Topband: Skywave vs. Earth Conductivity

[Richard Fry]

>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