The definition or range of the term "low frequency" was bounded by saying the cable was electrically short.
And the point is not to stop eddy currents from flowing, but to maximize and control where they flow. Both of these functions are achieved by using a high conductivity shield with low impedance shield terminations. Mr. Tang is correct that once the shield becomes electrically long the magnitude of a localized shield current is less dependent on the shield termination impedance. But that is the case that was covered at the beginning of this thread, before magnetic fields were specified as the culprit to be controlled. The reason for terminating the shield at both ends at high frequencies (cable electrically long) is to contain the eddy currents on the interior of the equipment enclosure, of which the shield is an extension, just as a tunnel or skyway between two buildings becomes an extension of the buildings it connects. > From: "George Tang" <[email protected]> > Date: Thu, 15 Sep 2005 13:49:00 -0700 > To: "Ken Javor" <[email protected]>, <[email protected]> > Subject: RE: When is a cable shield not a cable shield? > > "Low frequencies" is really a relative term these days. Depending on the > dimension of the shield, a 1GHz eddy current can drive the shield into a > monopole (if one end grounded, the other end floating) or dipole (both ends > floating). Eddy currents still flow, and opening one or both ends of the > shield does not stop the eddy currents. This is why fields and waves are so > interesting. > > George > > > > -----Original Message----- > From: [email protected] [mailto:[email protected]]On Behalf Of Ken Javor > Sent: Thursday, September 15, 2005 12:37 PM > To: [email protected] > Subject: Re: When is a cable shield not a cable shield? > > > Mr. Woodgate is correct in the general sense of shielding against all forms > of radiation at all frequencies, but in the specific case of magnetic > radiation (which to me at least implies low frequency, an electrically short > cable) the magnetic field circulates around the cable. That means that eddy > currents will flow in a loop which is penetrated by that vector. That means > that the shield has to form a loop end-to-end to allow eddy currents to flow > and oppose the field which caused it. > >> From: John Woodgate <[email protected]> >> Date: Thu, 15 Sep 2005 18:25:32 +0100 >> To: [email protected] >> Subject: Re: When is a cable shield not a cable shield? >> >> Ken Javor <[email protected]> wrote (in >> <bf4f1089.2c189%[email protected]>) about 'When is a cable >> shield not a cable shield?', on Thu, 15 Sep 2005: >>> 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 >> >> It's only open-circuit in one direction. In two other orthogonal >> directions, it's a closed circuit. But one open circuit is enough to >> kill it. Think 'balloon'. >> -- >> Regards, John Woodgate, OOO - Own Opinions Only. >> Deadlines are 90% of deadliness. >> http://www.jmwa.demon.co.uk Also see http://www.isce.org.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
