[TowerTalk] Measuring RF ground characteristics

[Lux, Jim]

On 2/13/21 10:57 AM, Richard (Rick) Karlquist wrote:
> Just a heads up:
>
> Gerry Burke, of LLL/NEC fame had some discouraging news
> about their ground measurements.  He told me that in
> some cases there was no configuration of NEC parameters that would
> correlate with some real life ground measurements.


Alan Christman's PhD dissertation talks about this too:
https://etd.ohiolink.edu/apexprod/rws_etd/send_file/send?accession=ohiou1172605318&disposition=inline


Burke, G. J., "Recent advances to NEC: Applications and Validation", 
UCRL-100651, 3 March 1989


>
> The other disclaimer is that NEC only models a uniform ground.
> I know my ground is far from that.  After 1 to 5 feet of pure
> clay, I hit "cemented hardpan" waterproof clay.  About 40 feet
> down I hit sand, according to the well diggers.  No way to
> model that with NEC. 

This is very much so. What I do is model with NEC and use several 
different ground properties, varying both sigma and epsilonr, and see 
how much difference it makes.

I did some analysis for the  OVRO - Long Wavelength Array, which is a 
field of hundreds of fan dipoles.  They had some buried probes that 
measured actual RF properties, and data for about half a year (long 
enough to see rain storms, snowmelt, etc.).  What we were interested in 
is whether the "ground screen" under the dipoles made a big or little 
difference.