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Re: [TowerTalk] Feedline (choke) question

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Feedline (choke) question
From: Gedas <w8bya@mchsi.com>
Date: Fri, 26 Sep 2025 13:16:28 -0400 (EDT)
List-post: <mailto:towertalk@contesting.com>
In case someone out there was wondering what would happen if you wound your 
coax through a toroid core WITHOUT cutting it in half first I just ran some 
very quick measurements for you. 

I took a couple of images showing the test setup and how the CMC's were wound. 
In one case (the normal way) I wound 10 turns of stranded wire through a small 
type-75 core. It's CMR can be seen in one of the photos as the yellow trace. 
The sweep is from 1 MHz to 20 MHz, 5 dB/div normalized at the very top 
horizontal reference. 

I then took the same wire, made it twice as long, did not cut it in half, but 
instead folded it in half and wound 10 turns of both halves. I knew what the 
results were going to be but I figured this might be of interest to some 
others. The purple trace is the CMR of 10 turns of the "dual wires". I could 
have used some small dia coax to make it more realistic but I did not want to 
waste it. There are differences in the CMR of wire vs. coax, due to stray 
capacitance which I documented in a short YT video where I tested the CMR of 
the same number of turns but through different numbers of the same core 
material. I can provide a link to that YT video if anyone would like it. 73 
Gedas W8BYA 

Folded wire CMR: 
[ 
https://drive.google.com/file/d/1Y03KsqaspBF6GvCVH92fWq39YuLjkE3l/view?usp=sharing
 | 
https://drive.google.com/file/d/1Y03KsqaspBF6GvCVH92fWq39YuLjkE3l/view?usp=sharing
 ] 

Normal CMC: 
[ 
https://drive.google.com/file/d/1_wfrNAWcu2nqL5nhspk1za8ASeg17npf/view?usp=sharing
 | 
https://drive.google.com/file/d/1_wfrNAWcu2nqL5nhspk1za8ASeg17npf/view?usp=sharing
 ] 

Folded Wire CMC: 
[ 
https://drive.google.com/file/d/1gy-4kV1n95DOXhYnVejO6zjICEKOKKwM/view?usp=sharing
 | 
https://drive.google.com/file/d/1gy-4kV1n95DOXhYnVejO6zjICEKOKKwM/view?usp=sharing
 ] 

CMR Plots: 
[ 
https://drive.google.com/file/d/1teuarHr7Vyra8oM0dPL9JhzdMJd9QdPy/view?usp=sharing
 | 
https://drive.google.com/file/d/1teuarHr7Vyra8oM0dPL9JhzdMJd9QdPy/view?usp=sharing
 ] 




From: "Brian Beezley" <k6sti@att.net> 
To: "[TowerTalk]" <towertalk@contesting.com> 
Sent: Friday, September 26, 2025 12:22:01 PM 
Subject: Re: [TowerTalk] Feedline (choke) question 

On 9/26/2025 8:34 AM, Joe Subich, W4TV wrote: 

> One would need at least a dozen clamp on cores to achieve a 
> significant (> 1K) choking impedance. 
Thanks for looking up the data, Joe. I have an old K6NA bead choke 
somewhere in the garage. As I recall, it's about a foot long. Maybe it 
uses a dozen beads. 

Clamping a dozen beads still sounds easier to me than severing the coax 
and installing connectors. You could use a split toroid to accommodate 
multiple turns, but I don't see an easy way to hold it all together 
under stress. I can imagine some schemes, but they are pretty Rube Goldberg. 

Incidentally, I used some bare clamp-on cores outdoors for several 
years. When I finally took them down, I was surprised that the core 
material looked like new. I mention this for interest, not because I 
recommend it. 

Brian 

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