Sunday, 13 February 2011

Short Wave AM Transmitter


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The main feature of this trans- mitter is that it is free from the LC (inductor, capacitor) tuned circuit and operates on a fixed frequency of 12 MHz which is extremely stable. An LC based tuned circuit is inherently unstable due to drift of resonant frequency on account of temperature and humidity variations. The circuit is very simple and uses only a few components. The figure shows the complete circuit diagram of the transmitter. Resistors R1 and R2 are used for DC biasing of transistor T1. The capacitor C1 provides coupling between the speaker and the base of transistor T1. Similarly, resistors R3, R4 and R5 provide DC bias to transistor T2. Resistor R5 also provides negative feedback which results in higher stability. The oscillator section is a combination of transistor T2, crystal Xtal, capacitor C2 and resistors R3, R4 and R5. The crystal is excited by a portion of energy from the collector of transistor T2 through the feedback capacitor C2. Thus the oscillator circuit generates the carrier frequency at its fundamental frequency of 12 MHz. Any crystal having the frequency in short wave range can be substituted in this circuit, although the operation was tested with a 12MHz crystal. Transistor T1 serves three functions:

  • It provides the DC path for extending +Vcc supply to transistor T2.
  • It amplifies the audio signals obtained from speaker.
  • It injects the audio signal into the high frequency carrier signal for modulation.

The loudspeaker converts the voice message into the electrical signal which is amplified by transistor T1. This amplified audio signal modulates the carrier frequency generated by transistor T2. The amplitude modulated output is obtained at the collector of transistor T2 and is transmitted by a long wire antenna into space in the form of electromagnetic waves. The transmitted signals can be received on any short wave receiver without distortion and noise. The range of this transmitter is 25 to 30 metres and can be extended further if the length of the antenna wire is suitably increased along with proper matching.

1 comment:

  1. I built this thing using NTE 128 and 129
    crosses and it worked every bit what is
    described. I used heat sinks and run it
    at 18vdc continuous and it sounds fantastic
    and scopes to a beautiful envelope with
    an MP3 player driving it. Try different
    speakers for best result. Smaller the better
    and 8 ohms works just fine.

    Only 2 modifications were needed. Eliminate
    R1 and R4 and also put a choke of 20uH between
    the PNP and NPN. Pull the antenna off on the
    NPN side.

    I drive an Ameritron Amp for 2 watts out so
    it puts out about 200mW.

    27 years on Broadcast Radio RF
    Chicago, IL 27