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
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