[TowerTalk] How to mount choke balun?

Tom Rauch W8JI@contesting.com
Sat, 17 Nov 2001 11:17:36 -0500

> I know that aluminum does little or nothing to block electromagnetic
> fields from nearby transformers, so I guess I was figuring the
> interaction would be minimal, or at least a better option that
> strapping the balun to the boom where all of the turns rest right up
> against the metal boom. Again, that is just a guess and it would be
> interesting for someone to actually measure the effects of the various
> mounting schemes.

Aluminum is a very effective shield if it is thicker than a few skin 

Anyway, easy enough to measure something I already have laying 

I just measured what I used as an 80-meter 4" air core choke balun 
with RG-8X and it had a common mode impedance of about 10 
ohms 500j at 4 MHz in open air.  It went to 10 +280j when I slipped 
a piece of 2" aluminum tubing in the middle. A 2" thin steel tube 
had the same effect. 

It looks like a balun might still be OK if you start with enough 
impedance, but I'd measure the actual balun and be sure! You 
could even do that do that with a cheap antenna analyzer, by 
clipping it across the shield from end-to-end in the balun coil before 
connecting it to the antenna terminals.

73, Tom W8JI

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