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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 11:51:40 -0400
List-post: <>
> A 'balun' is used to go from 'BALanced' to 'UN-balanced'
feed types.  A
> 1/4 wave vertical over a ground plane is already
un-balanced and so it
> coax so no, a balun is not what you need.  An rf choke on
the feedline
> may help to uncouple the shield from the radiation.

A current balun is a choke, so a current balun would work
perfectly well.

Contrary to what many people think verticals with small
radial systems, like a traditional 4 radial vertical
groundplane, have substantial current on the feeder.

This leads people to all sorts of mistaken conclusions about
vertical antennas, like the idea that adding radials changes
the wave angle substantially. What the additional radials do
is reduce common mode feeder currents and that reduction is
what has the most significant effect on the system.

Another thing that is interesting is pretty much everyone
proposing small elevated counterpoise systems has included a
warning buried in the text that the feedline needs
decoupled, but everyone ignores it. When I was measuring
elevated radial systems I found adding a choke or current
balun increased the FS of the elevated system by as much as
1dB or more, and I wasn't looking for a worse case

If the idea of using a current balun is troublesome, get a
magic marker out and write the word "isolation choke" where
it used to say "balun". Then it won't confuse the RF. ;-)

73 Tom


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