The distance at which a direct radio wave can reach a receiving antenna of given height from a transmitting antenna of given height. Note: The radio horizon range in nautical miles, R, is given by the relation R = 1. 23 (ht1/2 + hr1/2,) where ht and hr are the heights of the transmitting and receiving antennas in feet. The radio horizon range, R, in nautical miles is also given by the relation R = 2. 23 (ht1/2 +hr1/2,) where ht and hr are the heights of the transmitting and receiving antennas in meters. The effective Earth radius, 4/3 times the actual Earth radius, is used in deriving the formulae. Second-order differentials are neglected. They are of the order of 0. 1%.