More Reader Comments on Aluminum Computer Cases
Posted by Kathy Smith
Date: June 10, 2002
In response to Bill Brier's comment on an aluminum Cooler Master Case review, "The one "con" missed in this review was that aluminum has almost no EMI shielding capability. Any PC built with an aluminum case would violate FCC radiation regulations (the PC wouldn't even make class A) and would probably irritate close-by neighbors with herringbones on the TV screen, whining in the radio, etc." Readers had these additional reactions.
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Metal computer cases need not be made of steel. Any conductive metal can act as an emi shield rather nicely. None of the radiated energy is in the form of magnetic fields. The ancient concept is that of the Faraday shield, a conductive metal layer ground to earth that intercepts radiated energy. EMI is actually a mis-used word, implying electro-magnetic energy as would be produced by a permanent magnet moving around rapidly or an unshielded ac transformer. Computer citcuitry produces RFI energy that is easily absorbed by conductive metal surfaces.
A deposited film of aluminum that lets light through can, if properly applied, do the same job.
Low frequency energy is radiated by the power supply, but that usually is in a plated steel case.
nils
just an old man
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Your assertion is true for magnetic ( inductive ) radiation, but the effect of magnetic fields are localized as stored magnetic energy and are greatest at low frequency.
For electromagnetic radiation ( more likely to be produced by your computer circuits ) aluminium is as good as any other conductor when used as a grounded shield.
Yes, it is possible that a low-frequency magnetic source could cause havoc for such a system. But I can't see how that's really such a problem, unless you plan to stick an unshielded powered subwoofer ( < 100Hz response ) right next to the case, or wrap a few meters of loaded 120v AC power cable in a loop around your case. I mean, really, how many sources of low-frequency strong magnetic field can you name off the top of your head?Gabriel Sashkin [sashkga@speakeasy.net]
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In the comment after the Cooler Master case review, someone wrote "The one "con" missed in this review was that aluminum has almost no EMI shielding capability."
Actually aluminum is used TO provide EM shielding. Coax cable is often wrapped (shielded) in aluminum.
I found a number of references on the web recommending aluminum screens or aluminum coated panels for EM shiedling.
Aaron
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