Dear John, LF Group,
No doubt a high resistance soil would lead to increased earth system
resistance, but remember that whatever "earth" resistance you measure is a
combination of several sources of electrical losses. In large professional
antennas, the earth is normally is the dominant source of loss, hence the
big ground mats and radial systems generally employed. But with small
amateur antennas, there are other substantial sources of loss as well. The
antenna is approximately a capacitor, one plate being the antenna wire, the
other plate is the ground. The dielectric of the capacitor is the air
between, plus all the stuff in the air subject to the electric field of the
antenna, such as trees, buildings, etc. All these poorly-conducting
extraneous objects will contribute to the loss of the capacitor, along with
the losses of the ground "plate" itself. This is compounded by the amateur
antenna having a much higher reactance than much bigger professional
antennas, so having higher voltage for a given antenna current. Further, the
height above ground of the amateur antenna is much less, and so the electric
field between the "plates" is higher due to the smaller separation, for a
given antenna voltage. This all adds up to relatively large electric fields,
and so higher dielectric losses, contributing a greater proportion of the
overall antenna resistance.
I suspect that dielectric losses are the dominant component of the antenna
resistance for many small amateur antennas. A while back, I did an
experiment comparing the resistance of my 10m high home QTH TX antenna with
a nearly identical antenna erected in an open field, on the same type of
ground, using as near as possible the same earthing system (6 x 1m ground
rods). The home QTH antenna, which was surrounded by several trees a similar
height to the antenna wire, had a loss resistance of about 50ohms. The
open-field antenna had a loss resistance of only 8.5ohms. This demonstrated
that the environment surrounding the antenna had a very major effect on the
antenna losses, more than the actual antenna and earth system components
themselves. I would be interested to know what the environment around your
antenna is like.
I also tested several quite elaborate ground systems at the home QTH
antenna, including large numbers of ground rods distributed over a wide
area, various numbers and lengths of insulated ground radials, with and
without balancing and decoupling arrangements to ensure the current was
distributed reasonably evenly, and areas of wire mesh. None of these things
made more than a few ohms difference to the overall loss resistance - it
seems you quite quickly reach a point of diminishing returns, where the
ground system is only a minor contribution to the loss resistance. One would
expect this to be different where the ground conductivity is very poor - my
antennas were on rather wet clay soil, so the ground might make rather more
difference in sandy or rocky areas.
Cheers, Jim Moritz
73 de M0BMU
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