A floating shield is different than a floating enclosure in one very important respect. A sealed floating enclosure is a Faraday shield. If the enclosure sides are several skin depths thick, it will be an effective shield.
A floating shield, on the other hand, is open at both ends. Whatever has coupled to the inside is also on the outside. No shielding in that case. In fact it is not the concept of floating vs. grounding that is important here. It is the concept of enclosing the radiating circuit. Terminating the shield to an enclosure at both ends helps to enclose the radiating elements, even if the entire enclosure is floating. > From: Scott McMorrow <[email protected]> > Date: Thu, 15 Sep 2005 11:52:06 -0400 > To: "Hudson, Alan" <[email protected]> > Cc: "EMC-pstc (E-mail 2)" <[email protected]> > Subject: Re: When is a cable shield not a cable shield? > > Alan > > This would be no different than a floating metalic enclosure around any > piece of electronic equipment. You have parasitic capacitance > everywhere, and, in the case of a cable with such a metal jacket, the > coupling is quite high. Thus, everything is coupled to everything > else. In addition, you have yourself a very nice floating dipole > antenna, which will radiate all of that coupled energy very > efficiently. Essentially, the manufacturer has created a capacitively > coupled antenna system. > > > regards, > > Scott > > Scott McMorrow > Teraspeed Consulting Group LLC > 121 North River Drive > Narragansett, RI 02882 > (401) 284-1827 Business > (401) 284-1840 Fax > > http://www.teraspeed.com > > Teraspeed® is the registered service mark of > Teraspeed Consulting Group LLC > > > > Hudson, Alan wrote: > >> G'Day! >> >> Came across an oddity today which has failed to stimulate any memories (from >> about the time the old King died!) of basic electromagnetic theory. A >> well-known computer Company supplies what it calls "shielded ribbon cables". >> When we examine one of these we find it's essentially a metallic braid >> sleeve around a standard ribbon cable. The braid isn't connected to anything >> (not even a drain wire at the connector ends). >> >> So what level of shielding is this going to give - if any? >> >> I could imagine signal lines being capacitively coupled to the braid and >> then capacitively coupled to any 0V lines in the ribbon. But I can also >> imagine signal lines being capacitively coupled to other signal lines. >> Similarly with external signals hitting the braid - they could go to either >> the signal or 0V lines - yes/no? >> >> I know I should know the answer to this, but obviously, today at least, if I >> had a brain I'd be dangerous! >> >> :-) >> >> Regards, >> >> Alan >> >> > > - > 2005 IEEE Symposium on Product Safety Engineering > 3-4 October Schaumburg, IL > http://www.ieee-pses.org/symposium > > ---------------------------------------------------------------- > 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 [email protected] > > 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: > > Scott Douglas [email protected] > Mike Cantwell [email protected] > > For policy questions, send mail to: > > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc > - 2005 IEEE Symposium on Product Safety Engineering 3-4 October Schaumburg, IL http://www.ieee-pses.org/symposium 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 [email protected] 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: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
