Sunday, May 30, 2010

Full Duplex QRSS TRX Idea

This has been in my head for some time... What about in-band full duplex QRSSing?
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
I have no idea if this would actually work...
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
The high crystals for 40m and 10m may be a little difficult to find, check DigiKey. All others are either standard, or available at GenesisRadio. Remember, we need a crystal for every QRG, at least for the notch filter!

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.

1 comment:

  1. Taking into account that Eddie G3ZJO does full duplex on 28.322/28.126 (qrss/wspr) all the proposed frequencies should work quite well. Probably 3500/3579 should work nicely, too (one would then be the standard Warbler80 kit.

    BTW I have a pile of 28000 crystals on the way, so 28000/28322 could work, too.