[TowerTalk] Effects of Oxidation on Copper Antenna Wire

[Michael Tope]

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..........