On 3/24/20 8:35 AM, Richard (Rick) Karlquist wrote:
This was pretty well studied by George Hagn at SRI in the 60s/70s in
the context of antennas for field use of HF and VHF in Vietnam and
there are some publications out there. In fact, that's what led to the
development of the Open Wire Line (OWL) technique.
Gerry Burke (of NEC fame) told me that the OWL technique doesn't always
work. IIRC, he said that there was no combination of NEC ground
parameters that produce simulations that agree with the measured data.
Yes, I've heard that also. There's also the real problem that soil isn't
uniform with depth - NEC assumes a uniform slab. Neither the dipole nor
the OWL, nor the single probe, nor the open wire line laid on the
surface techniques will really match.
I've been working with a variety of approaches to modeling non-uniform
soil (regolith, technically) - without going to the full on FEM
approaches - for wide bandwidths (100kHz to 40 MHz) - none are great,
but what I'm really interested in is not the precise values, but what
the range of effects might be on HF (and LF) antennas laying on the
surface of the Moon.
I am convinced that for ham applications, a "rough estimate" is as good
as you need - run your models for a range around what you
measured/estimated, and see if the antenna design performance falls
apart with small changes. If it's "robust", then, you'll need to adjust
it in-situ for the performance (if you're doing something like a 4
square and you want good null performance).
I started out trying to measure the soil properties on a 1 meter grid in
my yard using both a loop and a OWL approach, and got so much variation
that I thought, OK, it's either an instrumental effect or my soil really
does vary (which is quite believable - the moisture content varies).
It would be nice to have some sort of rapid survey approach - sort of
like they do with the ground penetrating radar - something you could
drag across the surface, and would be able to use multiple frequencies.
A compact loop would be one possibility, but it's inherently narrow band.
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