Found an interesting post and video about doing soundcard based radio direction finding: http://dangerousprototypes.com/2012/01/27/28c3-soundcard-based-radio-direction-finding/
The talk is a little awkward but he gets the point across. The major takeaway is that he is feeding the audio from the radio into one channel on his soundcard, say left channel, and then the other side is a pulse to indicate the antenna switcher has switched. Then these signals may be processed using DSP techniques to figure out what the phase of the audio signal is and match it to which antenna is active from the pulses on the other channel. It seems like a lot of expensive processing to do something that can be done in hardware but a computer gives you the opportunity to send the data elsewhere over packet or wifi. Coordination could also be accomplished.
The other interesting bit I liked from his video is his use of an antenna switcher IC meant for cellphones. It can work from 300khz to 3.5ghz so it has plenty of range. It's a tiny surface mount chip but he did find it in SSOP-16 which is easier to solder than a QFN. The SSOP-16 was mounted on a SparkFun carrier board to allow easier access to the pins for breadboarding.
I've wanted to experiment with RDF for awhile. Not only could you apply it to Ham Radio uses, such as finding interference or foxhunting or other roles.. but it could also be handy for scanner enthusiasts. Imagine scanning through a frequency range, hearing some traffic and being able to locate the traffic right to the building it's coming from. This would be very handy for maintaining lists of frequencies for different factories and other workplaces.
