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Re: [TCML] primary tubing



Thanks for that info Jim! Your in depth analysis is much more insightful than a simple "don't do it".

Drake



On Nov 8, 2009, at 6:32 PM, jimlux <jimlux@xxxxxxxxxxxxx> wrote:

Drake Schutt wrote:
Thanks fir the insight Brian, hopefully that clears some things up for some people. The only discrepancy I have is that I thought Al simply oxidized in air, maybe you could also call it anodization but I thought that was adding pigment to the outside oxide layer. Could anyone comment on the resistance ratio of Al vs Cu? If it was say 1.5:1 would you simply need 1.5x the calculated length of copper for primary R to be equivalent?


That's about right for the conductivity ratio. It's a bit trickier when talking RF and skin depth. The tricky part is that if you just scale up the area, it doesn't really help, because electrically, it's more of a tube. The skin depth goes as square root of conductivity, so if you wanted to have a tube that has a wall that is say, 5 skin depths deep, the wall thickness would need to be more than for copper.

At 100kHz, skin depth is about 0.26mm for Al vs 0.21 for Cu. (e.g. 20% more). 5 skin depths is a good number, so wall thicknesses of 1.25 mm (0.05") would be needed. If the tube (or wire) is small in diameter compared to say, 10 skin depths, all the equations change (because the current distribution is different... the current on one side of the wire is affected by the current on the other side, so the "approximation" of considering it a tube doesn't work). So don't go using aluminum wire for an antenna!

And then, because that "layer" has higher resistivity in the first place, you need a "wider" strip (that is, the circumference of the tube needs to be more). Assuming the wall thickness is "big enough", then you want the diameter to be about 60% more, for the same AC resistance.

that is, a 3/8" OD aluminum tube should be about the same as a 1/4" OD copper tube.

Given the general hassle of trying to make good connections to aluminum, it's probably not worth it. 50-100 ft of copper tubing just isn't that expensive, compared to all the other costs of a tesla coil.
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