I first read about these kinds of devices in John Alexander's 1999 book Future War (Non-Lethal Weapons In Twenty-First-Century Warfare). Anyone interested in this kind of stuff should definitely pickup this book.

HERFs, or high energy radio frequency weapons (particularly microwaves), will become the weapon of choice for many global guerrilla operations (see Homemade Microwave Weapons for more). There are signs that these weapons are just starting to make it into the mainstream. David Giri, of ProTech, is currently building HERFs for the Marine Corps and the LA Police department. Here is more on the device, which has trialed at up to 50 meters away (source: Global Guerrillas)

I might have to pickup his newer book: