Radio Transmission beyond the Horizon in the 40-to 4,000-MC Band

Reliable signals have been received at distances of several hundred miles at frequencies of 500 and 3,700 mc. The median signal levels are 50 to 90 db below the free-space field, but are hundreds of db (in one case 700 db) stronger than the value predicted by the classical theory based on a smooth spherical earth with a standard atmosphere. Antenna gains and beam widths are maintained to a first approximation and no long delayed echoes have been found. The experimental results are compared with other available data, and it appears that roughly the same median signal levels are obtained for frequencies from 40 to 3,700 mc.