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We are talking about devices that are designed to pump out quite a bit of power in the MHz. Admittedly the chips have comms protocols so they don't spew all the time, and the resonant circuits are very high Q, using the near field. However, might there be any problems in an electronics development lab?

How far do these fields extend in a measureable sense?

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closed as unclear what you're asking by Chris Stratton, Elliot Alderson, Voltage Spike, Edgar Brown, StainlessSteelRat Mar 1 at 17:45

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  • \$\begingroup\$ If you are doing work where this might be an issue, presumably that work means you have the gear to measure this. And probably the expertise to make a quieter supply if you find you need one. And you'd also be thinking about the phone radio signals... it will do network housekeeping even when it is not being "used". \$\endgroup\$ – Chris Stratton Feb 27 at 16:54
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    \$\begingroup\$ I don't think this question is answerable. No one can possibly know the RFI and EMI characteristics of every possible USB power supply (I assume that's what you really mean). If you are concerned, banned the suspect devices. \$\endgroup\$ – Elliot Alderson Feb 27 at 17:01
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    \$\begingroup\$ There is no way to know how a "silly person" can screw up your sensitive measurement with any equipment which can emit EM waves. So indeed a simple ban is clear for everyone. Of course everything is relative, in a lab where drives for electric locomotives are tested the EMI from a wireless charger is probably insignificant. In a lab where you're testing low frequency sonar equipment it's a whole different story. \$\endgroup\$ – Bimpelrekkie Feb 27 at 17:05
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    \$\begingroup\$ Even if there were standards, how could you be sure that everyone purchased USB supplies that met those standards? What happens if a compliant devices is later damaged so that it radiates like mad but still provides 5V? \$\endgroup\$ – Elliot Alderson Feb 27 at 17:07
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    \$\begingroup\$ I have a wireless phone charger on my desk. It interferes with the nearby VGA cable or the monitor, creating stripes and lines and things. \$\endgroup\$ – tomnexus Feb 27 at 20:20
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How far do these fields extend in a measureable sense?

That's a hard question, because I don't know what you're measuring with, and for how long!

But: if properly designed, these are (luckily) not radio emitters, but just the primary side of a transformer. Magnetic flux (what's measured in Tesla) drops with the square of distance, but what really matters is that Gauss's Law for magnetism says that the integral over any closed surface, for example the one enclosing the charger with phone in operation, is zero.

That really means that no flux escapes – just like you putting a permanent magnet on your fridge doesn't have any electrical effects on your phone (unless you move any part of this).

Energy could be radiated if the sizes involved would allow for a perpendicular E-Field to form. They don't, usually – but as Elliot said, don't trust anything to be perfect. Especially, the switch-mode controllers for these transformers won't produce perfect cosines; expect harmonics that also couple into cabling and if high enough radiate. Again, up to the sensitivity of your measurement, whether you notice. But: these are exactly the same problems that any SMPS has, as Elliot pointed out!

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