This week at an arsenal in Huntsville, Ala., defense researchers are testing a new high-power microwave (HPM) bomb—one that creates an electromagnetic pulse capable of disabling electronics, vehicles, guided missiles, and communications while leaving people and structures unharmed. The tests mark the first time such a device has been shrunk to dimensions that could make it portable enough to fit in a missile or carried in a Humvee or unmanned aerial vehicle.

Microwave weapons have been sought for decades, but the problem until now has been the portability issue. The bomb to be tested, developed at Texas Tech University, in Lubbock, with U.S. Army funding, is a 1.5-meter cylinder with a diameter of about 15 centimeters. These dimensions were the most difficult of the three metrics the Army asked Texas Tech to meet, according to Larry Altgilbers, Texas Tech's contract monitor and an engineer at the U.S. Army Space and Missile Defense Command (USAMDC), in Huntsville. The bomb also had to operate under its own power and, of course, generate lots of microwaves.

"It's a big deal" that an HPM bomb has been shrunk to this size, says Edl Schamiloglu, a professor of electrical engineering at the University of New Mexico and a noted expert in the field of high-power microwave sources. The military would be able to actually use these."
IEEE Spectrum: Portable E-Bomb to Be Tested

Currently there's no known real defense against EMPs that could protect entire cities, so they're all completely unprotected.