Brian,
Isn't the measured probe result only useful for near-field system efficiency
analysis? By near-field I mean to obtain system efficiency within a
wavelength or so of a vertical radiator.
But for skywave propagation field strength, don't we also need to know more
about the ground conductivity much further out to more than 1km on 160m? I
can't find it now, but Frank, W3LPL once referred to a U.S. government
publication that addressed this very topic.
Paul, W9AC
-----Original Message-----
From: TowerTalk <towertalk-bounces@contesting.com> On Behalf Of Brian
Beezley
Sent: Saturday, December 3, 2022 11:46 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] 16om vertical and the number of radials
Rob, K5UJ:
"For US hams wanting to get a rough idea of where they land in the ground
conductivity lottery, you can look at the high def. FCC map and either cheer
or weep. But, keep in mind, the boundaries were drawn a long time ago so
use as an approximation."
Rob, that map is fascinating to ponder. It's fun to try to correlate contour
lines with known soil types or terrain features. But the map has limitations
besides age. First, it is for the AM broadcast band. Ground conductivity
typically differs greatly at other frequencies. Second, conductivity varies
a lot faster than the contour lines suggest. The map resolution isn't nearly
fine enough to tell you the conductivity in your neighborhood.
A ground probe can yield more accurate results. It has its own limitations,
mainly limited depth sampling. Antenna-induced ground current typically
extends deeper than the ground probe rod length. Your antenna sees ground
the probe doesn't. If the ground is inhomogeneous, results may be
inaccurate. But I think it's still a lot better than using a map. Plus a
probe gives you permittivity.
Brian
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