This small am transmitter circuit worth trying if you had failed to build radio transmitter. It uses Schmidt trigger 74HC14hex as the oscillator, and for better stability, you can use capacitor for low cost goal of quartz crystal. Quartz crystal value around 1.6MHz is recommended since it need shorter antenna for better radiation.

Small AM Transmitter Circuit Schematic Diagram using Logic Gates

Small AM Transmitter Circuit Schematic Diagram using Logic Gates

You can also replace the quartz crystal with a 100pF capacitor, thus the frequency can be adjusted using the 25k variable resistor which connected in series with 5.1k resistor. Square wave signal will be generated by the Schmidt and then amplified by the first transistor configured as a class C RF amplifier. The modulator is acted by the second transistor. This modulator is used to variate the amplitude of the RF signal together with the audio signal level that need to be transmitted.

The voltage at the output level will be around 6V (with no modulation). The current flowing through the antenna will about 20mA if properly tuned. Antenna tuning is done by 140uH inductor and 30-300pF variable capacitor. The inductor can be made using a 2 inch length of 7/8 inch PVC pipe wound with 120 turns of #28 copper wire. To indicate antenna current and optimum settings, a small grain of wheat lamp is used. The radiated power at 20mA can be around 14 microwatts because the antenna will be much smaller than 1/4 wavelength (depending on the antenna size). [Source : Bill Bowden's circuit collection]

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