# Tilted Beam Fabry-Perot Antenna with Enhanced Gain and Broadband Low Backscattering
## 3 Simulation Results and Experimental Validation
### Antenna Results  
The simulated vs. measured reflection coefficient (S\({}_{11}\)) plot of the proposed F-P design is shown in Figure 13a. The \(-10\) dB impedance bandwidth (BW) is 240 MHz (3.65%), extending from 6.45 GHz to 6.69 GHz, and a good impedance match is achieved at an operating frequency approaching \(-23\) dB. The measured result follows the trend of the simulated plot; the discrepancy is attributed to manufacturing tolerances. The frequency response of gain for the proposed antenna is also given in Figure 13a. The maximum achieved gain is 11.5 dBi. The 3 dB gain bandwidth is ~750 MHz (11.76%) and extends from about 6 GHz to 6.75 GHz. The measured response is in satisfactory agreement with the simulation results. The left shift of the maximum gain frequency could be because of the final height of the cavity, which had been slightly varied in the simulations to fine tune the tilt angle/gain, the side lobe levels (SLLs), and antenna matching. The plots in Figure 13b display reflection coefficient, and gain vs. frequency, of the reference antenna (antenna without the PRS and the absorptive FSS). When reflection coefficient responses of the proposed and reference antennas are compared, it is observed that both antennas are well matched at operating frequency. When gain responses are compared, it can be seen that the presence of the superstrate structure above the reference antenna increases its gain by 5.24 dB.  
The radiation efficiency of the reference antenna at 6.6 GHz is 95%. The radiation efficiency of the proposed antenna reduces to 79% (which is expected). This reduction in efficiency is attributed to the radiated wave's energy losses within the conductors, dielectric, and resistors of the superstrate structure.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Gradient Patch Lengths (mm)** & 8.2 & 10 & 10.2 & 10.7 & 11 & 11.8 \\
**Reflection Phase Values** & \(158.51^{\circ}\) & \(105.61^{\circ}\) & \(88.37^{\circ}\) & \(-27.28^{\circ}\) & \(-83.8^{\circ}\) & \(-125.03^{\circ}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Gradient patch dimensions (v) with associated reflection phase values.  
Figure 12: Antenna structure. (**a**) Simulated model; (**b**) Fabricated prototype; (**c**) PRS top side; (**d**) PRS bottom side (gradient pattern).  
The radiation patterns of the proposed design at operating frequency are given in Figure 14. The E-plane radiation pattern is displayed in Figure 14a. As can be seen, the peak radiation is deflected from the boresight. The main lobe appears at a tilt angle of 13\({}^{\circ}\), with SLLs below \(-12\) dB. The measured pattern agrees well with the simulated pattern. The tilt appears in the axis in which the gradient has been implemented, i.e., the \(x\)-axis, as per Figure 12. The beam deflection occurs towards the negative \(x\)-axis, as patch elements from the positive \(x\)-axis towards the negative \(x\)-axis show gradual phase lag, realized by gradual increase of patch length [29]. Simulated and measured H-plane patterns are shown in Figure 14b. The half power beamwidths (HPBWs) for E-plane and H-plane are 23\({}^{\circ}\) and 20\({}^{\circ}\), respectively.  
To highlight how the radiation pattern changes with frequency, Figure 15 illustrates radiation patterns plotted at three different frequencies. Satisfactory main lobe deflection can be observed over a bandwidth of \(\sim\)100 MHz, with more or less similar tilt angle. With an increase in frequency, SLLs decrease; however, at the same time, pattern broadening occurs.  
Figure 13: Reflection coefficient and gain vs. frequency response. (**a**) Proposed antenna; (**b**) Reference antenna (without superstrate structure).