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Re: [TowerTalk] CMRR

To: towertalk@contesting.com
Subject: Re: [TowerTalk] CMRR
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Thu, 25 Sep 2025 14:08:05 -0700
List-post: <mailto:towertalk@contesting.com>
On 9/25/2025 5:15 AM, Brian Beezley wrote:
After further reflection, I think CMRR is a misleading term for what I'm calculating.

Yes. Common mode behavior of a feedline must be understood as the feedline becoming part of the antenna, by virtue of imbalance in the feedline/antenna/surroundings. That imbalance causes common mode RF on the feedline, whether coax or 2-wire line, to be coupled to the antenna, and back down the feedline to the receiver as a differential mode signal.

Because the feedline has become part of the antenna, common mode voltage and current along it vary according to its antenna behavior. When we apply a serious common mode choke at the feedpoint, it's very high resistive impedance forces a current minimum at that point, but more important, disconnect the feedline from the antenna in the common mode circuit, preventing the coupling of common mode current to the receiver.

With that choke at the feedpoint, that feedline becomes a long wire, open at the end. If grounding and bonding is not properly done, it can act as a receiving antenna, and cause all sorts of mischief in the shack.

My antenna farm includes six dipoles at 100-125 ft, with feedlines that are 140 - 180 ft long. Soon after moving here, I observed that, even with chokes at the feedpoints, those feedlines were acting as parasitic elements to my 160M Tee vertical, screwing up its matching impedance and strongly affecting its pattern. I added chokes roughly halfway down each feedline so that they look like an open half-wave on 160M.

BTW -- my background includes having worked for Pete Johnson as a college EE student. Pete was a broadcast consultant who with Carl Smith (CREI in Cleveland) wrote the FCC AM broadcast Technical Rules after WWII. Pete made his living trying to fit new stations into a band that had been full for 20 years. My gig included computing the patterns of multi-tower AM arrays he was designing and plotting them on aero maps. So I have a pretty good understanding of these interactions.

73, Jim K9YC

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