I think there are a couple other effects here. The lid projection below the lip of the box forces some of the radiation to be reflected and the rest to take a right angle bend to get in the box. I don't know how to quantify those effects, but they exist. Another effect is quantifiable, you have increased the length of the path the radiation takes between lid and box. The lid and box and fasteners form a waveguide. Below the waveguide cutoff frequency, there are simple equations that predict the loss. There is even an equation that predicts the sum total shielding effectiveness of an array of slots, such as you get in honeycomb, or a long lid/box seam interrupted by fasteners.
If you really wanted to quantify the attenuation due to the bend, you could probably get a feel for it by looking at waveguide design, looking at the relationship between bend radius and vswr. > From: "Chris Maxwell" <chris.maxw...@nettest.com> > Reply-To: "Chris Maxwell" <chris.maxw...@nettest.com> > Date: Mon, 17 Nov 2003 11:03:07 -0500 > To: "Jeff Chambers" <j.chamb...@ndirect.co.uk>, <emc-p...@majordomo.ieee.org> > Subject: RE: Leakage at Enclosure Seams > > > You may expect a small improvement because the overlap forms a weak capacitor, > which will short circuit some high frequency currents across the gap. > > I really can't fathom a guess at how much improvement. It would be an > interesting mathematical experiment to model the capacitance of the seam. > > Chris Maxwell | Design Engineer - Instruments Group > email chris.maxw...@nettest.com | dir +1 315 266 5128 | fax +1 315 797 8024 > > NetTest | 6 Rhoads Drive, Utica, NY 13502 | USA > web www.nettest.com | tel +1 315 797 4449 | > > > > >> -----Original Message----- >> From: owner-emc-p...@majordomo.ieee.org >> [SMTP:owner-emc-p...@majordomo.ieee.org] On Behalf Of Jeff Chambers >> Sent: Monday, November 17, 2003 10:22 AM >> To: emc-p...@majordomo.ieee.org >> Subject: Leakage at Enclosure Seams >> >> >> I have been asked to comment on what improvement might be expected in >> changing the design of an equipment lid from: >> >> A flat lid sat on the edges of the box, where the gaps between fasteners can >> be modelled as a simple aperture, where the attenuation decreases linearly >> with log(f) to zero at a half wavelength. >> >> To: >> A stepped lid, with the lid sat on the edges, and with the projection >> extending below the inner edge of the box.This removes the 'line of sight' >> gap into the enclosure. Does this improve the attenuation? Intuitively it >> should, but if the leakage occurs because of the interruption in shielding >> conductivity and hence current flow at the seam, it won't. >> >> Does anybody have any references to analyses of the above, or comments, >> please? >> >> (No emi gaskets are used btw). >> >> Thanks, Jeff Chambers >> >> ---------------------------------------------------------- >> Dr Jeff Chambers >> Westbay Technology Ltd >> Main St >> Baycliff >> Ulverston >> Cumbria LA12 9RN >> England >> Tel: 01229 869 108 >> Fax: 01229 869 108 >> http://www.westbay.ndirect.co.uk >> >> >> >> ------------------------------------------- >> 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: >> Ron Pickard: emc-p...@hypercom.com >> Dave Heald: emc_p...@symbol.com >> >> For policy questions, send mail to: >> Richard Nute: ri...@ieee.org >> Jim Bacher: j.bac...@ieee.org >> >> Archive is being moved, we will announce when it is back on-line. >> All emc-pstc postings are archived and searchable on the web at: >> http://www.ieeecommunities.org/emc-pstc > > > ------------------------------------------- > 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: > Ron Pickard: emc-p...@hypercom.com > Dave Heald: emc_p...@symbol.com > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > Archive is being moved, we will announce when it is back on-line. > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc > 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: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
