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Design parameters of UHF/VHF ground-to-air communication systems are discussed with particular reference to propagational reliability. Included are results of some flight trials made over Western Europe using UHF transmissions to four different ground locations with the object of studying the repeatability of signals and effect of different terrain conditions. These experiments were performed under winter conditions when signal variability due to atmospheric refraction was much reduced. In many cases, the plot of field strength vs. distance differed markedly from the theoretical smooth-earth value. However, near the optical horizon the field strength obtained was in reasonable agreement with the theoretical value at all four locations. The fading recorded at long range was no greater than about ±1dB. Both from theoretical arguments and from experimental results, it seems that only the near-in part of the first Fresnel zone is important. The Fresnel zones do not have the same significance as in the usual optical case. Experiments showed that, even in the case of a very irregular foreground, the field at extreme ranges was relatively unaffected. The dominating factor for long-range communication is therefore the degree of atmospheric refraction. By using a known probability distribution of the atmospheric refraction, it is possible to construct curves showing the probability of obtaining a given range with a particular set of system parameters. The paper concludes with curves of this nature for frequencies of 40, 120, and 300 MHz, and for a variety of antenna heights.