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Intermittent, Cyclic Interference

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Subject: Intermittent, Cyclic Interference
From: (Pete Smith)
Date: Thu, 23 Jan 1997 00:20:01 -0800
My contribution to the knowledge base:

I used to live in a townhouse area - despite which, I decided to try 80
meter DXing.  I quickly discovered a strange sort of intermittent
interference, centering at 3513 and again up at 3670 or thereabouts.  In
each case, a very loud, approximately 60-Hz impulse noise extending down
from the peak frequency, but not up.  At times, it was 20-30 dB over S9 but
would then quit abruptly, only to start up again.

Walked around the area with a portable shortwave receiver, and was able to
localize the noise to one of two houses.  Knocked on the door of the first
house and explained what I was up to.  Very nice guy, let me in to sniff
around for the source.  Discovered it was loudest along the common wall with
the house next door, so went over and repeated the process.  Again well
received, homeowner as curious as I was.  Snooped all over the house,
unplugging TV sets, etc.  Suddenly, the noise quit.  The homeowner had
turned on a bathroom light.  Turned it off, and the noise resumed.

I have no idea what the physical mechanism was, but the interfering signal
was being generated in a simple, three-bulb incandescent circuit - no
fluorescent, no dimmer, nothing - just three bulbs and a standard, 70 cent
silent light switch.  Sniffing around, the signal seemed to be coming from
the switch, so with permission I replaced it with a new one.  Noise gone!

A month or so later, I discovered another similar source in another nearby
house.  In that case, the circuit involved 2 3-way silent switches.  Again,
when I replaced the switches. the noise went away.

I suppose there was probably some sort of arcing going on inside the
switches.  The part of this I can't explain is the frequency spectrum they
generated.  But the lesson is, pray for friendly neighbors!

73, Pete Smith N4ZR 
West (bigawd) Virginia

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