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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