Ground Systems

Author: Edmund A. Laport

It has been known theoretically since the works of Pierce and Ballantine that, for the condition of perfectly conducting flat ground, a very short vertical radiator will produce, within about 6 percent. the same field strength as a radiator one-quarter wavelength high for the same power input. This is illustrated in Fig. 2.1. For greater heights, the field-strength gain increases very slowly to well beyond three-eighths wavelength. Only as the radiator approaches the optimum heights previously discussed does any real gain occur.

One important conclusion one draws immediately from Fig. 2.1 is that there is very little difference in the performance of a radiator in the range up to about 120 degrees from the standpoint of field strength. If we compare a 60-degree radiator with one of 120 degrees, the gain of the latter is trivial with respect to the increase in cost for a structure of twice the height. However, the bandwidth requirements of an antenna may dictate the use of higher radiators without regard to the comparative radiation efficiency.

The question naturally arises: Why not use very short radiators? Several factors make this impractical in most cases. The shorter the radiator, the lower its radiation resistance. Practical ground systems can be constructed to have very low resistance, but as radiation resistance becomes very small, the ground resistance becomes an increasingly important factor in the circuital efficiency of the antenna system. For this reason it is difficult to realize the desired over-all circuital efficiency, with the result that short antennas are usually very inefficient.

Furthermore, very short antennas have very high reactance, so that high reactances are needed for tuning. The inductor loss therefore becomes large with electrically short radiators. The low-resistance and high-reactance systems have relatively small bandwidth, also. For these reasons a radiator for medium-frequency broadcasting is seldom made less than 60 degrees high.

Since stations of low and medium power do not usually require anti-fading antennas, it was obvious that it would be desirable to investigate synthetic perfectly conducting grounds for obtaining optimum efficiency from electrically short radiators. Experimental research and theoretical studies of earth currents near radiators of various heights yielded a simple and practical ground system that fully satisfied the requirements. This work6 established immediately a uniform ground-system design for broadcast stations in the medium frequencies. A system of 120 radial wires, spaced 3 degrees and having a length of about one-half wavelength, approaches the condition of a perfectly conducting ground within about 2 percent for radiator heights of 45 degrees or more. Ground rods at radial ends and various other departures from simple straight buried wires are of negligible benefit in such a system at the medium frequencies.

Diligent research and experiments have been conducted for other possible broadcast principles that might equal or surpass those disclosed by Ballantine.3 Various natural and unnatural current distributions have been studied and tried, as well as circles of radiators,25 controlling the velocity of propagation and using great heights. Some such devices produce equivalent performance at much greater cost and design complication - others are definitely inferior. Only one form, the uniphased antenna developed by Franklin for high-frequency use, holds promise of surpassing the straight vertical antenna of uniform cross section and of height 190 to 225 degrees. The Franklin antenna is realizable at medium frequencies by extremely high structures, insulated at the current nodes and tuned to produce uniphased currents on each side of such current nodes.

The medium-wave broadcast radiator is thus in that happy state where, so it seems in the light of present knowledge, a standardized optimum design exists. Also, the optimum ground-system design exists. These optimum designs are practical, as proved by extensive application at hundreds of stations whose performance has been carefully measured. The design formula is very simple. The performance is predictable with very high accuracy, and this performance is very close to the theoretical maximum. Furthermore, the cost of such systems is within economically practical values.

While one may wonder, in reviewing this story of progress, why it took so long to solve such a simple problem, it can be said that it is seldom in technology that such an important problem is so completely solved in such a short time.