Experiments on the electric strength of air at centimetre wavelengths

The paper is concerned with the electric strength of air when stressed by alternating fields of wavelengths 10.7 and 3.06 cm (2800 and 9800 Mc/s, respectively). The spark gaps used for the experiments consisted of either coaxial-line or waveguide transmission systems, and the electric stress was applied in the form of impulses lasting for one microsecond and repeated 400 times per second. Stress determinations were made by the relationship, afforded by Poynting´s vector, between stress and power conveyed along the system. The method of measurement is described. The behaviour of the spark gap is shown to be dependent on the irradiation conditions of the gap, but a minimum value of breakdown stress exists which is independent of irradiation. In all cases this minimum stress was compared at atmospheric pressure with the continuous direct breakdown stress of the gap, and was found to be approximately 70% of the direct value, irrespective of the gap length and frequency. Differences with respect to gas pressure are shown to exist between the microwave behaviour of such gaps and the corresponding direct-voltage behaviour.