How to find out which coax is which to DA?

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Fran3
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How to find out which coax is which to DA?

Post by Fran3 » Tue Nov 29, 2016 5:03 pm

Have a discontinuity in one of two 7/8 inch coax cables between phasor cabinets and ATU's.

We are temporally operating at reduced power on one tower while we try to find the problem with the coax.

There is a long underground run to each tower. The cables run east south-east for a long ways before they branch off to each of the two towers.

About 40 feet from the transmitter we have both cables unearthed and they are (at that point) side by side heading toward the two towers (This is where our "Rig Expert" TDR tells us the likely problem is located.)

After a good visual inspection we can not see any issues but at that location there are a couple of splices in each coax. The splices have been there for a year with no problem but it is possible that one of them is causing the issue.

Also, we have had no rain for weeks and are dry as a bone so that seems to mitigate water getting into the line.

The system has been working perfectly for months and months so we are unsure what changed.

The question is which of the two unearthed coax cables is which. We can't dig up the entire cable run to make this discovery.

We have the cable with the issue disconnected at the ATU and on the output of the phasor.

Any ideas on identifying the cable carrying the rf vs the disconnected cable at the point we have them unearthed?

Thanks for any help.

grich
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Re: How to find out which coax is which to DA?

Post by grich » Tue Nov 29, 2016 5:14 pm

Since the bad cable is disconnected at both ends, try hooking a cable locator to the bad line.

Dollars to doughnuts one of those splices failed.

Fran3
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Re: How to find out which coax is which to DA?

Post by Fran3 » Tue Nov 29, 2016 5:43 pm

Good suggestion.

Issue is a couple of guys are trouble shooting this and feels that we should disconnect the ground at each end of the coax in order to use the cable locator... and that ain't easy in the back of the phasor cabinet.

I'm not convinced we do need to disconnect the coax shield from ground. I assume the cable tester "transmitter" outputs an ac signal and it doesn't care if onside of the pair it is connected to is grounded... right?

Also, I would assume that the cable tester output circuit is designed such that if it is connected to a cable with a short some where along the line... it will current limit and protect itself... right?

Wonder if a Harbor Freight cable tester will work or should we consider renting or buying an "industrial" unit?

Another question... for a 7/8" 50 ohm coax... what are some typical readings should we get with a "megger" insulation tester?

And should that test be performed with the "megger" connected to one end of the coax and the other end of the coax open? (This is how I would imagine it being done.. so correct me if I'm wrong.)

thanks again for any help.

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kkiddkkidd
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Re: How to find out which coax is which to DA?

Post by kkiddkkidd » Tue Nov 29, 2016 6:17 pm

Most cable locators will work much better if there is a hard ground on the far end of the cable IF, IF you directly feed the shield of the cable with the generator on the other end.

IF you are using an inductive clamp, then leave both ends of the cable connected to ground. You are trying to induce current flow in the cable to be located.

This assuming that you are using a MetroTech or Progressive Electronics /Greenlee locator. Of which I have both...

If you disconnect both ends of the cable, there is no path to ground hence little-to-no current flow, hence a very hard locate.
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RGORJANCE
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Re: How to find out which coax is which to DA?

Post by RGORJANCE » Wed Nov 30, 2016 5:48 am

I used a telephone beeper and audio sniffer to find a problem many years ago, but the difference was the line was physically cut.

If you hook the beeper to the outer conductor, and if the splice is readily available, you can touch the sniffer to the connector and possibly "wait for the beep".

Fossil

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KPJL FM
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Re: How to find out which coax is which to DA?

Post by KPJL FM » Wed Nov 30, 2016 8:51 am

Splices in both lines, buried underground? Both lines are bad. Or will be soon. Rain or Shine. The 'cheap install' has run it's course.
Trim to fit, paint to match, tune for minimum smoke.

Fran3
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Re: How to find out which coax is which to DA?

Post by Fran3 » Wed Nov 30, 2016 10:44 am

Thanks for all the suggestions.

In response to the last post there is nothing cheaply aka "poorly" done here. The 7/8" coax cable splices were professionally done. They were also properly supported and deliberately not put back underground so is to mitigate sitting in potentially wet or saturated soil.

That said we still suspect that one of the splices my have developed a problem and further efforts will be taken this afternoon to resolve the problem.

With the tips we received here we are hopeful that by redoing the cable locator test we can determine which of the two cables has the problem and then do a more careful TDR measurement to more precisely locate the discontinuity... and then execute a more careful visual inspection... and then... if necessary... rework the cable splice(s).

Wish us luck :-)

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PID_Stop
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Re: How to find out which coax is which to DA?

Post by PID_Stop » Wed Nov 30, 2016 12:10 pm

Here's another idea: take a look at the printing on the cable jacket. A lot of cabling has the running footage printed every several feet, and you can often tell which cable is which from that. (Just took a look, and the LDF on our satellite dishes is actually printed in meters, not feet... but the same principle applies.) This is especially true if both runs originally came off the same reel...

-- Jeff

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kkiddkkidd
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Re: How to find out which coax is which to DA?

Post by kkiddkkidd » Wed Nov 30, 2016 12:19 pm

Do your TDR measurement from both ends for confidence.

