Fellow RFI folk:
I walked into the shack this morning and started listening on 30 meters. A
familiar and unpleasant sound emanated from the speaker.
As I tuned across the band I could hear the swept buzzing sound. On the
IC-756ProIII's bandscope, the buzzing sound presented itself as a pair of
wide squarish pulse-peaked waveforms. I had seen and heard this before.
Curious, I tuned the receiver down to 1.8 MHz. The noise was loud and clear
with the same waveform. Checking all ham bands up to 15 meters revealed the
same sounds and sights.
In September of 2006, I had experienced the same unpleasant discovery.
Using a simple portable shortwave receiver I was able to determine that the
interfering signal was originating from my next door neighbor's home. My
neighbors and I get along very well. They allowed me into their home and I
walked around for a few minutes and noted that the interference got
stronger in the living room where a mobility scooter was parked and plugged
into a wall outlet. My neighbor told me that the chair was purchased from a
friend only the day before. I unplugged the chair and the interference ceased.
The chair is made by Pride Mobility Products Inc., an international company
based in Exeter, PA. Its battery charger is integrated into the scooter's
interior. The user needs only to plug a standard AC power cord into a wall
outlet and let the chair sit for 8 hours or so to recharge the onboard
battery. The battery charger is a switching power supply that produces a
pulsing, sweeping interference that can be heard 200 feet away or more on
frequencies that span approximately 2 to 30 MHz.
There was no way that I was going to try to do anything to the chair or its
internals. I figured the power cord had to be radiating the interference. I
decided to make a new power cord for my neighbor.
I took an 8 foot long standard power cord and wound it through 2 Fair-Rite
2631803802 Type 31 material Ferrite "donuts." I wound the cord through the
first core and secured the cord with cable ties. I wound the cord through
the second core so that the two cores were very close together. The
resulting "filter" was a total of about 1 foot long so I plugged it into a
very short extension cord I had. I went back to the neighbor's house and
plugged the scooter back into the wall with my new power cord/filter while
monitoring on my portable receiver. I could still hear the interference but
it was so weak I lost the signal after I walked a short distance away. The
neighbor appreciated the fact that I was able to solve the problem at no
cost and no inconvenience to them.
When I returned to the rig, the bands were clear and all was well. That was
last September 2006. This morning, it was deja vu all over again. I
wondered, "Had my next door neighbor abandoned my filter/cord?" A quick
walk next door with the receiver clearly showed that the signal was NOT
coming from there. A short walk across the street with a directional
antenna connected to my Icom T90a conclusively revealed the new source. I
didn't trespass. Since I knew for certain which house was the source I
decided to leave a note asking the homeowner ( a neighbor I didn't know )
to call me to discuss the matter. Not having any paper or a pen with me, I
went home to compose my note.
When I walked up the neighbor's driveway and approached the porch and front
door, there it was. Another Pride Mobility Products mobility scooter parked
on the porch with a 15 foot extension cord plugged into a wall outlet. I
smacked my forehead, left the note in the door and went home hoping this
neighbor would be cooperative. An hour later the phone rang. The neighbor
identified himself and I asked him if he had just taken delivery of the
mobility scooter I saw on the porch. He had. I explained to him that when
plugged in to the wall outlet, his scooter created severe interference to
my radio. He unplugged the chair and the noise disappeared from my
receiver. I was lucky. He understood what was happening and agreed to allow
me to install a filter/cord on his unit.
Once again I prepared a 2 core AC power cord and trekked across the street
with a handheld radio. I demonstrated the interference to my neighbor by
having him plug the chair in, listen and unplug the chair. He was visibly
surprised at the potent level of the scooter signal. We installed my
filter/cord and once again the noise was audible but only near the scooter.
Once I left the porch the signal was inaudible. Back in the shack the
receiver was free of the plague.
I chose the ferrite cores mentioned above because I had just taken delivery
of 24 of them from Dexter Magnetic Technologies a short time before the
September 2006 encounter occurred. I ordered these specific cores based on
the recommendation of a note I saw posted on the RFI reflector. They have
proven to be very useful! Details about the cores can be found on page 71
of the 15th edition of the Fair-Rite downloadable catalog. Here's a link to
Dexter that will allow you to get the catalog:
These round cores measure 2.4 inches in (outer) diameter with a 1.4 inch
circular opening and they are .5 inches thick/deep. I can wind a standard
AC power cord around and through the core about 6 times. The size of the AC
plug will determine how many times you can wind through the core. 2 cores
work effectively. Adding one or two more doesn't appreciably improve signal
Of course, the manufacturer, Pride Mobility Products, Inc. of Exeter, PA is
at fault for allowing such a negligent battery charger design to be used in
their products. I haven't approached the manufacturer about the
interference because I don't think it would do any good. They probably
don't get enough complaints to give the matter serious consideration.
Ultimately I blame the FCC for no longer enforcing rules and regulations
that would insure proper designs.
As an interference investigator for a large electric utility, I have run
across a wide variety of consumer electronic devices that produce
interference to the AM Broadcast band and HF spectrum. Locating these
chairs was easy. Luckily solving the interference problems was equally
easy. I suppose I would consider being more aggressive with the
manufacturer if the problem were more difficult and costly to correct.
Experience is showing me that users of the HF spectrum will face a
veritable tidal wave of interference in the years to come as manufacturers
continue to shave cost by eliminating those few parts that make a product a
better electronic citizen.
Just in the last year, I have encountered televisions, cordless phone
battery chargers, elevators, sprinkler timers and other unexpected
electronic devices that have produced potent interference on HF. Nearly all
of these have switching power supplies. It is becoming apparent poorly
designed switching power supplies have become the new electronic
interference epidemic. This problem is only going to get worse. Power line
interference represents only a small percentage of the sources I locate. I
predict that the most popular guy in the local ham radio club will be the
one who can find and identify these HF interference sources because they
will become ever more numerous and annoying.
I'm posting this so that those who search the archives will have at least
one reference to mobility scooters as a source of interference and a simple
solution that can be used to solve the problem. I suspect that this
solution will work for many types of switching power supply interference
Frank N. Haas KB4T
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