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Re: [TowerTalk] New N6LF Ground Probe Designs

To: "Richard (Rick) Karlquist" <richard@karlquist.com>, towertalk@contesting.com, topband@contesting.com
Subject: Re: [TowerTalk] New N6LF Ground Probe Designs
From: Brian Beezley <k6sti@att.net>
Date: Sat, 2 Mar 2024 12:10:09 -0800
List-post: <mailto:towertalk@contesting.com>
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
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