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It is shown that there is an optimum ground-station antenna height for use in typical air-to-ground communications systems. When antennas lower than this optimum are used, the maximum distance range is reduced at all aircraft altitudes. When antennas higher than this optimum are used, the interference between the direct and ground-reflected waves causes gaps to occur in the coverage at the higher aircraft altitudes. The minimum altitude at which the gap in coverage occurs decreases with increasing frequency and with increasing ground-station antenna height. The optimum antenna height decreases with increasing frequency and this, in turn, reduces the maximum distance range for satisfactory comzmunication as the frequency increases. The curves presented are based on values expected from theory for a smooth spherical earth. Such curves have been found experimentally to approximate very well the average practical operating results.