The Ground Wave

Author: J.B. Hoag

Energy is radiated outwards from an antenna in all directions. That part which passes along the surface of the earth is called the ground wave. For frequencies above 1,500 kHz, its intensity is practically the same day and night, winter and summer. But it is different for different frequencies and for different surface conditions. The curves of Figs. 9 A and 9 B illustrate these effects.¹

Fig. 9 A. Strength of the ground wave over sea water when the radiated power was 1 kw.

Fig. 9 B. Strength of the ground wave over land from a 1 kw. transmitter

It will be noticed that a wave of given frequency travels farther over sea water, whose conductivity is comparatively great, than over the ground. Obviously, less energy is absorbed from the wave in a body of greater conductivity. As the frequency is increased, the small currents induced in the body by the wave become larger, the heat losses are greater and the wave does not travel as far. At about 2 MHz, the reliable range of transmission from a 1/2-kw. transmitter is about 200 miles over land and 500 miles over sea water. Above 4 MHz, the ground wave can only be used for short distances.

The ground wave is vertically polarized, i.e., its electric field is perpendicular to the earth. This is so because the conductivity of the earth tends to short circuit any electrostatic component parallel to its surface. The curvature of the ground wave around the bulge of the earth is due to diffraction, a bending which occurs when a wave grazes an object. The curvature is also partly due to refraction or bending of the wave caused by a slightly higher velocity of propagation in the upper than in the lower parts of the atmosphere, a process like that which causes light waves to bend as they pass through a glass prism or lens.

1	Selected data from the report of the Committee on Radio Wave Propagation, Proceedings of the Institute of Radio Engineers, Oct., 1938.