[TowerTalk] W7PUA Tiny Ground Probe

[Jim Lux]

Today, you could probably do the measurement with a swept tone and UAVs along the radials.
But that doesn't give you much of subsurface structure (it would be a very complex "inversion")


On Wed, 4 Mar 2026 10:31:09 -0800, Jim Brown <jim at audiosystemsgroup.com> wrote:

On 3/4/2026 6:58 AM, RT Clay via TowerTalk wrote:
> Non-contact measurement of ground conductivity is a standard method in geophysics and archaeology.

As a young EE student, I worked for Carl Smith, who co-authored the FCC
AM Technical Rules after WWII, in his practice designing antenna systems
for new applications. In support of his own application for 10 kW
daytime on 680 kHz in Charleston, WV, second adjacent channel to WLW
about 200 miles away, he ran radials plotting field strength in that
direction to prove non-interference. He eventually got the license.

I strongly suspect that method was used after WWII to construct the FCC
maps that are still on the FCC website. To run a radial, field strength
measurements were made with standard test equipment at many points at a
single azimuth from a station operating on the frequency in question.
Ground wave attenuation is strongly related to frequency, especially at
LF, and multiple FCC curves for attenuation vs distance based on that
data in the AM Rules are for small segments of the AM band, and the map
generated using those curves.

I've downloaded the map (which is in pdf segments) to my website.
http://k9yc.com/FCC-GroundMap.zip

73, Jim K9YC


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