[TowerTalk] New comments on old threads

[Kurt Andress]

To All:
After browsing through some of the threads in this archive I've found a
couple of thoughts that didn't appear.

Weatherproofing:
When I was a relatively new ham, an oldtimer (now SK), who worked for
Motorola, installing systems, taught me how to weatherproof coax
connections. He gave me a can of 3M Scotchkote Electrical Coating P/N
14853, and told me never to use anything other than vinyl electrical tape
with it. That PVC stuff is for watering the lawn!
He explained that the instructions on the can worked pretty well to seal a
connection, but when he really wanted a bulletproof seal (like underwater)
he did it differently. 
After assembling and tightening the connection, brush an even layer of the
Scotchkote directly onto the connection and at least 1" onto the coax
jacket. Allow this to dry for 5-10 minutes, depending on the ambient
temperature. When it's really cold it could take an hour. The idea is to
let all of the solvents in the coating evaporate (I.E. doesn't smell too
much). It will still be slighty sticky.
Next, wrap the connection with tape, overlapping each wrap by 1/2 the width
of the tape. Pull the tape very firmly over changes in diameter to
eliminate as many wrinkles as possible. Wrinkles in the tape are death to a
good seal!
Then, coat the connection again with Scotchkote and allow to dry. The
coating should extend past the ends of the wrap by a 1/2".
At this point you have two options. If the connection is going to hang in
the air, coat again and dry, and you are done. If you want to put the
connection in standing water, wrap it and coat it again.
I have had many single wrap connections out for 5-8 years, 7 miles from the
ocean and found them perfectly clean when disassembled.
The down side to this procedure is that the connections are hard to get
apart. One usually has to carefully cut the tape off of the connection, the
coating really bonds the tape to the connectors!. The connection then has
all of this dark brown stuff stuck to it, so it isn't very pretty, as if
that was important! But the inside of the connection looks like the day you
put it together. I think that's important! You can find the 3M coating at
any electrical supply house.

Aluminum bonding:
How many of us have experienced problems with trying to make anything stick
to aluminum? 
How about paint? Everybody knows that if you paint your antenna, the paint
will start cracking and come off after a very short time.
There are some very good reasons for these occurences.
First, aluminum is one of the most active elements on the galvanic chart
(see towertalk post 4/98). This means that it readily oxidizes in the
presence of moisture. When it oxidizes, the surface of the material becomes
contaminated with the aluminum oxide. This material prevents effective
bonding. 
In reality, aluminum is one of the best materials to get a good bond. The
term "bond" relates not only to structural adhesive bonding, but also
electrical bonding. The reason aluminum is such a good material to bond to
is the same reason it can be difficult to bond to. The ony difference
between easy and difficult is the oxidation on the surface.
Boeing Aircraft spec's for structural bonding to aluminum require a mild
sulfuric acid etch to remove the surface oxidation. Then the bond process
must be performed within 2 hours. There are many other parameters involved
with the spec, but the basic principle is very clear. Aluminum is so
reactive, that one has to remove the oxide from the surface and not allow
it to be exposed to the environment for very long before making the bond.
This means that any antenna connection you want to make must be prepared no
more than 2 hours before you make the connection and seal it off from the
atmosphere. We don't have ready access to sulfuric acid to prepare the
aluminum, but that is ok! Our antenna isn't going to fly to Japan, at least
we hope it isn't, if we did the rest of our homework! 
All that is needed is to remove the aluminum oxide from the surface just
before we coat it with the proper anti-oxidant paste (see same 4/98 post).
There were many good ideas in the archives about how to clean the inside of
a tubing section. Another one, that I did not find is this:
Cut the head off of a 1/4" dia bolt and hack saw a slot into the end of it
about 1" long. Cut a strip of 320 grit sand paper, about 12" long and
engage it into the slot and wrap around the bolt. Put other end of bolt
into drill motor and insert the wrapped end inside tubing. Turn on drill
motor and work the sandpaper back and forth like a cylinder hone to strip
the oxide off of the inside of the section. When the paper becomes worn out
or loaded up, tear off the bad portion and continue honing. 

After honing is done clean the surface with a clean dry cloth (on a stick)
to remove the dust. 
Do not blow the dust off with compressed air, as most compressors not fit
with sophisticated filters, have very contaminated air (mostly moisture &
oil).
DO NOT WIPE THE SURFACE WITH A SOLVENT DAMPENED CLOTH! All solvents, even
Reagent Grade (chemical lab grade, super clean), have contaminants (oils)
in them that will degrade your bond.The hardware store grades are worse!

Preparing the OD of tube sections is a "no brainer." Just use some fine
Scotchbrite or 320 grit until all of the gray appearance is gone. It should
appear silvery white. Remove the dust with a clean dry cloth and you are
ready to put on the paste. Be sure to work it into the aluninum with a
vigorous action.

Why your silicone doesn't stick to aluminum.
How about silicone? We have all seen that it looks good the first few weeks
or months, but later it seems that the silicone is peeling away from the
aluminum and we can't get it off of the stainless hardware.
Most commom grades of "silicone", or more accurately named RTV (Room
Temperature Vulcanizing), elastomers have a very curious curing mechanism.
The elastomer ONLY cures in the presence of moisture. The moisture reacts
with the Acetic Acid in the compound. The Acetic acid is the stuff in
"silicone" that makes your eyes water, and your nose burn! 
Well, no surprise, the reason why "silicone doesn't stick to aluminum is
caused by the chemical reaction between the "active aluminum" and the
moisture and acid. The curing mechanism oxidizes the aluminum, while the
silicone is curing, to prevent it from bonding. There are dozens of
"silicone" compounds that will not cause this problem. They are simply not
sold at the hardware store. You need to go to an industrial supplyer that
carries, two part (not one part) silicones. The two part silicones require
fairly accurate mixing of the silicone resin and curing agent. Or, you can
request and "Electrical Grade" silicone. These compounds have a much less
aggressive chemistry, that is designed not to harm delicate electrical
circuitry.
Sounds like this silicone stuff is in the "Too Hard" box!
Okay, let's do something easy, go to the hardware store and get some
exterior grade (UV stabilized) latex caulking compound. Clean the aluminum
properly, put some of this goop on your connections, and check them every
2-5 years.

I saw some posts about antenna preservative coatings. The same preparation
requirements exist for these. You wanna make something connect to aluminum?
Get rid of the surface oxides and any contamination (like your oily
fingerprints)!

I learned all this nonsense from 20 years of designing and building
hardware for somebodys favorite uncle, "Sam". 

73, Kurt

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