Computer Shelves (RF EMI/EMC issue)

Robert G. Brown rgb at
Tue Feb 20 11:30:27 PST 2001

On Tue, 20 Feb 2001, Jim Lux wrote:

> I doubt that metal shelving would have any beneficial EM properties, and, in
> fact, might actually make things worse by being a capacitively coupled
> radiator.  Trust in your FCC Class B cases...
> Actually, does anyone have any evidence of Electromagnetic Compatibility
> problems in a cluster?  Lots and lots of computers, all with switching power
> supplies, running off fairly long extension cords all strewn about (neatly,
> of course), and the conditions are certainly ripe for EMC/EMI problems.
> Fortunately, the computers themselves are probably fairly immune, but what
> about other devices (i.e. does your cell phone still work standing next to
> your cluster?).

Back in oh, 1983 or 1984 my 4.77 MHz IBM PC would radiate like all hell
on FM 100 or thereabouts.  I could even hear it "work" in the static.
I'm sure that any good phreaker could have told you character by
character what I was typing from fifty feet away.  Messed up my ability
to listen to music and work.

Shortly thereafter, those Class B cases started to be required and
motherboard designs improved (probably fewer sharp turns and more
attention paid to conductive path shapes and lengths) -- haven't heard a
peep for well more than a decade out of anything except the sound card
of the system itself, which DOES sometimes register a bit of system
state-dependent noice.  Given that this is an amplifier with all sorts
of resonant loops sitting INSIDE the box I'd have to say this is pretty
darn good.

I honestly don't think that metal shelves or the lack thereof make any
real difference to cluster stability or performance these days.


Robert G. Brown	             
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at

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