[SGVLUG] Calling the brightest of cal tech and other nerds!

[Dustin Laurence]

On Fri, Dec 22, 2006 at 04:26:24PM -0800, Christopher Smith wrote:
> Dustin Laurence wrote:
> > 
> > "Enough like"?  I await the explanation of why a conductive enclosure is
> > not *exactly* a Faraday cage.
> 
> I'm guessing it's things like holes for drives, cables, and expansion
> cards, but I'm at a loss to see how that kills the effect. Admittedly,
> this is stuff I haven't studied in well over a decade, so I'm probably
> wrong, but I'd love to relearn *how* I'm wrong.

Well, to start with, since "Faraday cage" clearly applies to mesh
enclosures, those don't make it not a Faraday cage.  They do presumably
identify the wavelength below which the exclusion begins to drop in
effectiveness, but that is also true of the mesh spacing for a
mesh-based cage.  Real Faraday cages with maximum openings of size l
ought to be essentially high-pass filters with a knee frequency around
c/l.  If they aren't aiming a klystron tube at it, I imagine it's not a
problem. :-)

I thought the idea might be some near-field effect if the components are
too near the surface of the container.  For a superconductive chamber I
don't think there could be any such near-field effect, but there might
be one for finite conductivity.  That's no more than to say that
real-world cages aren't Ideal, though.  There should also be a thickness
effect for real materials (i.e. the filter attenuation isn't infinite),
but if that is relevant here they should move the case out of the
cyclotron. :-)

Umm.  Hey, Matt--there aren't any enormous superconducting coils
anywhere near this computer that you just didn't notice, are there? :-)

That said, I have great difficulty seeing how this could be related to
the original problem.  Then again, I have great difficulty understanding
how the original problem could happen at all, so maybe that's not
relevant consideration.  Desperate times call for desperate measures, I
guess.

Dustin

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