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Re: [TowerTalk] How to best determine the need for a balun

To: "David Robbins K1TTT" <>,"'reflector -tower'" <>
Subject: Re: [TowerTalk] How to best determine the need for a balun
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Sat, 16 Oct 2004 21:01:16 -0400
List-post: <>
`> What kind of geometry was this with??  What about a case
where the
> feedlines of a 4-square with elevated feed points is in
the same plane
> as the radials??  Would the outside of the shield be any
different than
> any other radial in this case?

My initial thought was a ground in parallel with the small
radials system would only help, but I was wrong. I found on
several systems I tested that not isolating the shield
reduced FS with no other changes made to the system in the
test. That means it is definite, and I have to say I was
fascinated when I saw that efffect. The shield provides a
path to ground, and often carries enough current to cause
problems unless a dozen or more radials are used.

When I read the early papers *closely* on the models, the
models also predicted that effect. The only disagreement was
I found system loss under best case conditions was much
higher than the model predicted, even with the shield

The more radials I added, the higher efficiency became. The
less important the choke was.

Many people assume four radials makes a perfect ground.
That's far from true, even when the radials are a wavelength
above earth. There is substantial voltage exciting the
shield of the feedline with four or less radials, because it
is definitely not a "zero ohm" ground. A groundplane antenna
with sparse radial system is nether perfectly balanced no
perfectly unbalanced. It is someplace between.

73 Tom


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