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Bad feedline? Measurement question

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videobruce

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I thought I had a 'bad' antenna, but it turns out the feedline is bad, or so it seems.

I installed a known good dummy load (tests good attached to a Bird 43) at the antenna end and checked fwd & rtn readings on VHF and UHF and I get around a 50% return reading (50% reflected). I checked the cable w/ a DVM and it shows 2.5 ohms with the other end shorted (out and back), a 50' length of RG-8 (Carol 1180).

I changed the antenna fitting (both SO-239 each end) which looked fine. No corosion, water etc. though there was a little oxidation of the braid. This didn't change anything.
The radio end is a BNC fitting (for RG-8) which looks good (no shorted braid).

The center conductor is solid with that hollow space around the dielectric with that sprial nylon piece that acts as a spacer. I have had a few somewhat sharp bends in the run, but nothing exessive.

This runs to on a antenna above a TV antenna on a rotor so it does twist, but there is an ample loop for this.

Again, NO sign of mosture ingress, no cracks in the outer insulation. Any ideas why the cable tests bad w/ a Bird meter??
 

rescuecomm

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Braid oxidation indicates that sometime in the past water has wicked into the jacket. I had this happen to a run of Andrews heliax and had to cut off about 20 feet of it to get the reflected power reduced. The copper wrap was oxidized and was the giveaway. Cut off a few feet until the color goes normal and retest.
 

videobruce

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The oxidation was only the exposed part. I replaced the connector, the braid was good, but no change.

This is Carol 1180 w/ that 'helix' designed dielectric where there is a air space between the solid copper center conductor & the nylon 'tube' that holds the braid at distance coupled w/ that sprial warp throughout the cable.

I talked with someone from Bird and he suggested (as I already thought) that the center conductor was bent and the impedence has been changed somewhere in the line (because of spacing). I told him I just can't beleive it would cause a 50% reflection.
 

jim202

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A quick question about looking at the cable with an Ohm meter. If you
left off the load and didn't short the cable, do you see any resistance?
Do this in the highest scale on the Ohm meter. Looking for any leakage
in the order of meg Ohms. If you have anything, throw the cable out.

The 50% reflected power indicated that there is a major problem with
the cable anyway. I would almost think it is shorted someplace. At only
50 feet long, you don't have much invested in the cable. The connectors
are costing you more.

Jim



videobruce said:
The oxidation was only the exposed part. I replaced the connector, the braid was good, but no change.

This is Carol 1180 w/ that 'helix' designed dielectric where there is a air space between the solid copper center conductor & the nylon 'tube' that holds the braid at distance coupled w/ that sprial warp throughout the cable.

I talked with someone from Bird and he suggested (as I already thought) that the center conductor was bent and the impedence has been changed somewhere in the line (because of spacing). I told him I just can't beleive it would cause a 50% reflection.
 

JohnWayne

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jim202 said:
A quick question about looking at the cable with an Ohm meter. If you
left off the load and didn't short the cable, do you see any resistance?
Do this in the highest scale on the Ohm meter. Looking for any leakage
in the order of meg Ohms. If you have anything, throw the cable out.
I am pretty sure his meter doesn't produce a high enough voltage to test that. It's either going to read infinity (good) or near zero (bad).

You really need a time domain reflectometer or any variety of SiteMaster style pieces of equipment to spot the impedance bump on the line. I have also seen circuits where you can use a 'scope and a signal generator as a TDR.

My money would be either be on water ingression that has made its way down the line, or mechanical failure due to the repeated movement of the rotor. If your bends exceed the specified bending radius, then that will cause an impedance bump, as well.

