The Imaginary Part

This is my newest radio. It's called a Pixie II. It also happens to be the first amateur-band transmitter I've ever made.

These things are pretty cool. It is a continuous wave (CW) (or morse code) transceiver operating at a fixed frequency determined by a crystal on an HF band. Mine's configured for 80 metres using a 3.579545 MHz colorburst crystal. To move it to another band you just need to change the crystal and the L and C parameters of the antenna pi network.

The associated circuit description claims that it puts out a couple of hundred milliwatts. I'm not sure exactly how much power it does produce as it doesn't move the needles on my SWR meter but I intend to do the theoretical analysis sometime.

So what is it? Refer to the schematic. It's a Colpitts crystal oscillator running permanently at the transmission frequency. When the morse key is closed this pulls down the emitter of Q2, allowing it to amplify the oscillator into the antenna. When the switch is open the received AC is multiplied by the local oscillation frequency (direct conversion) and the result is superimposed on the supply voltage and applied as the input to an LM386 amplifier. This amplifier then provides a fixed (!) gain of 200 to the multiplied signal and in my case is attached to a 1 W 8 Ω speaker.

Despite the simplicity of the circuit I'm pleased to say that it really does work. I've tested both transmission and reception with my much fuller-featured Yaesu FT-7B.

There are a few bits missing, which is very much the point. The intention of the Pixie II is to provide the basic operating radio so that the operator can make improvements. In this case I would like to add a proper audio power amplifier with volume control, an automatic gain control for the received signal and some tuning capacitance. I'll need to take some measurements but I'm hoping I can get a stable 20 kHz tuning range with appropriate loading capacitance on the crystal. I could also experiment with an antenna filter with a sharper roll-off.

When I make these improvements, coupled with a decent antenna there's no reason why I couldn't have proper QSOs with this radio or even run it in a contest. Being low power and on 80 metres, it is not going to get much more range than Tasmania unless we get some crazy sun spots so I'm hoping that I'll be able to find some other morse operators within the state. If you're a radio amateur you should build one of these.

Perhaps I should learn morse code too.