PROLONGED SIGNAL FADE-OUT ON A SHORT MICROWAVE PATH

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Abstract
During a period of 1 year, the incidence of signal fade-out has been observed in 2000 Mc/s transmissions over a 21-mile path near Ottawa. Fade-out durations varied from a few minutes to several hours, with the most frequent occurrence in summer and during the night. The cause of signal fade-out has been studied qualitatively through a ray analysis of the air refractivity profiles at the center of the radio path. This analysis indicates that fade-out is associated with a shallow, horizontal transition zone in air vapor pressure at a level near the antenna heights. Here, the vapor pressure changes through 1 millibar or more in a height interval of 100 ft. The signal fade-out is weak if the transition is from dry air below to moist air above, but is strong if the inverse transition occurs. Weak fade-out (a 'radio hole') is due to ray divergence, and strong fade-out (a 'radio antihole') is due to ray interference at the receiver.

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