Since replacement sounds imminent, you may want to consider a non-water-hose style of coax such as RG213 or LMR400. Be sure to use quality crimp connectors and sound waterproofing methods. Also, you may want to consider a flexible jumper between the main feeder; something like LMR240UF or LMR400UF would be fine. Some connectors are different for the UF coax, so be aware of that.
 

prcguy

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That sounds like a classic problem with 9913 type cable where water collects in the hollow tube that suspends the spiral spacer and center conductor. I had the same reflected power indications with a load at the antenna end and eventually poured water out after cutting off the connectors and raising the low point of the cable.
prcguy
 

videobruce

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If you left off the load and didn't short the cable, do you see any resistance?
No, I tested it that way also. On a 20 meg scale it was wide open.
At only 50 feet long, you don't have much invested in the cable. The connectors
are costing you more.
The cable was $.50', the connectors were $1. Even if there were $5. 'N' connectors, the cable is always more expensive unless it is a short 'jumper'.
you may want to consider a non-water-hose style of coax such as RG213 or LMR400.
213 is higher loss and LMR400 is out of my price range for what it is being used for (especially the connectors).
That sounds like a classic problem with 9913 type cable where water collects in the hollow tube that suspends the spiral spacer and center conductor.
I thought about that when I saw that type of cable for the first time. I have two other runs of the same type cable (one is another manufacture). I went with this because of the much lower loss and still watching the budget (including the matching connectors used with a higher grade of cable).

I cut a 8' piece out from the antenna end and retested. Tested OK. I sliced the jacket lengthwise, the steel braid and foil had spots with a powered substance (not the whole length and not at just the end either). I then cut lengthwise the inner dielectric and found the center conductor with NO breaks or sharp bends. Just tarnished (normal).
I then cut 6" off the radio end of the cable. This part had the worst oxidation (powered foil/braid). There was absolutely NO water in the cable or even moisture!.
It appears that condensation and/or a reaction between the braid and foil was the culpert. It was not the flexing of the line as I thought it was (as far as I could tell).

That 'hollow' center was not the problem AKAIK. I will call General Cable and ask about a possible reaction between the foil & braid. The only oher thing I can think is flexing of the jacket could of caused slight moisture ingress, just enough to cause some oxidation.

Can't you have oxidation w/o water contact??
 

prcguy

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I don’t remember any oxidation on my bad 9913 and the connectors were well sealed. The only indication of moisture was when I raised the lowest point of the cable on the roof, which caused water to pour out the end in the shack. Not a lot of water but maybe a tablespoon full.
prcguy
videobruce said:
Can't you have oxidation w/o water contact??
 

videobruce

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Here is the part where you all can say I told you so (as long as you did in the first place).
I received the replacement line from R&L (Jetstream 2015 I believe) and as I was pulling the old line up from the wall cavity in the attic, low and behold I bet you will not guess what came out of the end?

Hint: it dripped.............
Unbelievable. The cable was actually over 20 years old as I moved it once 10+ years ago so I guess it served it's purpose.

My guess it came in from the 4 solder holes under the barrell of the PL259 since that would/could be the only place water would of gotten in.

BTW, I measured the resistance on the new line (60' after being cut to length) and it was .1 ohm round trip (far end shorted).
 

prcguy

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Glad to hear you fixed the problem and good to know your test equipment was telling the truth from the start. I never found where the water entered my 9913 and I avoid using it now. You mentioned LMR-400 was out of your price range but it can be found for around 34c/ft and N connectors are under $3 each. You can sometimes find large spools of Commscope LMR-400 series in the 17c/ft range surplus.
prcguy
 

JohnWayne

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Make sure you waterproof the connections on your new cable installation. I like to use 2" electrical tape sticky side out, followed by a layer of butyl, then an overwrap of standard 3/4" electrical tape in a "shingle" pattern. Works like a champ.

Jeff
 

mancow

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What do you guys think about that gummy tar like tape at Radio Shack? I recently used a couple of rolls of that to seal everything on a new install on the roof. I completely covered the connections down to several inches of cable. The stuff is wicked sticky and reminds me of roof tar. I figure I will have to cut the cables and replace the connectors if I ever have to move anything since it looks like the stuff probably won't clean off after being applied.

I'm just curious if it actually seals like it looks like it does.
 

prcguy

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John Wayne described a method similar to how Andrew Heliax connectors are to be weatherproofed. The first layer of tape keeps the sticky rubber snot from clogging the connectors if you ever need to take it apart. Andrew says to use a normal layer of ¾” tape first then a layer of sheet sticky rubber then layers of 2” tape over everything. It will look like a football when finished. I’ve cut open connectors that were done 20 to 30 years ago and the connectors were as shiny and perfect as new.
prcguy
 
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