Ken, Your approach about eddy currents somewhat true, but not all. We would never be in need of hign-mu materials then. You forget about skin effect that limits your shielding effectiveness. As long as can provide enough material thickness >> skin effect depth you are home free. Otherwise eddy currents in your shield will not compensate impinging magnetic flux fully. Look at the skin depth formula h = ((2*r)/(2*pi*f*u))^0.5 where r-resistivity (Ohm*in), f - impinging wave frequency, u - magnetic permeability. As you can see u can enhance (lower) skin depth big deal. Other approach is to make r=0 (superconductor).
Best regards, Alex Gourari, From: Ken Javor [mailto:[email protected]] Sent: Thursday, September 16, 2004 9:54 PM To: [email protected] Subject: Re: Germanium & ITO Window shielding effectiveness What Mr. Mellberg says is narrowly true but interpreted broadly it causes mischief. If a magnetic field is impinging upon a barrier at some distance then Mr. Mellberg is correct. But if a shield can be arranged such that eddy currents can be induced to flow, the magnetic field can be shorted out using nothing but good conductivity - relative permeability identically equal to unity. Hence the copper bands around transformers, and the electrostatic shielding configuration for magnetic loops. Note that these shields circumferentially surround the windings, but there is a narrow gap so that the shield is not contiguous 360 degrees around the loop. These shields are obviously not made of any ferrous metal, or more accurately, they are made of high conductivity materials whose relative permeability is again identically equal to unity, specifically so that they do not absorb the field. But if the shield were 360 degrees contiguous, the field penetrating the plane of the loop would cause eddy currents to flow in the shield. By Lenz' Law, they would flow in a direction which would oppose the field that caused them. Hence the net field within the loop would decrease, with an attendant decrease in the potential output of the loop. That is magnetic shielding by nonmagnetic materials. The mechanism is different >from a high permeability material. The mechanism is reflection, not absorption. > From: "hansm" <[email protected]> > Date: Thu, 16 Sep 2004 14:30:13 -0700 > To: "Gary McInturff" <[email protected]>, "'Price, Ed'" > <[email protected]>, <[email protected]> > Subject: Re: Germanium & ITO Window shielding effectiveness > > In order to attain eny significant mangentic shielding, at least Fe, Ni or > Co must be in the alloy. All other metals simply provide electric field > shielding which for low frequencies (long wavelengths), do not do much of H > field shielding. Thickness counts and can be reduced by increasing the > relative permeability at frequency of interest. Hence mu metals. > > Hans Mellberg > Engineering Manager > BACL, a TCB and an EU CAB > 230 Commercial Street > Sunnyvale CA 94085 USA > 408-732-9162 x38 > 408-732-9164 fax > > ---------------------------------------------------------------- > 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: > > Ron Pickard: [email protected] > Dave Heald: [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 > 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: Ron Pickard: [email protected] Dave Heald: [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 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: Ron Pickard: [email protected] Dave Heald: [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
