# Tilted Beam Fabry-Perot Antenna with Enhanced Gain and Broadband Low Backscattering
## 1 Introduction
The frequency reconfiguration in [13] was achieved based on the technique presented in [14], in which varactor diodes were used within the capacitive grid of the PRS in order to achieve tunable reflection phases, and thus the tuning of the operating frequency. In another work [15], scattering of incoming in-band wave towards the non-threat direction and absorption of out-of-band wave were achieved by employing phase gradient metasurface (PGM) and MA, respectively. Excellent broadside peak radiation enhancement was attained for a patch radiator. In the same work [15], the design without MA was also presented, with gain increase even higher. It is important to note that the use of PGM in this work was primarily for scattering away, and not absorption, of the in-band incident wave. Although a good in-band RCS reduction was achieved, from a stealth point of view, absorption of incoming radar waves might be preferable over scattering them away. A different type of antenna is a tilted beam antenna. Instead of employing complex electronic phase shifting techniques or bulky mechanical methods for beam tilting, a gradient-based metasurface is a more practical method for achieving a beam tilt when a fixed angle is desired. It is useful in commercial and military applications where above/below the horizon communication is desired, such as in a base station (BS) [16], satellite communication platforms [17], side looking airborne radar (SLAR) [18], as well as other defense-related imaging and reconnaissance applications. The technique of achieving beam tilt is to introduce a gradient in the dimensions of the periodic elements which constitute the PRS. The designed phase gradient metasurface can direct the radiated wave away from the boresight to a tilted angle direction. The gradation in the constituent unit cell elements varies the reflection magnitude and phase along the inductive or capacitive grid, and this variation of transmission/reflection properties gives rise to a beam redirection effect. The principle was used in [19; 20; 21; 22] to achieve \(\pm 20^{\circ}\), \(33^{\circ}\), \(18^{\circ}\), and \(27^{\circ}\) beam tilt, respectively. Owing to a number of applications of tilted beam antennas [16; 17; 18], which also includes their use in military applications, an investigation of their RCS suppression is deemed important. According to the background literature, it is evident that a composite superstrate in an F-P cavity configuration has been utilized for wideband RCS reduction, as well as boresight gain enhancement. It can also be observed that a progressive phase delay mechanism introduced into a PRS can redirect the antenna beam. Therefore, leveraging the two concepts together in a single antenna system, to exploit the advantages of both, is worth exploring. In this work, we aim to design a phase gradient metasurface with an absorptive FSS, in an attempt to achieve wideband incident wave absorption for monostatic RCS reduction, as well as high gain and tilted beam operation in the transmission mode of the antenna system. The designed F-P cavity is excited by a rectangular patch antenna operating at 6.6 GHz (C-band). A beam tilt of \(13^{\circ}\) is achieved in the E-plane, with low side lobe levels (SLLs), and the achieved gain is 11.5 dBi. For x and y incident wave polarizations, 13 dB average monostatic RCS reduction is achieved, spanning over 5.6-16 GHz (96.3% bandwidth). The antenna is fabricated to verify the simulation results.