Here's an informal guideline that I use. If memory serves, I got it off the
Futurebus specification years ago. I imagine this guideline was based on
slower frequencies than we have nowdays.
1. Enclosure should have 20 dB of RFI attentuation at 5 Ghz.
2. Maximum gap shall be less than 3 mm in any direction.
3. Holes for indicators, switches, or connectors that are not shielded shall
be limited to the following:
A. No source of high frequency within one diameter distance of the hole.
B. The thickness of the hole wall shall be more than:
0 for a 3 mm hole
2.5 mm for a 7.5 mm hole
10 mm for a 15 mm hole
4. Larger holes shall have back-shielding behind the light, switch, or
connector, and tied to chassis ground through low-impedance gasketing.
I would be interested also in any cook-book or analystical information
anyone else might have.
Max Kelson
Evans & Sutherland
-----Original Message-----
From: Neven Pischl [mailto:[email protected]]
Sent: Tuesday, June 05, 2001 10:16 AM
To: [email protected]; [email protected]
Subject: Shielding Effectivness Question
I would appreciate if anyone could let me know if there are any references
(books, application notes, anythig ..) that deal with shielding efectivness
in cases when a source is close to an (electrically small) opening in a
shield (enclosure). In such a situation, the field will penetrate through
the hole and leak even if the size is much smaller than the wavelength. I am
particularly interested in situation when high-frequency source, such as a
PCB edge or a component operating at (say) 1 GHz and above is in proximity
of the venting holes, "small" gaps in the chassis etc.
All references that I have deal with uniform plane wave propagating incident
to a metal plane with a slot or hole, in which case it is enought o have
electrically small size of the opening (e.g. lambda/10) to efficiently block
any field propagation through the barrier. I can't find any useful reference
that deals in any analytical way with the situation I am intersted in.
I believe I might get some answers using some of the simulation programs,
but at the moment I am more intersted in the analysis of the problem than in
simulating it.
Thank you,
Neven Pischl
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