Hi All, A few years ago (2008, which is equivalent to yesterday for a 77 year old like myself) I decided to take some measurements to make up a tutorial for young engineers to show why they should use shielded type magnetic field probes as opposed to unshielded ones. What I ended up proving is that most of what I had been told about these kinds of magnetic field probes was actually wrong. The result is that the "shielding" of magnetic field probes only works if the field is symmetrical around the center line of the probe's loop, which is satisfied for plane waves (10x further from the source of the field than the size of the source of the field). In the near field, the shielding not only does not work (a shield does not keep "things" out, it is simply a transformer with the shield as the primary and the center conductor as the secondary as I show in my classes) but causes a really strange frequency response with a 30 dB dip in sensitivity at a much lower frequency than what should be the bandwidth of the probe.
In general, shielded magnetic field probes should not be used in the near field, close to a PCB for instance. They will have a really strange frequency response. If one worries about E-field sensitivity in an unshielded probe, it is generally very small and can be checked in three seconds by simply flipping the probe 180 degrees and observing the result. All this is covered in the three articles and a paper linked below on my website. * June 2008, The Square Shielded Loop - Part 2, Parasitic Coupling<https://emcesd.com/tt2008/tt060208.htm> (Electric Field Shielding of Magnetic Loops is Not Always Effective!) * July 2008, The Square Shielded Loop - Part 3, Parasitic Coupling Between Unshielded Wire Loops<https://emcesd.com/tt2008/tt070508.htm> * August 2008, The Square Shielded Loop - Part 4, Coupling to a PCB<https://emcesd.com/tt2008/tt080108.htm> (From Shielded and Unshielded Magnetic Loops) * Signal and Noise Measurement Techniques Using Magnetic Field Probes<https://emcesd.com/pdf/emc99-w.pdf> (~600K) * (1999 IEEE EMC Symposium paper) If you ever are near Boulder City, Nevada (in between Hoover Dam and Las Vegas) stop in for a tour of my facilities! If you like, I will take you on a sightseeing run (2 to 10 miles) of the area, possibly up and down mountains. Bring your running gear.... I have been running for 50 years now, running 3,100 miles last year. 1305 Arizona St., #101-103, Boulder City, NV 89005. Doug [https://lh4.googleusercontent.com/_HuR3Ky2TF_XhFHyxnYRmiq7nHQldnMsPNYFaLG6kb5T4y8MeCe-BDC_BscJtSFgszSSjssihHS-pjM3-jwNP8S0CwE-gN8fsRsPkojiAlmpBwb20vIVizS-siCUywW_jqrefbVr] - ---------------------------------------------------------------- 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 [email protected] All emc-pstc postings are archived and searchable on the web at: https://www.mail-archive.com/[email protected]/ Website: https://ewh.ieee.org/soc/pses/ Instructions: https://ewh.ieee.org/soc/pses/list.html (including how to unsubscribe) List rules: https://ewh.ieee.org/soc/pses/listrules.html For help, send mail to the list administrators: Mike Sherman at: [email protected] Rick Linford at: [email protected] For policy questions, send mail to: Jim Bacher: <[email protected]> _________________________________________________ To unsubscribe from the EMC-PSTC list, click the following link: https://listserv.ieee.org/cgi-bin/wa?SUBED1=EMC-PSTC&A=1
