> As it would be difficult to place probes at various depths
below the surface for taking measurements, I would suggest
driving a short vertical dipole at a specific low power, and
taking field strength measurements a couple wavelengths out.
The weaker the field strength, the more ground loss present.
Of course, other above-ground RF absorbers and reflectors
would have to be taken into account, as well as existing
radials. Standards of measurement could be developed. This
may be similar to Broadcast procedures already in use.
Broadcast stations use the slope of attenuation with
distance to estimate ground conductivity.
The steeper the slope, the poorer the soil.
The problem is attenuation levels vary widely as the person
making measurements moves out along a radial (direction).
Levels not only vary with distance over small changes in
distance, levels vary with how the meter is held.
If we tried to plot an accurate slope in a short distance
things would go all over the place. Even with experienced
people making FS measurements using expensive commercial FS
measurement gear, the conversion from data points to a curve
(that is matched to soil conductivity types) can be off as
much as 3dB depending on how the guy normalizes out the
flyers in measurements.
It seems most of us just can't get our heads wrapped around
the tolerances involved in measurements, how much soil
varies along a path even over short distances, and the fact
modeling wires near earth is probably the most inaccurate
thing a model will do.
The soil isn't homogeneous so loss isn't uniformly
distributed, and what we have at the surface might not be
what influences the antenna anyway. The change in the system
is generally small, so we are measuring small changes with a
rubber ruler. Impedance of radials also does not tell us
much about actual loss.
So why worry?
Why not just measure what we want to know directly? Why
would we measure resistance of an antenna feedpoint when we
really want to know FS change as we add wire?
The quickest way to sort this all out is to just directly
measure what we really want to know, the change as we do
things.
73 Tom
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