Topband: Near Field/Far Field

[Richard Fry]

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