Medium Wave Transmitter

Discussion in 'Wireless & RF Design' started by powzoom, Apr 8, 2009.

  1. powzoom

    Thread Starter Active Member

    Jan 18, 2009
    I'm working on a school project where I'm building an AM radio transmitter. The purpose is to make something like the iTrip but for AM, where you can plug your ipod in and hear it on the radio. I've made the oscillator, modulator, and amplifiers, but I'm not sure what to do about the antenna.

    It broadcasts from 550Khz to 750Khz. I know antennas for these wavelengths are huge and impractical for my purpose but is there away to at least increase the power so that it can be heard for about 20-50 ft? Right now it works but only to 3ft because there is no antenna. Thinking about a loop antenna but I'm not sure how to make one or hook it up.

    Also, is there a better way to modulate that doesn't use transformers - which I don't have and have no idea where to find. It can be single or dual sideband.

    Attached is block diagram and modulator circuit. Values on the modulator circuit are wrong on the schematic but right in my actual circuit.

  2. flat5

    Active Member

    Nov 13, 2008
    Don't know where you are and what the law is but attach a few feet of wire to the output and the range will be greater. Keep the wire away from grounded metal objects.
    Do use a proper tuned circuit on the output and use a smaller coupling cap to the antenna.
    That is one simple transmitter :)
  3. Audioguru


    Dec 20, 2007
    The ferrite rod in an AM radio could be used as the antenna and tuned circuit on your AM transmitter. It is directional.
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008

    I hope you don't REALLY intend on using 120 AC as your carrier generator!

    Anyway....a simple method of modulating at low power is SERIES modulation. You supply the collector of your R.F. output stage (which MUST be Class C) through an emittter follower. This emitter follower is modulated with your audio.

    YOu can also use a Gilbert Cell to generate A.M., which is really a form of seris modulation, but balanced. It's a bit more complicated, but very very clean.