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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