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Guidance on Installing coaxial cable underground
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by K0ZN on March 13, 2004
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Hi,
In the number of years that I have been in this hobby I have never needed to bury a coaxial transmission line, so I didn't pay much attention to the subject, but at this QTH I need to bury it. I am of the understanding that only coax with a non-contaminating jacket should be directly buried and that "ordinary" coax should be buried in some kind of conduit. Is this correct? (At this point, I don't think there is a concern about rodents/moles, etc. bothering it, but I guess I will find out!!) Is PVC conduit OK for long term burial? What about allowing for drainage of the conduit; is it necessary? I would think that any buried conduit would fill with water, and the coax would essentially be totally submersed most of the time. Is this a problem for common coax cables? How concerned should I be about drainage of the conduit? I would much appreciate comments/guidance, hints and suggestions in this area.
Thanks! Jon, K0ZN
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RE: Guidance on Installing coaxial cable undergrou
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by W4TYU on March 13, 2004
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Cannot advise as to type but will pass on a couple of tips used by others in this area.
Use "long sweep" elbow joints at each end with the final riser to the ends. Use two elbows or such to make an inverted "J" to help prevent water from gettig in. These ends can be closed with steel wool etc to keep bugs and varmints out.
Ole man JEAN
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RE: Guidance on Installing coaxial cable undergrou
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by K0IPG on March 14, 2004
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Jon, I'm no expert, but here's what I've heard:
To start with, only coax with a non-contaminating jacket should be directly burried. "Regular" coax should be put in a conduit of some sort. I believe regular PVC (sch 40) is perfectly acceptable for long-term coax burrial.
As far as drainage goes, I would definitely suggest drilling some small holes in the pipe. Recently, we uprooted some long runs of coax that had been ran through a long piece of 4" PVC burried underground, without any drainage holes in it. The pipe was filled with water, and the coax smelled VERY oddly. The coax, being a nice long run of 9913, was something I would have rather kept than thrown away (and since the MFJ 259 indicated it was still within specs...), but even after bleaching it and laying it out to dry for a few weeks, it smelled too bad to even bring a short run back into the shack! I was simply amazed that such a small diameter object could acquire such a potent smell. Nevertheless, definitely an argument for making sure water doesn't collect in your conduit!
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RE: Guidance on Installing coaxial cable undergrou
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by KB2FM on March 14, 2004
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I use buried PVC pipe and conduit extensively in my business (swimming pool renovation). I strongly recommend using the grey PVC conduit, 3/4" or larger sizes. As previously mentioned, the sweep type elbow fittings are the only ones suitable for burial, and multiple bends should be avoided. You must use PVC pipe glue to connect the conduit and fittings. Do not under any circumstances drill holes into the pipe for drainage, as water cannot get into a properly glued pipe and the holes would completely defeat the purpose of using water-tight conduit. The trench should optimally be at least 12 inches deep, but a shallower trench can be used for this application. You can assemble the conduit with the cable inside of it ahead of time, and then carefully glue the sections together, carefully avoiding getting the glue onto the cable jacket. If you must pull cables through a long run of conduit, you can use an inexpensive fishtape together with a cable lubricant (both available at the Home Depot electrical aisle)if necessary.
For a professional look you can terminate the ends of the conduit with a variety of matching gray electrical boxes, or simply pack the ends with duct seal, a grey putty which is also available at Home Depot. If you do inadvertently get water inside your conduit assembly, use a shop-vac with the hose on the blower side to blow the water out of the conduit.
You will find that the conduits are inexpensive, ( dollar or two for ten feet), and the combination should be cheaper than expensive direct burial coax. Also, the conduits can be easily refed if needed.
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RE: Guidance on Installing coaxial cable undergrou
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by AA4PB on March 14, 2004
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I wouldn't recommend drilling any holes in the conduit. That will just guarentee you get water in it. Use a couple of 90's at the ends to make a "U" and ensure the open end is facing downward so you don't get any direct rain into it. You'll probably still have some condensation. Don't glue the "U" to the main pipe so that you can remove it for threading the coax around the "U" (you'll never be able to pull coax around that sharp of a bend).
I threaded a pull string thru each piece as I assembled and glued the PVC. Use the pull string to pull in the coaxes after it is finished. Every time you pull a coax, pull a new pull string along with it in case you decide to pull another someday.
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RE: Guidance on Installing coaxial cable undergrou
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by N0RTU on March 14, 2004
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Hello Jon!
I'll agree with most of the other posts here about putting 9913 or similar coax in conduit. Even if you find a coaxial cable the has the "non-contaminating" jacket, I'd still put it in conduit for protection. I've seen too many times when direct buried cables got damaged because someone wanted to plant a shrub or tree or the like and thought, "I know for sure that cable is over here closer to the corner of house!" only to find out later it was right smack in the middle of the "new shrub". I've been putting power cable of all sizes and voltages in the ground for nearly 15 years now and my suggestion would be either Carlon or Cantex brand schedule 40 pvc(grey in color, its UV resistant). It can take a pretty good hit with a shovel or such and survive most of the time. I just won't direct bury cable any more. No more than the pipe costs, its just not good sense to do without it.
