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From: John R Walliker <jrwalliker@gmail.com>
Newsgroups: sci.electronics.design
Subject: Re: Estimation of magnetic field strength of radiated immunity test
Date: Fri, 28 Jun 2024 07:37:05 +0100
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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