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

Sun, 03 Mar 2002 07:39:12 -0500

At 03:18 AM 3/3/2002 +0000, Phil Atchley wrote:

Here is my sad tale of coax.  Once I had a vertical, and old CushCraft I 
believe, but that's not important.  It was mounted about 50' from the house 
on the ground, and fed with buried coax.  The coax was ordinary, not cheap 
RS or RG 213, but just standard Belden.  After a few years, I don't 
remember how many, I was having trouble getting out, so on a hunch, I 
checked the power at the antenna.  It was 10 watts at the antenna with 100 
watts out of the transmitter.  I dug up the coax and found water had gotten 
inside the outer jacket and the copper braid was coated with heavy green 
copper oxide.  Next coax I installed which was also buried was RG213 and it 
was inside a plastic water pipe.  Been there over 15 years, and the power 
at the antenna is at least 80 watts on all bands with 100 in.  There's a 
moral here somewhere.

On the UV hardening of the outer jacket.  I believe black is the preferred 
color for UV resistance, and I guess some are more UV resistant than 
others.  I use a little bit of 8X outside too, but it has black cover.

>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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73, Chas, W1CG