Elevated radial wires perform much differently than buried radial wires.
Elevated, horizontal radial wires having self-resonant 1/4 wavelength used
as all, or part of an elevated counterpoise for a monopole, tee, or inverted
L should NOT have a low-Z (direct) path to the earth at the operating
frequency at either end of those horizontal conductors -- such as provided
by wired connections using ground rods.
If so, then those elevated, horizontal conductors no longer will have the
characteristics of opposing, end-fed 1/4-wave conductors -- which have about
zero r-f current at the ends opposite their feedpoints and maximum r-f
current at their feedpoints, and which produce the best radiation
efficiencies from such antenna systems.
The reason for this is that the r-f current flowing in to and out of the
ground plane from the end(s) of such grounded, elevated radials constitutes
loss that otherwise would add to the fields radiated by that antenna system.
_____
Depending on their wavelengths, number, and geometry, buried radial wires
concentric with the base of a monopole provide a relatively low loss path
for r-f currents flowing in the earth within about 1/2 wavelength of the
monopole (due to its near-field radiation into the ground plane) to flow
between the r-f ground connection of the transmit system and the feedpoint
of the monopole.
The lower the r-f losses in that path, the greater the radiation efficiency
of that antenna system.
As shown by well-documented physical experiments going back to 1937 and many
subsequent real-world tests and measurements also accepted by the FCC, a
monopole of about 55-deg electrical height or more driven against a set of
~120 buried horizontal radials at equal azimuth spacing and each having a
free-space wavelength of about 90 degrees produces a peak inverse-distance
groundwave field at 1 km that is within several percent of the maximum
theoretical field possible for that applied power when radiated by a perfect
monopole of such electrical heights when driven against a perfect ground
plane.
R. Fry
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|