Sounds like the antenna may have suffered damage from lightning. Just a
possibility, that seems to correspond with the damage described..
73,
Riki, 4X4NJ
Pete Smith wrote:
> 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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