Behaviour of Coil-Loaded Antenna Counterpoise or Artificial GroundAuthor: R.J.Edwards G4FGQ © 20th January 2006
The primary purpose of this program is to calculate the number of turns on a loading coil of given length and diameter needed to resonate a wire to 1/4-wavelength. The wire length must therefore be less than 1/4-wavelength.
The behaviour of an antenna counterpoise, being located at a low height above ground, is quite complex and is aggravated by the uncertainties in the ground resistivity and permittivity. As a consequence, all program output data is approximate. It will only be in the right 'ballpark.' In the course of construction it will probably be necessary to prune wire length for resonance. In general, the higher the wire is above ground, the greater the accuracy of predictions. Greatest accuracy occurs when height is greater than length. On the other hand, at extremely low heights, Q is low and length is non-critical.
A counterpoise can be considered to be an artificial ground. Two or three can be used in parallel. The resulting ground resistance at resonance will be the individual input resistances connected in parallel. Artificial grounds are used only when there is insufficient space for full-length 1/4-wave radials, or when it is not possible to install a shallow-buried set of ground radials.
The overall length is automatically limited by program to less than 1/4-wavelength. The equivalent ground loss resistance is equal to feed-point input resistance. Counterpoise wires need not be straight or parallel with ground. Feed-point resistance remains relatively constant either side of resonance.
In practice, feed-point reactance either side of resonance can be tuned out simultaneously with antenna tuning. Several counterpoises of different resonant frequencies, running in different directions, can be used in parallel.
Loading coils need not be physically large, very high Q, because coil loss resistance is swamped by the inevitable ground loss and radiation resistances.
A counterpoise is usually used when buried radials are not possible. The ground under it may consist of concrete or other uncertain high resistance materials as may exist inside a building. Inside a building the effective height will be somewhat greater than actual height above floor level and the effective ground resistivity will be high due to the very low moisture content.
However, the calculated number of coil turns will usually be in the right ball park and when the working Q is low the length will be uncritical.
A counterpoise radiates and may change the true antenna's radiation pattern.
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