A High Performance Home-built HF Transceiver

Home construction used to be almost obligatory in amateur radio: high costs and scarcity of commercial equipment meant that transmitters were usually home-built, with government surplus or an Eddystone for the receiver. That seems a long time ago now, but whilst the commercial equipment of these days has certainly opened up the bands to many more people, it has meant that construction is no longer so popular.

But it is not quite dead; we were treated to an example of first-class home-construction with Steve R’s talk and demo of his HF transceiver. In terms of man-hours expended it was reminiscent of the G2DAF projects of the 1960s, needing the same level of commitment.

Steve’s project is based on the G4CLF kit of the 1980s, this using Plessey SL1600 series ICs. To this basic circuitry has to be added band-select filters for signal input to the first mixer and frequency generation of the first oscillator, also input to the mixer (7dBm at 50ohm). For transmit, a driver and PA are needed. Construction is modular, with BNC connectors handling RF interconnection between the screened boxes.

First we listened to the receiver side working on 80-metres, before Steve worked through a block diagram of the transceiver. As he did so, he passed around the audience each of the relevant modules.

As the project progressed, Steve encountered various problems, which meant design changes were necessary, which were first prototyped before final incorporation into the modules. PC boards were hand-built, each having its own ground plane. Each of the screened modules has a multi-way socket for the pin-diode switching of function and frequency.

The VFO is a 5-5.5 MHz FET Vackar, with a short warm-up time and very little subsequent drift. The VFO output is not input directly to the first mixer as this would only provide the 80 and 20-metre bands. Instead, the VFO is mixed against a bank of crystal oscillators, and this mixer output taken via a corresponding bank of filters for injection into the first mixer.

After Steve re-fitted the modules back, we listened to KB4HF calling CQ DX on 20-metres.

Receiver spec:

• 0.3 microvolts I/P for 10dB S/N
• Dynamic range 114 dB
• 3rd order intercept of +7dB

Transmitter spec:

• Carrier suppression 43dB