From Popular Science:
An enemy missile has no strategic value if its computer is down. A high-power-microwave emitter can disable a missile’s electronics on the launchpad, leaving bystanders unharmed — and now Texas Tech University engineers have a plan to scale down the truck-size tech.
To make strong microwaves, you need a lot of electricity, usually from a bulky generator. Instead the team packed small explosives into a five-foot-long, six-inch-wide tube. The detonation produces a burst of electrons, which power a microwave tube—similiar to the type in a microwave oven—that radiates microwaves.