One thing I will disagree with though is the point about water in the pvc pipe. No matter how well you glue your pipe together and seal the ends, you are going to have water! Here in the midwest, I've yet to see any pipe that didn't show evidence of water infiltration at some time. Here's what I like to do to allow water to drain away. Design your pipe run with one end lower than the other. At the low end where your 90 deg. elbow turns up, dig a "pit". That is to say dig a "box" say 20 inches square and about 4 to 6 inches deeper than the bottom of your pipe run. Run your pipe as normal, but where where the elbow of the pipe starts to turn up, drill several 1/4 in diameter holes in the conduit bottom.
Fill the "pit" about half full with CLEAN crushed 1 inch limestone gravel. (Usually about 3 or 4 five gallon buckets full will do the job nicely) I emphasize clean here because if you use rock with lots of fine particles in it, your "rock filter" will soon clog up with silt and your drain area will cease to work. The rock "pit will allow water to drain away slowly into the soil and your conduit will drain and remain essentially dry. Fill the top half of the "pit" with the dirt you excavated from the pit and tamp it well as you backfill. Except during times of extreme hard rains, the top dirt portion of your pit will turn most water away from your gravel drain area. Tamping the top dirt portion here is important. If you find you can't get the "antenna" end of your conduit run lower than that near your home, you can still use the pit concept. Just don't put the pit near basement wall or foundation. Stay at least 10 feet away. Some hams I know have put the pit in the middle of their run and let the conduit drain both from the shack end and the antenna end. This works quite well also. Good luck with your project!
73
Mike
N0RTU
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RE: Guidance on Installing coaxial cable undergrou
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by W8JI on March 14, 2004
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Hi Jon,
Non-contaminating has NOTHING to do with direct burial.
Non-contaminating simply means the plastics and other materials used in the cable do not leach chemicals into conductor areas over time. The chemicals would corrode or tarnish the conductors, and increase cable loss over time. This has nothing to do with the cable being suitable for direct or indirect burial. It only means the cable materials won't leech chemicals that deteriorate the cable over a period of time.
What you are looking for is a cable that is rated to be buried. That almost always involves a gooey flooding compound in the cable that seals the cable and/or displaces moisture. Without the "internal goo", almost any buried cable will eventually have problems with moisture ingress.
You can use buried conduits, but you also have to be sure they NEVER collect water or you once again have to use a flooded cable suitable for direct burial. Otherwise the water laying in a conduit will eventually leetch into the cable.
Solid copper shield bonded-foam cables like Heliax can be directly buried or exposed to constant moisture without ill effect, even though not rated for direct burial. Even if there is a small puncture, the bonded closed-cell foam dielectric and lack of woven braid prevents moisture migration through the cable.
I bury Heliax directly and pull it through wet underground pipes without worry.
If it has an ALUMINUM shield you should use flooded cable designed for direct burial, because any jacket flaws might allow eventual errosion of the aluminum shield as the aluminum turns to powder from electrolysis or exposure to soil chemcals.
What I do here is either buy direct burial cable (I have many miles of that in direct soil contact), use solid copper shield bonded-foam Heliax, or put cables in black plastic 4" drainpipe. I use the type of pipe with holes at low spots, and put the holes face down over a layer of material used to allow moisture to pass but not dirt. I often pull regular cables through that pipe, but I make sure it does not retain water in lowspots!
73 Tom
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RE: Guidance on Installing coaxial cable undergrou
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by KT8K on March 14, 2004
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I would probably only use conduit if I was doing a bunch of cable runs. My preference would be to use some good burial-rated coax "buryflex?" from cable-experts or wireman or somebody, and use a remote antenna switching scheme to reach my various antennas. It's a lot less work and, depending on the size of your antenna farm, maybe of comparable cost.
Otherwise, I, too, would use the gray PVC and seal the daylights out of it, with junction boxes at the ends, using oversize pipe (at least 4") and leaving a couple of "pull strings" in with the coax runs for when I want to add more.
Good luck and good reception! 73 de kt8k - Tim
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RE: Guidance on Installing coaxial cable undergrou
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by WB4QNG on March 14, 2004
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This is just my experience. I buried RG8 cable and used it for about 10 years without any problemns. Don't remember the kind wouldn't be surprised if it wasn't R/S. I didn't have it burried but about 6 inches in the ground just enough so I wouldn't cut it with a lawn mower. With this is mind I wouldn't be scared at all to get some decent type rated to be buried and go with it. Of course puting it in conduit would make a more professional job.
Good Luck
Terry
WB4QNG
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RE: Guidance on Installing coaxial cable undergrou
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by AC6IJ on March 14, 2004
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Hi all, can anyone explain how the water is getting into the PVC pipes that are sealed at both ends? Does this mean that water is going through the pipe somehow and the pipe acts like a sponge? If this is the case then would runs of metal conduit be better?
Thanks to anyone who can answer these questions. Bill
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