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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