[aprssig] Persistent EMI/RFI Problem Solved (Stephen H. Smith)

[Dave B]

Hate to say it, but leaving a SMPS idling with no output load is asking 
for trouble, and almost guaranteed to cause such odd EMI issues.   Even, 
if the same SMPS is clean(ish) when working into a load (check it just 
in case.)

The difference in radiated/conducted EMI between continuous and 
discontinuous conduction modes, combined with it's controller no doubt 
seeing over voltage events at the output so cycling the converter on/off 
in a random way.

You might find an improvement by aggressively filtering the input to the 
converter, but the only cure is to power it down when not used.

This is a common problem, especially with AC powered (wall wart) battery 
chargers for things like power tools and golf trolley’s, invalid carts etc.

And don’t get me started about low voltage LED lighting!   You'll be 
amazed at how far up the spectrum the EMI from that technology spreads 
at times.

Especially a problem, when the original high current electronic 
"Ballast" has not been replaced with one more suited to the much lower 
currents the LED's use.   Again, SMPS's working outside their intended 
operational mode, as a result, any EMI mitigation filtering is often 
rather ineffective.

Lastly, always go EMI hunting with a mk1 AM RX.   Unless it's a clean 
carrier, then SSB/CW mode is the order of the day.

73.

Dave G0WBX.