In the spirit of passing on stories of odd sources of RFI, here is one that
was recently solved at K8AZ:
For several years we had intermittent problems of interstation
interference at Tom's Multiop contest station, with the 20 meter station
interference on 15 meters. it wasn't always there, but was mostly observed
transmitting on the 20 meter stacks about 100 feet from the house. Not a
problem when using the multiplier station stacks on a rotating tower about
300 feet away.
During the CQWW SSB this year we noticed that this noise was appearing in a
20-30 KHz spread of garbage at 1.5 times the 20 meter frequency, i.e.
transmitting at 14220 would center the garbage around 21330.
After the contest I spent an afternoon on the tower checking connectors,
bypassing relay boxes for the 20 meter stack, even trying an alternate
feedline normally used on the 15 meter system. No success.
So we next took an IC-R10 scanner, using the rubber duck supplied with that
radio, through an ICE 15 meter filter to avoid any fundamental overload
problems. Lots of sniffing around, while reducing the transmit power to the
minimum amount that would cause the interference (about 40 watts with the TX
stack pointed at the east end of the house would do it). We found a hot
spot near the garage door opener, but totally disconnecting the opener did
not eliminate the problem.
I had pretty much run out of time that day, so I left, with Tom's next step
to unhook a number of wall warts run from a power strip in his bedroom,
located pretty much right over the garage.
With the strip unplugged the QRM disappeared. He then started plugging them
back in one at a time.
The one that powered an XM Satellite receiver. The receiver was originally
intended for automotive use, with a low power FM transmitter to couple the
XM to the car radio. The transmitter section was being grossly overloaded
with HF RF from 20 meters, causing it to put out RF garbage at who knows
With that radio out of service, the 20 and 15 meter ops at K8AZ were able
to very peacefully coexist in the CQWW CW test.
73 & HNY to all - Jim K8MR
In a message dated 12/27/2010 12:58:44 P.M. Eastern Standard Time,
On 12/23/2010 4:03 PM, Tim Duffy K3LR wrote:
Yes, this is truly excellent work. Required reading for anyone active
on the ham bands.
73, Jim K9YC
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