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# Near field antenna to tell if radiated emissions are better or worse after modification to circuit?

## Near field antenna to tell if radiated emissions are better or worse after modification to circuit?

(OP)
Hi,
We have an SMPS in an earthed metal enclosure A, which is wired to another SMPS (12W) in another earthed enclosure B.
B contains 4 SMPS chargers of 3W each. A is totally enclosed, with only a few holes of dimension much smaller than the wavelength of the problem frequencies of 50MHz to 200MHz.
B is half open (so people can take out the chargers).
We want to adjust the Y caps in the SMPS in A.
We then want to see if this makes the radiated emissions better or worse.
Can we do this simple analysis with a near field probe connected to a EMC Analyser eg SA1002A by Laplace instruments?

SA1002A

Should we use a B field near field probe, or an E field near field probe?

**..**..**..**..**..**..**..**..**..**..**..**..**..**..**..**..**..**..
Here is a better description of the system...

Please help our kiddies post-lockdown pager system game kit pass radiated EMC? I am doing this with a company in South Africa for charity. Its as attached.
It is an earthed metal enclosure A, which contains a 20W offline SMPS and a 12V battery. This enclosure feeds a 12V cable to enclosure B which contains a 10W , 12V to 5V SMPS. This 5V is then fed to each of four removable pagers, (so they get charged up) which the kiddies can remove and talk to each other. (they will be in a “hospitals and doctors” game)
It fails EMC at the moment. (Radiated emissions failure at frequencies from 80MHz to 190MHz) We wish to sell it into the EU, where they need some post-Covid relief as much as everyone else.
Should the cable between A and B be earth screened and the earth screen connected to both enclosure A and B?
The enclosure B is open at the front, so is it therefore a waste of time to earth enclosure B? As such, is it also a waste of time to take earthed 12V cable from A to B?
The wires running from the 12v_to_5v SMPS to each pager, should they be loomed as close as possible to the walls of enclosure B?
The enclosure B has to be open at the front so that kiddies can grab the pagers.
We are thinking we may need a common mode choke at the point in enclosure A where the 12V cable leaves A to go to B? Also, we could put a common mode choke at the input and output of the 12v_to_5v SMPS. (NiZn torroid based common mode chokes with just a few turns) We could also put a flux band round the offline flyback transformer in A. We could also increase the Y capacitor across the flyback transformer in A. At the moment its only 100pF. We don’t want PCB respins if poss as the customers don’t want to pay much.

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### RE: Near field antenna to tell if radiated emissions are better or worse after modification to circuit?

(OP)
It is well known that passing radiated emissions to EU standard with an SMPS that is not surrounded in a metal enclosure is pretty well impossible. Even if it is in a metal enclosure, it will still fail radiated emissions if a cable leaves the enclosure to go somewhere else. (as in the attached). In that case, a common mode choke and Y caps will be needed in order to stop the cable from radiating. Would you agree?

### RE: Near field antenna to tell if radiated emissions are better or worse after modification to circuit?

"It fails EMC at the moment. " - that's good, you have the first step, as long as you know how badly you are failing (X dB). I often use a near field probe to do before and after comparisons before I take a unit back to the test lab (\$). If your mod shows an X + 3 dB or better improvement you are probably okay, more margin is better.

Yes, chokes on cables are almost always required for EMC.

Z

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