[RFI] LG 12,000 BTU Model Advice

[Jim Brown]

On 2/1/2023 9:54 PM, Dave (NK7Z) wrote:
> Hi James,
> 
> If it were me I would do the following:
> 
> Visit a store which has the exact same AC unit I am interested in, in 
> stock.  I'd then ask to turn it on.  I'd bring a portable shortwave 
> radio with me, and a very short antenna on a coax cable. 

Great advice -- IF (and it's a VERY big IF) there aren't dozens of other 
stuff in the showroom making noise. Also, the radios suggested in my RFI 
tutorial may be a better choice. The TH-F6A has a loopstick in the base 
that's the default below 10 MHz, the SMA connector defaults above 10 
MHz, and a menu setting can toggle between them. The Tecsuns are also 
very nice radios. I would do my probing on the AC cable right at the 
unit, and on any other cables that might be present.

I would then
> probe for RFI, around the AC unit, and along the power cord of the unit, 
> while turned on, and while turned off.  Once I had a feel for the unit, 
> I would unplug it to see if anything changed...  If I found no RFI, or 
> very little RFI, I would buy that unit on the spot, and make sure I got 
> the one I was looking at, not a different one.
> 
> If I heard a little RFI, I would not worry too much about it, "a 
> little", is a relative term...
> 
> I would also bring a number of Mix31 ferrites with me that will go 
> around the AC cord of the AC unit, and drop them on at the AC unit where 
> the AC cord enters the AC units case. 

The weapon of choice would be the 1-in i.d. #31 clamp-on, and for HF 
noise, I'd carefully wrap 3-5 turns thru the core, being careful to wind 
the turns in sequence around the core (no crossovers). I'd listen on 
both sides of the choke -- noise should die moving away from the choke. 
Also, very important -- bring a suitable extension cord at least 20 ft 
long to use between the A/C and the outlet to simulate house wiring, and 
repeat the test, listening as you move the probe along the cable all the 
way to the outlet. If you do hear noise, the choke helps but you still 
hear it, add a second choke on the cable from the A/C, not on the 
extension cord.
  I would then listen to how much
> RFI I pick up on the AC power cord, and in general, all around the unit. 
>   Again, if little to no RFI...  I'd buy it.
> 
> If I discovered serious RFI at any of these stages, I'd move on...

A primary source of noise from anything with a motor is a variable-speed 
controller for it. I suggest avoiding any such animal, which operates by 
varying the speed or pulse width of a string of rectangular waves. In 
addition to being radiated from the AC cable, it often escapes by 
failure to treat the wiring carrying that pulse train as a transmission 
line, so the noise escapes both as a magnetic field and as an E-M field 
(that is, antenna action).

73, Jim K9YC