Dear Bryan, LF Group,
The ground here is a heavy clay, that remains pretty wet all year round, so
I imagine fairly high conductivity. The usual ground system I use with my
antenna (40m long single inverted L wire abt 10m high) is 6 x 1m long
ground rods, distributed over a few m radius around the antenna feed point.
This arrangement has a loss resistance of about 36 ohms. Using just a
single ground rod increased this to 56 ohms, so I estimate the resistance
of each rod is about 20 ohms. I have tried several larger arrays of ground
rods (it is then necessary to make arrangements to equalise the currents in
the ground rods) - but the best result I have achieved so far was to reduce
the loss resistance to 33 ohms - so a point of diminishing returns is
quickly reached.
I also tried a counterpoise system; covering the whole available area
(about 50m x 12m) in counterpoise wires about 2m off the ground and 1.5m
apart. The best result with this system was to reduce the loss resistance
also by about 10%. I think a counterpoise works in two ways - firstly, it
acts as a large area, capacitive ground connection. but as the ground rod
experiments showed, this does not seem to make a lot of difference. Also,
it acts as a screen between the antenna and the actual ground, which
prevents the electric field of the antenna reaching the lossy dielectric of
the soil, and provides a metallic return circuit for the ground return
currents. But for me, and I imagine most other amateurs, the available area
for a counterpoise is much too small to intercept a large proportion of the
field of the antenna. so it has a limited effect. The same kind of argument
would apply to buried radials.
It seems to me that the majority of loss in small amateur LF antennas is
caused by dielectric losses, partly in objects around the antenna like
buildings and trees, and partly in the ground itself under the antenna.
This is bourne out by measurements which show that loss resistance falls
with increasing frequency - as the frequency gets higher, the antenna
reactance decreases, therefore the antenna voltage for a given current
decreases, and so the electric field and the dielectric losses. This would
also explain why increasing the amount of wire in the air reduces the loss
- also increasing the height has a similar effect.
The weather conditions have a substantial effect on loss resistance - a
period of rain gives about 25% increase in loss resistance, icy weather
about 15% reduction.
From a practical point of view, beyond some minimum amount of ground
system, there is little to be gained by extending the ground system further
- the best I could get was about 0.5dB increase in radiated power. In my
circumstances, there is little to be gained by having more than the
original 6 rods - allthough I suspect this would be different in areas of
low ground conductivity. Any benefits are swamped by day-to-day weather
changes. On the other hand, the height of the antenna has much more impact.
putting a 13m high pole under the middle of the span of wire, making it
into an inverted V shape gives an immediate 3-4dB improvement in ERP. I
reckon the best improvement to be had from all the ground system mods is
worth about the same as a 0.5m height increase of the antenna wire. So the
first priority must be to get the antenna as high as possible, even if this
means less wire in the air. The trouble is the neighbors don't like it...
Cheers, Jim Moritz
73 de M0BMU
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