> Thanks for the expanded info RE: ground conductivity.
> I was interested in this so I looked in the ARRL
> Antenna book and in "Reference Data for Radio Engineers".
> Both of those books have nearly identical maps showing
> "estimated ground conductivity" in the USA. I trust
> that the measurements given there were the averages
> of actual low frequency AC (like 60 Hz) readings
> at various sites, not at RF.
Those conductivities are from data collected in the AM BCB.
They are estimated through measurements of the attenuation slope
of AM BCB signals. The poorer the ground, the more rapidly a
signal deteriorates when propagated via groundwave. The angle of
that slope is used to estimate conductivity, and by nature of the
measurement it is an average value that includes all factors
affecting propagation.
No one in the BC industry, including the FCC, accepts
measurements made at frequencies other than those taken at or
near the operating frequency. Anything else would be meaningless.
By the way, if I model my antennas on modeling programs my high
dipoles should totally wipe out my verticals at any angle. (Local
measurements at WBAF in Barnesville show I am on a 10 ms/m
estimated conductivity area.) Virtually all of the time the very
opposite occurs!
In order to get the same results on the model I get comparing
transmitting antennas on the air, I have to "inflate" conductivities
into the hundreds of ms/m. To get the same currents I measure in
a Beverage, I have to insert additional losses.
That's probably more important to know than anything discussed,
because it is a warning flag to trust TO angle and such for
antennas that rely on earth effects to form patterns.
73, Tom W8JI
w8ji@contesting.com
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