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
range.
73, Tom W8JI
w8ji@contesting.com
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