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 <rfry@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
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