Topband: "Missing" Buried Radials for a Monopole
mikewate at gmail.com
Fri Aug 24 12:46:42 EDT 2012
Ok, let's not make this black and white, then. :-)
It goes without saying that both halves of the antenna (radials and the
vertical) must be present in order for the bottom-fed vertical monopole to
radiate. But I maintain that any far-field radiation from the radials is
way down from the radiation from the vertical. It only exists at all
because in the real world, nothing is perfect and there will always be
losses, imbalance, and imperfect cancellation of fields.
Maybe this is not a good comparison, but there is also a strong field in
the immediate vicinity of a transmission line made from two parallel wires.
But as long as it is straight and balanced, the field rapidly decays as we
move away from it.
Below is the question I posed to the other forum (with very minor edits):
On another forum (somewhere), someone that I very highly respect for his
technical expertise once seemed to imply that an elevated counterpoise
under a vertical monopole radiates energy into space, the same as the
vertical element does.
But is that really the case?
In other words, BOTH a bottom-fed quarter-wave vertical antenna AND an
elevated counterpoise (consisting of ... symmetrical 1/4 wavelength radials
all connected together at the feedpoint under the monopole, ~6' high)
contribute to the far-field radiation.
I have always believed otherwise; that while there is certainly a field
around a symmetrical elevated counterpoise, that field actually very
rapidly decreases as we move farther away from it, as is the case with the
field surrounding a transmission line that consists of two straight
close-spaced parallel conductors.
If such an elevated counterpoise DOES actually radiate, then what about the
radials buried just below the surface of the earth under a vertical
monopole with its feedpoint just the above the earth --such as is installed
at many AM broadcast stations throughout the USA? Do those radials buried
in (or lying on the surface of) highly lossy soil radiate? Based on
everything I've ever studied, I simply cannot believe that is the case.
Now suppose that we were to raise that same buried radial system up and out
of the earth. At what point above the earth would they then begin radiating
a significant portion of energy that would contribute to the far field?
I say that such a counterpoise capacitively couples to the earth (the same
as non-resonant radials lying on the ground certainly do). If we walk under
an elevated counterpoise, then we become part of the dielectric of a
capacitor: one plate of which is the counterpoise, and the other plate of
which is the planet itself.
(We could take this one step further and make a counterpoise of an infinite
number of radials (a disk). Would such a disk radiate?)
Having said all this, I doubt that the radials on a VHF ground plane
antenna, many wavelengths above the earth, have much coupling to the earth
itself. Maybe a counterpoise radiates, after all. :-)
I don't have a problem with understanding that both legs of a 90 degree
center-fed dipole (such as with one element vertical and one element
horizontal) radiate. But everything I've ever studied seemed to indicate
that in the case of a symmetrical counterpoise (balanced, equally spaced
wires and with the monopole directly under their apex) parallel with the
earth and with a sufficient number of radial, >99% of the far field comes
from the vertical monopole.
What is actually the case here?
I don't pretend to understand this. I'm not sure that anyone does. And I
don't know what practical use knowing this would be. But it's something
that fascinates me.
On Fri, Aug 24, 2012 at 11:15 AM, Tom W8JI <w8ji at w8ji.com> wrote:
> The question is really all about proportions. ... Two radials can pretty
> much cancel in the farfield, but not perfectly. ... Near earth, there is
> an "image" that helps cancel radiation, but it isn't perfect. ... We have
> to watch making things black and white.
More information about the Topband