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.
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