I had limited time to create an Unconference badge. I came up with this, a Raspberry Pi serving as MP3 player feeding a super-low-power FM TX module. Power for the Pi came from a phone booster pack.
The joke was that it broadcast a chiptune Rickroll, thans to my friend [Matt "Gasman" Westcott].
The unusual bit is the antenna, it's a fractal design to pack an FM-radio broadcast antenna into the small space of a badge. It's made from fence wire, with the top of a pin from a British mains plug as its connector. No rocket science, just take a 1/4 wavelength and a pair of pliers, bend the wire into 4, then bend every straight edge into 4, and keep on doing so until your pliers no longer fit the edges.