[TowerTalk] Effects of Oxidation on Copper Antenna Wire

[jimlux]

On 4/25/18 8:16 PM, Michael Tope wrote:
> 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.
> 


I wonder if it ever gets *that* bad.  what you might get is a lot of 
partial connections, at random spacings. And, of course, the currents in 
all those strands are magnetically coupled, as well as capacitively 
coupled through the oxide. It's not like they're parallel transmission 
lines with idealized couplers.

An interesting modeling problem.  How much does a "pulse" get smeared.