A microwave heating system includes: a variable-frequency microwave source having a usable bandwidth about a center frequency; a waveguide with an input side connected to the source and an output side terminating in a launch structure; and, a metallic reflector plate facing the launch structure and perpendicular thereto and spaced therefrom at a distance of no more than twice the microwave wavelength at an operative frequency of the microwave source, the reflector plate including a recessed area of a selected size and shape located facing the launch structure so that a material to be treated may be placed between the launch structure and the recessed area for exposure to microwave energy. A related method is also disclosed.