On 4/24/18 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 wi
re
cause the RF to stay on the outside of the overall collection of strands, all of which have good contact with their adjacent strands?
Yes - and even if the strands are insulated from each other (this would
be like litz wire). It's the magnetic field that "pushes" the current
to the outside, and that's there whether it's one solid conductor or
multiple separate conductors.
When you start to get "semiconductive" layers, it gets more exciting, of
course.
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.
You'd have to be careful about measuring the Q of a tuned circuit -
parasitic L and C might have a bigger effect. About 15-20 years ago,
there were a bunch of Tesla coilers trying to quantify such effects on
their systems (if you wind your coil on PVC pipe, which is hygroscopic,
does the Q change with humidity) - the effects are noticeable, but
non-trivial to measure.
I'd build a two wire line and short the far end, and measure the
impedance. Short rather than open, because I'm thinking coax and
microwaves, and good shorts are easier than good opens, which radiate -
for this purpose, probably no difference.
It affects the loss term in the antenna radiation efficiency - probably
small (after all people don't go out and try to build Yagis out of
copper or silver tubing).
Where antenna resistance starts to really get you is when there are high
circulating currents - a compact magnetic loop is a good example. A
very high gain multi-element Yagi might be another. A very close spaced
W8JK array is another.
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