And, as the base of the ground plane vertical is further elevated above
ground
a smaller portion of the near field fringes through the ground, and the
efficiency
increases. In the infinite height limit (free space) all of the near
field is between
the vertical element and the radials. At about a quarter wave above
ground a single
radial is highly efficient, although the azimuthal radiation pattern is
asymmetric.
Such an approach may be used to provide some directivity in a direction
of interest.
Keep in mind that as the centroid (the base of a standard ground plane)
of the
vertically oriented current distribution increases above a quarter wave
above
ground, a larger fraction of the energy is radiated at higher angles.
I have obtained good results with the base at a quarter wave above
ground and 2
radials. In this configuration the input impedance of the antenna is
negligibly
affected by seasonal changes in ground moisture content (wet vs dry),
since coupling
to ground is weak.
Jack K1VT
That's true. But the best solution is to use raised radials a few feet
above the ground (I used 10 feet). Just 3 raised radials has been
show to
make the antenna as efficient as 10 to 30 ground radials. And you don't
have all the work of burying the radials.
Carl Moreschi N4PY
121 Little Bell Drive
Hays, NC 28635
www.n4py.com
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