For ham radio purposes, why not measure the effect of ground
conductivity instead of the ground conductivity itself? As Jim Lux
says, the conductivity isn't going to be uniform and since you can't
measure all of it without a backhoe you don't have the ability to
calculate a net effect anyway.
So ...
1. Erect a 20m vertical antenna with raised (tuned) radials.
2. Borrow a decent drone with self-positioning and install a stable
noise generator on it with a very short antenna hanging from it.
3. Position the drone a few wavelengths away from the vertical and take
receiver readings from the vertical as the drone is elevated straight
up. A bit of visual triangulation should be able to determine the
height above ground for the drone with reasonable accuracy.
4. Mathematically translate the readings taken vertically to spherical
data and compare to the theoretical lobe of a vertical over perfect
ground. EZNEC would let you model different ground conductivities for
comparison to the measured data.
Just a thought. If somebody really wants to know I suspect this method
would be as good as any, although certainly any nearby structures would
distort the results. And although the vertical would most likely be
single band, I don't know why you wouldn't be able to get relevant data
at other frequencies ... again by comparing to the EZNEC model.
73,
Dave AB7E
On 3/24/2020 9:47 AM, jimlux wrote:
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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