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
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