From: David Haedge ([email protected])
Date: Thu Aug 31 2000 - 13:42:28 PDT
SIer's-
The other effect that enters into the equation is the proximity effect,
which is frequency and geometry dependent. The best illustration
of this is a coax conductor with currents flowing in opposite directions
on the inner and outer conductors. Using skin effect only, you would expect
a high concentration of current to be the outside surface of the outer
conductor, when in fact the current concentrates on the interior
surface of the outer conductor, in direct conflict with the a "skin
effect only" computation. I have used field solvers (Ansoft Maxwell Eddy)
to examine a number of different geometries with some surprising
results. When you have ground planes involved, current will always
concentrate on the edge of the conductor nearest the ground return
plane and the return current tends to bunch up right below the
conductor.
Also remember that skin depth is not the depth where the current
stops flowing, it's the depth where the current is 1/e (.368) times
the current density on the surface. There is still significant current
beyond that depth.
As in most SI problems, you have to also consider the return path
to find the correct solution. My 2� for today.
David Haedge
Raytheon
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