It looked like most of the noise was leaking out through
the AC input of the LiftMaster units so I decided to
start there. Fortunately, my neighbor (Dave - K1GQV) had
a couple of Mix 31 torroid donuts left over from a
project last summer and he passed them on to me. The Mix
31 ferrites are useful as cores for chokes that can
block RF between 1 MHz to 300 MHz so they are a good
choice for blocking noise on the 160 meter band that was
passing into the AC line. I grabbed some unused AC cords
that were left over from old computers and chopped off
the ends that plug into the computer power supply. Then
I passed the wire through the cores a dozen times and
terminated the end of the cords with a 3-pin AC socket.
They looked neat and would easily handle the maximum 6
amp draw of the LiftMaster units when they were moving
the doors.
Fair-Rite Mix 31 Round
Cable Core available at Mouser: part# 623-2631803802
priced @ $4.89 each (October 2019)
When I installed the Mix 31 filters between the AC
outlets and the LiftMaster units the noise on 160 meters
was almost completely eliminated. In fact, it was only
barely visible on the receiver's panadapter during the
daytime when the band was empty and was definitely clean
enough that it would not be heard at all in the evening.
However, the AM BCB was still being obliterated, mostly
at the low end of the band. I ended up buying two Mix 75
torroids from Palomar as their useful RF blocking range
is from 150 kHz to 5 MHz. I had two cheap 3 wire
extension cords from Walmart that were very flexible and
so I cut 3 feet off one end of each cord and wound them
ten times through the mix 75 torroids. I terminated the
cut ends with a 3 pin AC socket.
.
Mix 75 cable core
available at Palomar Engineers: part# FT240-75
(RFI Range 150 kHz - 5.0 MHz) priced at $12.95 each
(October 2019)
I installed these filters in series with the Mix 31
filters and hoped that the overlap of coverage would be
effective. As it turns out, the series filtering is
extremely effective with the AM broadcast band now
perfectly clean on every AM radio in the house and the
160 meter band shows no sign of any noise from the
LiftMaster units. I had some Mix 31 split cores left
over from another project so I ran 3 turns of the wire
bundle for the safety sensors and the hard-wired up/down
motor controls mounted on the wall through a Mix 31
split core. I don't think that was really necessary as I
couldn't detect any noise on those wires but there's no
harm in putting them inline. One of the filtered
LiftMaster installations can be seen in the picture
below.
My final thoughts on eliminating the noise produced by
the LiftMaster model 8155W units:
1. If you don't listen to the AM broadcast band
and have no interest in the 160 meter band it looks like
these units are quiet on the 75 meter band and above
without adding any filters.
2. If you don't listen to the AM broadcast band
but 160 meters is important you can probably get
satisfactory results on 160 meters by installing only
the Mix 31 filters.
3. If you listen to the AM broadcast band and use
160 meters install series connected Mix 31 and Mix 75
filters on each unit. This produces the cleanest results
across the MW/HF spectrum.
The short 35 second video below shows what the 160 meter
band looked like with the LiftMaster units unplugged,
then how the band looked with the unfiltered LiftMaster
units powered up on standby, and finally how it looks
now with the filters installed and the LiftMaster units
powered up on standby. I don't know for certain what is
causing this particular RFI problem, however, from what
I have read there are garage door openers out there at
this time that use PWM motor controllers. The
implementation of PWM controllers in current HVAC
systems has caused lots of noise for hams and it's
certainly possible that my smooth-running LiftMaster
units are using PWM motor controllers in the 8155W
contractor-grade models installed here.