According to antenna engineering textbooks (Kraus, Balanis. Johnson & Jasik
etc), the free space, far-field radiation pattern is not a function of the
distance from the radiator, as it is in the near field. The
near-field/far-field boundary conventionally is defined as equal to
2L^2/lambda, where L = the greatest physical dimension of the radiator. So
for a 1/4-wave monopole on 160 meters, that boundary would be located about
20.25 meters from the radiator.
Unfortunately NEC software did not follow this convention, which may lead to
some confusion.
For example, the surface wave fields calculated by NEC are defined by NEC as
"near field" even when the calculation was made in the far field according
to the equation above.
The link below leads to a NEC surface-wave analysis of a 160m monopole at an
H distance of 0.1 km, or about 5X greater than the near-field boundary.
It will be seen that:
1) The greatest field is radiated in the horizontal plane.
2) The field radiated toward low elevation angles is greater than the field
at elevations at/around the "takeoff angle" described in a NEC or textbook
far-field pattern over real earth.
Of interest also is that the maximum field (0.89 V/m) shown in this analysis
for earth = 5 mS/m, dc 13 is 89.9% of the field that would be generated over
a perfect ground plane (0.99 V/m).
This analysis further develops those in the Topband thread "Skywaves from
Monopole Surface Waves."
http://i62.photobucket.com/albums/h85/rfry-100/160m_Monopole_ElPat_at_1km.jpg
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