My understanding is that HFTA is horizontal pol only (the reflection model is
simpler).
Over the years, I've given thought to how one could measure soil properties over a largish
area with enough resolution to be useful. There are orbiting sensors (SMAP, for example)
that measure it at L-band, and airborne data in P-band (UHF), but not down to
"meters" kind of resolution, more like 1 km.
And then you have the question of Lband vs HF - skin depth is much shallower at
higher frequencies, so the deeper layers are invisible.
The geophysical prospecting folks do use low frequency measurements - an
intriguing one is using AM radio stations as the source, and flying a receiver
on a drone over the area of interest. The prospecting folks use different
terminology (example: Induced Potential) for things, but ultimately, it's all
the same Maxwell's Equations.
On Tue, 14 Jan 2025 14:38:58 -0700, David Gilbert<ab7echo@gmail.com> wrote:
That's a very good point. The soil even on my own four acre lot is
HIGHLY variable, and I know that for a fact because there is a deep
ravine running across a portion of it. The variation in the composition
(soil versus rock formations of all kinds) is pretty amazing.
I have also compared HFTA plots for various distances from my home.
There is a mountain range about 15 miles east of me and the HFTA results
vary significantly whether I include that range or not in the terrain file.
So yeah, since there is a river in between I doubt whether it would be
possible to ever get accurate soil data for that kind of range.
By the way, I have also used my drone to plot an antenna pattern, albeit
it was for a 5 element wire log periodic antenna tilted upward at a
steep angle for radio astronomy purposes. It's not difficult to put a
small transmitter on the drone with a short antenna, and the GPS
position reporting capability of a decent modern drone lets you know
where it is accurately enough to get worthwhile data.
Dave AB7E
On 1/14/2025 1:42 PM, Jim Brown wrote:
On 1/14/2025 12:12 PM, David Gilbert wrote:
I suspect that professional modeling software like FEKO can do the
job, although I've never tried to learn it. It does have a free
version for individual use.
Yes and no. Dean told me that in our mountains, we should have data
out as far as practical (don't remember the distance he suggested). In
my experience, the result is as good as the accuracy of the model.
Vertically polarized antenna performance is quite dependent on soil
characteristics, whereas horizontally polarized antennas are not.
Having good data for soil would be an additional factor limiting
accuracy.
That doesn't mean it isn't worth trying. :)
N6BT took his test antenna to a mesa, set it up at various locations,
and probed the pattern using a drone. Then setting up a rig (probably
QRP) feeding the antenna close to the edge of the mesa, he worked
stations only in the direction of the drop-off, repeating for multiple
directions, and none in the center.
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