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From: Pete Smith[SMTP:n4zr@contesting.com]
Sent: Monday, September 28, 1998 2:46 AM
To: towertalk@contesting.com
Subject: [TowerTalk] Weird choke "balun" failure
Hello Pete:
That is very interesting, indeed. I have always been a big believer in the
bead current baluns for the past several years and have been making my own out
of surplus beads I have on hand. I also had one of the Force-12 baluns on each
of my Force-12 yagis and they were all the bead current baluns.
This summer I decided to fix my six stacked C3D yagis on the 190 ft. rotating
tower. After fixing all splices, replacing a 500 ft. run of hardline and
re-tuning the antennas at the feedpoint with the new CIA-HF Complex Impedance
Analyzer, I still had problems. Jay, WX0B kindly replaced the Dunestar Stack
Switch with his new Array Solutions Stack Match and that helped a lot with the
SWR problems. He then re-worked the Dunestar 50 ohm:25 ohm UNUNs which had a
bad SWR problem too. Thanks to all his work the SWR came into a reasonable
area.
While I was going through all this hassle, I talked with Earl Grandison at
Amidon and told him about my tale of woe. My stack was still not performing as
well as when I installed it. Earl told me that Jerry Sevick, W2FMI had
convinced him to replace his current bead balun with a Guanella balun on his
triband beam. When he did, he said his front-to-back came to life and he was
able to crack pileups.
Apparently, there is some controversy between Walt Maxwell and Jerry Sevick
over this one and those are two titans I would not want to get into the middle
of a technical argument with. But after thinking about things a little, it
appeared to me that maybe those beads do actually dissipate some power when
they stop spillover currents from coming down the coax. Based on that theory,
I replaced my all my bead baluns with Amidon Guanella baluns on the triband
stack.
All I can say is that I am getting the same old great results with the stack.
Your finding really makes me suspect that the baluns were a big contributor to
my problems Pete, thanks for the input!!! I am going to now look at replacing
the bead baluns on the rest of my antennas in the future!
73/Mike, N7ML
I use a homebrew common mode choke at the 40m. feedpoint on my C-4 -- it's
the fairly standard 50 beads of #73 material on small-diameter teflon
insulated coax.
I run about 1200 watts output, and the C-4's 40 meter driven element is
tuned for CW, so when I operate on phone the SWR is pretty high (though
still within the range of the SB-220's plate tank).
Yesterday, I was inspecting the choke after bringing the antenna down for
modification. To my surprise, there was a hole through the tape wrap at
about the middle of its length. I surmised that it might have been damaged
by the birds that perch on the antenna in great numbers, but when I removed
the wrapping I'm not sure. First, at the point where the tape was damaged,
2-3 beads are missing, the teflon jacket is open, and the braid is severely
disrupted. The cable retains continuity and is not shorted. Looking very
closely, the teflon appears to have been melted, and there is some evidence
of burning inside the jacket (though this may be discoloration from
oxidation of the exposed braid).
Even more mysterious, about 10 of the beads adjacent to the opening, on the
antenna side of it, have been shattered into 4-6 pieces each, even though
the tape outer wrapping is intact. There is no evidence of heating other
than the teflon right at "ground zero."
Anyone out there experienced a similar failure, or understand what's going
on here? Does it make sense that a failure due to bead heating would occur
in the middle of the string of beads rather than at one end? If this is
due to bead heating, does the high SWR on phone make the heating worse? Is
there a design change I can make (different beads, for example) that would
maintain adequate common mode choking on 40m. while reducing the heating?
Should I expect to find similar damage to the chokes at the feedpoints of
my 80 - meter array, which also operates at high SWR on the 75 meter end of
the band?
Thanks in advance.
73, Pete Smith N4ZR
n4zr@contesting.com
"That's WEST Virginia. Thanks and 73"
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