The FCC M-3 Map, although used for predicting coverage and interference for AM
broadcast stations at the application stage, uses theoretical soil conductivity
data from the 1950's. Although it will get you somewhat in the ballpark, the
actual conductivity can vary a good deal from actual measured values. For
example, The M-3 map predicts the ground conductivity for North Central Ohio as
8 mS/m. In the summer months I have measured values as low as 4 mS/m and no
higher than 6 mS/m in the winter months. In Western Ohio the theoretical
conductivity is predicted as 15 mS/m and the measured values agree closely with
the theoretical ones. So the map is an estimate which may or may not be
totally accurate.
Since the FCC now requires most applications to be filed electronically, you
may be able to obtain relatively recent and more accurate measured conductivity
values from an AM broadcast 301 application filed for a station in your area (a
301 application is an application for a construction permit for an AM Broadcast
Station). If you do a search of the FCC web site for AM broadcast applications
in your area, you may be able to find the measured values in pdf attachments to
the application. That might give you some insight on ground conductivity
values in you area. The applications are all a matter of public record so you
should have no trouble accessing and viewing them.
73.................de Goose, W8AV
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