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Re: [TowerTalk] Feedline (choke) question

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Feedline (choke) question
From: David Gilbert <ab7echo@gmail.com>
Date: Thu, 25 Sep 2025 20:27:51 -0700
List-post: <mailto:towertalk@contesting.com>

I can't claim all of the following perspective to be based upon much practical experience, but from a theoretical point of view I lean toward the following as an ideal case:

1.  A common mode choke at the antenna to keep RF that has been picked up by the coax from getting into the antenna and affecting the pattern or the match.

2.  A common mode choke at the output of the rig to keep RF that has been picked up by the coax from getting into the rig.  Choking common mode current at the antenna doesn't necessarily keep the coax run to the rig from picking up unwanted RF.  The coax shield is an antenna and making it look like an open at one end doesn't keep it from being an antenna.

3.  A common mode choke, like any other inductor, isn't going to have much effect at a current minimum, so placing common mode chokes at current maximum(s) along the coax seems like it would be good additional protection ... and they aren't necessarily located at either end of the coax.  Note that this would be the current maximums for the common mode current, not necessarily for the differential current.  It would probably require a field probe to find those points if they exist.

Thoughts?

Dave   AB7E


On 9/25/2025 7:55 PM, Tim Duffy wrote:
I agree with Jim , K9YC recommendations.

I also have had success installing high current mode chokes on the coax to
the shack right at the amplifier output. Also sometimes a high common mode
choke from the radio output before the amplifier input can be effective.
Always at the antenna - always at the phasing/matching box. These must be
high CMI chokes in excess of 4000 ohms at the frequencies of interest. Good
quality ferrites and good quality connectors.

73
Tim K3LR

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Jim
Brown
Sent: Thursday, September 25, 2025 10:24 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Feedline (choke) question

On 9/25/2025 4:01 PM, Gary Smith wrote:
Just connect the coax to the phasing bars directly or do as suggested by
byMosley which is make a six turn choke near the phasing bars or go without?

My detailed recommendations, based on nearly 20 years of research, are
here. http://k9yc.com/2018Cookbook.pdf

In general, I recommend the matching system or "balun" recommended by
the mfr be used, with a choke based on my designs added between that and
the feedline.

73, Jim K9YC


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