Evaluating RG-8X for water intrusion - Page 1
Forum section: electronics / rf-microwave

author: elroy
author_rank: Regular Contributor
timestamp: April 27, 2022, 12:25:22 am
content: I have a partially buried run (somewhere around 30m) of RG-8X with a BNC male connector on the far end that was supposedly sealed to protect it from weather over the past year or so. Just looked at it today, and clearly water managed to work its way into the plastic bag sealing the BNC on the end.I'd prefer not to chuck the whole cable away and start over with fresh coax, as it would be a hassle to bury.There's several meters of extra coax on the far end, so I'm hoping I can trim off a couple of meters and crimp on a fresh BNC connector.Any suggestions on how to evaluate the cable, and decide how much I need to cut off? A NanoVNA is available. The exact length of the buried cable is not known, however, which would complicate using the VNA to measure its electrical length and compare it to a physical length.The coax is for receive-only use with a mini-whip antenna. BNC wouldn't be my choice for an outdoor connector, but that's what's on the mini-whip. Thanks for any ideas.

author: dcarr
author_rank: Regular Contributor
timestamp: April 27, 2022, 04:37:26 am
content: I’d start by measuring the return loss of the cable near your frequency of interest. If you leave the far end disconnected, the measured return loss will be double the coax loss. Eg: 5db return loss on the VNA = 2.5dB coax loss.If the loss looks reasonable, then there probably isn’t too much water in the cable. You could also trim a bit off the wet end and see if the loss improves markedly.The magic wand for this job is TDR. You’ll be able to visualize the exact location of any issues with the cable. There may be software compatible with your VNA that will post process a full VNA sweep into the time domain (TDR). Some googling may be warranted there.

author: SeanB
author_rank: Super Contributor
timestamp: April 27, 2022, 11:25:35 am
content: Simplest is first to see how much spare you have, eg 5m. Then cut off the end plug, plus around 0.5m of cable, and strip the outer sheath, and see if it is wet or green. Then move back 0.5m for every time you get green or damp, till you either run out of cable, or find good. Then you solder on a new plug, and make sure, using some red rubber grease, followed by connecting a coupler, followed by wrapping the cable end of the coupler, the plug and 20cm of the cable, in self amalgamating tape, or silicone tape, followed up by overwrapping with 3M 33 tape to cover the tape and seal it, and then seal the loose end of the cable with a cable tie, followed by using some PVC weld to seal that end to the cable sheath. Not going to get water in again, providing the new cable you connect has a nice rubber grease blob in the plug that is going to keep the water away, and the same wrapping and sealing, with the overwrap on the new piece covering the connector as well.Only expensive tool free way, and the pieces of wet coax simply slit the sheath, strip out the copper and put in with the scrap copper.

author: A.Z.
author_rank: Super Contributor
timestamp: April 27, 2022, 06:19:11 pm
content: Quote from: SeanB on April 27, 2022, 11:25:35 amwell, Sean, you are correct, but I disagree about the way you suggest to seal the junction; imHo it should beconnectors and barrel adaptergreaseteflon tape (like the one used for water pipes)self amalgamating tapeUV resistant duct tapenylon tiers to block the tapethat, in my experience, works

author: El Rubio
author_rank: Regular Contributor
timestamp: April 28, 2022, 02:02:19 am
content: Ok, just to beat this horse to death, I’m going to give a suggestion. I will preface this with the fact that I work in the commercial wireless business and have been exposed to every form of cable/ connector sealing method out there. First, trim your cable several inches at a time to find dry braid.Replace your connector. Don’t believe any claims that certain connectors are weatherproof.Wrap the entire connector and several inches of the cabling with plain old electrical tape, nothing fancy is necessary. If you leave at this it will last3-6 months ( maybe longer, but this has been my experience)When it costs $1500-$3000 to get someone to climb a tower to replace a jumper or connector, they better use more than just tape. So on top of the tape, wrap with silicone or other self amalgamating tape. Butyl tape works very well and is cheap. You can get butyl tape at Home Depot in the area where they sell metal roofing supplies for around $5 for 25 feet. It is gray and tar-like. It will not melt or ooze. Silicone and othe specialized tapes can be very expensive. Wrap the butyl tape with more electrical tape. When completed, the cable should look like a snake that has just had a meal. I would add a drip loop if necessary. You don’t need tie wraps, glue, or other reinforcements to hold the tape. This method is not used professionally as much because there are specialized sealers that are more common, but it is extremely effective and will lasts many years.

author: antenna
author_rank: Frequent Contributor
timestamp: May 06, 2022, 03:49:32 am
content: Depending on your nano firmware, it probably already has the ability to do this. Its the transform function. The nano isn't the ideal tool for this, but it will give you an idea of where the problems are. Check this out.Also, if you have an oscilloscope, that same person has a video on how to build your own TDR using commonly available parts. That method will provide much better resolution!And if you want to understand the readings on that last video, here is a good paper on it.https://www.neetrac.gatech.edu/publications/CDFI/5-TDR_17_with-Copyright.pdfYou can just scroll through it looking at the pictures, it wont take long before you have read the whole paper! It even covers water ingressAnd as a bonus, here is a much more simple paper that has drawings showing what the typical step responses look like under various situations.https://www.edn.com/tdr-response-tells-a-story/And assume the velocity factor is 82% for that cable...

