AM Transmitter

Posted on Feb 4, 2014

An AM voice transmitter with variable tuning. The antenna circuit is also tuned and transmits via a long wire antenna. Please Note. It is illegal to transmit on the AM wavebands in most countries, as such this circuit is shown for educational purposes only. Please read the disclaimer on this site before making any transmitter circuit. It is illega

AM Transmitter
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l to operatea radio transmitter without a license in most countries. This circuit is deliberately limited in power output but will provide amplitude modulation (AM) of voice over the range 500kHz to 1600kHz with values shown. You can input values in the calculator below, remember to change drop down box to picofarads for capacitance and microhenries for the coil. The coil is fixed at 200uH, the capacitor values can be varied and resonant frequency found by using the calculator below. The circuit is in two parts, a microphone pre-amplifier built around Q1 and an RF oscillator circuit (Q2). The oscillator is a standard Hartley oscillator which is tunable. Tank circuit L1 and C1 control frequency of oscillation, the power in the tank circuit limited via emitter resistor R1. The transmitter output is taken from the collector, L2 and C2 form another tuned tank circuit and help match the antenna. L1, L2, C1 and C2 may be salvaged from an old AM radio if available. The antenna should be a length a wire about 10 feet or more. In the schematic I have shown coaxial cable to be wired to the "longwire" antenna, the outer coax shield returned to ground. Ground in this case is a cold water pipe, however even without a ground and coax cable a signal should still be possible. L2 and C2 not only help match the antenna to the transmitter, but also help remove harmonics and spurious emissions in the transmitter circuit caused by non linearity...

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