[Amps] Regarding skin depth and silver plaing of tank coils

[Dr. David Kirkby]

On 10/ 6/10 11:03 PM, LB3HC wrote:
> Thanks to all that have answered my post about skin depth and silver
> plating of tank coils.
>
> My conclusion is now that tank coil silver plating is not necessary
> for this appliaction. My tank coil will be polished but not silver
> plated.
>
> My working assumption about how the current RF behaves on a tubular
> tank coil is that it concentrates to the outer surface of the tubing
> (skin effect) and that the small percentage of the current that flows
> in the high resistance (oxidized, rough) layer of the surface may
> cause some loss that shows as heating. As long as this loss is within
> the limits I have assumed in my design, it will not cause overheating
> of the tank coil. Silver coating will not improve this condition much
> I now understand. Silver look is not a design criteria in this
> particular design.
>
> 73
> LB3HC

I think that assumption is probably invalid. If you covered the coil in heat 
shrink, which is a poor conductor, but much thicker than the skin depth, the 
current would not stay in the heat shrink, but would sit further down in the 
copper.

So to assume the current always sits on the outside is not true. As such, I 
think the value in polishing the copper to remove the oxide might be at best 
dubious, and at worst inadvisable, since you remove some copper and so therefore 
reduce the diameter of the coil.

In any case, an attempt to remove the oxide will be only termporary, as it will 
oxidise again.

If something is a poor conductor, then you are not going to be able to get much 
current through it, so the I^2 R heating loss will be low.

Of course, something which is a poor conductor at DC does not necessarily remain 
a poor conductor at RF, as there will be a loss tangent associated with the 
material, which will often rise with frequency.

But I think your overall conclusion, that it's not worth plating, is correct

dave