Low-frequency Ground Systems
Author: Edmund A. Laport
The principles of grounding low-frequency antennas differ from those used at higher frequencies for two main reasons: it is usually impractical to employ electrically long buried-wire systems (1) because of the relatively greater wavelengths and (2) because the low frequencies penetrate the soil to a relatively greater depth. This is in contrast with the situation at medium broadcast frequencies where radial ground systems of the order of one-half wavelength long are practical and economical. In such systems, most of the electrical flux that causes return ground conduction currents to enter the base of the antenna as antenna current is collected over the top of the ground system so that the current density in the soil beneath is very small. In the case of low frequencies, with electrically short ground wires, a considerable portion of the field is completed to ground beyond the limits of the ground system, and currents flow back to the antenna at considerable depth under the ground system. It then is important to collect ground currents in such'a way as to minimize current densities in the soil to reduce ground loss.
There are essentially three methods for the design of low-frequency ground systems: radial buried ground systems (Fig. 1.12); star grounds (Fig. 1.13); counterpoises (Fig. 1.14).