XBM80-2 An Experimental 80m CW Transceiver

XBM80-2 An Experimental 80m CW Transceiver


An 80m transceiver using just 14 parts plus crystal and earpiece

This is an experimental CW transceiver for 80m, although the same idea will work on all HF bands. So far I have only used it on the bench and not on the air. I have called it the XBM80-2 (80m two transistors – very original!). It has something of my XBM80 and designs by PY2OHH and K4TWJ.

The circuit uses a simple Colpitts VXO producing around 120mW when the key is pressed. On receive, audio is derived across the 15k emitter resistor (could be higher to reduce backwave more?) and this is amplified in a single stage of audio gain. Output is to a small high impedance crystal earpiece. Sensitivity has not been accurately measured and will depend on the earpiece itself, but I was able to hear the output of my RF generator on its lowest setting and with an additional 10dB pad in the coax lead. The offset between RX and TX is around 400Hz. This is a bit too small, so more can be obtained by shorting the trimmer capacitor in series with the crystal. C1 is a fixed capacitor chosen to give 400-800Hz between switch S1 open/closed. In my case 33p was best and gave around 600Hz offset.

Update 3.2.10: I’ve changed the values in the collector circuit and this has reduced the AM breakthrough from medium wave a lot (still get some from 75m BC band). Also, power out is now 120mW at 12V (was 60mW) and the RX is a little more sensitive too. The receiver is picking up a lot of stations now on 3.560MHz (G, GW, LA, DL etc).  Making R3 82 ohms increases power still further, but I may stick with 100 ohms finally.

So far I’ve only had local QSOs with the rig (599 both ways and able to copy a 3mW TX at 3kms OK on RX) but I was able to hear the TX (at 60mW) 430kms away on the on-line SDR receiver in Twente in the Netherlands at RST529 the other evening. It is only a matter of time before I work some reasonable UK distances with this transceiver. For serious use it would need a low pass filter unless the ATU used had some decent selectivity.

On the left is a suggested PCB layout for this transceiver. It fits on a small 1 inch (25mm) square single sided PCB. Please note I have NOT built the transceiver on this board; this was an exercise to see how easily it would fit on a small board. With resistors “on end” rather than laying flat it could be made even smaller – probably 20 x 25mm. With SMT parts it would be possible to make this in 12 x 25mm – tiny!
——————-Tnx Roger, G3XBM!, 73 de yo2bzv—————-

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