# Tilted Beam Fabry-Perot Antenna with Enhanced Gain and Broadband Low Backscattering
## 2 Proposed Antenna System and Its Operation
### Unit Cell Configuration
Geometrical dimensions can be referenced from Table 1. From Figure 5a, it can be observed that the absorptive FSS behaves more like a bandpass filter towards the edges of the band. Around the center of the band, the reflection tends to rise while the transmission tends to decrease, and based on their absolute levels, it can be inferred that the incident wave energy is experiencing a degree of dissipation. Likewise, for the PRS alone, the S\({}_{11}\) and S\({}_{21}\) magnitudes are shown in Figure 5b. The inset depicts the simulated model. Each curve corresponds to a particular value of the patch size (\(v\)). As expected, low transmission and high reflection is observed over a wide band. Therefore, the combination of the two layers (absorptive FSS and PRS) is mandatory in order to achieve absorption, i.e., low reflection and low transmission, as was shown in Figure 4. The behavior of absorption with respect to the intervening air gap between absorptive FSS and PRS is discussed in Section 2.3. It is also necessary to mention that the term absorptive, here, only refers to the inclusion of resistors to the frequency selective metallic loops. Electric (E)-field plotted on the top face of unit cell is shown in Figure 6. The plot is for the case of transverse electric (TE) incident field (y-polarized), at a frequency of 12 GHz, and for the gradient patch size (\(v\)) of 12 mm. A strong magnitude of E-field can be witnessed across the two resistor gaps that lie orthogonal (along the \(x\)-axis) to the direction of the E-field vector. This indicates the primary absorption mechanism, as the resistors within these gaps would dissipate the impinging field energy as heat. The transverse magnetic (TM) field plot would be similar because the unit cell geometry is symmetric, except that  
Figure 4: Reflection and transmission response of the negative \(z\)-axis incident wave. (**a**) Reflection magnitude; (**b**) Transmission magnitude. the intense E-field would now appear at the resistor gaps lying along the \(y\)-axis. The plot in Figure 5 has been shown only at 12 GHz for a wave incident towards the negative \(z\)-axis; identical plots are expected to be witnessed at other frequencies also, and therefore have been excluded for briefness. Figure 5: Reflection/transmission response of individual layers of unit cell. (**a**) Absorptive frequency selective surface (FSS); (**b**) Partially reflecting surface (PRS). Figure 6: Electric (E)-field overlay plot shown on absorber surface for y-polarized incident wave (plot shown in dB scale). Figure 2:0 Frequency response against loop width (\(w\)). (**a**) Reflection magnitude; (**b**) Transmission magnitude. Figure 2:1 Frequency response against fixed patch length (\(p\)). (**a**) Reflection magnitude; (**b**) Transmission magnitude; (**c**) Reflection magnitude for the positive \(z\)-axis incident wave. Figure 2:2 Reflection magnitude response for different resistance (\(R\)) values. Figure 2:3 Frequency response against air gap (\(g\)). (**a**) Reflection magnitude; (**b**) Transmission magnitude; (**c**) Reflection phase for the positive \(z\)-axis incident wave. Next, the variation of \(p\) is studied; its \(\mid\)S\({}_{11}\)\(\mid\), \(\mid\)S\({}_{21}\)\(\mid\), and \(\mid\)S\({}_{22}\)\(\mid\) are displayed in Figure 9a-c, respectively. From Figure 9a,b, it can be observed that some further fine tuning of absolute absorption can be achieved by tuning \(p\), particularly at the operating frequency. However, the bandwidth of absorption does not show significant dependence on the value of \(p\). Furthermore, from Figure 9c, it can be inferred that the tuning of \(p\) also controls the PRS reflectivity value, and thereby the antenna gain enhancement. At operating frequency, the variation of \(p\) varies the reflection magnitude from 0.55 (medium reflectivity) to 0.93 (high reflectivity). Thus, the parameter \(p\) primarily relates to the radiation property of the antenna.