Re: Estimation of magnetic field strength of radiated immunity test

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?
John