Low-frequency-wave Propagation

Author: Edmund A. Laport

Low-frequency radio waves are propagated by means of radiating structures which are, in terms of wavelength, electrically very close to earth. Thus all propagation which can be utilized is in this region close to the interface between earth and air, where the ground plays a very important role in the propagation physics.

The field in the space above the earth is accompanied by a wave of ground currents in the earth or water (hereafter referred to as "ground"). The lower the frequency, the more negligible the displacement component of the ground currents with respect to the conduction component. Both conductivity and inductivity of the ground determine the depth of penetration of ground currents. The density of such ground currents decreases exponentially with depth below the surface when the soil is homogeneous. (See Appendix II.)

Losses in the ground cause attenuation of the wave field in space immediately above the surface of the earth, since energy from this field is dissipated continuously in the ground as the wave passes over it. This loss causes the electric vector to be tilted forward in the direction of propagation, producing a component of electric intensity parallel to the direction of propagation and another normal to it and to the surface.

The wave mechanics at the interface between air and ground are very complicated even under the simplest physical circumstances. The solution of Maxwell's equations in this region has long been in dispute, and serious work on the subject continues.

Nothing can be done about the electrical characteristics of the ground or the topography along the path between transmitting and receiving antennas. By choice, it is possible to locate the antennas in areas of the best available soil conductivity, thus to increase the terminal efficiency to some extent, and to increase this efficiency still further by proper design of the grounding system.

Optimum ground-wave propagation is obtained over salt water because of its conductivity (many times that of the best soils to be found on the land) and its uniform topography. Undulations in the topography of land cause losses in propagation greater than the loss produced by conductivity alone because the impingement of a wave against a tilted surface creates wave reflections that produce scattering of the energy in directions other than the original direction of propagation. The energy loss due to scattering is dependent upon the electrical height and the slope of the surface undulations. The greater the electrical heights of hills and mountains, the greater the loss due to scattering and the greater the wave attenuation. Behind mountains greater than approximately one-half wavelength high, there may appear genuine shadows, but unless other obstructions occur, this shadow will gradually be filled in as distance is increased, owing to diffraction.

In choosing a site for a station operating on a low frequency, therefore, these general facts must be taken into consideration by using the best available land in terms of flatness, soil depth, and soil conductivity and by choosing the flattest available profile in the direction of dominant interest. Whenever possible, sites are located near the sea for overseas transmission or reception.