# Circularly Polarized High-Gain Fabry-Perot Cavity Antenna with High Sidelobe Suppression
## 3 Simulation and Experiment Results  
This section discusses the experimental results for a CP-FPCA, including impedance characteristics and radiation power pattern, which are measured using a vector network analyzer and anechoic chamber, respectively. The antenna impedance is measured on the vector network analyzer (Anritsu MS46122A, 40 GHz) using the short-open-load (SOL) (Anritsu TOSLKF50A-40) calibration method, which is depicted in Figure 9a. Measurements of the impedance characteristics in respect to the resonance cavity height variation are also shown in the Supplementary Materials [see Figure S4]. Here, the impedance bandwidth is well maintained around the operating frequency \(f_{0}\) = 10 GHz and the measured data shows that the \(-\)10 dB impedance bandwidth is 57% higher than the simulated result and is 9.7% of the fractional bandwidth. Overall, we can see that the measured data follows the simulated results relatively well. In regard to the peak realized gain, which is depicted in Figure 9b, we observe a 0.98 dB gain drop at design frequency \(f_{0}\) and a slight deviation in the gain at the 9.7 to 10.3 GHz frequencies. This gain variation is mainly because the AMC substrate and double-layered PRS superstrate [bonded with epoxy FR4] are not perfectly flat, which is a common issue of chemical etching processes. To address this issue, we employed 8 M3 plastic spacers within the cavity to keep the superstrate as flat as possible. Here, we can see that cross-polarization of the peak gain is less than \(-\)5 dB at the operation frequency \(f_{0}\) = 10 GHz, indicating that the antenna provides perfectly circularly polarized waves in the broadside direction. The main goals of the CP-FPCA are to achieve a high realized gain in the target direction and minimum AR and cross-polarization for circularly polarized waves. These goals can be evaluated based on the form of the radiation patterns in Figure 10a,b and the AR in Figure 10c. The radiation pattern of the CP-FPCA at operating frequency \(f_{0}\) = 10 GHz is illustrated in Figure 10a,b for the \(xz\)-plane and \(yz\)-plane. Here, we can see that CP-FPCA provides a peak gain of 15.4 dB in the broadside direction with 23.8 dB and 25 dB, respectively, of high SLS along the \(xz\)-plane (\(\phi\) = 0\({}^{\circ}\) and 180\({}^{\circ}\)) and the \(yz\)-plane (\(\phi\) = 90\({}^{\circ}\) and 270\({}^{\circ}\)). The measured radiation patterns closely resemble the predicted ones in both planes with half-power beam widths of 28.5\({}^{\circ}\) and 27.5\({}^{\circ}\), respectively. The 3D radiation power pattern is illustrated in the Supplementary Materials [see Figure S5]. Moreover, the cross-polarization level is less than \(-\)42 dB in both planes in the broadside direction at operating frequency \(f_{0}\) = 10 GHz, which is 55% more suppressed than the sidelobes in each plane. Figure 9: (**a**) Reflection coefficient S11, (**b**) peak gain of the CP-FPCA at \(f_{0}\) = 10 GHz. The minimum AR is shown in Figure 10c. The antenna exhibits a minimum AR of 0.12 dB at the design frequency \(f_{0}\) = 10 GHz and has a 3 dB axial ratio bandwidth of 233 MHz, but 0.53 dB loss is observed for the measured case at operating frequency \(f_{0}\). The measured 3 dB axial ratio bandwidth is 303 MHz, which is 23% higher than the simulated results. The Ex-field distribution within the CP-FPCA is depicted in Figure 10d, confirming the constructive interference, which satisfies [PERSON]'s beamforming condition to attain the peak gain in the broadside direction and high SLS in both planes [see Figure 10a,b]. The Ex-field distribution above the resonance cavity is also illustrated in the Supplementary Materials [see Figure S6]. The fabricated CP-FPC antenna and its anechoic chamber measurement setup are depicted in Figure 11. The proposed antenna size is 100 mm (3.3 \(\lambda_{0}\)) \(\times\) 100 mm (3.3 \(\lambda_{0}\)) \(\times\) 12.4 mm (0.413 \(\lambda_{0}\)), and the antenna is made up of 36 PRS and 32 AMC unit cells.