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Asunto:[dxcolombia] FW: Re: Topband: A Big Mess!
Fecha:Viernes, 17 de Agosto, 2007  11:19:21 (+0000)
Autor:David J. Sourdis <hk1kxa @.......com>






David J. Sourdis

 +34 699500005


From: W0UN -- John Brosnahan <shr@...>To: topband@...
Subject: Re: Topband: A Big Mess!
Date: Thu, 16 Aug 2007 19:03:07 -0500
At 17:57 16-08-07, Charles Bibb wrote:
>Hi, Topbanders
>These
>are electrical enclosures
>available at Lowes's, etc. They seal tight with a rubber gasket and
>have no pre-drilled holes.
>Sometimes, however, I found that when I opened one up for one reason
>or another, I noticed some
>beads of condensation had formed on the inside walls of the box, even
>though the seal is pretty tight.
>I place a 5 gram size perforated packet into two of my
>Beverage transformer boxes, sealed
>them back up, and forgot about them.
>Today, several months later, I had to go inside one of the boxes to
>tighten the nut on the backside of
>one of the ceramic feed-throughs after re-attaching a broken antenna
>wire. WHAT A MESS!
>73,
>Charles - K5ZK


Hi, Charles,

Welcome to the wonderful world of condensation. It is virtually IMPOSSIBLE
to seal out the moisture. I learned this a LONG time ago and yet I had
engineers design antenna matching networks in sealed boxes and I explained
to them that this was a bad idea. Their solution was to no longer invite
me to their design meetings.

They tried better and better epoxy sealing and still the boxes would fill with
water. The ONLY practical way to build "weatherproof" boxes for outdoor use
is to let them BREATHE. You need to make a small hole at the lowest point
in the box so that any collection of condensed humidity that collects inside
can have a place to drip out.

Humidity (even when it is low) gets pulled into the box THROUGH connectors
and AROUND connectors, as well as virtually all methods of sealing the box
seams. And this humidity will condense out when the box gets cooler at night.
And now it is liquid and it is trapped inside. And more humidity
will condense
into moisture the following night, and eventually, if the box is
pretty well sealed,
it will completely fill with water.

Ultimately their "superior attitude" cost the company a couple of
million dollars.
They even forced moisture through closed-cell foam hardline -- something that
Andrews said was impossible, but with enough hydrostatic pressure the sealed
box pushed moisture through the connectors and through the closed cell foam,
destroying a few hundred matched phasing lines, in addition to hundreds of
antennas and their matching networks.

Later this inability to seal out moisutre was confirmed to me from
experience by
the largest manufacturer of weather instrumentation. You cannot seal moisture
out (without going to full mil-spec hermetic sealing), all you can do
is trap the
moisture inside. You MUST allow the condensation to leak out.

Many have tried to beat Mother Nature but few succeed!

Silica gel is designed to absorb a little bit of humidity. Once it
is full of water
they added water just makes a "gel of a mess". (I LOVE it when the English
language provides me with a "perfect storm" opportunity to make a bad pun!)

73--John W0UN


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