Ground-system Design

Author: Edmund A. Laport

The radial disposition of wires in a buried or a surface ground system is dictated by the natural paths for returning ground currents. Meshes of crossed wires, which were once widely used, should not be used with vertical radiators, because the return paths are not direct and eddy-current losses in the closed loop circuits of the mesh can be appreciable.

If the radial wires are of optimum length, sufficient to have virtually zero current as one approaches the ends of the wires, there is no need to add ground rods. The test for the desirability of ground rods is to drive one ground rod, connect one of the radial wires to it, and measure the current distribution along the wire up to the end. If it is evident that any appreciable current exists at the end of the wire in using the ground rod, their desirability may be indicated. The same applies to the use of a circular bonding wire around the periphery of the system. It is only when the lengths of the radials are insufficient that there is justification for using peripheral bonding and ground rods.

The size of the ground wires has only a secondary effect on the performance of the system. Usually the wire size is that which will withstand the mechanical duty of the plowing-in process. If one desires to increase the amount of copper in the ground system, it is best to employ the copper in the form of more and longer wires rather than heavier wires, unless the antenna current has a very large value, causing high current densities in the individual wires.

FIG. 2.20. Example of radial ground system for a two-element directive array, on limited plot.

When the available plot of ground for the ground system is insufficient for a complete circular layout, certain compromises must be used. If the boundaries of the property limit some of the radials to less than optimum length, the use of ground rods at the ends of the short radials, driven in to a depth equal to the skin depth, will generally be beneficial in minimizing ground loss. If the property is so small as to limit radial lengths in all directions to those well below optimum, ground rods and peripheral bonding may be used to advantage.

In a directive array using several vertical radiators, each with a radial ground system of its own, the ground systems usually overlap. There is no useful purpose in overlapping the ground systems, and the ground radials may be terminated at the intersections, as shown in Fig. 2.20. In nonoverlapping regions, the radials of each system should be continued out to their optimum length. Bonding of radials at intersections is frequently used and is probably desirable.

The depth of the wires is immaterial, whether the soil be moist or dry. They may be placed on the surface except that they are then subject to injury and prevent the use of the land for other purposes. When the plot is to be cultivated, the wires must be far enough below plowing depth so as not to be injured by plowing.

FIG. 2.21. Design for elementary plow for burying ground wires.

However, it is desirable to bring the ground wires to the surface a short distance from the radiator base so as to form a good ground screen above the soil near the antenna base where the electric field strengths are high. The exposed portion of the ground system can be protected by a fence if necessary.

The burial of ground wires is a simple procedure in soils where a wire plow can be used. Such a plow is easily made from a piece of sheet steel from 3/8 to 1/2 inch thick, cut as shown in Fig. 2.21. The leading edge is sharpened so as to cut its way through the soil. The trailing edge has a small iron pipe welded to it to act as a guide for the ground wire. This pipe is turned backward on a radius that will not cause too much friction on the wire as it passes through the pipe. When a hand or tractor plow of this type is used, it cuts a thin groove in the soil at the desired depth into which the wire is fed as the plow moves along. The reel of wire is often mounted on the plow. The photograph (Fig. 2.67) shows another type of ground-wire plow drawn by a tractor.

The ground system is laid out from the center, using a transit to set the angles between radials. Stakes are then set at the proper distance to mark the end of a radial. The ground wires are plowed in by running the wires as directly as possible from center to one of the stakes or from the stake toward the center. The soil grooves will fill in a short time with the erosion of the sides of the groove.

The inner ends of the radials should center at the base of the radiator, and not to one side. They should be securely soldered to a ring or plate to which is connected the metallic parts of the lower end of the base insulator (see Fig. 2.65). From this common ground-wire junction, the ground connection to the antenna coupling network should be taken, and all other grounding wires to conduits and other metallic objects in the field of the antenna. Care should be taken not to introduce coupled loop circuits that often result from grounding several objects with the same ground wire. Separate insulated ground wires, connected back to the central junction for the ground radials, will avoid many of the instability troubles that occur in directive systems. Grounding connections should be securely bonded by welding or soldering so as not to be vulnerable to corrosion. The stability of the ground system and the various ground connections should be tested by measuring the antenna resistance with a sensitive balance indicator such as an impedance bridge. When one can touch, pull, or shake any grounded objects in the vicinity of the radiator base without observing a change of resistance, the system is likely to be stable.

The ground bus from the radial system to the coupling network should be of very low reactance. A wide strip of sheet copper is desirable. Alternatively, several insulated wires in parallel and mounted so as to simulate a sheet of conductors are very satisfactory.

Some of the ground radials are often impeded by the presence of the house for the antenna coupling equipment. Unless the ground radials can run under the house, the best practice is to bring the ground radials to the surface as they approach the house and to continue them in insulation around the house to the junction point. Insulating the wires will avoid erratic variations in impedance when the wires are cabled to pass around the house. Bunching several bare ground wires before they reach the common junction is to be avoided always.