[TowerTalk] Topband: Ground Conductivity

[David Gilbert]

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.