The E region critical frequency at night is around 0.45 MHz. That says any elevation angle less than about 45 degrees at 660 KHz does not get through the E region. The skywave plot showing the 0.25 mV/m RMS field strength contour line is for a one-hop E mode at a distance of around 1150 km at an elevation angle of about 6 degrees. Shorter distances from WFAN are still an E mode, until ground wave takes over.
The question comes down to "how far out from the antenna does ground impact an elevation angle of 6 degrees?" Al K3LC did a study of this on 160m, 80m and 40m using vertical monopoles, and presented his results in the September/October 2006 issue of NCJ. From his work, we can extrapolate his data (with some caution) - and it looks like ground is important out to about 2.2 wavelengths at 6 degrees elevation, which is just over a half mile on 660 KHz. That puts 'ground' to the east through south out in the Long Island Sound and out in the Atlantic Ocean. So I would expect the skywave plot to be elongated to the east through south at the 0.25 mV/m contour line. It appears that the data was generated using a signal strength prediction program as "50% of the time" was used in the qualifying remarks (50% implies a median value - which likely comes from our monthly median model of the ionosphere). But the prediction program didn't look at ground around the antenna. Carl K9LA _________________ Topband Reflector Archives - http://www.contesting.com/_topband
