Ground permittivity and conductivity are needed to model an antenna over
ground. Accurate values are particularly important for vertical antennas
and for horizontal antennas with low conductors. You can obtain values
for your local ground with a ground probe and VNA. But not everyone
wants to go to the trouble of constructing a probe and making the
measurements.
For many years I've used the table of ground permittivity and
conductivity values for various ground types in the ARRL Antenna Book. I
figured the values likely were reasonable generalizations for the
various types of ground described. But I just discovered that the values
are for the AM broadcast band. Since ground parameters vary greatly with
frequency, the ARRL values are not valid at HF. *&$#@!
Poking around the web, I ran across a fascinating paper on soil models
for lightning grounds. What was so interesting wasn't so much the
lightning aspect but that the author investigated seven different soil
models! These models extrapolate ground permittivity and conductivity
values from LF to HF. They account for the effectiveness of a grounding
apparatus to the HF components of a lightning strike.
I took one model, modified it somewhat, and created a table that
extrapolates the ARRL values to the HF ham bands. The extrapolation is
approximate and I'm sure I'll be revising it (I revise everything!), but
it should be much more accurate than the original ARRL values.
The 13/5 "average ground" I've used by default for decades (13/.005 if
you're not into mS) extrapolates to 4.8/9.6 on 10m. How disquieting. And
if you scrutinize the original FCC source paper, you'll discover that
13/5 is really 13/4. Is nothing sacred?
http://ham-radio.com/k6sti/gndp.htm
Brian
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