[RFI] Looking for explanation...

[Jim Brown]

On 6/26/2020 10:30 AM, Gary Peterson wrote:
> Next, he plugged the treadmill, via an extension cord, into a different outlet, without the EMI filter.  No interference whatsoever was heard or seen on the waterfall.

Several possibilities. Question: Did the extension cord carry the Green 
wire? Could one outlet have its wiring in steel conduit but not the other?

AC line filters provide maximum benefit when they are located right at 
the noise source AND have their case bonded to the case of the noise 
source. And even then they don't always work because the Green wire may 
not be bonded within the equipment.

I ALWAYS recommend winding a common mode choke carrying all three 
conductors -- phase, neutral, and ground -- through one or more 
Fair-Rite #31 toroids. Use the winding guidelines in my latest (2018) 
transmitting choke cookbook as a starting point. Or build a choke like 
this one that W6GJB built to my design for our county expeditions. It's 
in the new ARRL Handbook, and on page 23 of http://k9yc.com/RFI-Ham.pdf
Note that it should NOT be in a metallic enclosure, because that would 
detune the chokes, AND because connectors bonded to the enclosure would 
short out the choke on the Green wire, which is usually what is carrying 
the current that radiates.

What the choke does is prevent AC wiring from acting as a TX antenna for 
the noise.

73, Jim K9YC