Usually, the reflectarray realized by using a single metal plane structure is suitable for a single frequency case or a limited frequency band. Furthermore, it is difficult to work in a wide scanning angle case, as a wide phase range would be needed [1; 2; 3; 4]. In our current research, dual-frequency systems are required for modern multimedia communication applications. Additionally, the interval between the two operating frequencies is relatively large. Single substrate may be too thin to have a smooth phase range for a lower frequency resulting slope. In the meantime, it may be too thick for higher frequency to obtain enough phase range. Generally, a tradeoff between the smooth linear level and the wide phase range is needed.  
The design of dual-band reflectarray antennas with high performance is one of the major considerations for multimode reflectarray design [5; 6; 7; 8; 9; 10]. One of the most common approaches for dual-band application is to use a multi-layer reflectarray. However, the top layer blocks the signal form the other layer, and this technique has a probability that the materials will physically interfere with each other as their sizes are changed to obtain the required reflection phases. In [5], a dual-frequency reflectarray using a dual layer ring element working at 7.3 GHz and at 31.75 GHz was designed and reported by [PERSON] et al. The highest efficiencies measured were 46% at 7.3 GHz and 38% at 31.75 GHz, respectively. As described in the literature, the efficiency drop at the Ka band is caused by the interference of the C band layer. [PERSON] et al. proposed another dual-frequency reflectarray working at 7.3 GHz and at 31.75 GHz [6]. Backed-FSS is utilized as the reflection plane at the Ka band. Two reflectarray layers were used, which also cause the problem of interference with each other. Another technique is using elements of two working frequency bands printed on the same layer. By using this technique, the blockage of the top layer in a multi-layer dual-band operation can be eliminated. However, the reflection phase of a reflectarray relies on the thickness of the substrate sensitively. That means wide and smooth phase range can be hardly obtained at both frequencies simultaneously.  
In our research, a concept for a reflectarray design using a sandwiched FSS structure is proposed to solve the dual-band problem. To simultaneously obtain a larger reflection phase range, a smooth reflection phase curve and dual operating frequency bands, an additional FSS layer is sandwiched between the metal ground plane layer and the reflectarray element layer. For the incident wave, the FSS layer can play the role of the reflecting plane for a higher frequency band. Lastly, for the lower frequency band, this FSS layer is transparent and the reflection plane acts as the metal ground plane. A dual-band and dual-polarization reflectarray which comprises cross dipole elements and sandwiched FSS layer is designed and measured to validate this concept. Smooth and wide reflection phase ranges are achieved at both working frequency bands (12 GHz and 16 GHz). A 7 \(\times\) 11 element reflectarray is fabricated to demonstrate the performance.