Minimization of beam squint in microstrip reflectarrays using an offset feed

Microstrip reflectarrays utilize the advantages of planar antenna technology in providing an alternative to parabolic reflectors. The main barrier to implementing a microstrip reflectarray in many applications where reflectors have traditionally been used is that the gain bandwidth is typically only 5 percent (defined for a 1 dB drop from peak gain) for a microstrip reflectarray. Factors that limit reflectarray gain bandwidth are discussed. Reflectarrays can be designed with a prime focus feed, or an offset feed. Using a prime focus feed is the most straightforward approach, however this configuration results in aperture blockage effects. Also, in reflectarrays which are scanned off broadside, beam squint with frequency will occur for a prime focus feed. The offset feed is more attractive in that aperture blockage effects are minimized, at the expense of a slightly more complicated design and analysis procedure. A further advantage is that the effect of beam squint with frequency can be greatly minimized by choosing the correct offset position. In this paper a comparison of beam squint in prime focus and offset fed microstrip reflectarrays is investigated. A simple criterion for minimizing beam squint with frequency is presented, and results from an analysis of a linear reflectarray are discussed.