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Hear Amateurs on your FM Radio

If you're the sort who'd like to have an HF receiver in every room, but can't afford it, this idea is for you. It will also appeal to the beginner wishing to build the simplest possible direct conversion receiver. Using a readily-available FM wireless microphone kit, it is possible to broadcast the audio output of a homebrew direct conversion receiver around your property and beyond. This means that you can monitor amateur activity on your walkman, car radio or stereo system either at home or next door.

The idea makes use of a very simple direct conversion receiver on the band of interest. My unit uses a two-diode product detector and a one transistor VFO using a ceramic resonator. The audio from the product detector is fed straight into a $19.95 FM wireless microphone kit available from Dick Smith (K-5006). This is done by removing the electret microphone insert and connecting the output from your detector instead. Because all the audio stages are in the wireless microphone kit and the FM receiver, you need build no audio stages yourself. This means that by adding almost any VFO and diode, transistor or IC product detector to a wireless mic kit, you have the simplest imaginable direct conversion receiver. The block diagram gives the general idea. Note that no connections to the radio itself are needed.

Block diagram

Build the wireless microphone kit first, test it and then remove the electret microphone. Then add your favourite front-end, local oscillator and bandpass filter combination and feed the audio into the microphone input. I used a 3.58 MHz ceramic resonator VFO and a two-diode product detector (see schematic diagram). Appropriate adjustment (and componet values) of the front end allows reception of either eighty or forty metre signals on an FM receiver.

Circuit diagram

Depending on the range of the wireless microphone, you will be able to listen to amateur signals around the house, garden and adjoining houses on any FM receiver. This opens up a range of possibilities. For example, you could listen on a walkman whilst in the garden or next door. Or, tune in on the car radio. If you have a stereo system, you could try experimenting with selectivity by adjusting the graphic equaliser. Alternatively, use a clock radio so you never again miss your favourite net, sked or broadcast. The things you can do with this project are almost endless.



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