Medium wave Transmitter -constuction problems.

Discussion in 'Wireless & RF Design' started by squb, Aug 21, 2013.

  1. squb

    Thread Starter New Member

    Jun 29, 2013
    Hi to all forum users.

    I have problem with bulding a medium wave transmitter.

    I have designed circuit and put it together on bread bord.I using Epson programmable oscillator (4,57 MHz) and johnson decade counter in order to divide signal by 10 (457 kHz).I have a complementary power amplifier as a output stage.I connecting output stage with antenna by transformer( I trying with 21 turns on ferrite rod).

    My questions:

    -I need approximately 0.1W output power and 50 m range , but I don't know how to measure it?

    -Another problem is connected with proper antenna impedance matching. I couldn't find any formula for transformer to calculate how many turns of a wire are need to be made on a ferrite rod in order to get right impedance so I tried few random numbers of turns.Do somebody know how impedance matching should be ?

    - I made simulation in LTspice but results are not the same like in reality. So, if somebody could tell me-Do I biased transistors in good manner ?

    Circuit in attachment.

    Best Regards
  2. radiohead

    Senior Member

    May 28, 2009
    The BC640 PNP is improperly biased. The collector should be connected to the negative. Remember this rule of thumb...the arrow points to the negative. You should consider a current limiting resistor along the transistor negative path, otherwise they may get excessively hot.

    I don't see how this circuit could generate a radio signal without a tank circuit.
    Last edited: Aug 21, 2013
  3. squb

    Thread Starter New Member

    Jun 29, 2013
    Thank you for fast reply radiohead.

    I made mistake with PNP transistor connection.
    I improved my circuit scheme.I added current limiting resistor (1k) and connected BC640 PNP collector to the negative ,like it should be.

    You wrote-"I don't see how this circuit could generate a radio signal without a tank circuit."

    Could you explain where I have made mistake.I use the Epson chip as a oscillator and then divided signal is passing to the power stage amplifier which is connected to ferrite rod antena. Antenna is tuned to 457 kHz - according to formula f=1/sqrt(LC).

    Do youknow any useful formula or solution for antenna matching to power stage?

    What about output power ? How to calculate it ?
  4. bertus


    Apr 5, 2008

    Keep in mind that your 457 kHz generator produces a signal that will distort the reception of many AM receivers in your naberhood.
    This is because the 457 kHz is ver near to the MF frequency of many AM receivers, wich is 455 kHz in most cases.

    If you do not want to have trouble with the authorities, please stop your experiment.

  5. squb

    Thread Starter New Member

    Jun 29, 2013
    Hello Bertus

    Thank you for fast reply.

    I work on my project for studies ( Topic: " Security system for climbers and skiers. Design and build working prototype of avalanche transreciver for 457 khz "). I hope my experiment will not cause any problems because of very small transmitter power.

    I try many times with different circuits layout ,but I didn't manage to get sufficient results.I affraid that I made mistakes in calculations.I made simulations in LTspice and them shows that everything is all right.

    In attachment is reciver circuit. Sensitivity of my reciever is fairly small and I don't know how to increase it

    I figured out that it is also possible to use delta-sigma Ic to get RMS value. I need DC voltage signal which is proportional to distance from transmitter.

    What do you think. Two stage BJT amplifer and op amp integrator in line as a reciever circuit are good idea ?

    Could you tell me how to design matching network . I know about T-type matching network but I still don't know how to calculate a ferrite rod antenna impedance in order to calculate matching network components.

    Best regards