference. Each impulsive burst, however, gives rise to a burst of radio output. Cathode-ray oscillograms show these current bursts to be typical double-exponential impulses of very steep wave front, and in general, of short duration. High resistance in the discharge circuit near the ionization point has a suppressing action on the impulses and greatly reduces the accompanying interference.