[TowerTalk] common mode chokes, baluns and multiband doublets

k7wxw k7wxw at arrl.net
Tue Dec 12 17:05:17 EST 2017

Good morning - I am on the digest version of the list and it took a while to sort all the emails regarding my question out. 

Background:  I chose the multiband with a 450 ohm feedline and a Johnson matchbox as the best way to address my particular circumstances (multiband operation with a single horizontal wire, available antenna attachment points, location of shack, neighborhood powerlines, spousal boundaries, etc) while meeting my operating needs (regional nets, casual DX, mostly CW and digital modes, using less than 100W).  I understood the various compromises built into the design (which I won't belabor) and decided that I could live with them when operating as I operate. I have been generally right about that.

After a year of using this antenna system, though, it is clear that I have serious receive noise problems.  I addressed a lot of them by removing RF sources from my operating environment and incorporating the sorts of fixes that K9YC recommends. But I haven't got them all and I am guessing that my unbalanced "balanced system" creates a common mode path for noise ingress.  

That's why I asked about common mode choking. If my guess is wrong, someone just tell me that and I will go away quietly. ;)

Here's what I think I've heard or confirmed:

Designing a common mode choke that can withstand the mismatch caused by the wide variation of antenna impedances at the feedpoint of an multiband antenna *and* effectively choke RF noise is not impossible but the weight/cost to power ratio of such a choke is going to be very large.  

Designing a common mode choke that works across 3 to 30 MHz requires multiple chokes in series, with different core materials in each to insure adequate choke impedance across the frequency range with the consequent high weight/cost to effectiveness ratio.

So... even if my guess is right (about imbalance creating a common mode path for noise), designing a choke to solve the problem is very difficult. 

Does that sum it up?

BTW, as my wife will tell you, I have spent a lot of the last year with my noise buried in a stack of books, papers, and website screendumps on antenna design, impedance transformation and chokes. I am really grateful for all the work that you all have shared. I won't mention all the call signs because I would accidently leave someone out and piss them off but trust me, there's a highlighted and penciled copy of your stuff around here somewhere.

73 de bill K7WXW

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