On Beam Squint in Offset-Fed Reflectarrays

A full-wave receive-mode analysis is used to show that it is the shift in the location of the focal point with frequency (caused by nonconstant path delays over the surface of the reflectarray) that is principally responsible for the beam squint that occurs in offset-fed reflectarray antennas, and not the element type used or the lattice size. A similar transmit-mode analysis confirms that it is this focal point shift that implies noncoincidence of the focal point and the feed phase center at off-center frequencies, which results in a phase distribution over the reflectarray aperture with a slope other than that required to have the main beam in the desired direction. We further demonstrate this fact (computationally and experimentally) by showing that if the feed is physically moved to the shifted focal point at some off-center frequency, the main beam pointing direction at this frequency is restored.