Can you provide me with some theory in 3m method?

Can you provide me with some theory in 3m method? I am in urgent need of it. Thank you

Hi there,

It is quite ambituous to expect an answer on this question...
I have no clue what you are talking about. You need to give a bit more explanation on what you are looking for otherwise we are guessing what you want :|

You are reffering to 3m EMI measurment method?
//a

Yes, I am referring to 3m EMI measurement method. Because i wonder why 3m was selected,not 1m or 2m. What are its advantages?

Hi

Normally fieldstrength measurements should be done in the far-field because in the near-field the fieldstrengths are changing more rapidly and are less accurate.
100 MHz corresponds to a wavelength of only 3m.
Therefore measuring at 100 MHz and 3 m distance is at 1 wavelength which is still pretty close and will result in free space not to be considered 377 ohm but less.
Far field starts more from 1.5 ... 2 wavelengths.
For a few 100's MHz you can indeed consider the field to be more far-field.

Some FCC stds refer to a distance of 10meters which i even more in the far field for eg 100 MHz. The disadvantage is that of course you measure much less voltage at 10m then at 3m.

But if you want to check EMI or fieldstrengths at 900 MHz or 2.4 GHz you can measure at closer distances and therefore have more voltage on your recvr/analyser. One can also skip the more expensive preamps. In our lab I measure often at 1 meter for quick verifications although it is not standard and I then recalculate to 3 meters.

Hope this satisfies your question... you can also check the very basic std from IEC , I think is IEC106 on radiated emission measurements.

Kind regards
Mike