Jim Brown k9yc at audiosystemsgroup.com
Tue Dec 1 12:11:31 EST 2015

On Mon,11/30/2015 12:21 PM, Tony Brock-Fisher via CQ-Contest wrote:
> Most modern appliances now use switching power supplies to control motor speed,

Yes, these "variable frequency drives" (VFD) use modulated square waves 
to control the speed of motors, and are well known to be MAJOR 
generators of RF noise. I first became aware of them about 20 years ago, 
when a colleague discovered that the VFDs that ran the elevators in a 
major performing arts center were coupling trash into the audio system.

It seems to be a rule that these systems are badly designed, with strong 
loop currents in large loop areas generating strong magnetic fields. The 
key to doing it right is to make the loop areas very small. Sadly, the 
designers of these systems don't have a clue.

73, Jim K9YC

More information about the CQ-Contest mailing list