Perhaps a better way to visualize it is that an antenna is a transformer
between your radio and free space. That transformer circuit includes
both the antenna and the ground surrounding it. Dirt is a relatively
high dielectric constant, high loss material. Near the antenna, current
is fairly high. Going back to Ohm's Law, P= I^2*R; this means that
losses in the dirt go as the square of the current. It makes sense then
that reducing the losses near the antenna would greatly increase the
efficiency of the the system. Metal has low loss compared to dirt, so
replacing some of that dirt with metal will cut some of the losses.
73, Erich
N6FD, DM15dp
On 3/17/2015 12:49 PM, Joe Subich, W4TV wrote:
On 2015-03-17 2:26 PM, john@kk9a.com wrote:
If using elevated radials, I see no point in a buried ground system
unless they went well beyond the elevated radials.
It really depends on the degree of screening provided by the elevated
radials and the distance in wavelengths the elevated radials are above
the ground. If the screening is minimal (as is the case with two or
four radials) and the distance from ground is small (< 1/8 wave), a
dense ground screen can significantly reduce losses in "bad" soil.
Old studies done for the US military show "losses in the dirt" increase
at an accelerating rate as the "ground plane" moves below 1/4 wave.
Even the vertical dipoles (center fed half wave) at WWV had substantial
ground screens to reduce the losses close to the antennas.
Unless I was prepared to install a radial system that extended .4
wavelength beyond each element in an array, I would not use fewer than
6, preferably 8, elevated radials per element to minimize losses in the
soil.
73,
... Joe, W4TV
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