[TowerTalk] Topband: Ground Conductivity

[jimlux]

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.