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Re: Primary and copper (fwd)
---------- Forwarded message ----------
Date: Sun, 29 Mar 1998 14:07:53 -0800
From: "Antonio C. M. de Queiroz" <acmq-at-compuland-dot-com.br>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Primary and copper (fwd)
Bert, Jim Lux, Malcolm, all:
Thank you for the comments. I will see if I find or derive a better
expression for rac.
For awhile, what I am using is (MKS units, essentially the same as in the
Ref. Data for Radio Eng.):
sd=skin depth=sqrt(2*r/(w*u0))
where w=2*pi*frequency; u0=permeability=pi*4e-7; r=resistivity=1.724e-8 (Cu).
Assuming a tubular uniform layer with thickness=sd around the wire:
Rac=length*r/(pi*(diameter-sd)*sd)
This reduces to Rdc when sd=diameter/2.
The problem is that this alone doesn't give correct results for long coils. I have
a secondary coil, made with rather thin wire, where I get Rdc~=Rac=142 Ohms
(Rdc measured, Rac calculated) at the resonance frequency. Measurements of
the coil characteristics by several methods give a consistent result of
Rac~=1000 Ohms at the resonance.
I know that the formula is imprecise at this condition, and that the proximity
of other wires in a closewound coil increases Rac by a good factor, that
dielectric losses in the PVC form add some resistance too, and that there is some
radiation resistance. But I don't know yet how to take these effect in account.
Antonio Carlos M. de Queiroz
http://www.coe.ufrj.br/~acmq