In reality, NEC4 can produce quite accurate results when modeling buried
radial wires and groundwave propagation losses along a real earth path -- as
long as earth conductivity is known for that path and operating frequency.
The link below shows the value of the groundwave E-field at a range of ~28
miles produced by a NEC4 model of an AM broadcast station. Even though NEC
does not account for earth curvature, this groundwave path is short enough
for that error to be negligible.
The NEC field is rather a close match to the value shown for this set of
conditions when using the FCC's groundwave propagation curves for this
frequency, radiated power and earth conductivity (see the table at the
bottom of the graphic).
http://s20.postimg.org/tkg5hcfsd/WLS_NEC_vs_FCC_Field.jpg
R. Fry, CPBE
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