It works! I have a Raspberry Pi transmitting WSPR and it has been heard.
Map from WPSRnet
Using WsprrPi is easy- to transmit on three different bands in rotation with frequency calibration using NTP the command line I used is:
The vertical is not in good shape (broken counter poise wires and other problems) and I didn't check the SWR. Still, the 10mW signal from the Raspberry Pi was heard on 2 bands by 4 different stations. W0AY, on 30m in Montana, recorded the most reports (63) over the first 24 hours. Reports on 20m were from N0HAV, K0SQL53, and the distance winner (2041 km) VE6PDQ.
This has been great fun and I recommend that anyone with a Raspberry Pi and an Amateur Radio license give it a try.
Next I need to look into WSPR reception...