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[TowerTalk] Telrex balun stinks!

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Subject: [TowerTalk] Telrex balun stinks!
From: (Tom Rauch)
Date: Fri, 18 Jun 1999 17:22:38 -0400
If any has an old Telrex yagi, watch out for the balun. If the balun 
you have is constructed like the one my yagi came with, the balun 
will guarantee undesirable feedline radiation, a skewed pattern, and 
some loss of directivity.

I'm installing a 40 meter Telrex yagi and thought I'd test the Telrex 
balun. I removed the balun and tested it.

Dissecting the balun, it was wired and wound like a 1:1 voltage 
balun. (Voltage baluns are the worse kind to use anyway in most 
or all antenna applications). I was a little suspicious because it had 
a few turns of 50 ohm coax with a #12 insulated house wire air-core 
coil inside the coax winding for the "balancing" winding. No way 
could it have enough impedance or proper balance with a design 
like that.

With the balanced output terminated in a perfect 50 ohm balanced 
load,  the feedline terminal measured 81 +j36 on 40 meters! Worse 
than the impedance error (which could easily be fudged-out to a 
perfect match by adjusting the feedpoint hairpin wires), it had about 
15% voltage balance error on the balanced output!

Into the trash it goes, and now I have to fuss with the feedpoint until 
I get it right with a proper choke balun. Hard to believe a company 
that made such big expensive top-of-the-line antennas supplied 
such a crummy balun.

The hairpin loading system about five feet long had a measured Q 
of about 100, just over one ohm ESR. That translates to about 1/2- 
3/4 dB lost in heating the hairpin. As long as I'm redoing the 
feedpoint, the hairpin might as well be scrapped and replaced using 
conventional copper tubing inductors with Q's up in the 400-500 


73, Tom W8JI

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