Calculation of the dependence of the multimode structure of the field strength of a short-wave signal on distance

It is shown that the multimode structure of a short-wave signal is determined primarily by the size of the communication channel. The channel size in turn depends on variations of the ionosphere along the path and the wave frequency; the channel size determines the ray divergence and, consequently, the spatial attenuation of the signal. Absorption is the determining factor in daytime conditions.