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Re: [TowerTalk] Coaxial Moxon

Subject: Re: [TowerTalk] Coaxial Moxon
From: "Dan Zimmerman N3OX" <>
Date: Mon, 4 Aug 2008 21:59:04 -0400
List-post: <">>
> I have looked at the pics and dont see why the
> antenna would not work.

It does work, in a way.

The radiating elements are the *outside* of the coax shield.

The *inside* of the coax shield and the center conductor form a coaxial stub
that's in series with the outside.

Imagine building a dipole from aluminum tubing that's 66% normal length.

Now, you want to put some pretty big loading inductors at the feedpoint to
get it to resonate, and you don't feel like winding coils, so you take some
coax and make a couple of shorted stubs about 1/4 wave long (which have very
high, resistive impedance) and stick them in *series* between either side of
your feedpoint and the aluminum elements.  The shield of the coax is
connected to the aluminum element side  and the center conductor is
connected to the feedpoint side. Exactly 1/4 wave stubs are no good,  very
high impedance, but if you trim just a little bit off those coax stubs,
they'll go off resonance in the inductive direction.

At some point, you've trimmed the stubs so they show an inductive reactance
that just cancels the short aluminum elements' capacitive reactance and you
have a resonant antenna... but you've got these dumb stubs dangling down
near the feedpoint.  So you stuff them inside the aluminum antenna.  Well,
the space between the outside of the coax stub shield and the inside of the
aluminum element shield has no electromagnetic field inside of it, it's
shielded from the fields inside the coax and it's shielded from the fields
outside the aluminum element, so you just magically fuse the outside of the
coax shield and the inside of the aluminum tube together with metal to keep
the wasps out.

Now you've used magic and a lot of work to come to the realization that the
*outside surface of the aluminum tube radiating element* and the *coax stub
outer shield's inner surface* could have been two sides of the same pipe to
start with.

So you start the next one with coax cable as *both* the inner stub and the
outer element instead... same thing as a "velocity factor" dipole.  The
trick is that if you're just a little short but *close to resonance* with
your stub, you can find *any value* of inductive reactance you want, so you
discover that there's always a little tiny bit of trimming you can do at the
end of an "electrical quarter wave" worth of coax to make it resonant.

But it's a terrible way to load an antenna.  It's very, very lossy.

Give it a try if you want, but objectively measure it against a reference
dipole antenna for *gain* ...


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