[TowerTalk] "anodized" aluminum

[Roger L. Elowitz]

Hi Gang,

k6gt wrote in response to a question on antenna aluminum polishing:

>> I never thought anything special was done to aluminum tubing and it
was OK
>> to scrape, polish etc.  Am I wrong?  (I remember a thread on polishing
>> elements awhile ago, but I don't think this was addressed).
>
>you aren't wrong!  and as i remember, the element-polishing was 
>associated with april fool's tricks.

The polishing of antenna aluminum may have ended up as grist for an
April Fool's bit of humor although I can reassure you, it's original
intent was serious. Recalling the sequence of previous posts is out of
the question, unimportant and non-productive, but here's my simple take
on this subject since I have both originated some of the humor... as
well as "restored" several aluminum antennas.
........................................

First and foremost... K6GT IS CORRECT!  Most, if not all amateur
antennas ARE NOT IN ANY WAY ANODIZED.  In time, exposed to the
elements, the aluminum WILL become oxidized and, unlike Iron Oxide...
common reddish iron rust.... aluminum oxide will not flake off and
cause the further deterioration of the underlying metal. 

Aluminum Oxide develops a rather strong bond with the underlying
non-oxidized aluminum metal and prevents any further oxidation. Think
of it, if you will, as a self sealing process.

And, for what it's worth... this aluminum oxide coating is a poor
electrical conductor but this really has nothing to do with an oxidized
aluminum tube's ability to function as an antenna as long as electrical
contact is made to the non-oxidized surface. 

So... when restoring an older beam it is only important to check the
traps (if any) for damage and to improve the electrical contact of the
feed line, matching sections and tubing joints. (Of course there's more
too). When disassembling and cleaning of tubing joints... some people
caution against using steel wool or coarse sandpaper to "roughen or
scour" the aluminum.  I don't really know if doing so creates a problem
or not.

I personally have used steel wool as well as Scotchbrite scouring pads,
obtainable in most paint supply places.  I've put the abrasive pads on
an electric sander and not only sanded the joints clean and shiny but
also the rest of the tubing... removing the dull gray aluminum oxide
surface coat.

This point is clear.  Cleaning the tubing joints is WORTHWHILE!  This
means scouring or polishing the inside of the mating tubing sections
too. Then, coating the mating section with some conductive paste such
as Noalox or Penetrox or the like. It seems the material that DOES NOT
CONTAIN COPPER is preferred. The chief advantage to using these
materials is NOT to promote electrical conductivity.... but to prevent
oxidation of the mating surfaces which causes them to bond in ways that
make them nearly impossible to ever disassemble.

If the conductive pastes were truly conductive... then an ohmmeter
reading between test leads in a blob of paste would show some kind of
continuity.  It doesn't.  However... I'm told that at the point where
great pressure is applied between two coated surfaces... the metal
particles within the paste carrier... do promote conductivity. I can't
prove this point and so I take it on faith.

The real electrical contacts take place with a self tapping STAINLESS
sheet metal screw(s) or riviet(s) that lock the interleaving tubes
together. Hose clamps (all stainless ones I hope you use....not just
the ones with the stainless strap and otherwise crappy rusting worm
gear screw) are more for mechanical joining than electrical contact....
I'm told.

Now, back to cleaning the exterior of the rest of the aluminum tubing
elements.  This one subject is grist for much humor. The more
knowledgeable among us get great pleasure over watching well meaning
individuals (but otherwise totally naive) polishing their hearts out
shining up these tubing elements.  And, as truth would have it... I'm
one of those individuals who both laughs at others while doing my own
polishing.

I have restored many antennas like this and I confess I do it just so
that they "look new!"  I DO NOT PRETEND THEY WORK ANY BETTER!  And to
add ridiculousness to lunacy... I Krylon coat my aluminum tubes with
about three coats of Krylon Crystal Clear.

And one last point.... many of the older antennas were put together
with non-stainless steel screws, nuts and washers as well as hose
clamps.  I'd advise you to toss all the old hardware and buy all new
stainless stuff. Compared to the amount of labor and energy you are
about to invest in this project... it is foolish to deal with crappy,
old hardware.

Also, many,  older plastic parts (spacers, insulators, caps etc.) may
be either cracked or otherwise deteriorated by exposure to the UV in
sunlight. Most manufactures will have replacements on hand.  Buy them!
 Trap coil forms may be cracked likewise and they should be replaced.
Sheet metal screws holding trap coils ends to their respective tubes
may be loose as may rivets doing the same function.  These points need
careful attending to.

Finally, some people choose to insert ropes or dowels in the beam's
elements to absorb destructive vibrations or oscillations which promote
metal fatigue.  Some will swear by doing this.  Others just skip it.  I
think it's cheap insurance and so I do it.

Since I promised our esteemed List Administrator, Steve , I'd lay off
the humor for awhile... you may rest assured I'm keeping my word here.
The polishing and lacquering will do nothing whatsoever for your signal
or the rf that encounters you beam... either coming or going. When
restoring an old drab antenna the only considerations should be the
electrical and mechanical contacts of the interlocking tubing, feedline
and matching sections. All the rest is pure window decoration.

If you have the time... and the need for the exercise... (a worthy
effort here), then polish away and shine, shine, shine... and lacquer
coat everything a few times.  The end result is that you will have an
antenna that may be thirty years old and yet be INDISTINGUISHABLE FROM
ONE THAT IS BRAND NEW... and it will work as well as the day the first
owner put it together.

That is a really good feeling!  It's a labor of love and caring... not
unlike that derived from restoring the cabinet on an old radio or the
body of antique automobile or airplane.  The shiny ones don't work or
run better. But they make you feel good.

And finally, we come to the real world where you may well determine
that all the restoration effort you put into this project, when
translated into the dollars and cents you'd earn working for someone
else... brings the cost of this project to several times that what you
would have paid for a modern antenna of sophisticated computer design. 

The dx stations can't hear all the labor you put into your project.
They only hear whether the thing works or it doesn't.  If you've spent
"too much time" restoring the antenna then you've traded off the time
you would have spent on the air working the dx with a newer model right
out of the box. Does it make sense?  Only your wallet and your flabby,
out of shape body and your psyche can tell you what to do.

For me... the sight of that shiny aluminum farm glinting in the
sunlight gladdens my heart.  But whatever you do... do something and
don't sit around talking about it.  JUST DO IT.... and of course...
HAVE FUN.  It's a hobby... and any less effort is unworthy!

In all seriousness folks,

Roger, K2JAS..... Just Another Shiner



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