Tom, Using a double braided shield will improve the coverage to >99%. I would guess that having an insulating layer between the shields might allow return currents to pass along the inner shield with the outer providing an overall shield, but that would require some creative termination of the shields. Otherwise, I can see no benefit in having an insulating layer as the outer shield in contact with the inner will help to reduce porpoising on the inner shield. However, I am led to believe that the best form of shielding is not a double braid, but a foil and braid as the braid is more effective than the foil at lower frequencies and the foil more effective than the braid at higher frequencies so you get the best of both worlds. Also, a foil is even more effective (due to lower impedance at higher frequencies) at reducing the porpoising effect that can occur where currents coupled into the braid on the inside of the shield then pop out to the outside where they can re-radiate. If you look at the catalogues of some of the well-known manufacturers, such as Belden or Alpha Wire, you will find performance characteristics of a number of such combinations from which you should be able to select the optimum for your application. Best regards,
Neil R. Barker C.Eng. MIEE FSEE MIEEE Manager Compliance Engineering e2v technologies (uk) ltd 106 Waterhouse Lane Chelmsford Essex CM1 2QU UK Tel: +44 (0)1245 453616 Fax: +44 (0)1245 453410 e-mail: neil.bar...@e2v.com Web: http://www.e2v.com <http://www.e2v.com/> From: Donnelly, Thomas [mailto:tdonne...@act-sys.com] Sent: 13 October 2004 13:38 To: EMC-PST (E-mail) Subject: Cable Shielding Group, I am looking for information on cable shielding for a potential Radiated Emissions problem. Unfortunately due to equipment performance requirements we cannot filter the lines and must shield the cables. I am looking for a source of information on the difference in performance that can be expected using a single braided shield, a double braided shield with no insulating layer between, or a double braided shield with an insulating layer between. We cannot meet the emissions with a single shield and must use a second shield. We are trying to determine what the effect of an insulating layer between the shields on performance. We are trying to balance weight and performance concerns and don't want to add the weight unless required. All shields will be 95% optical coverage, and will be terminated on both ends with a good quality EMI backshells. Thanks, Tom tdonne...@act-sys.com ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Scott Douglas emcp...@ptcnh.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: http://www.ieeecommunities.org/emc-pstc ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Scott Douglas emcp...@ptcnh.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: http://www.ieeecommunities.org/emc-pstc
