Antenna modeling has become an important part of the antenna game.
Knowledge of the ground parameters is assumed as part of the modeling
process. Unfortunately, few of us have any real knowledge of our
local ground parameters. So What? Well, for horizontal antennas, a
quarter wave or higher, it really doesn't matter much. But for low
horizontal antennas and vertical antennas, both of which are
characteristic of the lower bands, ground parameters can have a large
impact.
We need to realize that the electrical behavior of soil at HF is
substantially different from what it is at BC frequencies and for many
common soils the change-over frequency is below 1.8 MHz. This is
related the relative importance of conductivity and permittivity which
changes with frequency.
What can we do about it? Well some feel it's a fools errand to start
with. They
may just be right but I'm not yet willing to concede. Fortunately we
don't need high accuracy data. Plus or minus 25% or even more is
perfectly acceptable. Also we can work with the average
characteristics
over an area. This can allow us to take scattered data and extract
meaningful average numbers. But whatever we do, the measurement
techniques must be
simple and convenient or they will be of little general use.
The low dipole technique has received a good deal of attention and has
some real possibilities. It yields an average of the ground
parameters in the immediate area of the dipole down to a skin depth or
so, which is fine. but we're learning that this technique has to be
done very carefully to get good results.
In addition the dipoles are large, at least on the low bands, and you
have to rescale if you change frequency very much. For surveys of
large areas it's too much trouble. It does not meet the basic
requirement of convenience.
Ground measurements with probe techniques can be simple and
convenient and are very practical for hams. But there are valid
concerns about measuring only the top bit of soil.
What I, and others, have been doing is to try to compare the two
techniques (and other means) using high and low accuracy
instrumentation to
see just how much faith we can put in the probe measurements. This
includes digging holes in the ground and making progressive probe
measurements as we go down. When I have some meaningful results, I'll
put it on
my web page and of course I would be delighted to hear from anyone
else interested in tilting at this particular windmill.
73, Rudy N6LF
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