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Re: Corroded antennas



Aluminum oxide is a nearly perfect insulator, in fact it is the dielectric in
electrolytic capacitors, and all aluminum acquires a thin coating of it soon
after the metal is worked. In fact the oxide film is so strong it usually
prevents further oxidation of the aluminum. That being said, I would suspect
that if your connections to everything are sound, that the oxide film would
have no more effect than a coat of paint, since current does not flow through
the film at all. That being said, I would suspect that heavy powdery oxidation
of aluminum  caused by salt water exposure could cause problems in a couple of
ways:

First, like rust it can eventually compromise the structural integrity of the
antenna, especially where it might be fastened to a mast or clamped together
with a different metal due to electrolytic corrosion.  As an experienced ham,
you are  aware of the problems with electrical connections already. You may
want to look for a product called DeOx, which is used by electricians who work
with aluminum wiring to help protect these critical areas..

Secondly, I suspect that over time corrosion etches  the surface of the
aluminum, which would destroy the smooth path that current flowing over the
skin of it takes, increasing the skin resistance of the conductor. I have
worked with CATV hardline on my antenna farm, which uses aluminum shielding
and an aluminum center conductor, but the aluminum center conductor is clad
with a very thin layer of copper to improve performance ,  so I suspect that
heavy etching that is more than a couple thousanths deep could have a
measureable adverse effect on  the antenna. This effect would increase with
frequency, being less of a problem at 2 meters than at UHF or Microwave
frequencies.

As a practical matter, you may want to polish at least the driven elements and
perhaps the reflective elements and a few of the adjacent director elements on
a beam with a material such as Scotchbrite or  fine emery cloth, and coat the
freshly polished aluminum with a clear lacquer to protect them. I wouldnt
worry too much about the boom or directors more than a few elements away from
the driven element.

Bob Bruninga wrote:

> Here is one for the experts.
>
> Living near the bay, most antennas are covered with a white almost sandy
> corrosion layer.  As routine maintenance, I occassionally re-do the
> connections and polish the metal-to-metal contact areas (to avoid RF
> mixing products), but I never thought much about the aluminum element.
> Hey, its still a conductor...
>
> Ah, but what about the skin effect?  If the RF is actually flowing only on
> the "outside" of the conductor, then maybe this crud level could be
> affecting my performance through I-squared-R losses in the "skin"..
>
> Has anyone seen an analysis of element corrosion on skin effect?
>
> de WB4APR@amsat.org, Bob
>
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