I think you'll find that making an antimatter bomb may be harder than it looks. I'm no expert, but I believe if you just dropped a blob of antimatter on the ground, it would not explode in a perfect conversion to energy. Rather, it would flash into an incandenscent cloud that would radiate light, heat, and nuclear radiation and particles for a considerable time as it interacted with the incandescent normal matter around it. Eventually, the reaction would slow and dissipate. We are talking more of a giant thermite bomb than anything else.

To get an antimatter bomb to work, the same kind of explosive confinement technique used in normal nuclear weapons would have to be used to keep the opposing materials in contact long enough to generate a true explosion. Whether the techniques used in atom bombs would work or not is a problem best left to better physicists than I am.