Ground systems

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More than copper in the ground

After AM broadcasting commenced, and became a commercially viable industry, a great deal of research took place. This was especially so in the 1930s when the Proceedings of the IRE carried many very detailed reports, confirming that a properly designed and constructed AM ground system involves far more than a copper rod stuck into the ground! They also showed how proper ground system configuration contributes to the efficiency of an antenna system by ensuring an adequate low loss path for the extremely valuable, and costly, antenna current. The fact that ground connections can affect radiation efficiency in another way is sometimes overlooked. Under some conditions, an unusually long wire connection to ground can increase radiation by virtue of adding the radiation from the "ghost" dipole in the ground.

A typical two-tower array layout

Figure 1. A typical two-tower array layout. The heavy lines between the towers and transmitter are 4" copper straps.

Figure 1 illustrates a typical two-tower DA ground system. A buried copper strap connects the two tower bases. Around the base of each tower are 120 radials each the same length in feet as the height of the tower. Each radial is spaced 3 degrees around the 360-degree circle. These radials will intersect midway between the two towers, and at this point a buried copper strap (at right angles to the copper strap joining the two towers) is brazed or hard soldered to the intersection points of the radials from the two towers. The intersecting radials need be no longer than the distance from an individual tower to the intersection point.

Ordinary soft lead tin solder should never be used. It disintegrates under earth chemicals and leads to disconnected radials and copper straps, and unexpected low efficiency.

In some cases, especially in very sandy or low conductivity soil, an expanded copper screen may be buried at the base of the tower. Squares of copper screen either 24'x24' or 48'x48' are commonly used depending on ground conductivity and transmitter power. These overall dimensions take advantage of the standard width of the copper screen and avoid excessive cutting and connecting.

An alternative to expanded copper ground screens is to bury 120 short, 50' radials interspersed at 3-degree intervals between the long radials. This also provides greatly improved low conductivity paths for the antenna current's return. Sometimes the periphery of the ends of these radials is comprised of copper strap forming a circle around the tower base. This is not essential, but it is helpful in low conductivity areas. There is very little to be gained by running a copper strap around the ends of the long radials because normally the current at that point is quite low.

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