[Antennas] Ground Planes

Thu Apr 7 09:52:23 EDT 2005

----- Original Message -----=20
From: Alex Eban=20
To: 'Dr. William J. Schmidt, II' ; pacificcommonwealth at excite.com ; =
antennas at mailman.qth.net=20
Sent: Thursday, April 07, 2005 3:31 AM
Subject: RE: [Antennas] Ground Planes
...William made a very important point!!
Actually what we call a ground plane can operate in one of 2 modes:
resonant and not resonant and the mechanism is fundamentally different =
in
each case.
For the resonant mode, known as counterpoise, the length is
critical. However, one should remember that any non symmetrical layout =
of
radials will distord the radiation pattern of the antenna! Soo, if you
choose to use a few resonant radials only, be prepared to accept some
distortion in the radiation pattern.
The other mode, the true ground plane is relying on its capacitive =
coupling
to ground to operate and due to its size, it also acts a current =
gathering
device for the those current that would otherwise floe through the lossy
ground.
Elevated radials usually operate in the resonant mode at the lowest
frequency of interest and it is a known fact that their being =
nonresonant at
the higher frequencies- although longer than a quarter wavelength- do =
not
detract from correct antenna operation. But, even then 3 or 4 radials =
only
produce a very definite, although minor deformation of the radiated =
field.
N6PL recommends at least 6 radials even high in the air. He should know: =
he
designed vertical or the Navy for a long time!
if you go for the counterpoise type, you might even consider adding
a ground wire tuner to optimize matters.
Alex 4Z5KS
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If you search the archives of rec.amateur.antennas, I think you will =
find ample evidence that in elevated radials, two (2) is both the =
minimum and maximum required. Redundant elevated radials may be popular =
(especially in military designs) but it lacks scientific support for =
efficiency.
Many designers accept the principles but still propose a third or fourth =
elevated radial as "insurance". Six elevated radials would be in the =
military-anal-retentive design mode, and have no bearing on the results =
achieved. Perhaps the effects of poor ground conditions, nearby objects =
and other elements nearly impossible to model have led some to believe =
adding more elevated radials helps. Such anectdotal evidence might have =
actually improved the tester's performance, but it should be explained =
as what it is, however interesting it sounds.
73,
Jack Painter
Virginia Beach, Virginia