dgsvetan at rockwellcollins.com dgsvetan at rockwellcollins.com
Mon Feb 19 12:38:12 EST 2007


An interesting post.  Well, I was a victim of power line RFI to my 2m FM
operation a number of years ago.  I was living in a downtown area, right at
the borderline between the commercial district and residential.  This was
in the Chicago suburbs and the National Simplex channel, 146.52 was (at
that time) a very active frequency.  I had two antennas on the roof of the
house:  a 5/8 wave ground plane and a 3 element Yagi that was aimed toward
the SE (toward Chicago, roughly 25 miles away).  Despite the limiters that
are inherent in a properly designed FM receiver, there were periods of time
in which the RFI from arcing and corona would totally obliterate even the
strongest signals on 146.52, or even local repeater channels.  The nature
of the interference was such that, being broadbanded in nature, the RFI
would not open the receiver squelch on its own.  However, when an
on-channel signal appeared, the squelch would open, complete with all of
the buzzing.

Without a steerable antenna, it was difficult to locate the exact pole or
poles that were nearest the noise source.  The RFI was stronger on the Yagi
than on the groundplane, so that told me the noise source was likely on or
near the poles across a sidestreet that fed 3 phase power to a supermarket.
I got lucky - one morning, when the racket was very bad, I left the house
to head into work and heard the very distinctive sound of a corona as I
went out of the door.  Humidity was high.  I went back into the house,
called the power company to give them a street adress and pole numbers, and
went to work.  When I came home that evening, there was roughly 500 feet of
new power line run on all 3 phases, plus new pole hardware.  The next
morning, an RFI tracking van came slowly down the street as a follow-up
check.  No problems there the remainder of the time I lived at that
location.  FWI:  distance from my antennas to the power lines was roughly
150 feet.

In the case of your customer, I presume the hand held FM radio is for
receiving broadcast FM, not communications FM (ham radio or public safety).
With a wide IF and no squelch, her FM broadcast portable may very well be
blasted and pass through the buzzing when the arcing and-or corona is very
pronounced.  You are correct about AM mode radios being more sensitive than
FM radios, but I can certainly vouch for the fact that yes, FM receivers
can be bothered by power line RFI.

73, Dale

             "Roeder, Landon"                                              
             <LRoeder at nespower                                             
             .com>                                                      To 
             Sent by:                  <rfi at contesting.com>                
             rfi-bounces at conte                                          cc 
             sting.com                 "Lee, Toby" <tlee at nespower.com>     
                                       [RFI] FM RFI?                       
             02/19/2007 10:43                                              

We have a customer who has been complaining of TVI for several months.
She has multiple TV sets all using rabbit ears (non-amplified).  We have
found several noise sources on our poles and have fixed them.  Honestly,
she usually cannot tell the difference between bad reception and TVI, so
she calls us quite often.

Recently, she has begun complaining about interference on her
battery-operated handheld FM radios.  She says that sometimes she hears
a little buzzing on it, and other times the signal will just go away
completely.  When she moves the radios, the signal comes back to normal.
We have not been able to verify that particular problem, so my question
is whether powerline interference would cause such a problem?  Most of
the time powerline noise is amplitude modulated and thus affects AM
signals more.  So, what would the typical effect be on FM?  I suppose
that if the source were strong enough (likely inside the house) that it
could overload the receiver, but we haven't been able to find a problem
in her home.

What do you guys think?

Landon D. Roeder
Engineer, Test Section
Nashville Electric Service
Office: 615-747-3309
Pager: 615-929-0093

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