[Antennas] Autopsy of a coax cable (Hustler 6BTV)

Sun, 3 Mar 2002 03:18:53 -0000

Hello
Tonight I performed an "autopsy" on the coax cable that was removed from my
Hustler 6BTV setup today. What I found was interesting....
To re-cap.
This antenna had deteriated to the point where it was nearly unusable, it
generated very bad IMD products from nearby broadcast stations at 4.00MHz
and across the tropical bands. Replacing the coax (and a minor
re-adjustment of the 20M trap) and the antenna is performing as it should,
with NO IMD.
This antenna had been installed about 2 1/2 years ago, using a good quality
RG-8X cable with 95% copper shield and grey plastic jacket. Supposedly UV
proof. (not Belden)
The autopsy revealed the following
1. The outer jacket had deteriated where it was exposed to the sun and ran
across the hot sheetmetal roof. BUT, the deteriation was only on the outer
surface. Probably not enough to cause the problem I was seeing as the
jacket appeared to still be weather tight (except as noted below).
2. The "choke coil" revealed something far different. This coil had been
wound and held in place by black sticky tape. I didn't have enough tape to
wind the ENTIRE circumference of the coil. It was then secured to the roof
(against the high winds) by BIG globs of roofing tar. After I pulled all
the tape off and unwound the coil it was discovered that where the tape had
been the Coax jacket was good. BUT, where there was no tape the roofing tar
had completely hardened the coax to a stiffness like unto a wooden pencil.
Stripping back the insulation revealed that the entire copper braid of the
coil was corroded where moisture had entered the coil somewhere. It ALSO
revealed brownish coloration of the inner foam insulation from the tar,
indicating the tar had leaked into the coax. (This roof gets 'really' hot
in the summer).
3. The short section of coax from the antenna to the coil was clean and
bright as new inside indicating that the "sticky clay" coax sealant (from
Radio Shack) had done it's job properly AND the fact that it was higher than
the choke coil prevented water damage from the choke.
Moral of the story. DON'T use roofing tar to hold your coax in place. In
fact, I would suggest trying to isolate the coax from any tar on your roof,
possibly running it through a pvc pipe or something. (Something I didn't do
this time either as I only discovered all this later)
73 de Phil KO6BB
DX begins at the Noise Floor!
[email protected]
Merced, Central California
37.18N 120.29W CM97sh
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