Perhaps FCC models don't take account of 'sea gain?' ITU models do, as I recall.
Bill Whitacre Alexandria, VA --- > On Feb 13, 2015, at 7:43 AM, Richard Fry <r...@adams.net> wrote: > > 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 _________________ Topband Reflector Archives - http://www.contesting.com/_topband