Hi Everyone,
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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.
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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.
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Here are some articles I have written on the subject.
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* 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:
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http://emcesd.com/pdf/emc99-w.pdf
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See pages 560 and 561 of the paper.
Â
Doug
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