Let's take two extreme cases of vertical antennas with ground radials: a
quarter wave broadcast band vertical with 120 buried radials, and a VHF
ground plane on a pole, with 3 or 4 quarter wave radials. Each antenna has
near 100% efficiency.
The ground plane serves two functions. One is to supply the "missing half"
of the dipole so that it is possible to achieve resonance, and the other
function is to act as a shield to isolate the radiating vertical element
from the lossy earth. If the base of the vertical is sitting on the earth,
a solid shield is needed for perfect isolation, which can be virtually
simulated with 60 or more quarter wave radials. The VHF antenna, many
wavelengths above the ground, needs only 3 or 4. It follows that any
practical Marconi-type antenna on 160M will lie between these two extremes,
and that the higher the ground plane is elevated above the ground, the fewer
radials we need. At any height of elevation, there must be an optimum
number of radials, beyond which by the law of diminishing returns there is
negligible improvement.
We don't want to bury the radials too deeply. The optimum depth would be
right on the surface. As we bury them more deeply, we are putting more and
more lossy earth between the ground plane and t he radiating element,
defeating the purpose of the ground plane in serving as a shield between the
antenna and earth.
Don K4KYV
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