An anti-hydrogen plasma in a magnetic bottle is the only practical implementation. Lower frequency radiation would come from crompton effects on any surrounding matter. Unfornatually, even a 1 ng of antimatter is unbelievably expensive and next to impossible to store. It would take the power generation output of the planet going for at least a year to generate, isolate, and store that much antimatter. Like the nuclear EMP devices, if practical, the device would be too destructive for EMP only use.
Antimatter is the ultimate explosive in efficiency, and energy density.