AM band Transmitter

Posted on Feb 5, 2014

The picture to the left is a high quality radio transmitter for the A. M. broadcast band. The transmitter legally operates with `micro-power` and will not set any distance records but, unlike simpler designs, the frequency stays put and the fidelity is excellent. Although the schematic looks somewhat complex, the circuitry is easy to build and adju

AM band Transmitter
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st for experimenters with a little "tweaking" experience. A simple output meter confirms proper signal level and checks antenna tuning while "on the air". Add an audio mixer, tape recorder, and perhaps a CD player and have a near-professional micro-power station. Most values are not critical but a few choices must be made carefully for best results. The output tank is tuned to the crystal frequency by selecting the values from the chart above. For example, for a 1 MHz transmitter, the chart indicates 500 pf and 35 uh. A 33uH and 550pF (470 + 82, perhaps) would be a good start. This chart assumes that a 220 pf capacitor is already connected between the collector and base of the output transistor as indicated in the schematic so the indicated capacitance is in addition to the 220 pf. A variable inductor or capacitor will allow the tank to be fine-tuned for the maximum meter reading with no antenna connected (a few volts with a 10 megohm voltmeter or about 50 microamps with a current meter). After the antenna is connected, the loading inductor in series with the antenna is selected for the minimum meter reading (best antenna loading). (A 3 foot antenna will need about 820 uH for a 1. 6 MHz output frequency. ) Longer antennas or higher frequencies need less inductance and shorter antennas or lower frequencies will need more. The meter reading should drop by more than half with a reasonably good antenna but the reading can be...

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