On 3/2/2024 12:10 PM, Brian Beezley wrote:
I should point out the the FCC ground conductivity map is generally not
useful for hams. It applies to the AM broadcast band only, not to HF. It
does not show ground permittivity. Both conductivity and permittivity
influence antenna performance and both can vary greatly greatly with
frequency. In addition, the resolution of the map is way too coarse to
be useful. For example, on the BC band where the map is supposed to be
valid, Rudy measured ground conductivity 4-5 times higher at his QTH
than the map indicates. In this case the map is highly misleading.
The FCC map is ancient -- I'm guessing late '40s-'50s. It's also rather
coarse-grained. Development is the enemy of good soil conductivity.
To the extent that it's accurate or not out of date, I would expect the
map to be a decent first approximation on 160M for assessing a vertical
antenna's possible radiation efficiency in areas that are reasonably
homogeneous, but close to meaningless in areas that are well-developed.
By this I mean the extent to which soil in the far field does or does
not attenuate or reinforce the direct signal to result in strong
low-angle field strength. Simply put -- if the map shows high
conductivity, verticals have a good shot at being effective, if low
conductivity, try to rig horizontal antennas as high as practical.
A serious radial system can minimize loss for a vertical antenna's
return current.
Extensive modeling studies have shown that while horizontal antenna
impedance characteristics are dependent on the soil underneath them,
their radiation efficiency and vertical pattern are almost entirely
dependent on their height and on ground slope. In the last decade, N6BT
has done work to show that the field strength and vertical pattern of
vertical antennas is strongly dependent on ground slope.
73, Jim K9YC
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