Dear Ralph, Dick, LF Group,
At 10:29 16/12/2002 -0500, you wrote:
Dick, conventional wisdom says that the soil resistance becomes lower as
the soil becomes wetter. However your measurements indicate the antenna
ground loss becomes greater with increased wetness. Perhaps there is
another coupling mechanism here. For example, if one had a loop antenna
suspended in free space, would the ground loss resistance become zero?
The resistivity of the soil may well become lower as it gets wetter - but
the bulk resistance of the soil is only one of many factors contributing to
the loss resistance of the antenna. In "professional" LF antennas, so the
text books say, loss resistance is predominantly due to resistive losses in
the ground connection - but in the case of amateur antennas it is
different, since these are much higher impedance, have a shorter path for
the ground return currents to flow and are much closer to the ground. Under
these conditions, I think the dominant factor contributing to the loss
resistance is dielectric loss due to the RF electric field around the
antenna heating up the ground and buildings, trees etc around the antenna.
This is supported by observations showing that loss resistance decreases
with increasing frequency for most amateur verticals (with the same antenna
current, the antenna voltage will decrease with frequency, leading to lower
dielectric loss - you would expect it the other way round due to increasing
skin effect if the resistance of the ground was the major contributor), and
the fact that improvements to ground systems quickly reach a point of
diminishing returns, where adding further rods, radials, counterpoises,
etc. makes almost no difference to the loss resistance.
Almost everyone using verticals seems to find that their antenna loss
resistance goes up in wet conditions - I would imagine the same would be
true to some extent with loops, with increasing conductivity resulting in
larger eddy currents flowing in the ground, and so higher losses.
Cheers, Jim Moritz
73 de M0BMU
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