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Wierd FM radio noise

  1. Jun 6, 2003 #1


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    I'm not sure where to post this.

    On my FM radio (yes, I am pretty old fashioned), if you tune it to be slightly out of a station or if there is a weak signal or something, you can distinctly here a strange pattern of interference in the background. This consists of a rapid pulsing of white noise at about 100 bpm. As the radio is tuned further from the station, the length and volume of the pulses increase until all the sound is drowned out by static. This is probably reproducible on most radios. Now, I have no idea where this pulsing static comes from, and there seems to be nothing in the visible vicinity that may cause it. (no mobile, computer etc around) So where does it come from?
  2. jcsd
  3. Jun 6, 2003 #2


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    I'm by no means a pro, but I've played around with FM radios trying to transfer a small amount of power. I've seen/heard some interesting results.

    Do you have any fluorescent lights around? Possibly even the wiring in your home, being AC is producing the distortion.

    Or, and I may be mistaken, but radio waves bounce off of things and never really "disappear". Is it possible that its just a mix of radio signals, distorted from so many bounces that it sounds as you described?

    My only other thought on this is do you live near any tv or radio towers? My ex girlfriend lived in between 2 radio and 3 tv towers, all in about a 5-10 mile area, and watching tv or listening to radio at her house would be nearly impossible from time to time, due to a sound/or flashes of static, similar to what you described.

    Of course, you could be getting instructions from aliens to build a quantum gateway to journey to there universe, where they will sacrifice you to Allah, and avoid destruction. Think FZ+, you could be a hero.
  4. Jun 8, 2003 #3


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    Well, here's a 3 sec snippet of it. Should I send a copy to SETI? :smile:

    Attached Files:

  5. Jun 8, 2003 #4


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  6. Jun 8, 2003 #5
    Ok I just needed to refresh this thread to see the link. Ok the bandwidth of FM is regulated via FCC at around 200khts, there is a gap between these in order to seperate from station to station, also this depends on the wattage license of the station, and how far a station is in order to broadcast at the sam frequency. TV's use AM to transmit there video but the sound comes from an fm carrier but needs to be demodulated through the tv because it is superimposed onto the carrier. Since your dealing with fm instead of am the noise should be minimal because it is frequency change that causes it. The noise you are hearing is most likely caused at the radio station that is producing it as a normal noise problem that occurs in fm transmitters. You could have some bleed over from another station, like hearing two stations at once caused from differant atmospheric conditions. Here is a link that may explain better than I.http://murray.newcastle.edu.au/users/staff/eemf/ELEC351/SProjects/clement/FMNoise/FMNoise.htm
  7. Jun 8, 2003 #6


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    Thanks alot. That seems right...
  8. Jun 8, 2003 #7
    When I get my computer's audio working...

    ...I'll be interested in hearing that noise.

    When atmospheric conditions are just right I can pick up FM radio stations from over 1000 miles distant. They come in just as clear as you could imagine, and it is fascinating, making me wish they could stay. At these times I will listen all day to dozens of stations that I cannot normally receive. Some will fade and then come back while others remain steady.
    AM and Shortwave ‘channels’ behave like described, but FM is more unusual.
    Last edited by a moderator: Jun 8, 2003
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