[TowerTalk] HF Ground Parameters

[Jim Brown]

Thanks for this very valuable work, Brian!

73, Jim K9YC

On 3/28/2024 1:10 PM, Brian Beezley wrote:
> 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