[TowerTalk] New N6LF Ground Probe Designs

[Brian Beezley]

On 3/2/2024 9:51 AM, Richard (Rick) Karlquist wrote:
> I'm not trying to take away anything from Rudy's excellent work, but he 
> doesn't explain in the posted article what useful advantage we can
> derive from the measurements.


Rick, see Rudy's original writeup where he goes into this in great 
detail. His recent update is just an update on that earlier work. He 
doesn't repeat the basics.


> It only gives a spot measurement of the
> top foot of soil.  And the soil that matters is the soil at large
> distances from the antenna.  It is unlikely that this probe could be
> used to survey all that area of land that isn't owned by the ham with
> the antenna.


It's not necessary to measure the ground a large distances unless its 
characteristics differ significantly from local ground. This might 
sometimes occur, but it only affects the ground reflection coefficient 
at very low angles, not local ground losses. The software includes a 
utility that averages measurements. It makes processing many 
measurements a lot easier.


> Also, I would prefer Rudy's previous method using a low 1/2 wave dipole.
> It covers more area and presumably would penetrate into the soil more
> than a foot.


Yes, it measures the ground an antenna sees. That's a distinct advantage 
and the only real limitation of a surface probe. The disadvantages of a 
low dipole are that the measurement is good only for one band, it is 
very tedious and time-consuming to set up accurately, and it requires a 
large clear area. And unless you leave the test antenna in place, you 
can measure only once. Ground characteristics vary with soil moisture 
and temperature, and hence with the season. Finally, the modeling 
necessary to interpret the low-dipole results can be extensive.

If I were interested in just one band and I had the room and patience, 
I'd use the low-dipole method. Here's something I wrote about using it 
to calculate maximum legal power for 13.56 MHz DXing. It took several 
months to get the two-dimensional parameter interpolation to work 
correctly. The program uses the results of 100 antenna models:

http://ham-radio.com/k6sti/hifer.htm


> Another thing I would prefer is to simply put up a vertical and a dipole
> and A/B them for signal strength on various RBN stations.


The purpose of a ground measurement is to help you generate accurate 
antenna models without having to construct candidate antennas. The 
models can give you insight that on-air testing can't. I'd use on-air 
testing to verify an antenna design, not to design it in the first place.


> Finally, it is also possible to build a Beverage antenna and run
> a current probe along it as I have done and determined how fast
> it attenuates with distance.


How do you determine permittivity and conductivity from this 
measurement? If you can do that it might be really useful for people 
with a Beverage already installed. I'd like to know more about this 
method. Do you have a reference?


> I know very well that my QTH is over highly conductive ground without
> ever using an OWL probe.


How do you know that? Most people have no idea without measurement.

I should point out the the FCC ground conductivity map is generally not 
useful for hams. It applies to the AM broadcast band only, not to HF. It 
does not show ground permittivity. Both conductivity and permittivity 
influence antenna performance and both can vary greatly greatly with 
frequency. In addition, the resolution of the map is way too coarse to 
be useful. For example, on the BC band where the map is supposed to be 
valid, Rudy measured ground conductivity 4-5 times higher at his QTH 
than the map indicates. In this case the map is highly misleading.

Brian