Bandwidth Enhancement of Single-Layer Microstrip Reflectarrays with Multi-Dipole Elements

The gain bandwidth limit of a single-layer microstrip reflectarray is analyzed using multi-dipole elements. By observing the reflection phase characteristics of the multi-dipole elements with different numbers of dipoles, it is shown how the bandwidth can be enhanced by increasing the number of element resonant structures. The fundamental limit of the bandwidth enhancement is then derived by analyzing the coupling between the surface wave and the incident plane wave. It is shown that the surface wave can be excited even with the plane wave of normal incidence on the reflectarray and that the excitation frequency of the surface wave is dominated by array parameters and is almost independent of the number of resonant structures. The degradation of the element reflection characteristics caused by the surface wave, which limits the reflectarray bandwidth, is investigated. By comparing the radiation characteristics of three reflectarrays with three-, five-, and seven-dipole element, respectively, the analysis of the bandwidth enhancement and its limit is verified. The measured 1-dB gain bandwidth of the three-dipole element reflectarray is 25.12%, and the bandwidth is enhanced to 33.52% with the five-dipole element. However, because of the surface wave, no further bandwidth enhancement is achieved when the seven-dipole element is used, as predicted.