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Re: [TowerTalk] Feedline (choke) question

To: "towertalk@contesting.com" <towertalk@contesting.com>, Jim Brown <jim@audiosystemsgroup.com>
Subject: Re: [TowerTalk] Feedline (choke) question
From: Wes Stewart via TowerTalk <towertalk@contesting.com>
Reply-to: Wes Stewart <n7ws@yahoo.com>
Date: Wed, 1 Oct 2025 18:51:37 +0000 (UTC)
List-post: <mailto:towertalk@contesting.com>
 Jim,
I think you're missing Jack's very interesting point.  I've used an open ended 
cable as an example, but a mated pair of your favorite connectors is no 
different.
At the very end of the cable (or connector) there is no inside and outside of 
the outer conductor, there is just the conductor, hence there is no skin effect 
at that point.  I'm not smart enough to figure out how far down the cable the 
skin effect develops.  But this raises a question in my mind. We've all seen a 
thousand times the drawing of a coax-fed dipole, where current is "spilling 
over" the open end and becoming a common-mode current on the outside of the 
cable.  A smarter mind than mine needs to 'splain this to me.
Wes  N7WS



    On Wednesday, October 1, 2025 at 10:28:53 AM MST, Jim Brown 
<jim@audiosystemsgroup.com> wrote:  
 
 On 10/1/2025 7:46 AM, Jack Brindle via TowerTalk wrote:
> Connectors are very important in this system. They must be added to the 
> analysis. Without them, we have to question the validity of the tests.

No. Common mode and differential mode currents are a characteristic of 
transmission lines, and common mode can be present on 2-wire lines if 
the system that includes the antenna, the transmission line, and 
termination in the shack has imbalance. The mechanism by which common 
mode in coaxial line is on the outside of the shield is skin effect, and 
it's present in those connectors.

Soldered or crimped, the connector(s) is/are simply part of the 
transmission line, carrying the differential and common mode current 
that is in that system (antenna, line, shack). Depending on their 
construction, they can introduce some discontinuity in the differential 
circuit.

73, Jim K9YC




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