Forum, I apparently did not express myself as clearly as I would have liked, at least for Mr. Gourari. I said that IF a shield can be arranged so as to cause eddy currents to flow that you would get shielding out of it, even with unity relative permeability. That is a demonstrable statement of fact, as in the magnetic loop shielding example I gave. It is also true, as Mr. Gourari states, that the use of a high permeability shielding material of the same conductivity would increase the shielding effectiveness (SE).
One should not draw the conclusion from my earlier message that high permeability materials are never useful, but rather that they are not always necessary. One cannot always arrange a shield so as to induce the flow of eddy currents, and an eddy current-based shield cannot protect against dc fields. In both of these cases, if shielding is required then a high permeability material is necessary. Next, it is not true that an absence of several skin depths means no SE. It means little SE from absorption. Recall that Shelkunoff shielding theory defines total shielding as the product of three different types of shielding. Expressed in log space: S = R + A + B where all quantities are expressed in dBs. S = total shielding R = shielding from reflection (resistivity of shield material vs. wave impedance) A = shielding from absorption (skin depth) B= losses due to re-reflection within the shielding material Even if the value of A = 0, the value of S will still reflect reflection losses. Going back to my magnetic loop electrostatic shielding example, I said that if the shield were made contiguous the pickup of the loop would decrease significantly and that represents shielding without permeability. It is not clear to me what Mr. Gourari means by saying, "Otherwise eddy currents in your shield will not compensate impinging magnetic flux fully." Fully meaning infinite dB? Agreed. Fully meaning not as much SE as if there were absorption? Also agreed. It remains there is significant reflection-based SE when a high conductivity, low permeability material allows eddy currents to flow. Ken Javor > From: "Gourari, Alexandre" <[email protected]> > Date: Fri, 17 Sep 2004 13:37:53 +0100 > To: [email protected] > Subject: RE: Germanium & ITO Window shielding effectiveness > > 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, > > > > > -----Original Message----- > 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 > 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
