A scanning array antenna (300) produces a directional beam by differentially rotating two, co-axial, flat phasing plate assemblies (302, 304). Each plate (302, 304) consists of a phase shifting means designed to efficiently pass incident radio frequency (RF) energy while imparting a particular phase shift to the energy. The energy can be supplied by a feedhorn (406) located behind the plates (402, 404) or can be introduced above the plates (508) and reflected by a ground plane (506) on the bottom plate (504). A method for producing the directional beam using the scanning array antenna (300) determines (804) the desired beam direction, calculates (806) the appropriate plate rotation angles (326, 328), and rotates (808) the plates (302, 304) at the appropriate time.