Bill:
I wound an air core balun for my tribander a few years back. Mine
ended up as five inches ID with five turns of RG-213.
http://lists.contesting.com/archives//html/Towertalk/2004-07/msg00435.html .
You can measure the effectiveness of your balun with an MFJ-259
analyzer (possibly with others.) After it's wound, connect one end of the
balun's shield to the analyzer's ground post, the opposite end OF THE SHIELD
to the center conductor of the analyzer input connector. Then run the
analyzer through the frequency range of interest and note the impedances at
various points. I stopped fiddling with balun turns once I reached 250 Ohms
(>5X the nominal coax impedance of 50 Ohms.) You're measuring the impedance
of a coil made from the outer surface of the coax shield.
To weatherproof the end of the balun I followed the advice from WB4MDC,
to wit:
http://lists.contesting.com/archives//html/Towertalk/2002-10/msg00391.html
.
73 de
Gene Smar AD3F
----- Original Message -----
From: "Bill Turner" <dezrat@copper.net>
To: <towertalk@contesting.com>
Sent: Tuesday, August 21, 2007 5:42 PM
Subject: [TowerTalk] Baluns
> I'm thinking of replacing the toriod-core balun on my Cushcraft X7
> triband yagi with an air-core coax balun. I've searched the TowerTalk
> archives for comments on air-core baluns and there seems to be a mixed
> bag.
>
> What I would like to do is eliminate any possible trouble spots. I
> would like to run a single piece of coax from the antenna, coiled into
> a balun, and then all the way to the transmitter. I don't see the need
> for a coax connector up 80 feet in the air, just waiting to fail. Why
> not just strip the coax and connect it directly to the antenna,
> through the balun? Waterproffed, of course.
>
> I know there was some discussion of solenoid-wound vs scramble-wound
> coax baluns, but I don't recall which was considered the best. The
> ARRL handbook says "coil it like a rope", or words to that effect.
>
> Another alternative is to slip a bunch of ferrite beads over the coax.
> I don't seem to be able to find any beads which will fit over RG-213
> size coax, only the RG-58 size. Anyone know of a source?
>
> Comments are welcome.
>
> Bill W6WRT
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