# Dual-Band Frequency Selective Surface-Backed Reflectarray for High-Speed Ka-Band Satellites
## 1 Introduction  
To sustain the demands of 5G communication in terms of mobility and coverage, satellites and space technologies play a critical role [1]. To provide high-speed internet coverage even in remote and underserved regions and achieve unique advantages of seamless, high throughput, uninterrupted global connectivity, satellites serve as a backbone for 5G networks [2]. Progress in the satellite and launch technology pave the way for these demands, which would not be possible technically and economically with terrestrial communication that severely relies on buried fiber optic cables.  
Ka-band satellite is a proven technology that provides high-speed services with higher data rates by using smaller receiving antennas [3, 4]. Since antennas play a critical role in satellite communications systems, high-gain antennas are essential to operate at the receive and transmit frequencies of the Ka-band (20/30 GHz) simultaneously. Reflector antennas with high gain have served as the mainstream technology until the last decade. However, progressively increasing demands of the end users have triggered the advancement of innovative antennas in both spaceborne and user terminals. As the new generation of high-gain antennas, reflectarrays that consist of planar arrays of conductive elements are developed as an alternative to curved, bulky reflector systems [5]. Besides their superior physical properties, reflectarrays combine the advantages of phased arrays and reflectors. With the individually controllable phases of reflectarray elements, the beam of the reflectarray can be generated according to the predefined benchmarks. Although printedreflectarrays have narrower bands compared to reflectors, they offer a practical solution for multi band antennas [6; 7]. Multiband characteristic of a reflectarray can be achieved by using broadband elements [8; 9; 10; 11], multilayer unit elements [12; 13], interleaved array grids [14; 15], or stacked arrays [16].  
The method to be applied mostly depends on the separation of the bands and specifications on the physical structure [17]. Both multi band operation and dual linear polarization may be achieved with a single layer reflectarray [18]. However, altering the phase of the unit elements for different bands results in reduced efficiency and bandwidth performance due to the mutual coupling generated by the phasing elements of the bands on a single aperture [19]. In recent years, efforts on realization of dual-band and/or dual-orthogonal polarization has led to the usage of FSSs in reflectarrays [17; 19; 20; 21; 22; 23; 24; 25; 26]. The FSS behaves as a ground plane and reflects the signal at one of the operation bands, whereas the signal is transmitted to the bottom reflectarray at the other band of operation.  
In this work, an FSS is used to isolate the reflectarrays designed to operate at the downlink and uplink frequency of the Ka-band satellite systems. With the usage of a simple ring resonator that is designed to operate as a planar bandpass filter at the downlink frequency of the Ka-band satellites, the beam is directed towards predetermined direction at two distinct frequencies. Improved gain and aperture efficiency is obtained by suppressing the mutual coupling generated due to the elements of the two reflectarrays. Instead of using a separate printed circuit board for the FSS, the conductive pattern of the single-sided FSS has been etched to the bottom side of the upper reflector, which was designed to operate at 30 GHz. The phases of the two unit cells, designed for 20 and 30 GHz reflectarrays, are calculated to radiate 20\({}^{\circ}\) by considering the existence of the FSSs in each case. A double-ridged horn antenna that has been oriented to the center of the reflectarrays is used as the feed antenna. The full-wave analysis of the reflectarray is evaluated numerically by CST Microwave Studio [27]. A prototype is fabricated and the performance of the reflectarray designed for transmit/receive operation is experimentally verified.