author: antenna
author_rank: Frequent Contributor
timestamp: May 06, 2022, 04:12:01 am
content: Here is a shot measuring my RG-8x coax going out to my VHF ground plane antenna (which is currently laying on the ground, hence that characteristic capacitive discontinuity).This is from my nanoVNA using the step response: (edited post with picture having better scale settings on nano)[ Specified attachment is not available ]And this is from my TDR I made following W2AEW's video using the 74AC14 schmidt trigger:If you have water ingress, it will affect the characteristic impedance and create a reflection that will distort that nice flat part of the trace.One of these days, I'll open up that schmidt trigger TDR (it's in a small aluminum case) and compensate out that pesky ringing, but not today:))

author: antenna
author_rank: Frequent Contributor
timestamp: May 06, 2022, 04:16:37 am
content: And I understand that you do not know how long the cable is, but you sure can find out (not that you need to unless testing from the dry end). Just change the load at the other end (like short it out) and see what disturbance on the step response changes. That will be the end, the water ingress reflections wont change. Use 82% as VF and count ns and you'll know the overall length. From there, identify the other disturbances, do the same math you did to find the overall length, and try to eliminate those problem spots.I forgot to mention, that is round trip ns. So divide by two

author: elroy
author_rank: Regular Contributor
timestamp: May 06, 2022, 03:38:01 pm
content: Thanks to all for the suggestions, both on evaluating the coax as well as on sealing junctions. Much appreciated.

author: coppercone2
author_rank: Super Contributor
timestamp: May 06, 2022, 04:12:33 pm
content: Maybe its a good idea to evaluate the conductor to make sure you can't use another transmission line like ladder line to work with the signal. Then you can use silicone grease and screw terminals etc.I figure its not acceptable for your application but it really might be a better more robust choice for certain signals. If I could get away with it, I would defiantly prefer screw terminals and standard hardware then anything coaxial.

author: elroy
author_rank: Regular Contributor
timestamp: May 06, 2022, 05:57:41 pm
content: Yes, I'm a ladder line fan too, in some situations. I use it now and then when I put up a dipole. In the case of the coax I posted about, it's frequently used with active antennas that receive +3vdc over the coax. So rigging that for ladder line would be tough.

author: coppercone2
author_rank: Super Contributor
timestamp: May 06, 2022, 07:52:48 pm
content: I kind of wonder what would happen if you just put another wire zip tied to one of the ladder line feet, or glued to it.. if its correctly positioned maybe it wont effect it too much?But I can see that 4 conductor ladder line is more hassle then coaxial cable. But I would still maybe think its more robust for the interconnects.

author: cdev
author_rank: Super Contributor
timestamp: May 07, 2022, 10:53:15 am
content: My nanovna2 4.4 GHz as it came had TDR capabilities..I am pretty sure its the default.Post processing shouldnt be necessary. The TDR should show him which parts of the coax are compromised, assuming it is. (the outside end, at least, sadly.)He'll need to key in the VF for the cable to get the exact lengths.. it should be around .7ish 70 percent. The manufacturer should provide this info.Quote from: dcarr on April 27, 2022, 04:37:26 amGood luck! If you need any more info, PM me.This has been shown in videos too.How best to prevent this, I am curious? Self Amalgamating tape? Plumbers grease? I see the list above, by nylon do you mean nylon cable times?

author: cdev
author_rank: Super Contributor
timestamp: May 07, 2022, 11:00:15 am
content: One could set up a bias tee for ladder line fairly easily. Assuming the ladder line wires were both the same.. metal wise.. I would guess they likely would be.Assuming HF it seems as if the parameters to do this would not be difficult to establish, whatever they are.This situation isnt so esoteric. One basically just needs to supply the far side with an appropriate DC voltage and block RF out of the power supply with a capacitor and possibly an inductor.Quote from: elroy on May 06, 2022, 05:57:41 pm

author: cdev
author_rank: Super Contributor
timestamp: May 07, 2022, 11:09:20 am
content: Since you have a preamp on the end you could use Cat 6 Ethernet cable. Chris Trask shows how to do this with a magnetic loop. Its a good design, for receiving. Its tunable.Quote from: elroy on April 27, 2022, 12:25:22 amWhat do people think are the best means of waterproofing RG-8 size coax? So it lasts years?

author: cdev
author_rank: Super Contributor
timestamp: May 07, 2022, 09:39:49 pm
content: Quote from: elroy on May 06, 2022, 05:57:41 pmThere are baluns that offer fixed balanced impedances which might be useful in antenna tuning. I'm going to attach an image of one. Sometimes this is all thats needed for a balanced antenna. They can help a lot, thats for sure.Note the blue wire which is almost invisible in the drawing. This top balun is very good. Ive built it a number of times and sizes. in the 1;1 setting its useful for measurement. The bl2 one with balun cores is better. Very efficient.

author: elroy
author_rank: Regular Contributor
timestamp: May 07, 2022, 10:57:39 pm
content: Quote from: cdev on May 07, 2022, 09:39:49 pmCan't say I've built one close to that, but I have a switchable 1:1 / 2:1 / 4:1 balun sold by Buddipole that's similar. Handy when trying out various horizontal and vertical designs.