Hi Everyone, We often hear that shielded magnetic field probes reject electric fields and therefore are better. This, however, is not generally true. The E-field rejection only happens for fields symmetric around the centerline of the loop. This is satisfied in the far field but not in the near field, say when held up to a PCB to "sniff" sources of emissions. A coax structure is just a transformer where the shield inductance equals the mutual inductance between the shield and the center conductor, for example as shown in Henry Ott's book. In fact, the presence of the shield causes resonance problems at frequencies well below the self resonance of the loop itself than cause the performance of an unshielded loop to be better for some applications. During my seminars I do an experiment to show this effect and how to easily measure the E-field component of the output of a magnetic probe, shielded or not. In fact, there is a lot of "common knowledge" that is not quite right that I cover in my classes. Here are some articles I have written on the subject.
* June 2008, The Square Shielded Loop - Part 2, Parasitic Coupling (Electric Field Shielding of Magnetic Loops is Not Always Effective!) * July 2008, The Square Shielded Loop - Part 3, Parasitic Coupling Between Unshielded Wire Loops * August 2008, The Square Shielded Loop - Part 4, Coupling to a PCB (From Shielded and Unshielded Magnetic Loops) Also see my IEEE EMC paper relating to this effect: http://emcesd.com/pdf/emc99-w.pdf See pages 560 and 561 of the paper. Doug - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
