See response to Mr. Woodgate. Shorted fibers are not an issue, my argument holds if the shield material is a solid copper pipe. You won't get eddy currents flowing in that pipe until the ends are terminated so currents can flow in a loop. The magnetic vector flows around the circumference of the pipe, but that is not the path the eddy currents nee to take. They must flow longitudinally down the pipe. The cable becomes the center of a coaxial transmission line; the shield is the coaxial transmission line shield and return conductor.
> From: "Robert A. Macy" <[email protected]> > Date: Thu, 15 Sep 2005 11:28:26 -0700 > To: Ken Javor <[email protected]> > Cc: [email protected] > Subject: Re: When is a cable shield not a cable shield? > > Yes, it can. > > The fibres are all shorted out making it a pseudo-metal > plane. > > Plus, the loops of interest are pretty much contained in > between the two ends of the shield. > > To see how much effect there is measure the inductance of > the loop inside shield and ouside the loop. > > - Robert - > > On Thu, 15 Sep 2005 11:50:33 -0500 > Ken Javor <[email protected]> wrote: >> I have a problem with this as well. A non-magnetic >> shield material protects >> against magnetic field radiation by providing a path for >> eddy currents to >> flow in. The eddy currents flow in such a way as to >> cause a magnetic field >> that opposes the originating field (Lenz' law). In order >> for the eddy >> currents to have sufficient magnitude to cause field >> cancellation, the >> shield must provide a very low impedance short circuit. >> An open-circuited >> shield braid cannot perform this function. >> >>> From: "Robert A. Macy" <[email protected]> >>> Date: Thu, 15 Sep 2005 09:16:09 -0700 >>> To: "Hudson, Alan" <[email protected]> >>> Cc: [email protected] >>> Subject: Re: When is a cable shield not a cable shield? >>> >>> Alan, >>> >>> Think also in terms of magnetic fields, or "area >> loops". >>> >>> The area defined by the small loop between signal lines >> and >>> ground lines now have rather opaque metal plates over >> them. >>> Starting around 10KHz the conductive "shield" kills all >>> the magnetic fields coming out. Which were probably >> more >>> significant than the E-Field contributions. >>> >>> - Robert - >>> >>> On Thu, 15 Sep 2005 16:22:31 +0100 >>> "Hudson, Alan" <[email protected]> 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 >>>> -- >>>> Principal Engineer, >>>> BAE Systems Limited, >>>> UK >>> >>> - >>> 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 > > - > 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
