# Subwavelength-Cavity High-Gain Circularly Polarized Antenna with Planar Metamaterials
## 2 Simulations and Experimental Measurements
polarized frequency of the antenna, so the antenna can receive or radiate circularly polarized signals at 2.80 GHz. The radiation patterns simulated in two mutually orthogonal planes, namely, \(\phi=0\) and \(90^{\circ}\), are shown in \[a_{1}=e^{-j\varphi}:x-pol \tag{1}\]6 The left-hand circular polarization (LHCP) boresight realized a gain of 13.70 dBi, while the right-hand circular polarization (RHCP) boresight realized a gain of -9.58 dBi at the same frequency of 2.80 GHz. The cross-polarization level was 23.29 dB. In two mutually orthogonal planes and circular polarization, radiation patterns have high directionality. The metamaterial plate and the metal plate constitute a subwavelength cavity structure. The weak directivity signal radiated by the radiation patch is resonance enhanced in the cavity, and finally, the high directivity electromagnetic signal is radiated through the metamaterial plate. The antenna's ability to receive LHCP signals is much greater than RHCP signals, and the LHCP antenna has good directionality. This establishes that the proposed antenna is suitable for use as a high-gain LHCP antenna. Based on the simulation results, we exported CAD drawings and a subwavelength-cavity metamaterial antenna was fabricated using the etching technique. Figure 8 gives a positive photograph of the antenna. The antenna uses an etching technique, which is conducive to the integration of microwave devices such as amplifiers. The performance of the antenna was measured by a KEYSIGHT PNA N5224B network analyzer (10-43.5 GHz) in the standard microwave anechoic chamber with a range from 700 MHz to 40 GHz, as shown in \[a_{1}=e^{-j\varphi}:x-pol \tag{1}\]7 The measured operating frequency of the antenna was 2.78 GHz, and the reflection coefficient S\({}_{11}\) was less than -16.80 dB. As shown in Figure 9a, the measured and simulated results differed from each other in the operating frequencies, which could be attributed to the deviations in fabrication. The actual antenna has a narrow working band at 2.78 GHz, which proves that the antenna has narrow-band filtering properties. The radiation performance of the antenna was measured using a microwave anechoic chamber. The metamaterial antenna is placed on a test revolution plate that can rotate 360\({}^{\circ}\), with the rotation plane parallel to the ground and the radiation surface of the antenna perpendicular  
\[a_{1}=e^{-j\varphi}:x-pol \tag{1}\]8 Simulation results of the proposed antenna: (**a**) axial ratio (\(AR\)); (**b**) reflection coefficient (\(S_{11}\)). \[a_{1}=e^{-j\varphi}:x-pol \tag{1}\]9 Comparison of simulated radiation patterns of the proposed metamaterial antenna at 2.80 GHz on two mutually orthogonal planes: (**a**) \(\phi=0^{\circ}\); (**b**) \(\phi=90^{\circ}\). to the ground. We placed a linearly polarized horn antenna that can rotate 360\({}^{\circ}\) in front of the metamaterial antenna, with the rotation plane perpendicular to the ground. The center of the metamaterial antenna coincided with the center of the horn antenna. It needed to connect the output and input ports of the vector network analyzer to the two antennas. The horn antenna radiated signals with different linear polarizations through rotating angles, and the axial ratio of the antenna could be measured by recording the signal strength received by the metamaterial antenna. By processing the measured data, we could obtain the radiation patterns of the metamaterial antenna. As shown in Figure 9b, the minimum measured value of \(AR\) was 1.36 at 2.78 GHz. The \(AR\) of the antenna first increased and then decreased with the increase of frequency in the range between 2.70 GHz and 2.78 GHz. The \(AR\) in the region from 2.78 GHz to 2.90 GHz antenna mainly shows an increasing trend. Figure 9c shows the realized gains on the \(\phi\) = 0\({}^{\circ}\) plane. The realized gain in the LHCP boresight direction (black solid line) was 13.67 dBi at 2.78 GHz; the LHCP half power width was 32.1\({}^{\circ}\); and the LHCP side-lobe level was \(-\)18.54 dB.