>-----Original Message----- >From: Chris Maxwell [ mailto:[email protected]] >Sent: Monday, November 17, 2003 8:03 AM >To: Jeff Chambers; [email protected] >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 [email protected] | 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: [email protected] >[SMTP:[email protected]] On Behalf Of Jeff Chambers >> Sent: Monday, November 17, 2003 10:22 AM >> To: [email protected] >> 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
Jeff: If the radiation were leaking out by propagating through the seam gap, which would have to be very high GHz emissions, as the gap is small, then a more labyrinthine gap would help a bit. At lower frequencies, where the gap creates an impedance discontinuity, and the radiation is caused by current across the impedance, then I don't think the gap shape would matter very much. OTOH, even if my guess is right, somebody will want quantitative data, so test a couple of cover gaps yourself. Regards, Ed Ed Price [email protected] WB6WSN NARTE Certified EMC Engineer & Technician Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (Fax) Military & Avionics EMC Is Our Specialty
