I pulled a pill bottle out of my junk box, it had a single layer of solid enameled wire wound on it, and it measured 85 microhenrys. So I added the transistor, resistors and the capacitors to get it to oscillate at the low end of the AM BCB, around 540 kHz.
The schem is the same as the one here, but with different values.
The circuit is the standard Colpitts with the tap between the two capacitors connected to the emitter and the hot end of the coil/capacitor is connected to the base. The coil must have a 0.1 uF DC blocking capacitor to prevent the base bias from shunting to negative. The supply voltage is 9 volts, but it will oscillate at much lower voltages.
I soldered the coil wire to the wrong point, putting the coil across the power supply wires. I put it on the power supply I made from a wall wart, and I was puzzled why the power supply output went to zero. I think the short burned out the fuse inside the wall wart, because after I fixed the coil short, the power supply didn’t put out any voltage. So I have another thing to repair.
The oscillator can be heard strongly when it’s a few feet from the receiver antenna, but I moved it across the room and the signal quickly disappeared into the noise. I when added a few feet of wire to the emitter to make an antenna, the signal was better, I could barely hear it across the room.
I lengthened the short antenna I had connected to the emitter, to extend the range, but I didn’t hear it on the receiver. I think the frequency changed due to the longer antenna.
More experimenting with this soon.
