Weekend Project
20/80m CW QRP Transceiver
Oktober 2019, DL6ZB

This a 20/80m imaging CW QRP transceiver.

Imaging is a transceiver technique which was widely used by homebrewers during the 60's and 70's. Usually a 9 MHz IF was used for imaging. A 5.0 to 5.5 MHz VFO results in 3.5/14 MHz input/output at the same time. The desired band has to be band filtered.

The output power of the described transceiver is about 1.5 watts into a 50 ohm load.
Design goal was to make it simple and unexpensive with most parts from the junk box and with as many BJTs and simple OPAs as possible.

Most of todays homemade projects uses micros and more or less sophisticated frequency engines. No doubt, these projects are of great value when it comes to learn more about modern RF technologies.

However, there still are a lot of homebrewers searching for more traditional projects. If that's what you're looking for the you're all right here with that project.

Here's the final design. The receiver is a single conversion with an IF of 9 MHz. A junk box 2,7 kHz KVG XF9B SSB crystal filter was used here. Any cheap CB radio SSB crystal filter will do the job.

That receiver has a 2,4 kHz bandwith so it isn't very selective for CW use. Therefore I added a simple but suprisingly effective 300 Hz bandwidth OpAmp AF filter. Since the IF amplification is not too high, this filter works most of the time. A better solution would be a CW crystal IF filter. However, I hadn't one in my junk box. The OP-AMP must do the job.

A small speaker or a high impedance headphone may be used. A low impdance headphone would require either an additional AF stage or a plug for connecting it to the loudspeaker output.

The pictures below are from a prilaminary stage. I designed the transceiver "manhatten style" or "dead bug style". This allows modifications at any time.

Click on schematic for a high resolution PDF version ( version October 2019).

circuit diagram ABACUS-2

Front View.

Top view.

- VFO with a 1:36 planetary gear
- IF and AGC amplifier (open case)
- S-Meter
- Crystal Filter
- AF amplifier

Bottom view.
- Front Plate
- RF TX pre and power amplifiers
- AF pre-amplifier and AF filter
- 9 MHz RX&TX crystal oscillators
- Bandpass and TX low pass filters
- T/R switching