Ultra-Short-Wave Transmission and Atmospheric Irregularities

Abstract

Results of an ultra-short-wave fading study are here reported. Transmission was carried out in the range of 1.6 to 5.0 meters, over a 70 mile (112.6 kilometer) ocean path, on 106 days during a period of two years. Both horizontal and vertical polarizations were used and during part of the time a 6-megacycle amplitude, 120-cycle, frequency modulated transmission was added, for the cathode-ray tube observation of the frequency characteristics of the radio path. On 45 mornings records were taken, on vertically polarized radiations, during the flight period of the Mitchel Field Weather Bureau plane. Fading was found present practically all of the time. Amplitude changes up to 40 db and fading rates up to 5 fades per minute were found. Simultaneous transmission of the same wave in two polarizations, and of two waves of different wave-length in the same polarization showed that the horizontally polarized component was practically always, and the shorter wave-length one was usually the worse fader of the pair. The greater part of the time there was no correlation between the fading of these radiation pairs; occasionally, however, and for the slow, smooth amplitude, undulating type of fading, coincidence was observed. The frequency sweep patterns showed multiple signal components to be present, with various degrees of relative phase retardation. A tentative explanation is proposed for these phenomena. This theory assumes the presence of a refracted-diffracted signal component, transmitted along the earth's surface and calculable in the manner of Wwedensky, Van der Pol and Gray, and one or more signal components reflected from air mass boundaries. The airplane results are shown to be in reasonable agreement with the frequency sweep observations. Boundary heights from 5.5 kilometers down to 1.9 kilometers are measured; below 1.9 kilometers other boundaries are indicated. The receiver band, flat over two megacycles, sets the low height limit of resolution of reflecting bound- ries at 1.9 kilometers. Most of the boundaries are at the lower heights.