# Tilted Beam Fabry-Perot Antenna with Enhanced Gain and Broadband Low Backscattering
## 1 Introduction  
Stealth targets are platforms which absorb or scatter the impinging electromagnetic waves, and hence are low detectable targets on a radar screen. These air/sea/land-based platforms have low radar cross section (RCS) areas and are essential for military and defense. Usually, to communicate with these platforms, antennas are mounted on them, and as antennas are a big source of radar signature, the overall RCS of the platforms increases significantly, easing their detectability on a radar [1, 2]. Therefore, the design and development of antennas with low RCS for stealth targets is attracting more and more attention. One of the promising methods, based on metasurfaces, for achieving RCS reduction of a planar antenna is to utilize a frequency selective surface (FSS), either as a radome [3, 4], or as a modified ground plane [5, 6, 7]. In both configurations, the intended antenna operation (in-band) is preserved/least disturbed, while it is ensured that out-of-band frequencies are either absorbed or transmitted through (band passed), respectively. Recently, wideband RCS reduction including both in-band and out-of-band frequencies was investigated in [8]. A band pass FSS was combined with an absorbing FSS (AFSS) and placed at a certain heightabove a 2 \(\times\) 2 patch array antenna. This achieved out-of-band absorption. The surface acted transparently for in-band frequencies. For in-band absorption, patch resonators were employed, co-planed with array elements. The antenna characteristics were unaltered. A double-layered resistive metasurface was investigated in [9], to reduce in-band as well as out-of-band RCS of a patch antenna. The upper layer held above the patch antenna reduced out-of-band RCS, whereas the in-plane lower layer loaded around the patch antenna reduced the in-band RCS. The total band of RCS reduction covered 2-15.5 GHz of frequency range, and \(\sim\)2.8 GHz was the patch antenna's operating frequency range. The gain loss was 0.6 dB. When metasurfaces are employed in the configuration of a Fabry-Perot (F-P) cavity, backscattering can be reduced, and peak radiation of the underlying source antenna can also be enhanced. In Reference [10], enhancement of gain along with wideband RCS reduction was achieved based on a composite superstrate design that utilized absorptive FSS (resistive square loops) on top and a partially reflecting surface (PRS) on its bottom. This superstrate was held at a certain height above the patch antenna in order to form an F-P cavity. The transmission and reflection characteristics of the composite surface were designed in a way that in incident mode, the out of band frequencies were being absorbed, while in-band frequencies passed with slight attenuation, thereby reducing both out-of-band and in-band RCS simultaneously. In radiation mode, the in-band wave was intensified due to PRS effect of the composite surface, enhancing the radiated gain. Using a similar technique, the RCS reduction from 2-14 GHz was achieved in [11], along with an antenna gain enhancement of 4.3 dB; however, the in-band RCS reduction was inferior. The in-band RCS performance of [11] was improved in [12] by employing a metamaterial absorber (MA). In Reference [12], out-of-band frequencies were absorbed by absorptive FSS and in-band frequencies were passed without being attenuated. Once reaching the antenna surface, in-band wave was absorbed by a metamaterial absorber layer surrounding the antenna with an intervening gap. In the radiation mode, a periodic surface below the AFSS formed a quasi-PRS in conjunction with the ground plane of the antenna and enhanced the radiated gain. In this way, wideband RCS reduction (in-band + out-of-band) and gain enhancement were achieved simultaneously. Similarly, gain enhancement, wideband RCS reduction, as well as operating frequency reconfiguration were achieved in [13].