You say you're not worried about capacitive coupling, but if there is
any effect at all that is exactly what happens ... capacitive coupling
to the channel. Possibly you're worried about the channel shielding the
RF, but that isn't what happens either. Whatever coupling there is to
the channel simply makes it become part of the radiator. You are
confusing the boom with "ground" ... it isn't. That driven element could
just as easily work fine if it was bonded (I won't use the term
"grounded here") to the boom, and if RF gets coupled to the channel it
would simply re-radiate just as if the element was connected to it.
Please explain the difference to me if you disagree.
Secondly, your analogy to a whip antenna on a car is completely flawed.
With a whip on a car you are feeding the whip against the car body ...
with the Moxon you aren't feeding the driven element against the boom.
If properly balanced, there is zero potential between that channel and
the boom.
73,
Dave AB7E
On 8/3/2014 2:09 PM, L L bahr wrote:
Most of the radiation comes from the current part of the antenna which is going to be
close to the center point of the driven element. If you are using a U shaped channel 2
foot long at this area of the antenna, you will be pumping RF into the channel at the
high radiation point of the antenna for 2 feet! Looking at a whip antenna on a car, if
you place the whip low on the rear bumper and have a lot of it length within near
proximity of the auto body, it will in no way work as good as if the base of the antenna
was "in the clear" say at the top of the trunk. A lot of the radiation is going
to go right into the car body. I'm not so concerned about the capacitive coupling.
Maybe this is part of the reason the center is 50 ohms. I'm concerned about the RF being
shorted and grounded thus reducing radiation efficiency with this long 2 foot piece of
aluminum channel.
Lee, w0vt
----- Original Message -----
From: "David Gilbert" <xdavid@cis-broadband.com>
To: towertalk@contesting.com
Sent: Sunday, August 3, 2014 3:42:26 PM
Subject: Re: [TowerTalk] Building a W6NL Moxon 40 Meter Beam.
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.
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