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Subject: [TowerTalk] FIBERGLASS TAPE vs. "NORMAL" TAPE
From: (John Brosnahan)
Date: Sat, 6 Dec 1997 08:15:09 -0700 (MST)
>For a lot of these applications wouldn't fiberglass tape and resin, 
>such as you get at automotive supply stores work?
>As but one example, wrapping 40-2CD traps in fiberglass tape 
>so as to isolate them from the elements. The end result would 
>resemble an epoxy encapsulated assembly, completely waterproof, 
>and almost as well sealed as hermetic enclosing.
>I realize that some situations for sealing might require special 
>considerations, such as the possible need for flexibility, repeat 
>removal and re-use, etc., but for those really serious sealing 
>applications, the fiberglass tape and resin would seem to be a 
>natural, but often overlooked, product that might readily fit some 
>very real amateur radio antenna sealing applications.


Please don't "TRY" to hermetically seal anything.  It NEVER works!
(This advice was not heeded by a company I had previously owned and it
destroyed millions of dollars of antennas over a couple of years--partially
resulting in their downfall.  They just "knew" they could make it work--I
would have had the last laugh except that a number of people lost their
jobs when things like this bit the company in the backside and the new
owners closed the doors.)

True hermetic sealing is very difficult.  What happens when one "tries"
is that any humidity in the air is sealed inside and when the temperature
drops it condenses.  As barometric pressure changes many materials
including fiberglass can breathe enough to draw in humid air and later when
the temp drops it condenses also.  Nothing every evaporates out and
you have a "water diode" that allows moisture to flow one way but not
the other.

The above mentioned company (that had some good engineering
talent) ended up with everything they tried to hermetically seal with
fiberglass/epoxy full of water. In fact there was so much water that
it ended up being forced into closed cell foam hardline, ruining the
hardline for the first few feet.  And, because once the air spaces
were completely full of water, the pressure from temperature
increases provided a lot of force to the water--forcing it through 
connectors and deep into coax.   Andrews said it was impossible to
get water into their coax--but with enough hydrostatic pressure
you can manage to do anything! 

Put drain holes in everything--otherwise they are guaranteed to fill
with water--and all of this "test data" is from DRY Colorado--the effect is
much faster in humid climates.  Things must breathe and the 
condensation must be allowed to drain.  Over three decades I have had
numerous people try to prove me wrong and they have all failed!

It is even more difficult to take a rigid substance like most epoxies and
make a decent seal around something like an antenna element
due to both the constant mechanical flexing of the element which 
distorts its shape and the differential rates of temperature expansion 
coefficients between epoxy and aluminum.

If you are on the east coast please disregard this information and
seal everything with epoxy.  The additional losses due to water
might finally give me the needed edge to beat you!

73  John  W0UN

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