The Bell System Technical Journal 



Vol. XVII October, 1938 No. 4 



Ultra-Short-Wave Transmission and Atmospheric 

 Irregularities 



BY C. R. ENGLUND, A. B. CRAWFORD AND W. W. MUMFORD 



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 radia- 

 tions, 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 polar- 

 izations, and of two waves of difi"erent 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 fre- 

 quency 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 com- 

 ponent, 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 air- 

 plane 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 kilo- 

 meters other boundaries are indicated. The receiver band, flat over 

 two megacycles, sets the low height limit of resolution of reflecting 

 boundaries at 1.9 kilometers. Most of the boundaries are at the 

 lower heights. 



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