On 4/24/2018 2:43 PM, jimlux wrote:
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.
Jim, et al:
The individual conductor strands that make up the shielded braid of a
coax cable follow a helical pattern with half the conductors wound CW
and the other half wound CCW around the dielectric. I never really
thought about it before tonight, but there is probably some common ratio
(greater than 1) between the length of a individual conductor and the
length of the cable. If the individual conductors in the braid became
truly insulated from one another, I would expect that the DC resistance
of the braid would increase by a factor somewhat close to the
aforementioned shield conductor strand length to cable length ratio.
73, Mike W4EF..........
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