In article , John S wrote:

Suppose my square loop antenna oxidizes. Does that affect its
characteristics?

Yes. Its color changes :-)

Beyond that - as far as RF characteristics go - the answer is probably
"Yes, but probably not enough that you would be able to detect the
difference in practice."

What I believe would happen:

(1) As the surface oxidizes, its resistance rises. This will cause
some amount of additional signal loss, as current flows through the
resistive layer.

The actual amount of added loss will depend on the depth of the
oxidized layer, its actual resistance per square compared to
that of the underlying copper, and on the frequency (which
influences the *skin depth" of the material).

Interestingly, the greatest amount of added loss does *not*
occur when the oxide layer's resistance is at a maximum (near-
infinite), since little or no current will flow through the
oxide... it becomes "invisible", and the current will flow
through the metallic copper under the oxide. The losses are
highest when the oxide layer is (1) of moderate resistance, and
(2) fairly thick, compared to the skin depth. At this point,
most of the current will be flowing through the oxide, and
excluded from the metallic copper by the skin effect.

I don't know the actual numbers involved here (i.e. point of
greatest loss, for any given frequency). In practice I don't
think you'll come anywhere near these conditions for a copper-wire
antenna at UHF frequencies.

(2) Copper oxide is not electrically linear - it behaves somewhat
like a semiconductor (you can actually make a diode out of it).
This nonlinearity may result in the generation of some second-
and higher-order harmonics, and possibly some intermodulation
products, if a significant amount of current is flowing through
the oxide layer.

I doubt that this will be detectable.

(3) If you manage to get oxidation into your actual metal-to-metal
contacts (and not just on the surface of the metal) the problem
in (2) becomes a lot worse, since you can have *all* of your
signal power flowing through what amounts to a diode. Harmonics
and IM products can become much more of a problem.

In practice, I'd say that if you make sure that (3) is not a
problem... that is, all of your actual metal-to-metal contacts are
soldered or welded (or gooped with a conductive anti-oxidant paste
prior to assembly) you'll be OK, electrically. Surface oxidation
on the loop is unlikely to be a problem.

You could always scrub down the loop with Brasso, wash it
carefully and dry it, and then spray it with an industrial-
strength outdoor-rated UV-proof clearcoat. That would leave it
shiny and bright for quite a long time.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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