Greetings Neven,
There are a couple of things that need to be considered when you have an
rf source close to a shield with apertures. First is the "shielding"
properties of the shield as people have said this is very dependent upon
the field impedance when you are in the near field. Second and no less
important is the direct coupling between the shield and source
conductors, the presence of the shield greatly affects the properties of
the radiating source.
With everything so close together there could well be resonances which
result in windows in the shield at lower frequencies than you would
expect. Some years ago I published a paper with my wife (Colin and
Bronwyn Brench) on the effects of conductors close to apertures in a
shield which illustrates this effect. The paper used very simple
geometries not real ones and used a field source not a local one - but
the premise was the same. It can be found in the EMC Symposium record -
sorry I don't know what year - it won't help you directly, only show
what behavior you might expect. I believe that to get an answer to your
question you can only go so far analytically and that to get a detailed
answer you will have to resort to a computational electromagnetics tool.
It is important to accurately represent your EMI source, physically and
electrically (I.e. source impedance) to get accurate results.
good luck with this,
Colin.
-----Original Message-----
From: Neven Pischl [mailto:[email protected]]
Sent: Tuesday, June 05, 2001 12:16 PM
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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