There are 2 kinds of splices in coax. Those that have failed and those that are going to. If you use flange connectors and keep them above ground they are more likely to survive but screwing Type-N (or UHF) connectors together or making a "butt" splice without connectors at all is a recipe for frustration in the long term. Thankfully yours aren't buried.

I have spliced a few lines using the proprietary splice connectors and it never worked better than just screwing mating connectors together. I have done them all different ways and then redone them and redone them again, etc.

Replacing the lines is the only real long term fix. And that ain't cheap... Which, I am sure was the point of KPJL's comment.

Good luck,
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grich
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Re: How to find out which coax is which to DA?

Post by grich » Wed Nov 30, 2016 12:21 pm

Fran3 wrote:... and deliberately not put back underground so is to mitigate sitting in potentially wet or saturated soil...
Hope you don't mind me asking why the splices are there...is there a groundhog loose on the property?

We've had critters gnaw on direct-buried 3-phase feeds to an equipment shelter. Presumably, they would chew it until they got blasted, then water and acids from the soil would do their magic on the aluminum until pffft...single-phase!

One of my sites elected to extend a 7/8 buried (in conduit) line by 100 feet to a new building. I also was not thrilled with a buried connection. The splice point was where a backboard structure was being built for some well pump equipment behind the old building (now gone), so we took advantage of that.The cables were brought up to a NEMA box above ground and joined in that. We had EIA flanges on hand already and used them.

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kkiddkkidd
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Re: How to find out which coax is which to DA?

Post by kkiddkkidd » Wed Nov 30, 2016 2:58 pm

grich wrote:The cables were brought up to a NEMA box above ground and joined in that. We had EIA flanges on hand already and used them.
That is exactly how I have had the best success.
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Fran3
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Re: How to find out which coax is which to DA?

Post by Fran3 » Wed Nov 30, 2016 5:47 pm

We found the problem and a repair will be completed tomorrow. I'll explain after I attempt to answer the history question...

As I understand it the two cables were originally spliced because of suspected water intrusion. That was a number of years back.

A little over a year ago we completely redid (and replaced part of) that section and created our own splice kit.

It sounds like a jury rig but the TDR said they were working perfectly.

I won't get this exactly right but the basic procedure was...
- take an approximate 6 to 8-inch long section of ~1-inch PVC pipe (with a pvc compression fitting on each end) and slide it over the coax.
- Starting with two clean cuts... remove the dialectic so that each center conductor stuck about 3/8's to 1/2 inch.
- The two center conductors were then neatly and firmly soldered together.
- Next a section of dialectic was split and wrapped around the center conductor at the joint and sealed with a little silicon.
- then a section of split copper tubing (copper plumbing pipe I think it was) was wrapped around the dialectic at the joint and the split in the copper tubing was carefully soldered to seal it back together.
- then the copper braid that had been peeled back from the end of each cable was pulled forward over the copper tubing/pipe and soldered.
- then a silicon sealant was placed around the entire repaired section
- next the PVC assembly was slid over the repaired splice and the compression fittings were snugged up to the coax.
- there may or may not have been some silicon sealer filling the pvc pipe... I can'tr remember as I did not personally do the repair.

After the repairs were done the cables had concrete blocks slid under them to hold them off the ground so they would not be submerged in potentially wet or saturated soil.

Now what happened to cause the problem... well the two repaired cables were, as explained, placed on top of concrete blocks to keep them up off the ground. The blocks were of course laying on their sides so the cables were maybe ~8-inches off the ground for a run of about 15 feet,... then they went back underground and out to the two towers.

The problem occurred when the guy who keeps the field mowed ran his tractor tire across the cables just before they went back underground. That put pressure on one of the splices and pulled it every so slightly apart. Apparently there was... at least for some time... some sort of connection remaining but ultimately it failed completely.

We should have this completely repaired tomorrow and will TDR the cable again to verify no other problems.

Again, it sounds jury rigged but it worked perfectly and deferred the expense of replacing two long cable runs to another day... but of course... sooner or later we will install new cable... but not until we must :-)

I'll report back when the repair is done and tested.

PS - What we failed to do was properly protect the above ground section of cable. We will fix that by laying down a commercial grade weed guard fabric under and around the above ground cable run and place concrete blocks or some such around the area to hold down the weed guard and protect the cables from the mower guy. Maybe this won't be the exact protection scheme but some such will be done to prevent a repeat :-)

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Deep Thought
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Re: How to find out which coax is which to DA?

Post by Deep Thought » Thu Dec 01, 2016 10:40 am

Fran3 wrote:Maybe this won't be the exact protection scheme but some such will be done to prevent a repeat :-)
I think I would also replace the mower guy.
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Fran3
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Re: How to find out which coax is which to DA?

Post by Fran3 » Fri Dec 02, 2016 7:02 am

Roger that :-)

Lee_Wheeler
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Re: How to find out which coax is which to DA?

Post by Lee_Wheeler » Fri Dec 02, 2016 4:55 pm

Deep Thought wrote:I think I would also replace the mower guy.
At least the towers are still standing. :D

...Lee

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