Some preconditions we have to look at. For a successful slow QSO, we need to assume condx of a single band, so that both stations can receive the counterpart equally. TX during RX in one band at the same time is a challenge even on the 2m-band. So, what can be done?
The problems, I figure, are more on the receiving side of the design. So here are some preliminary ideas that came to my mind:
- frequencies in one band, the furthest possible apart
- a very narrow-band RX antenna, e.g. a high Q magnetic loop
- a crystal notch filter for the TX frequency
- a crystal front end filter
However, for experimentation, there are a couple of possible frequencies (cheap crystals available).
- 80m: 3.500MHz & 3.6864MHz
- 40m: 7.000MHz & 7.15909MHz
- 20m: 14.000MHz & 14.31818MHz
- 10m: 28.188MHz & 28.636MHz
For me, the obvious choice for making a full duplex QRSS transceiver would be K1SWL's 80m-Warbler. I could show that the Warbler is quite potent on 40m when modified. I would assume that the kit could equally modified for the 20m band.
Based on the Warblers, one would need two per band. The Warblers make nice transmitters too...
Another approach could be the use of a superhet receiver with a (switchable) notch filter in the front-end, remember, it is all about keeping the TX out of the RX when going full duplex.