Help~ Long range FM transmitter

Discussion in 'The Projects Forum' started by youngboss, Oct 5, 2012.

  1. youngboss

    Thread Starter New Member

    Oct 5, 2012
    12
    0
    I'm just hoping someone would help me understand each and every part of a long range amplifier
    starting from the mic-audio amplifier-RF Oscillator- RF Amplifier- Antenna
    pretty sure that's how the order goes
    but please feel free to correct me also
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,442
    3,361
    What is your definition of "long range"?
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    My simple low power FM transmitter has a range of across the street to a cheap "radio" from The Dollar Store.
    Its range is about 300m to my cheap Sony Walkman headphones radio.
    Its range is more than 2km to my very sensitive car radio and home stereo.

    It is illegal to cause interference to radios receiving signals from a licenced radio station.
    It is illegal to cause interference to ambulance, fire department and police communications.
    It is illegal to cause interference to airplanes navagation and communications.

    Apply to your government for a radio station licence. It is very expensive and the certified transmitter is also very expensive.
     
    youngboss likes this.
  4. youngboss

    Thread Starter New Member

    Oct 5, 2012
    12
    0
    What I mean by long range is that it has a rf amplifier to go about 1k distance
    I'm guessing that's what rf amplifier is helping me todo
    Please correct me if I'm wrong
    Right now I'm trying to figure out how to calculate all the components
    I looked online there are alot of circuits to look at but don't seem to explain in depth for me to understand fully
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    If by 1k distance you mean one(1) kilometer you need to say that. An unlicensed transmitter of that power with an antenna of suitable height in the commercial FM band from 88-108 MHz. is going to get you arrested and thrown in the jug; at least in the US.

    You'd be better off getting an amateur radio license (one 35 question multiple choice test) then your experiments would be completely legal. You still can't cause intentional interference, but you won't be constantly looking over your shoulder either.
     
    youngboss likes this.
  6. DickCappels

    Moderator

    Aug 21, 2008
    2,653
    632
    What you are looking for is on Harry Lythall's website.
    http://www.sm0vpo.com/tx/v7_proj.htm

    Harry's admonition is:
    "NOTE - This project is illegal to use or build in many countries. I accept no responsibility what-so-ever for any ilegal use. This circuit is provided solely as an educational project"
     
  7. youngboss

    Thread Starter New Member

    Oct 5, 2012
    12
    0
    Yes I figured that I'm not allowed to go certain frequency
    But what of I change the distance to 200~ 500m??
    I didn't realize I had to go up in frequency if I wanted to go further
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Go certain frequency?? We are not talking about frequency, we are saying that a transmitter with a far range will cause interference to other radios which is illegal.
    The title of your thread says FM transmitter so we assumed it is for the 88MHz to 108MHz FM broadcast band.

    The signal must not go further than the boundaries of your property.
     
    youngboss likes this.
  9. youngboss

    Thread Starter New Member

    Oct 5, 2012
    12
    0
    [​IMG]
    please correct me if im wrong please
    so the first transistor is the audio amplifier and the second transistor is my RF amplifier??
    i know that the C5 and the inductor right under the antenna is what sets my transmitting frequency
    C1 and C3 are de-coupling caps
    and are C4 and R5 my Low Pass filter
    and also if anyone can answer what's R6, C6, and C7 doing???
     
  10. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    The first transistor is indeed an audio amplifier.
    The second transistor is a RF oscillator.
    You are wrong with the parts.
    C1 and C3 are coupling capacitors.
    C2 is a decoupling capacitor for the audio amplifier.
    C4 is a decoupling capacitor, making the circuit common base for RF.
    C5 is the tuning capacitor.
    C6 is the feedback capacitor for the oscillator.
    C7 is the decoupling capacitor for the power supply.

    Keep in mind that using a transmitter can be illegal in your country.

    In my country, the use of a transmitter is illegal.
    When they find you, the transmitter and everything connected to it will be taken and you can get a fine of more than 1000 euro.

    Bertus
     
    youngboss likes this.
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The FM transmitter circuit is so simple that the frequency changes as the battery voltage runs down. The frequency also changes if something moves toward or away from the antenna since it is connected directly to the tuned LC circuit (the capacitance changes).

    The circuit does not power the microphone so a normal electret mic will not work.
     
    youngboss likes this.
  12. youngboss

    Thread Starter New Member

    Oct 5, 2012
    12
    0
    yes i am well aware about the FCC regulation and i will stay out of jail, thank you very much for all your heads up :)
    right now im trying to make my transmitter transmit carrier frequency of around 49MHz
    i just bought a baby monitor that has 49.83 to 49.89 Mhz carrier frequency
    4 channels being 20Khz apart
    i know that now i have to calculate C5 and the inductor to carry that frequency
    but does it matter what value i choose for
    C1,2,3,4,6,7??
     
  13. youngboss

    Thread Starter New Member

    Oct 5, 2012
    12
    0
  14. youngboss

    Thread Starter New Member

    Oct 5, 2012
    12
    0
    sooo audio guru what ur saying is that my antenna is too close to my tuning cap that it'll be hard to get the right frequency?
    what if adding a RF amplifier second transistor and connecting the antenna to the RF amplifier section??
    i hope im making sense for u to help me
     
  15. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    If something moves toward or away from the antenna then the capacitance changes at the antenna. The antenna is directly connected to the tuned LC so the frequency changes.

    Yes.
    That is why I added an RF amplifier transistor in my FM transmitter to stop this problem.

    For 49MHz increase the value of C6 (use 15pf to 22pF).
    The capacitance of C4 is much too small. Use 1nF.
     
    youngboss likes this.
  16. youngboss

    Thread Starter New Member

    Oct 5, 2012
    12
    0
    [​IMG]
    thanks alot
    i'm really appreciating everyone's help on this web site, first time ever using a forum and ppl are great even with stupid. questions
    well so here i found a circuit with an rf amplifier
    i get that first transistor is the rf oscillator than it goes in to the rf amplifier
    am i suppose to make it same value for (C5,L2) equal to my oscillator (C4, L1) to get my maximum output??
     
  17. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    C4 sets the radio frequency and C5 is peaked for maximum output power.
    They are set differently because the antenna and its stray capacitance affects the setting of C5. Make a simple field strength meter with the brightness of an LED to see the power peak which is not sharp, it is broad.

    The very simple FM transmitter does not have a voltage regulator so its radio frequency will change as the battery voltage runs down. I used a low dropout 5V regulator for the oscillator and mic preamp in my FM transmitter to fix that.

    The very simple FM transmitter will sound muffled when heard on a normal FM radio because it is missing pre-emphasis (treble boost) that all FM radio stations have and all FM radios cut treble (and then cut hiss).
    My FM transmitter sounds crisp and clear because it has pre-emphasis.
     
    youngboss likes this.
Loading...