On 9/4/13 7:26 PM, David Gilbert wrote:
That's probably because none of us have a true idea just where our
effective "ground" is. It's not like there is some high conductivity
zone in the top few inches of soil, or that everyone's soil conductivity
profile is at all similar. I'd bet that in some low conductivity soils
the effective ground is several meters below the surface, and I'll also
bet that it is at least to some extent dependent upon the frequency of
interest. I use and depend upon EZNEC and HFTA all the time, but I
don't pretend they are any more accurate than the data I feed them.
There was some work back in the 70s by George Hagn (at SRI then) to
measure soil parameters by measuring the Z of a dipole close to the
ground, or laying on it. He found it doesn't work very well, which is
why the "open wire line" (OWL) technique was developed (basically drive
two rods forming a parallel transmission line into the soil).
You're right that very few hams know what their local soil parameters
are, and, of course, they vary pretty dramatically over a small
distance, depending mostly on soil moisture content, but also on the
relative concentration of sand/loam/clay.
This is why trying to get "a few degrees" phasing on something like a
4-square with a fixed network is well nigh impossible without a lot of
instrumentation and tweaking. It's also why some people swear by a four
square and others swear at them. A fairly small inhomogeneity in the
soil can turn a 20 dB null into a 5 dB null.
Where NEC comes in handy is seeing the effect of changes in soil
parameters. If your antenna has radical changes in behavior when you
change the conductivity or epsilon, watch out.
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