[SBE] FCC R & R 73.318: FM Blanketing Interference
mlangner at swcp.com
Sun Sep 20 13:19:56 EDT 2009
Hello Dennis !
I've had similar problems in a lengthy broadcast engineering career.
The most interesting one was a similarly powered station with only a 150
foot tower in the middle of our community, Albuquerque, New Mexico, about 40
It put so much RF into its own building, never mind the neighbors'
buildings, that when it turned on its transmitter, its own audio
equipment -- consoles, cart machines, etc. became useless.
So in a word, it could have its audio equipment working, or it could have
its RF equipment working, but not both at the same time. After their tech
folks couldn't make anything work, about 10 days of frustration for them, I
got a call from the station management asking for help. We had everything
working after one long day. And we'd gone through about 4 dozen 100 pf
What we did you will find successful -- but with one strong caveat. Once
you touch a member of the public's equipment, you'll be blamed for
everything that goes wrong with it for the lifetime of the public member's
Please bear in mind that "blanketing interference" is not audio
rectification. In the FCC's own words, "Mobile receivers and non-RF devices
such as tape recorders or hi-fi amplifiers (phonographs) are also excluded."
Still, being a "good neighbor" may make station management willing to assist
anyone who is being affected by the station's presence.
Your new best friends are going to be 100 pf disk ceramic capacitors and
Computer speakers: I've stopped both AM and FM from a number of them with
ferrite beads/toroid cores over the wires. Some have required 100 pf
capacitors at the input pin of the lowest level audio IC.
Electronic telephones: again ferrite beads/toroid cores, and in some cases
I've bought more robust telephones for members of the public.
Garage door openers: usually operate at 315 or 27.255 mHz. Often they have
essentially no front ends. Stub "suckouts" can help -- please see the next
FM receiver overloading/desence. A "suckout" stub across the receiver
antenna terminals can work wonders. They're easy to make and are quite
cheap. My favorite use of the stubs was to remove the "beeping interfering
carrier" from cable TV signals that had this kind of intentional
interference on some pay channels, such as Showtime and HBO. Worked great!
You'll probably want 300 ohm balanced and 75/50 ohm unbalanced stubs
The plethora of cheap "no front end" tuners has made "blanketing
interference" more difficult to deal with.
Still, most of the complaints you'll get will not be "blanketing
interference," but will be audio rectification. Again, it's up to your
station management whether or not that's something the station should fix.
Keeping your synchronous AM down to an absolute minimum will also help!
Excellent reference for getting RF out of equipment -- the Amateur Radio
Relay League's "Radio Amateur Handbook."
Mike Langner, CPBE
From: sbe-bounces at sbe.org [mailto:sbe-bounces at sbe.org]On Behalf Of Dennis
Sent: Sunday, September 20, 2009 10:45 AM
To: 'sbe member discussion mail list'
Subject: [SBE] FCC R & R 73.318: FM Blanketing Interference
Hello to the collective brain:
I am now, for the first time in a nearly forty-year career, dealing with
FM RFI complaints from a new FM station facility (15kW ERP/ 400' HAAT).
These complaints include demodulated audio in PC speakers, garage door
openers that no longer work, as well as the customary "I can no longer pick
up a station I used to hear that's 95 miles away," etc. My experiences in
this area have all been with AM stations as the facilities over the years I'
ve had with FM have been operating from towers in wide-open farmland or in
the swamps of Florida (I always seemed to be the C.E. for an AM with its
site located in densely-populated residential areas). AM RFI in some
respects, is easier to resolve inasmuch as good grounding can resolve most
I'd like to hear of the experiences/solutions from all of you on this
board. I believe sharing these would be of benefit to all of us.
Dennis L Sloatman
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