# A Low-Profile Dual-Polarized Transmitarray with Enhanced Gain and Beam Steering at Ku Band
## 1 Introduction  
Modern high frequency wireless networks require highly directive antennas to leverage beam forming and beam steering to enhance the information channel capacity. These types of antennas are hugely used in satellite systems, wireless power transmission, and 5G networks. The suitability of phased array antennas in different application areas is well known but require complex feed networks [1, 2, 3, 4, 5, 6, 7]. In high-gain applications, significant losses are common because the complexity and insertion loss in the feed network increase with the number of radiating elements. Additionally, losses and costs are increased due to the employment of numerous phase shifters. To overcome this problem, transmitarray antennas are proposed in several studies as a promising alternative to conventional phased array antennas [8, 9, 10, 11, 12, 13, 14, 15, 16]. A transmitarray (TA) generally consists of a single feed antenna and a metasurface capable of controlling the phase and amplitude of the feed signal. A TA system can be used for effective beam forming if it consists of a metasurface which has the capability of low loss transmission with a control of 360\({}^{\circ}\) transmission phase. A metasurface can be equipped with components such as varactors, diodes, MEMS, and liquid crystals (LC) for the purpose of beam steering in a desired direction, though these active elements increase fabrication complexity and losses [17, 18, 19, 20]. On the other hand,moving the position of the feed source also leads to beam steering [21; 22; 23]. The performance of the TA depends on two essential factors, the size of the metasurface (D) and the focal distance (F) of the metasurface. The ratio F/D influences the power transfer to the metasurface [24]. As a result, the spillover and taper efficiency product depend on the F/D ratio. The increase of focal distance (F) causes an enhancement of taper efficiency but a decay in spillover efficiency, whereas enlargement of the size of the metasurface (D) improves the spillover efficiency but makes the metasurface less compact. Low-profile beam-steering TA antennas can be designed using a phase gradient metasurface. For instance, low-profile beam-steering TAs for 5G mm-wave communication were developed with multiple feed horn antennas in [25; 26; 27; 28; 29]. Another important feature of next-generation wireless communication systems is the dual-polarized operation which increases the reliability of the radio link and enhances the overall system capacity. Continuous phase tuning, dual-polarized TAs have been reported in the recent literature, via dual-frequency selective surface layers [30; 31; 32; 33; 34], the cascade of three metallic layers [35; 36; 37; 38], and quantized phase compensation [39; 40; 41; 42; 43]. This paper proposes a novel monolithic unit cell design that achieves low profile, dual-polarized operation, and 3-bit phase compensation with minimal transmission loss, allowing to obtain performance comparable with the state of the art in terms of gain, side lobe levels, and scan loss. These results are reached with a novel symmetrical geometry, making it intrinsically suitable for dual polarized radiation. An equivalent circuit model (ECM) of the proposed UC is then developed and the scattering parameters derived from that model are simulated, showing a good match with full-3D numerical simulated results. Based on the developed UC, a proof-of-principle test TA is designed and fabricated, its characterization confirming the expected gain and side lobe levels. Moreover, the developed TA also enables efficient beam steering over tens of angular degrees with minimal scan loss, demonstrating its potential for advanced beam-forming applications. The paper is organized as follows: after the Introduction, Section 2 describes the UC design and its simulated performance. Section 3 presents the ECM of the UC and compares the corresponding scattering parameters with those calculated via the full-3D numerical simulation.