To use:
Connect a low-pass filter to GPIO pin 4 and ground on your Raspberry Pi. Connect an antenna to the LPF.
Compile by doing:
gcc -o wspr-pi wspr.c
Usage: wspr-pi <callsign> <locator> <power in dBm> <frequency in Hz>
e.g.: wspr-pi MD1CLV IO74 30 137500