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another FM transmitter project

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by hannobisschoff@gmail.com, Apr 14, 2011.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    What creates output power? Voltage swing times current swing.

    The 2N3866 will have almost the same output power as my 2N3904 transistor unless the supply voltage to it is increased. Then the output current will also be increased and the output power will be much higher.

    The higher power FM transmitter will be very illegal in almost every country in the world:
    1) It might cause radio and TV interference.
    2) It might disrupt airplane to air traffic controllers communications and cause a crash.
    3) It might interfere with police, ambulance and fire department communications.
     
  2. Nepaliman

    Nepaliman Member

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    -Can't we make your 3904 based transmitter as a driver for 2n3866?
    -Exactly how much power (mW) output has in your transmitter? (I think, 100mW RF should have to provide to 2n3866 for 1W output.)
     
    Last edited: Apr 26, 2012
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Don't you understand that the output power depends on the supply voltage and current in the output transistor? When they both use the same circuit and supply voltage then their output power are almost the same. The 2N3866 transistor has a maximum allowed current of 400mA but then its current gain might be as low as only 5. It can be tuned to use high current to produce a higher output power.

    I think my transmitter has an output power of 10mW to 20mW. It is legal when its supply is only 5V but then its range is only 200 meters to a very sensitive radio with nothing in between.
     
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  4. dave

    Dave New Member

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  5. Nepaliman

    Nepaliman Member

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    Even if I make 5 km transmitter, nothing may will be harmful becouse I live in remote village, almost 70 km far from airport and 30 km far from crowd town. I just want to make and experiment becouse it makes me so so so much happy. My intension is not to transmit all the time,and I'm not a abuser. I too much interested in such kind of wireless transmission system.and won't harm to any other system or public.

    look this transmitter: www.electroschematics.com/5594/four-stage-fm-transmitter/#comment-251921
    -May I use 3904 as Q2 and Q3? Coz I connot find exact BF200 and 2n2219. Give the alternative way.
    -Is that reliable circuit?
    -Why Q2 is necessary as current amplify, 12 v and 1 amp battery is not enough?
    -Can we buit it of 1watt without Q2?
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The circuit you found from that website in India is extremely old and uses old transistors. It might not work.

    You can look up the spec's of those old transistors and compare them to a modern 2N3904.

    If it works, T1 oscillator has an output of about 7mW, T2 has an output of about 50mW, T3 has an output of about 250mW and T4 has an output of about 1W.
    If you do not use T2 then the output power might be only 140mW.
     
    Last edited: Apr 27, 2012
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  7. Nepaliman

    Nepaliman Member

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    If so, do you have any circuit or URL (of 1Watt) which you most liked and easy to build like this or like your 3904 based??
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I live in a modern city where it is illegal to cause radio interference. So I do not have a powerful illegal FM transmitter circuit except for the one I made with 2N3904 transistors (it is also illegal).

    There are some FM transmitter kits made for FM radio pirates who transmit from a ship at sea in International Waters:
    http://www.freeradio.org/index.php?pagename=store/frb_kits.html
    Many of their schematics and parts lists are on the internet.

    Ramsey also makes FM transmitter kits for radio pirates. Find Ramsey on the internet.
     
    Last edited: Apr 27, 2012
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  9. Nepaliman

    Nepaliman Member

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    Look this one too:-
    www.electroschematics.com/1041/quality-fm-transmitter/

    - Here, the voltage and transistors are almost same as your 3904 transmitter. Your transmitter can produce only 20 mW! But how this can produce 200mW? Is it true?

    -How much diameter has Intermediate Friquency Transmitter ferrite core (IFT) of L4?

    - Are L1 and L2 in same core or different core at near top end of L1? Here, BC548 can face 12 voltage?

    - In this 200mW circuit, Can we add power output stage of 2n3866, 12V (for 1 watt) used on this circuit>>> www.electroschematics.com/5594/four-stage-fm-transmitter/#comment-251921
     
    Last edited: Apr 29, 2012
  10. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The output transistor conducts an average of 25mA. It has an average voltage of 8V across it. Then it HEATS with 25mA x 8V= 218mW. Its radio output power is about 10mW to 20mW.

    L4 is simply an RF choke. The IF transformer ferrite core is from an old radio and they are all about the same size.

    I have never seen an air-core transformer. The coils are probably close together end-to-end.

    No.
    Then its current will be much too high. A BC548 is a low current transistor.

    It is a mono FM transmitter. Its audio will sound awful because it is missing the pre-emphasis (treble boost) used in all FM radio stations so it will sound muffled when received by an FM radio.
    My FM transmitter has pre-emphasis so it sounds crisp and clear, not muffled.
    Why bother because nothing matches. The 2N2219 little power transistor is needed to drive the 2N3866 little power transistor.
     
    Last edited: Apr 29, 2012
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  11. Nepaliman

    Nepaliman Member

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    If so, In your 3904 transmitter, I can add 2n2219 stage and 2n3866 stage with 12 voltage from this design for 1 watt www.electroschematics.com/5594/four-stage-fm-transmitter/#comment-251921
    Is it possible or not?

