Counterpoise

Author: Edmund A. Laport

The counterpoise is an insulated net of radial wires assembled above ground to form a large capacitance with the ground. From the earliest days of radio the merits of the counterpoise as a low-loss ground system have been recognized because of the way in which the current densities in the ground are more or less uniformly distributed over the area of the counterpoise. Any tendency toward nonuniformity of current distribution in the ground will increase the portion of ground current toward the edge of the counterpoise. It is inconvenient structurally to use very extensive counterpoise systems, and this is the principal reason that has limited their application. The size of the counterpoise depends upon the frequency. It should have sufficient capacitance to have a relatively low reactance at the working frequency so as to minimize counterpoise potentials with respect to ground. The potential existing on a counterpoise may be a physical hazard which may also be objectionable.

All three of these ground systems require exposed over-ground wires near the antenna base. The buried radial ground system with the wires brought above ground near the antenna is possibly the best choice at stations where there is ample land for an extensive buried-wire system. In this system, the over-ground wires are not dangerous since they are at ground potential. The buried radial system accomplishes current-density reduction and decreases ground losses out to the distance of the buried radials. The over-ground portion forms an excellent ground screen as well. In restricted areas, the star system seems to offer the best possibility of obtaining low ground resistance without the inconvenience and exposed potentials of the counterpoise. However, if the disadvantages of the counterpoise can be tolerated, it may be superior to the star system for low ground resistance. Figures 1.15 and 1.16 show useful details of counterpoise construction. ¹

FIG. 1.14. Counterpoise (capacitance) ground.

These comparisons are not to be regarded as absolute, for they have not been proved quantitatively over a sufficient range of conditions to be considered as fact. They are the author's opinion from the information at his disposal. The soil conductivity and the frequency for any particular case may modify the controlling factors sufficiently to affect the final choice. For frequencies from 15 kilocycles to 500 kilocycles and soil conductivities from 10^-14 to 5,000·10^-14 electromagnetic unit (sea water) the conditions vary a great deal.

The depth of penetration of ground currents at the low frequencies (see Appendix II) makes it important to consider the nature of the subsoil to the depth known as the "skin thickness." The search for a station site should include an examination of the subsoil characteristics with the purpose of obtaining soil of best available conductivity to a sufficient depth.

FIG. 1.15. Ground screen construction - center detail. (Photograph courtesy of W. M. Witty, consulting engineer.)

FIG. 1.16. Ground screen for a vertical radiator. (Photograph from Radio Station KTBS, courtesy of W. M. Witty.)

A thin covering of good-conductivity topsoil overlying a base of poor conductivity is usually a poor location for a low-frequency station.

1)	Figures 1.15 and 1.16 are photographs of an electrostatic ground screen and not a counterpoise. However, the mechanical construction of a counterpoise can be exactly as illustrated in these figures except that the inner ring of Fig. 1.15 should be fully insulated from ground. There should not be any connection to actual ground in the antenna circuit when a counterpoise is used.