On Sun, 30 Nov 2008 04:20:33 -0800 (PST), Richard Fry
wrote:

But looking at a NEC far-field analysis
this would seem impossible, due to the greatly reduced fields in this
sector that NEC shows for a vertical monopole over real earth.

NEC is not a propagation modeler.

However, resourcing the top engineers of the AM field for their
observations of sky-wave and ground-wave field strengths (a typical
service application) where they combine destructively (the "fading
wall"); at a distance of 70 miles, for 50% of the time, both signals
are equal (with propagation variations of phase accounting for
fading).

The graph you supply suggests that this 70 mile distance is obtained
by a launch angle (for the sky-wave) of 60 degrees. The NEC far-field
analysis for the BL&E antenna of 70 foot tall radiator in a field of
113 135 foot radials over average ground has a response of -2.32dB @
60° and -1.61dB @ 1° which shows a pretty close accord with field
reports from Laport (Fig 2.7).

73's
Richard Clark, KB7QHC