1. There is nothing inherently wrong with "coupling to ground" at the
feedpoint. As long as whatever capacitance that may be there (which I'm
not convinced would be significant anyway) is relatively low loss it
would just be part of the overall tuning system. Besides, coupling to
anything is not uniform along an antenna. The net effect of capacitive
coupling is going to vary as a function of the voltage at that point,
and the voltage at the center of a "dipole" is low while the current
there is high. That's why loading coils (which require current to be
useful) are placed more toward the center of a typical halfwave element
(see note below) and capacitance hats (which require voltage to be
useful) are placed near the ends.
2. As best I remember, a properly designed Moxon antenna does indeed
present close to a 50 ohm impedance at the feedpoint. In any case, the
choke balun is there for balance and not anything to do with SWR. I
suggested before that you research Cebik's work on Moxon antennas ...
I'm going to suggest again that you do so.
73,
Dave AB7E
Note: Loading coils are rarely placed at the center of half wave
elements because it takes current to radiate a signal and if we cram it
all in one place that doesn't work so well. Proper placement of loading
coils is a compromise between spatially spreading the current and
locating them at a point where there is sufficient current for them to
actually act like an inductor.
On 8/3/2014 1:06 PM, L L bahr wrote:
As mentioned on another post and subject, I am going to build a 40 meter W6NL
Moxon Beam. The center of the Director is split and is not at ground potential.
I see a lot of builders have used a two foot long aluminum channel to mount the
Director and Reflector to the boom. They use a plastic or fiberglass sleeve to
isolate the director from the grounded Boom. I can see this would make for a
very strong support.
1. All this to me means, 2 foot of the radiating part of the Director is close
coupled to ground. I'd think this is Bad!! Seems to me the use of a Delrin
spacer rod or fiberglass sleeve with just the use of U-bolts without the 2 foot
of aluminum channel would dramatically reduce the Director coupling area to the
grounded aluminum channel.
2. Also, the feed point on all the versions I have looked at couple an RF
choke right to the center point area of the Director. When I built a couple
plain Yagi antennas, I used a Gamma Match or T match thus tapping the Director
element at a 50 ohm point rather then at say at a 15 ohm point or so right at
the center of the director. Why is this not done on the W6NL building
examples? Is it because they think and hope the RF balun choke is going to
keep the mismatch SWR in check? Can anyone explain this to me? I'm not sure,
but I suspect the impedance right at the center of the Director is not 50 ohms
or even close to being 50 ohms.
Anyway, I thought I would throw these two concerns out for you experts to
comment upon. Am I thinking clearly and correctly or am I wrong somewhere?
Lee, w0vt
Houston
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