10m Lesser Chirpy Transceiver

A handful of parts makes a full CW transceiver for 28MHz

This is probably the simplest CW transceiver possible for use on 10m. Based on my XBM80-2 design for 80m, this is essentially the same circuit redone for a 28.060MHz fundamental crystal. I had named it the XBM10-2 but now decided a more apt name was Lesser Chirpy. Power output is around 100-200mW, which is more than enough to cross the Atlantic on a good day. Unfortunately the first version (called Chirpy) produced a fair amount of chirp on the CW. The derivative Lesser Chirpy overcomes the chirp by FSK keying the oscillator. Lesser Chirpy has been copied in the USA.

Circuit Description

Lesser Chirpy is a crystal controlled Colpitts oscillator used as a TX oscillator with the key down and as a direct conversion receiver with a single stage oscillator-mixer on RX and separate audio gain stage with the key up. Keying shifts the oscillator frequency by altering the reactance in series with the crystal

Very inexpensive, ubiquitous, 2N3904 transistors are used. One can argue whether this receiver is a direct conversion or a regenerative one: in a single stage it is probably not possible to say it is one or the other. At least the receiver is crystal controlled and does not need a reaction control, so on balance it is more likely best described as a direct conversion receiver. The receiver audio output into the high impedance crystal earpiece is low, but it can hear down to around 2uV (-100dBm) in a quiet room. Your ears may be better than mine. This sensitivity has been checked on 3 different examples built. There is no real audio selectivity so you will need to use your "ear-brain filter" to select the 800Hz-1kHz received audio signal. Broadcast breakthrough does not seem to be an issue at all, which is surprising as there is little front end selectivity or rejection of out of band signals. The transmitter really needs the simple 3 component low pass filter adding for serious use, but this was omitted in the basic "no frills" version. It is possible that an ATU will provide the additional filtering in some set-ups. Don't try using a mains PSU as audio hum will be a problem.

The basic idea is to switch to TX by closing S1 (increases the oscillator output to around 150mW) then key a capacitor in series with the crystal. As the oscillator is on at full power with the key off or on (FSK shift) the chirp, that was an issue on the earlier design, goes. There are a couple of disadvantages (a) the rig is no longer full break-in, and (b) when the key is up there is the "opposite" morse about 1kHz up the band, which may confuse. Still, this is a "for fun" rig and, with virtually no chirp, a much better radio. A small low pass filter on the output is advisable unless your ATU provides this.

I also adjusted the turns on the main collector inductor finding that a tap 3t from the cold end worked better. You will almost certainly have to experiment with the values of capacitance around the crystal and key to get the right amount of TX-RX offset.

This is a "for fun" rig, so don't expect incredible performance, but it does work. On the first day of use 2 DXCC countries were worked. On receive it has copied plenty of stateside and European signals on the QRP calling frequency.