[TowerTalk] HF Ground Parameters

[Brian Beezley]

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