Software defined radio (SDR) is one of the things I am most curious about. There is just so much data being exchanged via radio these days. It’s strange to think about the constant complex pattern of broadcasting happening all around us.
This video gives a bit of a taste of what is happening in one part of the world and across a fairly narrow range of frequencies:
It’s pretty cool that he is able to identify and analyze Chinese over-the-horizon RADAR. It shows some of the possibilities SDR opens up for hobbyists.
Much of the hardware required to seriously experiment with SDR is expensive. Interestingly, though, someone has figured out how to do the job for the 64-1700MHz frequency band using an $11 digital TV tuner chip.
You could do some very cool stuff with this: set up your own infrastructure independent computer networks, explore what sort of communication is happening around you, conduct intrusion detection (looking for interception devices broadcasting), and experiment with the security of your hardware, such as the Bluetooth chips in your phone and laptop.