[Antennas] End-fed half-wave religion

2002年7月14日 11:46:37 -0500 (CDT)

Gotta love the world of antennas--it's science, yes, but there's
unmistakably a little bit of that good old fashioned religion mixed in
there, too.
Would anyone care to dispute the notion that end-fed half-wave antennas
(whether J-Pole, or fed with some other technique) in free space, or high
in terms of wavelength above real ground, do not electrically require a
ground plane or counterpoise? It seems to me that an end-fed half-wave
could be considered electrically complete.
On the other hand, Danny, K6MHE, argues that ground-mounted end-fed
half-wave (how's that for hyphenism?) verticals do require a ground plane,
lest there be substantial losses. That may be very different from stating
that an end-fed half-wave is electrically incomplete without the ground
plane (or counterpoise).
A quarter-wave conductor, for example, is different--it is clear that
we must feed it "against" something. The something may be the other
half of a center-fed half-wave dipole, or it might be some sort of a
ground plane. If we try to use real ground as the ground plane, we
will get disappointing, lossy results. A ground plane or counterpoise
is an absolute must, for a quarter-wave antenna element, is it not?
(I'm half expecting someone to come up with some exotic exception to
this statement, which will be fun in and of itself, but I imagine we'll
all still agree with what I'm saying here, as a general principle.)
End-fed half-wave antennas are not so readily understood. Through various
examples, we hear that end-fed half-wave antennas can work beautifully
fed against... "nothing". This seems to defy logic, on some level,
but it almost makes sense, too--if the only requirement is to develop
voltage, and current will be essentially zero, what necessity is there
to provide for anything to "go" anywhere? If we develop voltage and
attach the end of a half-wave antenna at that point, we'll get current a
quarter-wavelength down the antenna, which is good--we'll have radiation.
But apparently it's not critical to attach the "other side" of that
voltage-producing feed scheme to any sort of ground plane or counterpoise.
At least, that doesn't seem to be a requirement in free space, or anything
remotely resembling it.
Where the ground-mounted end-fed half-wave vertical is concerned, I
have seen enough assertions by respected experts that a ground plane
is required in order to avoid losses in the neighborhood of 3 dB that
I accept this notion as fact.
But what about other end-fed half-wave antennas? Is an end-fed half-wave
without any sort of ground plane or counterpoise electrically complete?
What about a horizontal end-fed half-wave, or an end-fed half-wave in
free space? There's no need for a counterpoise of any sort to make any
of these function correctly and efficiently, is there?
73,
Jim WB5UDE