DIY FM Transmitter works only if I touch the antenna wire.

Discussion in 'Wireless & RF Design' started by Keeri Vasu, Apr 28, 2016.

  1. Keeri Vasu

    Thread Starter New Member

    Apr 27, 2016
    Hello everyone,
    I built a FM transmitter (schematic and board attached) and found that it is transmitting at 99.9 MHz currently. I used a 10 cm wire as antenna (taken from cat6 utp cable). However I'm able to hear the voice in radio "only if I touch" the antenna wire. Without a touch there is full of noise.
    I tried putting the circuit inside a plastic enclosure (as seen in the picture) and kept it away from human presence just to avoid any capacitance of our hands, with no luck. The voice is very low with full of noise. The moment I touch the antenna wire or battery clip the transmission and voice is clear and loud.
    Please advise what else need to be done to make it right.

    I did a search on existing threads but couldn't find a matching problem.

    Thanks in advance.
    FM_transmitter.png FMT1.JPG FMT2.JPG
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  2. SLK001

    Senior Member

    Nov 29, 2011
    I suspect a broken solder joint.
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  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    When you touch the antenna, you become the antenna. I think the little green coiled wire is not a good antenna. It will be much better if it is straight (not coiled). If straightening the wire does not help, Try with a 75cm antenna (straight wire). This is a "quarter wave" of your 99.9 MHz signal.
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  4. bertus


    Apr 5, 2008

    Also your antenna is very close to the transistor.
    Bend it away from the transistor.

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  5. DickCappels


    Aug 21, 2008
    Just a guess, but there is a good chance that your transmitter is not tuned to the receiver's frequency except when you touch the wire. The added capacitance of your hand on the antenna lowers the frequency of oscillation. Solution: tune the receiver to a higher frequency and/or re-tune the oscillator coil.
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  6. Keeri Vasu

    Thread Starter New Member

    Apr 27, 2016
    Thank you everyone for the findings and advises. I'm now checking all these points.
    You guys are awesome !! Much appreciated.
  7. seanstevens

    Well-Known Member

    Sep 22, 2009
    This kind of transmitter is very low power and generally very unstable unless left somewhere. Normally when these types of transmitter actually work, when you touch the antenna the transmission frequency changes drastically.

    Having said that, I agree with DickCappels that it might be on a different frequency most likely lower end as C4 & C5 seem to be a bit on the higher values and when you touch the antenna you detune/change frequency.
    Keeri Vasu likes this.