> 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
depths.
Anyway, easy enough to measure something I already have laying
around.
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
W8JI@contesting.com
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