[TowerTalk] More Interference Pattern

[Brian Beezley]

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