[TowerTalk] More Interference Pattern

[David Gilbert]

Hi, Brian.

Yeah, I almost always use the phrase "effective ground plane" when I 
talk about this stuff but I didn't there.  Also, I know that it's 
actually a spatially distributed effect, but that gets even clumsier to 
say each time  ;)

I know that you've done a ton of really good work on ground conductivity 
and ground effects and as you say I am no doubt seeing some very complex 
effects given the ground conditions I have here ... very dry soil on 
top, probably more moist soil down deep, and composition that ranges 
from sand and fine particles to rocks the size of a bus at varying 
depths.  I have a drone that I have once or twice used to plot the 
elevation pattern of an antenna (it works surprisingly well), and I may 
some day model a couple of antennas and compare their actual measured 
elevation plots (using the drone) to see what I really have and how much 
that varies across my lot.

Lots of cool stuff to do ...

73,
Dave   AB7E


On 4/15/2026 6:16 PM, Brian Beezley wrote:
> AB7E said:
>
> "Keep in mind that the RF ground plane is almost certainly not at the 
> surface of the physical ground."
>
> Dave, the notion of a ground plane makes sense for perfectly 
> conducting ground. And it's not far off for seawater. But for other 
> types of ground, it can be misleading.
>
> Electrical ground is right where you see it. But it's only part of the 
> story. For generic desert soil like yours, skin depth is 157 feet at 
> 14.2 MHz and 533 feet at 1.8 MHz. That's where antenna-induced ground 
> current has decayed to 37% of its surface value. It's still 13.5% at 
> two skin depths. A lot can change over such distances, especially 
> moisture content, which greatly affects ground permittivity and 
> conductivity.
>
> For any type of soil, the Fresnel reflection coefficient is nonzero at 
> the air/ground interface. Some signal reflection occurs there. For 
> low-loss soil like yours, significant reflection also may occur at 
> deeper soil layers. An upward-going signal reflected from a deep 
> stratified layer may get re-reflected at the surface. It can bounce 
> back and forth within the layer, dissipating power as it travels. This 
> effect can occur at multiple layers. Each upward-going signal that 
> reaches the surface transmits some power into the air. The result can 
> be quite complex. A mathematical treatment is here:
>
> https://msp.org/memocs/2016/4-2/memocs-v4-n2-p03-p.pdf
>
> Some of your inconsistent phase results may stem from subsurface 
> reflections with long path lengths. Unwrapping phase may help.
>
> A fascinating effect is the resonance that can occur for low-loss soil 
> when the distance between subsoil layers is near a multiple of a 
> half-wavelength. I calculated that a sandy aquifer 66 feet below a 
> desert surface (a half-wavelength in ground) can increase effective 
> surface conductivity by a factor of 30 at 3.7 MHz. I suspect such 
> effects seldom occurs in practice because everything has to be just 
> right. You can download a calculator to explore resonance and other 
> stratified ground effects for two soil layers here:
>
> https://k6sti.neocities.org/sg
>
> Subsurface soil can affect both antenna impedance and far-field 
> patterns. NEC models uniform soil only. Its results may not be 
> realistic at your QTH because of the large exposure to subsurface 
> effects for desert soil.
>
> Brian
>
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