The counterpoise is a form of capacitive ground which is quite effective. Its use is particularly beneficial when an extensive buried system is not practicable, or when an ordinary pipe ground cannot be made to have sufficiently low resistance, as in rocky or sandy soils.
To work properly, a counterpoise must be large enough to have considerable capacitance to ground, which means that it should cover as much ground area as the location will permit. No specific dimensions have to be used, nor is the number of wires particularly critical. A good form is an approximately circular arrangement using radials wires with cross-connectors joining them at intervals, as in Fig. 7-7. There is no particular necessity for extending the radius of a circular beyond a half wavelength, nor do the lengths of the individual wires have to bear any particular relation to the wavelength. Rather, the intention is to have the counterpoise act as a pure capacitance instead of exhibiting resonant effects. The capacitance of the counterpoise will be approximately equal to that of a capacitor consisting of two plates each of the same area as that of the counterpoise, with spacing equal to the height of the counterpoise above the ground.
The shape of the counterpoise mat be made anything convenient, square or oblong arrangements are usually relatively easy to construct and will work satisfactorily. There should not be too few wires, but on the other hand separations between wires up go 10 or 15 feet will do no harm on fairly large counterpoises, and 5 to 10 feet on smaller ones. It is a good plan to join adjacent wires with jumpers at intervals about equal to the wire separation so that resonance effects will be minimized.
The height of the counterpoise is not particularly critical. It is best to construct high enough to be out of the way, which ordinarily means from 6 to 10 feet above the ground. Remember that the height of the antennas is reduced by the amount of counterpoise height.
Satisfactory results have been secured with counterpoises simply lying on the ground, or with large screens of chicken wire similarly laid under the antenna. However, the best performance will be secured, as a general rule, when the counterpoise in insulated ground ground. When in contact with the ground surface, the losses are likely to be higher because the counterpoise tends to act either as a poorly conducting direct ground or as a leaky-dielectric capacitor.
