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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 03:33:38 -0700
List-post: <mailto:towertalk@contesting.com>
On 9/25/2025 8:27 PM, David Gilbert wrote:

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.

Hi Dave,

You're right about pickup on the feedline, BUT: that should NOT matter if 1) Proper bonding and grounding is done for electrical safety and lightning protection per NEC, which includes, but is not limited to, everything bonded to everything and feedlines grounded and lightning protected at a bonded entry panel; and 2) the Station, especially the operating desk, is properly bonded and grounded. If it helps, there's something seriously wrong with Bonding Grounding.

I think I read into your observation that you're talking about a feedline that goes straight to the amp or the radio without that proper bonding and grounding. >
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.

Yes. EZNEC will show that if you model enough of the antenna system to include it. That observation is the basis for my tutorial on the placement along the feedline of stubs to reduce amplifier harmonics, and why what W2VJN published on that topic missed the mark, and why I published a more correct method in NCJ a year later (and confirmed by measurements in my station).

It has been my experience that if you give NEC ALL of the information, it will provide very accurate results. The challenge is to SEE all of the information, which requires both visual powers of observation and a thorough understanding what things can contribute to the system. N6LF published a great piece maybe 15 years ago showing that even a tower that is relatively short as a fraction of a wavelength can become parasitic to a nearby vertical antenna. Rudy's work is some of the most important stuff published in the last 30 years on antennas and radial systems.

73, Jim K9YC



Thoughts?

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