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The double-layer pillbox is a microwave parallel-plate system in which the image space and the object space relative to a two-dimensional internal reflector are electrically separated by a metal septum. The double-layer prevents the shadowing effects and impedance mismatch that result in a single-layer pillbox when energy from the reflector reenters the primary feed. It also allows correction of the optical aberrations of the system in the image space, the object space, or both. The pillbox with semicircular reflector has wide-angle scanning properties. Its inherent spherical aberration can be reduced by such elements as dielectric lenses, geodesic contours, quasi point-source feeds, and auxiliary reflectors. These techniques have been applied to the construction of pillboxes whose wide-angle scanning properties and improved radiation characteristics substantiate the theory.