[TowerTalk] Choke on feed point of dipole

[Brian Beezley]

K9YC wrote

"Horizontal antennas care NOTHING about soil quality but EVERYTHING 
about height."

I often use this notion myself, but it is a simplification. It is not 
true in general.

Some time ago I was surprised to see differences up to 1.5 dB (as I 
recall) at low angles for some horizontal antenna models as I varied 
ground constants. It surprised me because I had always thought such 
differences were on the order of tenths of a dB.

I have also seen ground dependence at very low angles (1 degree or less) 
for horizontal antennas. I don't think this is significant because I 
doubt propagation takes place this low on HF. Maybe occasionally on 6m.

Finally, ground characteristics can affect the efficiency of low 
horizontal antennas. This can come into play for portable antennas.

NEC assumes homogeneous ground. This is probably unrealistic at many, if 
not most, locations. Subsurface soil can affect antenna impedance, 
antenna efficiency, far-field gain, and far-field patterns. For example, 
using a published stratified ground model, I calculated that a sandy 
aquifer 66 feet below a desert surface increased effective ground 
conductivity by a factor of 30 at 3.7 MHz. This two-layer model is 
probably an oversimplification itself for a real desert. Still, this 
result should give anyone who relies on NEC pause. How do you know 
what's down there and how does it affect your results?

Electromagnetics involving ground is complicated. Not all relevant 
aspects can be measured, reliably estimated, or modeled. It's good to 
keep this in mind when evaluating antennas empirically or with models.

Brian