15 Jul 2012
My objectives are to make the design simpler than the original 2010 design (no expensive SBL1 double balanced mixer!), produce more power and improve the RX performance. Also I want to reduce the need for special parts such as toroids.
The design evolving is, like all my designs, a mixture of my own ideas and the best from other people. When the design is ready I will put it here and on my main website. I've moved away from 28MHz IF (too high, too much start-up drift) and instead gone to 3.7MHz using a 3.2MHz crystal available from GQRP inexpensively. The TX mixer (none is needed on RX as it is uses the FT817 in split mode receiving directly on MF) is a simple bipolar stage followed by a low pass filter. For the PA I have largely copied GW3UEP's excellent designs for a 25 (and later 100W) amplifier. Initially I am using an IRF510 from 13.8V and getting around 10W RF out. The next stage is to look at the RX front end. I want a little gain ahead of the FT817 but with good large signal handling and decent filtering. The FT817 works OK at 500kHz but is a little deaf.
So, a satisfying few hours building.