Shielding

[mmotyka at lsil.com]

One simple comment.

Below.

Bill Stewart wrote :
>At 04:45 AM 08/23/2001 -0700, David Honig wrote:
>>Faustine, look up Faraday cages, TEMPEST, and search the archives.
>>As if you didn't know.  Succinctly, the electron gas in metals shields you
>>from the electromagnetic antics of distant, radiating electrons, by
>>shorting the
>>ripples in the aether they make -and this shielding makes it harder to listen
>>to your emissions, too.  The problem is that cables and ventilation vents
>>are antennae,
>>for sending and receiving both.
>>
>>Testing is key.  If you don't measure, you don't know.
>
>This stuff was a *lot* easier when computers were slower.
>I used to test my TEMPEST room at 450MHz, since that was high enough frequency
>to cover any realistic level of emissions from the upper harmonics from the 
>VAX,
>and it was also a short enough wavelength that leaks were pretty detectable.
>It doesn't take much to get a leak - copper foil on a joint wearing out,
>or the copper mesh we'd stuff inside gaskets getting set unevenly.
>The waveguides we used for fiber or air vents were typically 1/8 inch wide
>and an inch or two deep - and if you pushed a paperclip halfway through you'd
>twang the leak meter.
>
>Well, that was fine for computers that were around 10MHz.
>These days, when 1GHz is slow, there's tons of stray energy above that,
>and that stuff is much more penetrating, plus you've got all the
>
The skin depth is proportional to f^(-0.5).

The skin depth for Cu at 100MHz is about 0.00026". 
At 1600MHZ it should be ~0.000065

I think maybe 'sneakier' ( because of its smaller wavelength ) is closer
than 'more penetrating' ( it is actually less penetrating in a conductor
).

Mike

>100 and 133MHz memory and disk bus stuff.
>Fortunately, the equipment runs at much lower power levels;
>you can run on batteries instead of 208-volt 3-phase (:-),
>but I'm still glad I don't have to design a room or even a box
>for that level of tightness.