The code uses the hardware on the raspberry pi that is actually meant to generate spread-spectrum clock signals on the GPIO pins to output FM Radio energy. Just add a quarter wave length of wire to GPIO 4 to act as an antenna.
Usage: program wavfile.wav [freq] [sample rate]
Looking at the source, it seems simple enough to narrow it a bit:
In pifm.c look for:
float dval = sample*15.0; // actual transmitted sample. 15 is bandwith (about 75 kHz)
Obviously this isn't a clean transmitter implementation, so don't expect much more than an autofox.
For a low power foxhunt it might amusing to wire a motion sensor to the other GPIO to adjust your transmit and announce timing cycle, when folks are getting close.
Apparently some people think like me:
Plenty of folks using the basis of this for Weak Signal Propagation Reporting
It would also be interesting to use it as a Part 15 FM transmitter with modified code to allow a GPS for transmitter phase locking.
Alessandro, IK1PLD released a binary to the Yahoo Raspberry_Pi_4-Ham_RADIO group:
Narrow Band FM transmitter derived from pifm. It is able to transmit FM from VLF to UHF with a settable deviation and pre-emphasys, frequency correction and subtones. It runs under debian distr. In a SSH session type ./nbfm.out for help.