# Tilted Beam Fabry-Perot Antenna with Enhanced Gain and Broadband Low Backscattering
## 2 Proposed Antenna System and Its Operation
### Unit Cell Configuration  
The unit cell of the proposed design is displayed in Figure 2a. A metallic square loop with four lumped resistors (of 100 \(\Omega\) each) constitutes the top absorptive surface. The lower part is a composite PRS that has a fixed size metallic patch on its upper side, and its bottom also has a metallic patch, but its size is variable. Both patches are square in shape. From the literature, it is well known that a surface of periodic resistive metallic loops etched on a dielectric that has a full backing metal plate functions as a wideband absorber of the electromagnetic (EM) wave [25]. Due to this backing metal plane, no EM wave can transmit/pass through it. When the goal is to design a low RCS F-P antenna using the said absorber, its (absorber's) backing metal plane is modified or replaced with a partially reflecting surface. This modification is essential, as a full metallic plate directly above the antenna would disrupt the antenna's operation by fully reflecting the EM wave, thus, not allowing the transmission and reception of the signal through the antenna. Now, coming back to the proposed unit cell as shown in Figure 2, our designed PRS is dual sided. Since our objective, in addition to RCS reduction, is also beam tilting, the lower surface of the PRS has been constructed from graduated patches (gradient surface). The purpose of upper surface of the PRS is to decouple the resistive surface from the lower phase gradient surface; otherwise, the absorption of the incident wave would suffer. Thus, the fixed size PRS patch acts as an intervening layer that is necessary to ensure parallel achievement of wideband absorption of the incident wave, as well as the beam tilting of the radiated wave. The frequency responses of transmission and reflection for a wave incident on the absorbing surface (from the positive \(z\)-axis towards the negative \(z\)-axis), and the frequency response of reflection for a wave striking the PRS (from the negative \(z\)-axis towards the positive \(z\)-axis) are sufficient to design and understand the unit cell operation and are discussed next. A high-frequency structure simulator (HFSS) was used to model the unit cell. The simulation was performed by applying Floquet port excitation and master/slave boundary conditions, as indicated in Figure 2b. The detailed frequency responses are presented next. For an incident wave towards the positive \(z\)-axis, the reflection coefficient (S\({}_{22}\)) in terms of its phase and magnitude response is shown in Figure 3a,b, respectively. These plots would determine how the transmitted wave from the antenna is going to interact with, primarily, the surface consisting of variable-sized patches, the phase gradient surface. A number of curves can be seen for each plot; each curve corresponds to a particular value of patch size (\(v\)). The values for the unit cell dimensions (\(g\), \(m\), \(R\), \(w\), \(l\), \(r\), and \(p\)) are shown in Table 1. The patch size varies from 8 mm to 12 mm. It is evident from Figure 3a that an appropriate phase gradient has been achieved at 6.6 GHz frequency. The range of the reflection phases is from +158\({}^{\circ}\) (for \(v\) = 8 mm) to \(-\)130\({}^{\circ}\) (for \(v\) = 12 mm), approximately. This totals to 288\({}^{\circ}\) of phase gradient space. Thus, the variable length patch surface effectively forms a phase gradient metasurface. From Figure 3b, the magnitude response at 6.6 GHz shows high values of reflectivity, all above 0.8, and this implies that the gain enhancement of the radiated wave would be achieved after determining the proper cavity height (resonance condition). The S\({}_{22}\) reflection phase plotted against varying patch size is shown in Figure 3c. This plot is also shown for the 6.6 GHz frequency. It reiterates that the reflection phase decreases (lags) when the gradient patch size is increased.