FM/Aircraft Receiver

Discussion in 'Wireless & RF Design' started by doug08, Mar 17, 2011.

  1. doug08

    Thread Starter Member

    Jan 30, 2011
    153
    2
    I just built this circuit, but I cannot get any frequencies to tune in with the trimmer. Any idea what the potentiometer is used for if the trimmer adjusts the frequencies? Limit the circuits current? I used a 2n3819 instead of the bf245(pin config was different).....but it should work...correct? The RFC is 40 tight turns of .4mm wire(26ga) wrapped around a small permanent marker(.42" diameter). Instead I used a tiny black 8.5uh coil...is that ok? According to the air inductor coil calculator my coil is very close to the same value...do I have to have that particular coil? Or it makes no difference if the values are the same. When I turn on the unit, I can hear the static just like a normal radio, and it changes slightly as I adjust the trimmer...but not one transmission is heard. Any ideas? Thanks.

    I had to translate everything from Indonesian to English.

    http://adibaduts.files.wordpress.com/2009/11/vhf-airband.jpg
     
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  2. Audioguru

    New Member

    Dec 20, 2007
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    It is the strangest radio circuit I have ever seen. It seems to be a crystal radio but the Jfet has no gain and the input signal is fed to the drain pin which is the output of the Jfet. Then the circuit looks like a lousy super-regen but they self-detect without using the diodes in that circuit.

    The value of your 8.5uH choke is much too low. 30 turns of wire make hundreds of uH.
     
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  3. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,789
    945
    Just google up 'retuning FM radio for airplane frequency'. The FM band is just right below the airline freq's. If you know your way around radio circuits, you can 'retune' the oscillator and peak the input coupling for new, higher freq reception.
     
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  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,692
    901
    Aircraft com radios are AM.

    John
     
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  5. Audioguru

    New Member

    Dec 20, 2007
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    Aircraft radios use AM so that an aircraft in trouble can "break in" and yell MAYDAY, say what is wrong and be heard. Frequently an FM radio communication cannot have a "break-in" due to the capture ratio of an FM radio not allowing it.

    A super-regen "radio" detects AM but FM can be heard with distortion if it is tuned to one side of an FM station and use slope-detection with its AM detector.
     
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  6. RRITESH KAKKAR

    Senior Member

    Jun 29, 2010
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    HI,

    why using FM communication cause distortion in signal but not in AM??
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    A super regen "radio" detects AM which is changes in the amplitude of the radio carrier. If an AM radio is tuned to the middle of an FM signal's bandwidth then it detects nothing because an FM carrier is always at one amplitude. But if an AM radio is tuned to one side of an FM signal then it detects the FM by the slope of its tuned bandwidth. Its output signal is higher when the FM swings closer to the peak of the AM radio's tuned circuit and its output signal is lower when the FM swings away from the peak of its tuned circuit. It causes distortion.
     
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  8. radiohead

    Active Member

    May 28, 2009
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    Last edited: Apr 8, 2011
  9. Audioguru

    New Member

    Dec 20, 2007
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    A super-regen AM "radio" is not a radio, it is a novelty.
    Very cheap radio-controlled toy cars use a super-regen "radio".
     
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