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Measurements were carried out on 6.4-cm. and 17- cm wavelength on two optical paths, 54-km and 82-km long, stretching over sea nearly East-West and starting at the same point. For the greater part of the measurements, height-spaced receivers were used. The bulk of the statistical data comprises distributions of field strengths for every day of measurement. Curves for single receivers and diversity combinations of two receivers have been worked out. Distributions for every hour of a day as well as distributions of fade durations for a few days with special propagation conditions were obtained. A study of special fading phenomena with almost coinciding fades on the receivers in operation has been made. The data reveal that the field strength distribution for single receivers on days with a great number of fades generally approximates the Rayleigh distribution irrespective of wavelength, path, and antenna height within the range of height considered. The field strength distributions for diversity systems approximate the diversity Rayleigh distribution, which is derived from two uncorrelated Rayleigh distributed signals. Deviations from appropriate Rayleigh distribution towards more serious fading conditions seem to be more frequent and more pronounced for diversity systems than for single receivers. Distributions of fade durations are found to be log-normal. Measurements on three-height-spaced receivers on 17-cm wavelength indicate that the simple two-ray theory is insufficient to describe the ladings on a path over sea.
Date of Publication: July 1957