Eric Seifert sent in some information on creating a home brew radio modem using a standard PC sound card, an iPod FM transmitter, and an FM radio. He has his current revision working at 9600 BAUD for distances in the neighborhood of hundreds of feet.
The sending side outputs an amplitude encoded data stream to the sound card, which is connected to the FM transmitter. On the receiving end, an FM radio receives the transmission, and outputs it to the receiving soundcard’s line-in, where it is then decoded. The hardware set up is extremely simple. It’s the software for reliably encoding and decoding the data (and handling error conditions) that’s the tricky part.
Eric released some example encoding/decoding software on his site, so you can start with that and take a swing at improving its error handling ability. You’ll need a Linux box with the ALSA and SDL libraries to compile it.
Radio Modem – Link