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  • Estimation of magnetic field strength of radiated immunity test

    From Klaus Vestergaard Kragelund@21:1/5 to All on Fri Jun 28 00:44:25 2024
    Hi

    Normally when testing a product for radiated immunity one would apply a
    field of 10V/m at a distance of 3m, to determine if the product is
    susceptible to external incoming radiated fields.

    In my specific case, we have a 3MW converter which has quite high
    internal fields, so I am doing an alternative test.

    In this test, I have measured the magnetic field at a certain point in
    the cabinet next to the sub-block I am working on. That way I know the
    magnetic field at this point, and the idea is then when we need to go to
    the approval body to test radiated immunity, that we will set the level
    of the antenna to match the recorded level next to the sub-block.

    Specifically I have used a near field probe to and measured a H field of
    30A/m at this sub-block at 1MHz.

    The near field probe voltage was recorded with a scope and in my lab I
    wound a coil and ran a 1MHz signal through the coil until I got the same
    level, then calculated the field with H = Iapplied*nturns/coilheight

    In the approvals test, the antenna is 3m away. Normally they use a level
    of 10V/m and for far field approximation using 377ohm, the H field from
    the antenna is H = 10V/m / 377ohm = 0.03A/m.

    At 1MHz, the wavelength is far above 3m, so the antenna will be in the
    near field, so the resistance is 60ohms, and the field decays with 1/r^2 distance since it's close to a magnetic field.

    Anyway, it seems the field needed at the antenna for this test would be
    VERY large to generate H field of 30A/m 3meter from the antenna.

    How do I calculate that level and any inputs into if my reasoning above
    is correct?

    I know that when you move into the near field, it's hard to predict the
    far field level, but in this case is seems since I am operating at 1MHz
    that both the antenna and DUT is in the near field...

    Thanks

    Klaus

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  • From Klaus Vestergaard Kragelund@21:1/5 to John R Walliker on Fri Jun 28 22:04:10 2024
    On 28-06-2024 08:37, John R Walliker wrote:
    On 27/06/2024 23:44, Klaus Vestergaard Kragelund wrote:
    Hi

    Normally when testing a product for radiated immunity one would apply
    a field of 10V/m at a distance of 3m, to determine if the product is
    susceptible to external incoming radiated fields.

    In my specific case, we have a 3MW converter which has quite high
    internal fields, so I am doing an alternative test.

    In this test, I have measured the magnetic field at a certain point in
    the cabinet next to the sub-block I am working on. That way I know the
    magnetic field at this point, and the idea is then when we need to go
    to the approval body to test radiated immunity, that we will set the
    level of the antenna to match the recorded level next to the sub-block.

    Specifically I have used a near field probe to and measured a H field
    of 30A/m at this sub-block at 1MHz.

    The near field probe voltage was recorded with a scope and in my lab I
    wound a coil and ran a 1MHz signal through the coil until I got the
    same level, then calculated the field with H = Iapplied*nturns/coilheight

    In the approvals test, the antenna is 3m away. Normally they use a
    level of 10V/m and for far field approximation using 377ohm, the H
    field from the antenna is H = 10V/m / 377ohm = 0.03A/m.

    At 1MHz, the wavelength is far above 3m, so the antenna will be in the
    near field, so the resistance is 60ohms, and the field decays with
    1/r^2 distance since it's close to a magnetic field.

    Anyway, it seems the field needed at the antenna for this test would
    be VERY large to generate H field of 30A/m 3meter from the antenna.

    How do I calculate that level and any inputs into if my reasoning
    above is correct?

    I know that when you move into the near field, it's hard to predict
    the far field level, but in this case is seems since I am operating at
    1MHz that both the antenna and DUT is in the near field...

    Thanks

    Klaus

    Why are you testing at 1MHz when radiated immunity testing is normally
    only done at frequencies higher than 80MHz?

    Correct, radiated immunity is from 80MHz to 6GHz. We know that the
    field, 10V/m is smaller than the internal EMC. So we detect the largest possible disturbing signal frequencies, and replicate the EMC test,
    monitoring the performance of the DUT.

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