[RFI] RFI in the HF spectrum from variable speed Mitsubishi HVAC systems

[Jim Brown]

On 6/22/2014 9:34 AM, Karin Johnson wrote:
> the egress of
> wiring will couple RFI out of the enclosure
> and if the loops of wiring create resonance in the ham bands we hear it.

Several comments. First, common mode current on wiring external to 
equipment is one of the most common mechanisms.

Another common cause is magnetic coupling from the field established by 
noise current that returns via a "ground" path rather than the neutral. 
That common mode current usually appears on external wiring as the 
result of poor design inside the box(es).

It is quite common for the pulsed motor current to be carried on 
external wiring from a motor controller. That wiring should ALWAYS be 
twisted pair (or twisted triplet), with the ground conductor, if there 
is one, bonded to the shielding enclosure at both ends.

Your use of multi-turn #31 common mode chokes was the right solution. 
Their clamp-on, if a single turn, probably did nothing below about 50 MHz.

In general, even the best AC line filters do nothing to kill common mode 
current unless they are installed inside equipment, with both their case 
and the power line green wire bonded to the shielding enclosure. Indeed, 
the same result would be likely without the filter, but with the green 
wire properly bonded.

As to the thermostat -- I would use the same sort of multi-turn choke on 
#31 and a good unshielded twisted pair cable.

73, Jim K9YC