how can I recognize measured fields whether reactive or radiative field from a near

[doooo]

When I measure near field using a near field probe from antenna, PCB, ICs etc. (simply H-probe).
I can see some peaks through Spectrum Analyzer(SA) according to clock frequency (in case of digital ICs).
By the way, if the measured field is reactive field it may not be harmful as a EMI source because the field will be bounded near the component. In contrast, if the field is radiative field it must be a EMI source.
Normally, if peaks which level is higher than a certain level are shown on the screen of SA, we determine the component can not be passed becuase of EMI regulation.

So, my question is how can I recognize measured fields whether reactive or radiative field from a near field measurement results?

Thanks.

 

[vfone]

In reactive near field the electric and magnetic fields are not orthogonal and any object placed in this region will distort the antenna pattern.
This is one clue that could give information where this field stops when moving far from the antenna. For doing this, have to do a far-field antenna pattern measurements, before and after placing an object.
Second option (which is relative easier to implement) is, because in reactive near field the coupling is totally inductive, means that any changes of the impedance of the coupled device would change the impedance of the antenna.

 

[doooo]

In reactive near field the electric and magnetic fields are not orthogonal and any object placed in this region will distort the antenna pattern.
This is one clue that could give information where this field stops when moving far from the antenna. For doing this, have to do a far-field antenna pattern measurements, before and after placing an object.
Second option (which is relative easier to implement) is, because in reactive near field the coupling is totally inductive, means that any changes of the impedance of the coupled device would change the impedance of the antenna.

Thanks, you gave me a intuition with 'orthogonal' !!
Could you explain more about 'reactive near field the coupling is totally INDUCTIVE' ??

 

[vfone]

Totally inductive I mean there is almost no capacitive coupling, and predominates only the inductive induction (or magnetic coupling).
For example NFC is based on this kind of coupling situated in the reactive near field of the antenna. Also NFC use "Load Modulation" which is based on the principle that changing the impedance of the coupled device (receiver) would change the impedance of the antenna (transmitter).

 

[doooo]

It is almost same as the principal of wireless power transfer.
So, although the measured field is reactive field the field would be a EMI source?

 

[jiripolivka]

It certainly will. The near-field reactive fields, however, do not transfer energy= this is why they are named reactive. But when a foreign object like a measuring probe is present, these reactive components couple a real power into the structure.
The general advice for measurement procedures is to avoid the "near-field" zone, if an antenna is tested on an open range or in an anechoic chamber.
Local reactive components cause a huge interference (and EMI) if a CW signal is used. Use noise if you want to map the local fields, and a wideband test system.