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Re: [TowerTalk] Feedline (choke) question

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Feedline (choke) question
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Fri, 26 Sep 2025 15:40:36 -0700
List-post: <mailto:towertalk@contesting.com>
On 9/26/2025 3:21 PM, Jack Brindle via TowerTalk wrote:
Jim’s method of installing verticals is to use a single piece of coax, remove the jacket and 
shield for 1/4-wave to form the top portion of a dipole, then add a choke 1/4-wave down from the 
“feed” point. No connectors needed until it gets down to the shack entry. Large 
toroids are very useful here. Having said that, I am pretty sure Jim placed connectors at the lower 
end of the dipole.

That's only one method I've shown for building a vertical, and that choke is placed 1/4 wave (common mode) down the feedline from where the shield is removed. The result is a center-fed vertical dipole, eliminating the concern of the feedline distortion the pattern.

The antenna doesn't provide a predictable feedpoint Z without the choke, which is really the bottom end insulator. The required choking impedance depends entirely on the transmit power -- because it's at a very high voltage point, dissipation can get high enough for destructive failure choking Z is not high enough.

Here's my applications note on that design.
ttp://k9yc.com/VerticalDipole.pdf

73, Jim K9YC



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