On Tue,2/9/2016 9:41 AM, Donald Chester wrote:
Those maps, based on random samples, are virtually useless for predicting the
soil conductivity at any specific site location.
How do you know that, Don? I worked in Pete Johnson's consulting office
in the '60s.Pete, along with Carl Smith (of Cleveland Radio and
Electronics Institute) wrote the FCC's technical rules for AM
broadcasting after WWII.
I learned that those contour maps were developed by running field
strength measurements of MANY radials on many broadcast stations,
plotting the results on a log-log scale of field strength vs distance,
and comparing the slope of those curves with theoretical curves that are
part of the FCC technical regulations. These plots clearly show the soil
conductivity in the region that the radial crosses. One must, of
course, avoid taking measurements at points where nearby conductors
(like power lines, communications towers, water tanks, etc.) are likely
to skew the result.
This technique can still be used today if you have a suitable,
calibrated field strength meter for the AM broadcast band. Search on
Potomac Instruments.
73, Jim K9YC
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