Investigation into Bandwidth Limitations of Microstrip Reflectarrays

The paper describes theoretical investigations into the bandwidth limitations of a microstrip reflectarray which uses variable size antenna elements for its phasing. Three main factors limiting the bandwidth are considered. The first one is related to the phase compensation that is required to convert a spherical wavefront launched by the feed into a planar wavefront. The second one is linked to the limited phase range of typical microstrip antenna elements. The third one is related to the match between the required phase as a function of frequency and the fixed size elements´ phase characteristics. The three factors are responsible for the reflectarray phasing errors that reduce its gain as the frequency departs from the centre frequency. It is shown that the first factor puts an upper limit to the reflectarray operational bandwidth, while the second one has a less profound impact. The third factor is influenced by choice of the element´s shape. It is shown that circular and square patches offer better match than printed dipoles to the required phase slopes.