Many amateur radio operators and others appear to believe that a
vertical monopole with its base near the earth radiates zero relative
field in the horizontal plane, and very little field at low elevation
angles above the horizontal plane.
Probably this assessment is due to many years of looking at
far-field patterns of a vertical monopole with its base near the earth that
are calculated for an infinite distance over a flat, real-earth ground
plane. However that analysis does not fully describe the fields actually
launched by that radiator -- which require using the near-field capabilities
of NEC.
Amateur radio operators apparently aren't concerned by the zero gain/field
shown by NEC in the horizontal plane in a far-field analysis, thinking that
only the "skywave" is important to them. However the significant radiation
at very low elevation angles from a real vertical over real earth produce
significant skywave, which can serve the greatest single-hop distances from
the radiator. The comparison of the space and surface wave from
the same monopole system linked next below illustrates this point.
http://i62.photobucket.com/albums/h85/rfry-100/Space_Surface_Wave_Compare.gifIt is undisputed that NEC-2 cannot model buried conductors. But that does
not limit its usefulness in calculating surface-wave fields over real earth,
using a properly constructed model. The considerations for doing this are
shown in the clip below, taken from the NEC-2 Users Guide.
The surface wave fields from a vertical monopole calculated using the
near-field capability of NEC-2 compare quite well with those fields shown in
the medium wave propagation charts of the FCC for the same applied power,
radiator, frequency, and earth conductivity. This is illustrated in the
propagation chart linked below for systems on 1700 kHz.
http://i62.photobucket.com/albums/h85/rfry-100/PropChart1700kHz.gifThe NEC surface wave in these attachments was calculated via the
Sommerfeld-Norton method, using a vertical monopole driven against
horizontal wires used as a counterpoise. The entire model was isolated
from, but physically near the earth.
Perhaps this information will add a different perspective when using NEC,
and evaluating the patterns it produces for this application.
Discussion is invited.
Richard Fry
+ + +
3. Modeling Structures Over Ground
Several options are available in NEC for modeling an antenna over a ground
plane. For a perfectly conducting ground, the code generates an image of the
structure reflected in the ground surface. The image is exactly equivalent
to a perfectly conducting ground and results in solution accuracy comparable
to that for a free-space model. Structures may be close to the ground or
contacting it in this case. However, for a horizontal wire with radius a,
and height h to the wire axis, [h^2 + a^2] ^1/2 should be greater than
about 10^-6 wavelengths. Furthermore, the height should be at least
several times the radius for the thin-wire approximation to be valid. This
method doubles the time to fill the interaction matrix. A finitely conducting
ground may be modeled by an image modified by the Fresnel plane-wave
reflection coefficients. This method is fast but of limited accuracy
and should not be used for structures close to the ground.
A wire ground screen may be modeled with the Sommerfeld/Norton method
if it is raised slightly above the ground surface.
