Crystal AM Transmitter

By Pradeep G.

20613
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Here is the circuit of a medium power AM transmitter that delivers 100-150 mW of radio frequency (RF) power.

AM transmitter circuit

Crystal AM transmitter circuit
Crystal AM transmitter circuit
Oscillator coil
Oscillator coil

At the heart of the circuit is a crystal oscillator. A 10MHz crystal is used to generate highly stable carrier frequency. An audio signal from the condenser mic is amplified by the amplifier built around transistors T1, T2 and T3. The amplified audio signal modulates the RF carrier generated by the crystal oscillator built around transistor T4. Here modulation is done via the power supply line. The amplitude-modulated (AM) signal is obtained at the collector of oscillator transistor T4.

By using matching dipole antenna and coaxial cable, the range of signal transmission can be increased. For maximum range, use a sensitive radio with external wire antenna.

The circuit works off a 9V-12V battery. For oscillator coil L1, wind 14 turns of 30SWG wire round an 8mm diameter radio oscillator coil former with a ferrite bead.

Modulation transformer
Modulation transformer

For modulation transformer X1, you can use the audio output transformer of your old transistor radio set. Alternatively, you can make it from E/I section transformer lamination with inner winding having 40 turns of 26SWG wire and the outer winding having 200 turns of 30SWG as shown here.

Construction

Assemble the circuit on a general-purpose PCB and enclose in a suitable cabinet.


The project was first published in October 2010 and has recently been updated.

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4 COMMENTS

  1. Very good and simple design. I am also going to make a crystal AM transmitter of around 200 mw output. Could you tell me how many turns is the inner and how many turns is the outer of the modulation transformer ? If I use an E/I iron core of the size about 2x2x2cm, the audio amplifier use 4 transistors with the last tow of push-pull circuit, in high frequency part ,I use one transistor as oscillator and another as final amplifier and collector modulator. Thank you very much!

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