Topband: Number of radials - elevated vs grounded

[Donald Chester]

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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