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All electromagnetic compatibility standards around the world for immunity to electromagnetic fields (radiated susceptibility test) mention the strength of electric field (V/m), but not the strength of magnetic field (T). An EM wave will have both fields while traveling in free space, so why do standards only mention electric field strength?

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ocrdu
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Ronnie
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    In the far field, the two fields are coupled, and the strength of one describes the strength of the other. – Kuba hasn't forgotten Monica Sep 07 '22 at 18:31
  • The standard should specify how far from the DUT the field generator will be placed. 3 meters is common iirc. Also 20V/m is a summer breeze compared to some field strengths required by industrial and military standards :) – Lundin Sep 09 '22 at 12:02
  • @Lundin Yes. 3m or 10m are mostly for automotive. Like MIL standard requires 100V/m with 3 meter distane. – Ronnie Sep 09 '22 at 14:41

3 Answers3

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EM wave will have both the fields while traveling in free space then why standards mention only electric field strength

Because the amplitude of the E-field (the electric field strength in volts per metre) is 120×π larger than the H-field (the magnetic field strength in amps per metre) in a vacuum or air when measuring in the far field.

Andy aka
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  • Why only magnetic field amplitude gets reduced by 120 x Pi and not both in free space ? – Ronnie Sep 08 '22 at 06:23
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    This answer is interesting, but requires more details. I assume the value is related to https://en.wikipedia.org/wiki/Impedance_of_free_space . Also comparing V/m vs. A/m, it also depends on the measurement setup what the sensitivity is. – jpa Sep 08 '22 at 06:48
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    @Ronnie the ratio of E to H is 120 pi. Both get attenuated with distance in free space but the ratio remains the same. – Andy aka Sep 08 '22 at 07:22
  • @Andyaka Got it . Thanks – Ronnie Sep 08 '22 at 09:57
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They're not -- IEC 61000-4-8 Power Frequency Magnetic Field Immunity Test for example is specified in terms of A/m. (Well, if I ignore the parenthetical specifier, anyway.)

Most RF probably specs chose V/m out of consistency, i.e., to choose one unit and avoid confusing conversion steps. Possibly there exists meeting notes concerning this choice (if anyone can chime in with these, that might be illuminating).

Tim Williams
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  • Power frequency magnetic field test is near field test to DUT ? – Ronnie Sep 09 '22 at 14:44
  • Like I said, if I ignore the "*(radiated susceptibility test)*". :) (Arguably this is an accurate answer; if you meant to stipulate that class of tests, I'd expect it not placed in parenthesis, or placed in the title as well.) – Tim Williams Sep 09 '22 at 19:33
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In the far field, the electric field strength is directly linked to the magnetic field strength via the "free space impedance" of ~377 Ohms. So by specifying the electric field strength the magnetic field strength is specified implicitly.

feynman
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