COAXIAL CABLE - CAUTIONS FOR USE
Should you bury your coax cable?
Well that depends. Some coax cables are designed for direct burial and others are not. There are also two types of burial processes that can be used; direct burial and burial in conduit.
Keep in mind that the two biggest factors that cause coaxial cable to fail are sun exposure (yes they get sunburnt just like we do) and moisture. Let's look at both of those topics a bit closer.
If you direct bury a cable that is not designed for direct contact with dirt, stones, salts, and whatever else is in your ground then you are asking for trouble. You might not have it right away but without a doubt, your cable starts to deteriorate right after you fill the hole, unless it is designed for direct burial. Most direct burial cables have "direct burial" marked every couple of feet or so on the jacket. If no such label is there, you can assume that you had better not bury it. Of course any detrimental effects from direct burial of the wrong type of cable gets worse as you go higher in frequency. Just as general cable losses increase with higher frequency, so do losses from deterioration.
The jacket of a coax cable not suited for direct burial starts to break down and over time is allows moisture and other contaminants to enter the cable. This can result in corrosion of the outer shield and center conductor, moisture and other types of absorption into the center conductor insulating layer. Foam types of insulation are one of the lowest loss insulations available but they are also the worst for absorbing things you don't want in your cables.
You can also bury regular or direct burial coax cable inside conduit. Here you have the choice of regular metal electrical (not recommended), the gray plastic electrical conduit (PVC) or even regular white PVC water pipe. The two types of plastic pipe are both PVC so it makes little difference which you use. Let price and availability be your guide. One consideration though; at both ends of the conduit the pipe is most likely going to come out of the ground where it will be visible. For this reason, the gray electrical PVC pipe might be a better choice since it will be less obvious. Make sure you use a conduit size that is plenty large enough for the cable, or cables, you are going to put through it. Ideally, the inside of the conduit will stay dry but moisture build-up can still be a problem. If the conduit has extra room, some moisture inside the conduit might work its way outside. Also arrange the conduit, using 45 or 90 degree bends, so that no matter what, rain and melting snow can not enter the conduit. An exception might be a fairly short run of conduit that goes downhill which would allow water to drain out at the bottom end. You also want to attempt to keep bugs and mice out of there as well yet still allow some air flow through the conduit.
So the bottom line is to know what type of cable you have if you are going to bury it in the ground.
What about moisture absorption in coax cable?
We have discussed keeping moisture out but why is that important? For a number of reasons; excessive moisture (there is always some) simply increases losses in cables. It can do this in a number of ways but I'll talk about only a couple of them. If the insulation absorbs moisture it may well swell in size which in turn would move the center conductor around and it would no longer be centered within the outer conductor. This changes the characteristic impedance of the cable causing VSWR increases and additional signal losses. Moisture might also cause the center insulation and/or center conductor to begin breaking down. This could result in the center conductor not only moving but possibly even shorting to the outer conductor. Any accumulated corrosion on either the inner or outer conductor would not be as good of a conductor as the bare copper further adding to losses. These are basic concepts but can be caused from a number of specific effects from moisture. Again, as frequencies increase, so do the losses.
If a center conductor insulation is breached it might also allow an arc to form when you are transmitting at high power. And here, when I say high power, I mean relative to the frequency in use. As frequency increases, coax cable will carry less and less power before overheating and failing. Overheating can cause the same problem with the center conductor moving as moisture can. As mentioned above, the cables with foam insulation are some of the most prone to moisture problems and also melt at the lowest temperatures.
I can't overlook some of the "air" dielectric cables that have come on the market in the last few years. These generally use a PVC center conductor insulation that is a thin spiral around the center conductor. So air insulation is not 100% accurate but the majority of space around the center conductor is in fact air. Guess what is good at absorbing moisture? Of course - air! A leaking connector can allow rain to literally run into the cable and with air line cable, the water has a large area to fill. Now your coax cable has a center conductor filled with water. It's impedance is no where near 50 ohms, losses can reach to nearly infinity and you can't hear a signal on any band. You think your receiver is deaf, maybe even dead. Well it is deaf, no signal can make it from the antenna to the receiver, the water has created a short circuit.
This brings up one last topic, coaxial connectors. Now I have never - ever - been a fan of UHF connectors but we as hams are sort of stuck with them, at least in commercial equipment. They are cheap but then they are also cheap! If you must use them, at least get those with Teflon center insulators and preferably silver plated. Don't use the UHF connectors with the light brown phenolic insulation. Nickel plating is not that bad, especially at HF.
Now I said they were cheap and also cheap. To define that better, they don't cost much and they are fairly low quality for RF use. They are not 50 ohm connectors and they can leak water like a sieve, or absorb it like a sponge. I have often removed a UHF connector from a commercial radio, where it is practical and replaced it with a type "N" connector. Anything I build will have different and better quality "N" connectors. These are 50 ohm connectors and can be waterproof. So if you must use a UHF connector outside, get out the sealant and make sure you waterproof them. Do not use everyday silicon sealant that is made for use around the home. The acetic acid in it will attack the outer jacket and begin the processes discussed above. There is special silicon sealant sold (hard to find though) for RF applications. There is also a commercial coax connector sealant sold by places like HRO, AES and Cable Experts that is easy to use and works very well, even in cold weather.
So there you have it. Be very careful how you use your coax cables. Their use is detrimental to their life span! Just don't help the inevitable failure of a cable along by making things worse then they are. A good quality cable can last many years if used in the right way.Go back
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