[TowerTalk] Blackened Sheild on Belden Coax

[Dick Green]

Tom Rauch <w8ji@contesting.com> wrote:

> Any time I've had "blackened braid", it has been from water.

That's been my experience, too.

Case #1: A few years ago, I picked up an old B&W dipole center insulator
with 70' of high-quality coax integrated into the assembly. The center
insulator is a hollow triangular metal shell made up of two pieces separate
by a rubber gasket and held together with many screws. After a few years 65'
up in a tree, I started to get really wierd SWR readings on my 80M inverted
vee -- sometimes it was normal at low power with a low duty-cycle signal,
but a continuous carrier would cause the SWR to slowly creep up.
Transmitting at high power would immediately result in high SWR. A lot of
testing and replacing of intermediate parts of the run took place before I
realized it was the integrated coax. When I disassembled the insulator I
found quite a bit of moisture had seeped in past the gasket. The gasket had
originally been glued in place, but the glue had disintegrated in several
spots. The water had make its way into the open end of the integrated coax,
down between the jacket and braid. The shield was black at least a foot down
the coax. Replacing the coax and regluing the gasket with Pliobond fixed the
problem (for now.)

Case #2: Critters periodically gnaw through the jacket of a piece of coax I
have running through the woods to an antenna. The run usually works until
enough water seeps in to turn the braid black.

I have never seen coax turn black from high power applied with high SWR.
I've seen the dielectric melt through the braid on thin coax, but never the
jacket.

73, Dick WC1M


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