[TowerTalk] CMRR

[Jim Brown]

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