[TowerTalk] Verticals on sloping ground

[David Gilbert]

Yes, I'm aware that HFTA is usable for horizontal polarization only.  I 
was only trying to make two points ... that HFTA results are indeed 
affected by terrain features far in the distance, and that in my case 
soil conditions are so variable that, as Jim says, even FEKO probably 
would not be able to give good results for vertical polarization.

The use of a drone to empirically plot the actual pattern would probably 
be required for an accurate assessment of a vertical antenna on a 
specific hillside.

Dave  AB7E


On 1/15/2025 10:33 AM, Jim Lux wrote:
> 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 at 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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