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[TowerTalk] HF Ground Parameters

To: towertalk@contesting.com
Subject: [TowerTalk] HF Ground Parameters
From: Brian Beezley <k6sti@att.net>
Date: Thu, 28 Mar 2024 13:10:59 -0700
List-post: <mailto:towertalk@contesting.com>
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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