Bandwidth Considerations for a Microstrip Reflectarray

The paper describes a theoretical investigation into a limited bandwidth operation of a microstrip reflectarray. Two main factors limiting the bandwidth are considered. One is related to the requirement of phase compensation to convert a spherical wavefront launched by a feed into a planar wavefront. The other one is linked to the limited phasing range of microstrip antenna elements. The two factors contribute to the reflectarray phasing errors that in turn reduce its gain as a function of frequency. Simple formulas for an upper bound of gain bandwidth are derived, assuming the phase compensation by the elements is independent of frequency changes and verified against the results produced by other researchers. It is shown that the phase errors incurred in the path equalization to obtain conversion from spherical to planar wavefronts have a more profound effect on the reduction of operational bandwidth of the reflectarray than the phase truncation implemented on the required phase from each element.