[TowerTalk] Effects of Oxidation on Copper Antenna Wire
W4EF at dellroy.com
Tue Apr 24 22:19:26 EDT 2018
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
>> 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..........
More information about the TowerTalk