    -If i use any 12voltage suitable transistor like-BF200 instead of Q2 2n3904, the output power will be more, isn't it?

    (Sorry for asking such general Question Frequently)
     
  12. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The BF200 transistor is so old that I cannot find its datasheet. The largest electronic parts distributors in North America (Digikey and Newark) do not have any and have never heard of it.
     
  13. Nepaliman

    Nepaliman Member

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    - From datasheet, how can we know that a particular transistor can face max voltage in circuit.

    -And how can we calculate transistor's current output and watt output in circuit? I got very different between the value of dadasheet and and actual value on the circuit.

    - Does dipole antenna mean- '80 cm antenna and rf output is connected at centre point of this 80cm antenna from coax???'
     
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I found my Philips 1968 databook from when I worked at Philips.
    The old BF200 transistor had a max VCE of only 20V, a max collector current of only 20mA and a max dissipation of only 150mW.
    At 100MHz its typical power gain was 28dB which is a lot.

    I don't remember. I went to university a long time ago and never designed a high power transmitter.

    Look in Google at Dipole Antenna. It is two separate 80cm rods (71.5cm each in Google). Sometimes a balun is used to mate the unbalanced coax cable to the balanced antenna.
     
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  15. Nepaliman

    Nepaliman Member

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    Next transmitter circuit please-

    www.electroschematics.com/2664/long-range-fm-transmitter/

    - Is it really 1 watt?

    - May I replace this difficult oscillator stage with simple oscillator like your 3904? If i replace, will output power decrease or not?

    - What is silvered copper coil? Can't I use normal enamelled copper as coil?
     
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You found another very old FM transmitter schematic.
    The article says if you use different old transistors then its output power will be from 400mW to 2W. It might be true.

    Its oscillator has more output power than mine. Then it has a 4 transistors RF amplifier.

    Silvered copper is copper with a silver coating. You won't see the difference caused by the silver.
     
  17. Nepaliman

    Nepaliman Member

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    I think, at time of circuit designing, the most important thing is biasing. And I think, at time of biasing, the most important thing is required voltage and value of resistors.

    In a circuit, if positive power supply is +9V and a Base of transistor requires +0.8 V. In this situation ''How to determine the value of resistor which step downs +9V to +0.8V?'' Any equation to solve this?? Or any formula to calculate??
     
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A transistor with a resistor as its collector load is not biased with a single resistor because its DC current gain (hFE) can be low or it can be high. If the current gain is low then the transistor might be cutoff. If the current gain is high then the transistor might be saturated. A cutoff or saturated transistor cannot amplify properly but some RF amplifier transistors are cutoff and the tuned circuit smooths the output.
    Usually an emitter resistor is added and a voltage divider provides base voltage and current so that the transistor is biased properly.

    If the collector load is a tuned circuit then a single resistor can be used to bias the base if the hFE has a small range like a 2N3904 that has a range from 100 to 300 or a BC547x where the "x" determines the hFE.

    My FM transmitter has a 2N3904 as its RF amplifier with a tuned LC circuit as its collector load. Its base bias resistor is 47k and the datasheet shows that its base ON voltage is about 0.75V. Then with a 9V supply a transistor with an hFE of 100 will have a collector current of [(9V - 0.75V)/47k] x 100= 17.5mA. A transistor with a typical hFE of 200 will have a collector current of 35mA and a transistor with an hFE of 300 will have a current gain of about 52mA.
    If I used an emitter resistor plus a voltage divider then ANY 2N3904 transistor would have almost the same collector current.

    Notice that I am biasing the base with a suitable current, I am not "stepping down 9V to the base voltage".
     
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  19. Nepaliman

    Nepaliman Member

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    Some irritating questions for you please:

    In datasheet of 2n3904:
    V cbo= 60 V
    V ceo= 40 V
    V ebo= 6 v
    P. Diss.= 625mW

    - Then how you findout that in your transmitter, 3904 cannot face 12 voltage? Is there any equation?
    - The datasheet shows that the Max Power Diss. is 625mW, but why your transmitter can produce rf signal lower than 20mW?
    - What is gain (related with transistor, antenna...). And what is dB (dBi, dBm)?
     
  20. audioguru

    audioguru Well-Known Member Most Helpful Member

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    It has an average hFE of 230. its collector has a low resistance tuned LC circuit connected to the positive supply. Its emitter connects to 0V. It has a 47k base bias resistor. Then its base current is (12V - 0.7V)/47k= 0.24mA and its average collector current is 0.24mA x 230= 55.2mA. Its power dissipation will be 12V x 55.2mA= 662.4mW and will be more if its hFE is higher. It will melt.

    My circuit does not match its high output impedance to the low impedance of the antenna.

    I don't know how to calculate the loss in my output transistor. A simulation shows an output power to a 75 ohm antenna of 200mW.
     
    Last edited: May 7, 2012
  21. Nepaliman

    Nepaliman Member

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    What should have to do to match its high output?
    Is it possible to make 100mW rf using 3904?
     

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