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From: Klaus Vestergaard Kragelund <klauskvik@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Estimation of magnetic field strength of radiated immunity test
Date: Fri, 28 Jun 2024 00:44:25 +0200
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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