Topband: Feedlines

John Kaufmann
26 Jul 2001 11:29:40 -0700

On Thu, 26 July 2001, "Bill Hohnstein" wrote:

> I've generally use flooded cables for my Beverages.  While an occasional
> critter may take a quick bite, I haven't seen one continue for a meal.  The
> cable flooding prevents water from future rains from significantly wicking
> in and the ruining the cable via causing high attention.  Plus, it "prevents water 
> migration by using a customized polymer adhesive over the center conductor."  
> "Protection against moisture ingress and other underground contaminants is 
> assured by using a  unique highly flexible polyethylene jacket on all 
> underground products."                                    
Let me describe a problem I encountered which I think is related to water migration.  I feed my Beverages with RG-6.  Over a period of time one of my Beverages started to exhibit extremely high noise levels, not related to atmospheric noise.  I found the noise persisted even after I disconnected the feedline from the Beverage itself.  Eventually I traced the problem to a section of feedline which was essentially lying on semi-swampy ground.  I replaced that section and all was fine.

I examined the bad section I took out but couldn't find any obvious signs of jacket damage or water ingress.  However, when I connected a voltmeter across the center conductor and shield, I measured 0.7 VDC with the far end open-circuited!  I verified this with another meter, too.  If I shorted the far end, the voltage went away, but returned as soon as I removed the short!  My coax had apparently become a dielectric battery which produced a lot of RF noise.  The swamp water migrating into the coax had to be the cause.

A few weeks later--guess what?  The new feedline became noisy again and it was the same problem!  I replaced it but this time I elevated the feedline off the ground and everything has been fine since.  

I don't know the make of this RG-6 because there is no brand name on the jacket.  I can only guess it's some cheap junk.

Moral of the story:  use good quality feedline, even for Beverages or avoid laying feedlines in swamps!

73, John W1FV

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