[Towertalk] Direct Burial Coax

[k4oj]

Thanks Roger - I have always wondered what they were hinting at - bottom 
line appears to be if the jacket is water tight it truly is direct 
burial - and just because it is plastic do not think it won't pass 
water!  Interesting.....

73,

Jim, K4OJ



Roger Borowski wrote:

>When I investigated this about 5 years ago, I found at that time only two 50 ohm coaxial cables rated for direct burial. The "key" for direct burial rating seemed to be the high density polyethylene jacket, which is impervious to moisture. Most other cables use a polyvinylchloride jacket. Many of these are rated "non-contaminating", which is quite different than direct burial as moisture can and does penetrate PVC jacketed cables. 
>The two direct burial cables were "Davis Buryflex" and "Times LMR400DB" with the "DB" suffix indicating direct burial. The Times direct burial cable is flooded with a moisture resistant goo between the jacket and the shield and there is a foil shield in addition protecting the foam dielectric from contamination of the flooding material. The flooding goo must be thoroughly cleaned of the shield with a solvent like alcohol prior to soldering and does require a bit of extra effort to install connectors to it due to its flowing back into the area being heated. Once you get the hang of it, its not really a big deal. The Davis direct burial cable costs a bit less and does not incorporate this flooding agent and it will likely provide ample service when buried but should anything cause an invasion of the jacket, it will likely become quickly contaminated. This can happen from insects and rodents chewing on it, gardeners hitting it with a spade, etc., abrasion by rocks, or even tree roots growing into it, etc. It should be OK if buried in a PVC pipe but that isn't "direct burial" in my opinion. 
>I chose and used Times LMR400DB for all my underground direct burial needs in hopes of not needing to replace it for as long as I will care about it. 
>My real purpose of this posting is to make it general knowledge that if a coaxial cable jacket is rated non-contaminating, it does NOT, by any means, indicate that it will provide ample service life in a buried in the ground environment. Non-contaminating is not direct burial! It only means that the jacket chemical composition will not degrade or contaminate the inner foam dielectric insulating the center conductor to the shield. When the foam becomes contaminated, losses are increased and the cable becomes more of a dummy load than a feedline. Beware that even when protected in a conduit or PVC pipe underground, direct burial cable should be used for maximum service life. This would be a perfect situation to use the Davis Buryflex. All the PVC jacketed cables, including LMR400 and LMR400UF allow for moisture penetration through the jacket when submerged, accelerating contamination and losses. Direct burial cables have high density polyethylene jacket material. Check the specs if in doubt.  73, -=Rog-K9RB=-
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