I agree that the seams and apertures cut into a case are the main avenue
for radiated emission to penetrate or escape from a shielded case.
However, a plated plastic casing e.g. Ni with a thickness of 20 um would be
many skin-depths (delta's) in the 100 MHz and above zone, so the SEE (
shielding effectiveness-electric) can be in the region of 80+ dBs. However
Ni may not be a good example in this case because this metal breakdown
rapidly (in the presence of moisture?). Ni can turn into it's amorphous
form and within a short period of time the shielding can be compromised.
As an aside, I would recommend that the metalwork is made using a
corrosion-resistant technique. All metalwork should be electrically
'bonded' in a manner which does not rely on hopeful electrical conduction
through anti-corrosive treatment like anodised surfaces (aluminium) and/or
'paint'. Careful attention to the assembly process will weed out things
like ball-bearings races, nylon runners and coasters, or other insulating
materials/parts. Conduction through painted panels should not be
dependent on the gripping action of star washers. The design should be
such that no intentional currents should be flowing in any part of the
metal work. The objective is to ensure that any part of the metalwork can
be relied upon as an effective electrostatic (and hopefully there are not
too many 'large' apertures that are penetrated by un filtered or protected
conductors, it can act as a RF) screen and not the reverse, a unintentional
radiator (antenna).
:-)
Tim Foo,
(or just call me 'Tim')
E-mail: [email protected]
ECE, School of Engineering,
http://www.np.edu.sg/ece/ Tel: + 65 460 6143
Ngee Ann Polytechnic, Fax: + 65 467 1730
535 Clementi Road,
Singapore 599489
"Ken Javor"
<ken.javor@emccomplianc To: John Harrington
<[email protected]>, [email protected]
e.com> cc: (bcc: Wan Juang
Foo/ece/staff/npnet)
Sent by: Subject: Re: Shielding
Effectivness
owner-emc-pstc@majordom
o.ieee.org
09/29/01 06:51 AM
Please respond to "Ken
Javor"
For all practical purposes the SE of commonly used metals is infinite
except at audio frequencies where skin depth becomes a problem. The
intrinsic SE of the material is always dominated by seams and apertures
that are inherent in any enclosure design. If you think that steel is less
conductive than Cu or Al and therefore should give lower SE, consider that
almost all shield rooms are made of steel and depending on construction
meet SE requirements of 100 dB or better. Steel actually gives better
magnetic performance at low frequencies, again because of skin depth
considerations. The absolute best doors for SE are not CU or brass knife
edge into Cu or brass fingerstock but rather the sliding steel doors in a
pocket with internal bladders which force the sides of the door against the
inside of the pocket much like the brake cylinder forces drum brakes
against the brake housing. This is steel on steel with tin flashing to
provide a soft, deformable mating surface.
----------
From: "John Harrington" <[email protected]>
To: <[email protected]>
Subject: Shielding Effectivness
List-Post: [email protected]
Date: Fri, Sep 28, 2001, 3:11 PM
Would somebody please be kind enough to point me in the direction of
some information on the shielding effectiveness of such metals as
copper, aluminium and steel.
Thank you
John Harrington
RF Group Manager
Nemko Canada
[email protected] <mailto:[email protected]>
Tel: +(1) 613 737 9680 ext 229
Fax: +(1) 613 737 9691
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