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Big Problem with small FM transmitter

Discussion in 'General Electronics Chat' started by PeTkO, Mar 14, 2008.

  1. PeTkO

    PeTkO New Member

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    First I want to apologize everybody for my bed englesh, because, I'm Serbian!

    I bulid simple FM transmitter.Shematic is here: http://www.electro-tech-online.com/attachments/fmtransmitter3-gif.3307/

    Transmitter is work fine.But, range of visibility is very small, cca 20cm.When I move antenna 30-40cm from reciver, signal is very bad.Antenna is telescopic, length about 40cm.

    Power supply for this transmitter is 9V battery.


    Please help me!
     
  2. mneary

    mneary New Member

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    You must have a very good receiver to achieve even marginal performance. Even with the best receiver, 1/4 mile is extremely optimistic for this power level. The range is reduced further if anyone else is using this channel or one near this frequency.
     
  3. mvs sarma

    mvs sarma Well-Known Member

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    See the FM Tx Mod 4 schematic moderated by Audioguru. it works fine. see HERE

    A revised PCB layout for Toner Transfer method, and component overlay are attached in PDF format. Perhaps one has take a LASER print at 1:1 ratio to be able to use it.

    I have added a series resistor and an electrolytic cap or the supply to Electret MIC circuit.
    If not needed they could be avoided by a shorting link for the resistor and leave the capacitor.
     

    Attached Files:

    Last edited: Mar 20, 2008
  4. dave

    Dave New Member

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  5. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Hi Sarma,
    Thanks for the pcb design.

    Hi PeTco,
    Somebody on another electronics chat website also complained about the circuit you found. I also built it to see and mine also didn't work. So I improved it in a few steps:
    1) I added an RF amplifier transistor to increase its range and to isolate its antenna from its oscillator so it doesn't change its frequency when something gets near or moves away.
    2) I added a low-dropout voltage regulator so that the frequency doesn't change as the battery voltage runs down and so that the preamp transistor remains properly biased (it didn't in the original circuit).
    3) I added pre-emphasis (treble boost) like FM radio stations have so that the sound quality is good.

    It overloads my Sony Walkman and clock radios which makes it difficult to tune because its signal is all over the dial. Luckily my Sony Walkman radio has a local-distant switch that attenuates strong local signals so I could tune it to an empty spot on the FM dial.

    Its frequency is so high that it won't work if it is built on a breadboard. I used stripboard with very short traces but a pcb is also good.
     

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