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COAXIAL CABLE FAILURE




 
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KB5MD
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« on: October 09, 2017, 12:25:48 PM »

I have a vertical array on 80m.  The coaxial lines are buried leading to the vertical elements.  All the antennas are tuned to a low SWR for the same frequency but the swr is different
on the opposite ends of the various coax leadins at the connecting points on the phasing box.  I am using a comtek box in the center of the array.  My question is what method to use to determine if the coax to each element is bad.  They are not shorted, just high SWR on two of the four and the remaining two are not equal in SWR.  All the coax leadins are custom made to exactly the same lengths and did work fine until the SWR went crazy.
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KD6VXI
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« Reply #1 on: October 09, 2017, 12:46:46 PM »

TDR or mfj259 or darn near any other vna would work.

--Shane
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w1vtp
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« Reply #2 on: October 09, 2017, 02:30:22 PM »

Roll your own TDR:

https://www.allaboutcircuits.com/projects/build-your-own-time-domain-reflectometer/
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WA2SQQ
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« Reply #3 on: October 09, 2017, 03:21:30 PM »

Even easier
https://www.youtube.com/watch?v=9cP6w2odGUc
OR
https://www.youtube.com/watch?v=I1gfUNh5PJQ

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W1ITT
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« Reply #4 on: October 09, 2017, 03:23:41 PM »

A simpler and dirtier method would be to put a dummy load at the far end of each  cable and measure the SWR. If the you don't have low level test equipment, you can jump the main transmission line from the shack into each of the feeders in turn, terminating them in the dummy load where the vertical element would be, and use the station exciter to measure.  It will involve a few back and forth trips but we can all use the exercise.   Remember, we are talking about 80 meters here, so some pigtail connections would be just fine.  Under "normal" conditions, a cable should be OK at amateur power levels unless it has gotten seriously wet, or bitten by a critter, but lightning just loves 4-squares, and Thor will have his way.  I like my TDR and network analyzers, but sometimes expedient methods are in order. Don't over-geek the problem.
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