# A Wideband Reflectarray Antenna for Satellite Application with Low Cross-Polarization
## 1 Introduction  
Reflectarray antennas (RA) have been studied intensively due to their promising features, owing to their low profile, reduced cost, and ease of manufacture and deployment [1, 2]. However, they suffer from narrow bandwidth, which is predominantly due to the nonlinear reflection phase response of the elements. Numerous techniques have been reported in the literature to enhance bandwidth performance, such as an air gap between the ground and substrate [3, 4], thick substrates [5, 6], multi-resonating elements [7], meander-shaped concentric squares [8], fractal elements [9], multi-layered multi-resonating structures [10], subwavelength elements [11], hybrid elements [12, 13], dual-cut-ring elements [14], and circular ring and diagonal patch ring combination elements [15]. The above techniques remarkably improve bandwidth. In some designs, an additional layer of air is used for a slow phase variation [3, 4], at which point it is hard to maintain a constant level of air gap across the wider aperture.  
Recently, in [16] a unit cell that comprises a multi-resonating structure with a cross dipole enclosed by cross-dipole loops generated a wide-phase variation of 802\({}^{\circ}\) that has a variable phase sensitivity with a maximum value of 300\({}^{\circ}\)/mm. In this design, foam is used between the substrate and the ground. Similarly, the authors in [17] proposed a unit cell that has a wider and linear phase range of 700\({}^{\circ}\) that is obtained by applying a Minkowski fractal-shaped reflector with an aperture-coupled phasing stub.  
Another important issue is the cross-polarization (x-pol) discrimination that causes the loss of energy. The x-pol can be reduced by providing a proper feed, and the design of the unit cell. The authors in [18] presented a single-layer structure that consists of a slotted rectangular patch with concave rectangular arms. The x-pol can be reduced by a special arrangement of elements that is \(-26\) dB. However, the phase variation of 360\({}^{\circ}\) is realized. Another design in [19] realized a wideband single-layer structure that comprises a slotted rectangular patch with concave rectangular arms. The structure features linear phase variations of 360\({}^{\circ}\) and x-pol level of \(-32\) dB. The authors in [20] presented a combination of a spiral dipole and four square rings that achieves a wider gain bandwidth of 35% (1 dB) and aperture efficiency of 55%. However, the cross-pol level is \(-30\) dB. A reflectarray basedon a windmill-shaped unit cell that features a 3 dB band of 35.7% with an efficiency of 51.7% is presented in [21]. The cross-pol level is \(-\)25 dB less than the co-pol level. High aperture efficiency of 62% is achieved with a better cross-pol level of \(-\)36 dB by employing a 0.15\(\lambda\) element in [22]. The x-pol can be reduced by designing the unit cell to have current in only a single polarization. However, it becomes difficult to realize the single polarized current when designing multi-resonating multi-layered unit cells, as the currents of closely multi-resonant elements are dependent on each other. In this paper, a new subwavelength element comprising a double-layer unit cell for a broadband RA is proposed. The unit cell offers a linear and wider phase response of 800\({}^{\circ}\). The slopes of the reflection phases at different frequencies are nearly equal, to realize a wide-bandwidth performance. Based on the proposed unit cell structure, an offset-fed RA is designed with the horn feed located at the mirror image of the beam at 30\({}^{\circ}\). The 3 dB gain bandwidth (GBW) performance of 32% is achieved. Furthermore, the RA provides a low level of x-pol that has been reduced by canceling out the current in an x-pol direction in the individual unit cell rather than the special arrangement of the unit cells in [18] on the RA aperture.