[RFI] 30m noise problem

Frank N. Haas KB4T kb4t at arrl.net
Thu Apr 22 12:56:31 PDT 2010

I had exactly the same problem. My source turned out to be the switching
power supply that powered the battery charger in my next door neighbor's
mobility scooter or power chair. 

I used a simple AM/FM/SW radio (Eton E5) and found that the signal was
strongest at my next door neighbor's house. We get along great with our
neighbors and they gladly let me walk through their home with the portable
radio. When I got to the living room the signal strength pegged the meter on
the radio and there was the chair. 

My neighbors agreed to use a simple filter I made from 2 ferrite donuts and
a 10 foot long specialty AC power cord (an extender to the standard IEC AC
power cord). The filter plugs into the AC power jack on the chair and the
chair's regular AC power cord plugs into the other end of my filter. Cable
ties kept everything together. The interference was reduced to a level low
enough that I never heard it again. 

6 months later, I had exactly the same problem AGAIN!!! This time my walk
around the neighborhood with the same radio showed the interference coming
from a neighbor's house across the street and two doors up. As I approached
the house, there was ANOTHER mobility scooter this time prominently parked
on the front porch plugged in and charging. I built another filter and quiet
returned to my rig.

The switching power supplies in these power chairs are potent broadband
transmitters. The signal warbles a bit and drifts up and down slightly but
they can be heard several hundred feet away from any decent ham radio
antenna especially if its resonant in any way on 30  meters. 

The filter consisted of 2 type 31 2.5 inch Inner Diameter toroid "donuts"
with the lengthy AC cord wound around first one toroid and then the next
with both separately wound toroids immediately adjacent to each other. This
type of filter reduced the level of the "transmitter" output by 30 dB (more
or less) and prevented me from hearing anything from the chairs. 

Of course the chair manufacturers would not even acknowledge any
communications to them inquiring about interference problems or suggested
fixes or kits. They chose to use the "ignore it till it goes away" approach
to these pesky inquiries. 

As I have said many, many, many times: Today's amateur radio operators must
become skilled direction finders if they hope to continue enjoying the use
of the HF spectrum and 6 meters. The HF spectrum is under siege from a
plague that has no end in sight. 

Not only must our technical skills be honed to perfection but our human
relations skills will need to be refined as we are forced to educate the
unknowing consumer population who neither cares nor is interested in
resolving the pollution problem. Just wait until you try telling some poor
schlub that s/he must unplug their Wobbulator 9000 because it's causing
interference to your LICENSED station!!! 

These situations can truly strain otherwise great neighbor relations. Of
course, should your ham operations interfere with their enjoyment of any
electrical/entertainment appliance, WATCH the SPARKS fly!!!! Where is Dale
Carnegie when you really need him???

This is only the beginning, friends. As the tsunami of junk electronics
continues to invade our shores, the rising noise floor and uncooperative
attitude of neighbors will force diehard HF radio enthusiasts to seek
quieter rural locations. Those who can't move will sell off their radio gear
and move over to the peace and tranquility of VOIP ham radio equivalents or
take up other hobbies. 

The almighty profit motive is the principal catalyst of this festering
disease infecting the communications landscape. Government has abdicated its
duty to protect licensed services so don't go complaining to the Feds. They
don't have the money, resources, personnel or interest to stem the tide of
unlicensed unintentional emitters crossing our borders in millions of
containers from overseas. You and your peers will be only front line
fighters in what has become a never-ending battle against a faceless enemy
with unlimited soldiers whose perpetual onslaught will eventually render the
HF and low VHF spectrum unusable in most populated areas in the years to

Enjoy HF while it lasts. 


Frank N. Haas KB4T
Professional Interference Investigator

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