From my reading of posts on many "ham" boards, the prevailing thoughts are
that the nighttime skywave field intensity received from a vertical monopole
is dependent on earth conductivity -- as well as on frequency, radiated
power, path length, and atmospheric conditions.
The plot linked below applies to the skywave from WFAN, a New York City
station on 660 kHz using 50 kW/24-7 and an omni vertical radiator. It shows
the FCC 0.25 mV/m RMS contour for the skywave received 50% of the time, six
hours after sunset in NYC.
There is no visible/useful difference in the radius to that contour over the
ocean than over the land.
This plot doesn't appear to be supported by a NEC far-field analysis of such
a system -- on which (apparently) most hams base their conclusions about the
skywave coverage potential of a vertical monopole for given values of earth
conductivity.
One reason for this difference is that NEC far-field calculations apply to
~infinite distances over a flat ground plane.
Just wondering what thoughts others have on this subject.
http://s20.postimg.org/f1z0o2e7h/WFAN_Skywave.gif
R. Fry, CPBE
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