We are planning on getting a balloon up this summer. I have been working on software running on a Parallax Propeller and it has been working very well on WSPR mode.
I did originally want to use an Arduino Nano. You have a good code base to work with. I have already made many changes to it and hope to be testing within the week.
The Propeller doesn't require a synthesizer module because I can devote a cog to the transmit function and generate the frequencies directly on an output pin. But the prop is more expensive.
We will weigh the pros and cons of Arduino vs Propeller later.?
Steve, w8an
Steve S