[TowerTalk] Expanding on water in coax-DAVIS RF Co. Response

Roger (K8RI) K8RI-on-TowerTalk at tm.net
Wed Feb 20 15:51:27 EST 2008


I'd like to expand on "water in the coax" and checking for it, a bit.
BTW I have never met Steve and have no business relationship with the 
company.


Steve Davis -Davis RF Co. wrote:
>  Hi Folks, I would like to echo Roger's K8RI note:  "just pitch" any coax
>  that has gotten water into it.  Unless you cut back the coax and check the
>  center conductor, the braid shield, and the foil shield (if there is one)
>  for contamination and get back to an uncontaminated point, you don't want 
> to
>  use the cable.  Also, when I say "check", this is a visual, very subjective
>  process. 
When checking for water (and finding some) you need to go back until no 
moisture is detected, Then remove at least  a couple extra feet.  The 
reason is that coax shield will eventually  corrode even in areas where 
only water  vapor is present.  The likely hood of  a piece of coax 
drying out is just short of nil.  IOW, it's not impossible,  but highly 
unlikely.  As Steve pointed out, at that point you really need to have 
the equipment to be  able to check for moisture in the cable.

The typical response is finding a wet braid that is still shiny, then 
going back to the point where no visible water is found and making the 
cut there.  Then a year or so later discovering the shield is corroded.

As for the foam, there are both open and closed cell foams. Closed cell 
should not allow water to migrate through the structure while open cell 
does. OTOH I know of no open cell foam used in coax. (Open cell example 
= the blue Styrofoam(TM) building material). OTOH  I  don't  know the 
effect water would have on the material itself.

73

Roger (K8RI)

>  Also, the dielectric:  if solid polyethylene (PE,  usually a 
> milky
>  grey color and used on RG-213,etc), likely water or moisture will not
>  penetrate it, but the presence of water/moisture poses an electrical 
> adverse
>  effect.  If the diel. is closed cell PE  (white foamed PE which has a 
> higher
>  VP) moisture that gets to it will migrate into the PE and you may see
>  discolorization but if not, you won't "see" the contaminating moisture. 



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