I’m fascinated by the WSPR (http://wsprnet.org) network and found out you can build a transmitter station with an Raspberry Pi.
So, because I have a few Raspbery Pi comptuers setting about doing nothing, it’s time to put one to work. The code for this is at (https://github.com/JamesP6000/WsprryPi). WSPR net has some documentation on it (http://wsprnet.org/drupal/node/5353). The output from the RaspPi is a square wave so lots of filter is needed. (http://www.gqrp.com/harmonic_filters.pdf). A kit is avaible at (http://www.kitsandparts.com/univbpfilter.php).
I also have a HackRF. I’m thinking I could use it to receive WSPR stations on a schedule. May take some work.