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Interference with FM radio, goes away when my hand is near.

Discussion in 'Electronic Repair' started by micky, Nov 19, 2012.

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  1. micky

    micky Guest

    Interference with FM radio, goes away when my hand is near.

    In the last six months I've had interference with two FM radio
    stations. I thought FM was immune to most interference?? So
    what might be doing it?

    It's so loud I can't hear the program,. But when I hold my fist very
    near the radio or touching the front, the noise goes away gradually
    and then completely when my hand is close enough. Is there some
    way to duplicate the effect of my hand being near the radio???????
    I tried a lot of things in a very different stituation and nothing I
    tried worked.

    Details.
    It only happens sometimes, often in the middle of the night, and when
    it's on one radio it's on two others also. Two have digital tuning
    and one, maybe two, have analog (One digital is expensive 25 y.o,
    the other maybe 10, one analog is transistor 40 y.o. or so. All of
    them work well otherwise.) It often lasts only 15 minutes or less,
    sometimes with interruptions with no noise. It may last longer other
    times but I don't have the patience to keep checking. When I find
    my mp3 player with FM radio, I'm going to walk around the n'hood and
    see if I can find the source.

    There's a bus with a 2-way radio that parks less than 100 yards from
    my bedroom, but the noise occurs even when it's not thjre. My n'bors
    all seem to keep regular hours, not up in the middle of the night,
    and none have SWave antennas visible, and the noise has no words.

    I live in Baltimore and it occurs on 88.5 and 90.1 MHz. FWIW these
    are both DC stations 30 or 40 miles away, NPR and C-SPAN raido.. 88.1
    which is local never gets interference. I don't know where other
    stations on the dial are,located sine these are the only 3 I listen
    to.

    It sounds like a foghorn, a consistent deep (loud) tone, but a little
    higher pitch than a real foghorn. .

    Thanks.
     
  2. gregz

    gregz Guest

    If it is in your residence, shut off the breaker to see if it stops on the
    portable.

    Greg
     
  3. Mark Zenier

    Mark Zenier Guest

    There is an older aviation navigation system, VOR, that runs in the range
    just above the FM broadcast band (108-118 MHz). It works by broadcasting
    a couple of tones at a low frequency, which, when compared, gives aircraft
    a compass heading of their location relative to the airport that it's
    located at.

    It could be an RF Image. The usual IF for a FM receiver is 10.7
    MHz, and the local oscillator is usually above the frequency, so a
    super-heterodyne receiver can also be sensitive to signals 21.4 MHz
    above the tuned station.

    It might be a nearby airport, or it could be at a farther distance
    than you would expect, especially now with the sunspots kicking up and
    strange radio propagation effects can occur.

    (The interference can go the other way, too, where the radio's local
    oscillator will leak out and jam a VOR signal 10.7 MHz above the tuned
    station's frequency. This is the main reason that personal electronics
    are supposed to be turned off on aircraft at various times. Now, largely,
    an obselete problem).

    Mark Zenier
    Googleproofaddress(account:mzenier provider:eskimo domain:com)
     
  4. Sjouke Burry

    Sjouke Burry Guest

    Your arm acts as an additional antenna, capacitively coupled.
    This suggests that the orientation and/or antenna size is
    bad for your location.
    Attach a wire of about 2 meter to the antenna, and play with
    the orientation.
    Even better is to find the interference source, any modern switching
    supply(computer, charger,GFL light,new fl light,senseo coffee) is a prime
    candidate, even if you think the divice is switched off,
    a lot of them still have an active computer chip and supply inside.
     
  5. micky

    micky Guest

    Thanks. I will try that. but it's pretty quiet here in the middle of
    the night**

    Anyone have any idea how to duplicate the effect of my fist or hand
    near the radio? That gets rid of the noise. I'm lying on a sheet
    and mattress -- I don't think I'm grounded. A bag hung in front of
    the radio filled with cooked chicken? Raw chicken? Metal scraps? I
    can try the last one, but I'm not made of metal. Paperback books?

    ** only the furnace, fridge, and one outdoor IR triggered light. None
    of them go on for a minute then off for a minute and back on. I can
    tell t he furnace is not on all the time because the hot air vent is
    right near me when the I'm in bed.
     
  6. N_Cook

    N_Cook Guest

    Is the interference a whine or a buzz?
    Does it change pitch as well as volume , when you move close to it, like a
    Therimin "musical" instrument
     
  7. amdx

    amdx Guest

    This is God's way of telling you to stop listening to liberal radio!
    Mikek
     
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