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: f...@np.edu.sg 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 <jharring...@ktlcanada.com>, emc-p...@ieee.org 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" <jharring...@ktlcanada.com> To: <emc-p...@ieee.org> Subject: Shielding Effectivness List-Post: emc-pstc@listserv.ieee.org 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 jharring...@nemkoca.com <mailto:jharring...@nemkoca.com> Tel: +(1) 613 737 9680 ext 229 Fax: +(1) 613 737 9691 ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.