I'm not sure what the effect on radiation efficiency could be, but I can
think of a possible way to see if it has any effect at all.
1. Put up a 20m dipole comprised of shiny new wire. Measure it's
impedance vs frequency curve.
2. Put up a second closely coupled dipole cut roughly 4% - 5% shorter
and made with shiny wire parallel to and spaced a few inches away from
the first dipole. Measure the impedance curve of the first dipole
again. There should be some difference.
3. Replace the second dipole with one made of heavily oxidized wire of
the same type/length and measure the impedance curve of the first dipole
a third time.
You'd want to make sure the lengths and spacings are almost exactly the
same in both cases, but if you are able to do so and the second and
third curves are not pretty similar there is possibly some effect due to
the oxidation.
Just a thought.
73,
Dave AB7E
On 4/24/2018 12:05 PM, Shawn Donley wrote:
Some recent posts on grounding reminded me of something I've always wondered about. How
is the radiation efficiency of a copper wire HF antenna affected by oxidation of the
copper over time? Empirical evidence is that any effects are small/negligible,
otherwise the dipole you put up last year would not work so well this year. My limited
understanding is that the two oxides of copper, Cuo and Cu2O, are semiconductors. So
after a while, the outside of the wire is covered by something approaching an insulator
(relative to clean copper conductivity). The depth of the oxide, as far as I could
research, is on the order of 100 nano-meters. OK...so the RF current is forced under the
oxide and follows the skin depth with frequency relationship. Not much effect on the
current or the "RF resistance" of the wire, if I can be forgiven for using that
term. But what about stranded copper wire? That's where things might get interesting.
Does the skin effect with clean copper wire
cause the RF to stay on the outside of the overall collection of strands, all of which have good contact with their adjacent strands? If so, what happens when all the individual strands are oxidized and not in low resistance contact with their partners? Anyone know of actual measurements of the effects of oxidation or how such a measurement would be done? Short of measuring the Q of a tuned circuit built with "clean" and oxidized wire inductors, I'm not sure how you could measure the effect and even less sure of how those measurements would translate to the original question...effects on the radiation efficiency of an antenna.
N3AE
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