VHF FM Transmitter

Discussion in 'Wireless & RF Design' started by rohs, Nov 10, 2012.

  1. rohs

    Thread Starter New Member

    Nov 10, 2012
    I'm using this schme for study, it's used to transmitt audio signal to vhf band 120 Mhz FM modulation to receiver (the receiver is SDR radio in the our lab).

    it's works very good, stable frequency, good voice quality, and low power consume.


    but the transmittion distance is not more than 40 meter in open field. I need to amplify the R.F power output of this transmitter. but i don't know how. could you please help me?

    the antenna is the audio cable itself. it's works great. long cable of about 1.5m . I also tried to use standard antenna and no better result.

    pleae note that i need to use this layout, i mean i can change and remove component, but i can't change the layout of the scheme because we already maked the PCB.

    note: this transmitter used for study purpose only.

    Thank you.
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    In the USA, the power limit for unlicensed FM transmissions is a signal strength of 250 micro volts per meter, measured 3 meters from the transmitting antenna. At this power level, stereo reception with a good signal to noise ratio is only possible within a 100 foot radius.
  3. rohs

    Thread Starter New Member

    Nov 10, 2012
    this transmitter used for study purpose only

    please help.
  4. Papabravo


    Feb 24, 2006
    Irrelevant. Are you aware that you could potentially interfere with a distress signal on 121.5 MHz. Would you want that on your conscience. As a student you should strive not only to learn your craft but do it in a legal and cooperative manner.

    If you were to get an amateur radio license a whole world of legal possibilities would open up to you.

    Don't ask us to help in what would would be a patently illegal scheme that could lead to loss of life. Have some respect for the community you live and work and study in.
  5. Audioguru

    New Member

    Dec 20, 2007
    The range of a radio transmission depends A LOT on the sensitivity of the radio receiver.
    My FM transmitter has a range of more than 2km to my very sensitive hifi stereo receiver and car radio. Its range is about 400m to my cheap Sony Walkman portable radio. Its range is across the street to a cheap "radio" from The Dollar Store. Its range might be 1m to a "crystal" radio.

    120MHz is used for aircraft communications. If you cause interference and a plane crash then you will be sued then hanged and shot. Keep away from these frequencies. Use 88MHz to 108MHz FM broacast band frequencies instead.
    If you cause interference with FM radios then you will be hanged then shot by the RF police.

    The capacitors to ground at the base of Q3 and Q4 are attenuators that reduce the output level a lot. Remove them.