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Author Topic: Repairing Damaged Coax Cable  (Read 517 times)

N8RFY

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Repairing Damaged Coax Cable
« on: April 03, 2023, 12:50:46 PM »

Hello,

One of my runs of coax (lmr240uf) has a damaged outer cover. It was ran up through the soffit into my 2nd story ham shack. Because I have aluminum siding, when I was pulling the line through the PVC cover in a couple of places was stripped and the outer conductor was exposed.

My question is, can I simply reseal the coax with electrical tape?

Any info will be appreciated!
N8RFY
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WB6BYU

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Re: Repairing Damaged Coax Cable
« Reply #1 on: April 03, 2023, 12:59:34 PM »

Depends how much water proofing you need.

If the problem is in the attic where it stays dry, then
electrical tape probably is sufficient.

But electrical tape isn't always good enough for extended
use out in the weather.  I'd use some of the rubber or
silicone sealing tape in that case, the type that you
stretch when you apply it and it fuses with itself.  Then,
especially for the rubber (which can decay in the sun),
add a layer of good quality electrical tape over the top
of it.

When you apply the electrical tape, cut it, rather than
tearing it, which reduces problems with the ends
coming unstuck and eventually coming undone.

If the shield braid looks at all corroded, you may have
gotten water into the cable - in that case, test the
cable for loss before repairing it.  Once the coax gets
waterlogged, there is no practical means to dry it out.

K7AAT

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Re: Repairing Damaged Coax Cable
« Reply #2 on: April 03, 2023, 01:16:49 PM »

I'll add one little thing to what WB6BYU said.  When you have completed the wrap of tape over your well sealed repair spot,  take a small black nylon tie wrap and tie down the end of the electrical tape.  This does help,  I think,  to keep it from coming loose over the years.
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N8RFY

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Re: Repairing Damaged Coax Cable
« Reply #3 on: April 03, 2023, 01:37:48 PM »

Thanks for the info! I ordered some new coax to replace this damaged stuff. I will test it as you suggested, and if it's not beyond repair, I'll use it for the antenna I'm putting in my attic.


Thanks again, it's much appreciated!
N8RFY
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N8RFY

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Re: Repairing Damaged Coax Cable
« Reply #5 on: April 03, 2023, 02:49:17 PM »

I'll do that, thanks!
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WA3SKN

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Re: Repairing Damaged Coax Cable
« Reply #6 on: April 03, 2023, 04:25:52 PM »

The answer is a firm "maybe".
If the electrical specs have not changed you could tape it or seal it up with silicone bathtub caulk and still use it (however, lmr270 is probably cheaper to replace).
Get or borrow an analyzer and a dummy load.  Terminate one end of the coax and sweep it from the other end with the analyzer.  Anything other than 50 ohms indicates a problem.

-Mike.
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N8RFY

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Re: Repairing Damaged Coax Cable
« Reply #7 on: April 03, 2023, 04:35:30 PM »

Thanks! 👍
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WB6BYU

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Re: Repairing Damaged Coax Cable
« Reply #8 on: April 03, 2023, 08:53:18 PM »

Quote from: WA3SKN

... Anything other than 50 ohms indicates a problem.




And 50 ohms may also indicate a problem.

When coax gets waterlogged, the loss increases,
but the characteristic impedance stays about
the same.  (Often the loss is due to the corrosion
between the individual strands of the braid, so
it doesn’t act as a solid conductor.)  The higher
loss will improve the SWR at the rig, because
there is less reflected power that survives the
round trip to the antennaand back.

One way to test a cable for loss is to measure
the SWR at one end with the far end open or
shorted (and preferably both, then average
the readings).  The lower the SWR, the higher
the loss in the cable.

W9IQ

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Re: Repairing Damaged Coax Cable
« Reply #9 on: Yesterday at 05:09:30 AM »

As a follow-up to Dale's well place comments, measuring coax loss is a trivial exercise with the nanoVNA or any other two port VNA/antenna analyzer.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

N8RFY

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Re: Repairing Damaged Coax Cable
« Reply #10 on: Yesterday at 08:04:26 AM »

Good to know, thanks!
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W9WQA

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Re: Repairing Damaged Coax Cable
« Reply #11 on: Yesterday at 11:35:00 AM »

tape, j f w i w,  i used hundreds of rolls doing tv ant work.
best by far was taiwan tape at 50 cents at ham fests.
no dont bother me with 3 m stuff.  i was on the roofs in winter, i know...
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N8RFY

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Re: Repairing Damaged Coax Cable
« Reply #12 on: Yesterday at 11:36:49 AM »

Thanks!
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