# Subwavelength-Cavity High-Gain Circularly Polarized Antenna with Planar Metamaterials
## 2 Simulations and Experimental Measurements
On the other hand, the realized gain in the RHCP boresight direction (red dotted line) was \(-\)5.31 dBi at 2.78 GHz on the \(\phi\) = 0\({}^{\circ}\) plane, and the cross-polarization level was 18.98 dB. Figure 9d shows the realized gains on the \(\phi\) = 90\({}^{\circ}\) 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 22.45\({}^{\circ}\); and the LHCP side-lobe level was \(-\)14.19 dB. On the other hand, the realized gain in the RHCP boresight direction (red dotted line) was \(-\)5.54 dBi at 2.78 GHz on the \(\phi\) = 90\({}^{\circ}\) plane, and the cross-polarization level was 19.21 dB. The measured results show that the antenna can receive LHCP signals and shield RHCP signals. The thickness of the subwavelength air-filled cavity was 18.0 mm, which was approximately one-sixth of the operating wavelength of 107.1 mm, and it is much smaller than that of traditional cavity antennas. For example, comparing the performance of the circularly polarized antenna with two cavities [46], the proposed metamaterial antenna of this work has only one subwavelength cavity and its MNM-PRS layer is composed of a circularly polarized unit cell. We used a subwavelength thickness cavity to enhance the radiation performance of the antenna and achieved a good conversion of linearly polarized signals to circularly polarized signals. \[b_{1}=e^{-j\varphi}:y-pol \tag{2}\]0 Photograph of the proposed LHCP metamaterial antenna. Next, the MNM-PRS plate was rotated horizontally by 90\({}^{\circ}\) to maintain the position of the PEC reflector and the radiation source fixed, as shown in Figure 10a. The position of the corner cut of the square metallic patch could produce a 90\({}^{\circ}\) deflection. Subsequently, the polarization selectivity of the MNM-PRS plate was reversed, and the LHCP antenna became an RHCP antenna. The measured parameters were as follows: The minimum \(AR\), as shown in Figure 10b, was 1.16 at 2.78 GHz. The \(AR\) of the antenna first increased and then decreased with the increase of frequency in the range from 2.70 GHz to 2.78 GHz. However, the \(AR\) at a frequency range from 2.78 GHz to 2.90 GHz antenna first increased and then decreased. Figure 10c shows the realized gains on the \(\phi=0^{\circ}\) plane. The realized gain in the RHCP boresight direction (black solid line) was 13.60 dBi; the RHCP half power width was 22.60\({}^{\circ}\); and the LHCP side-lobe level was -17.68 dB on the \(\phi=0^{\circ}\) plane. On the other hand, the realized gain in the LHCP boresight direction (red dotted line) was -9.78 dBi at 2.78 GHz on the \(\phi=0^{\circ}\) plane and the cross-polarization level was 23.38 dB. Figure 10d shows the realized gains on the \(\phi=90^{\circ}\) plane. The realized gain in the RHCP boresight direction (black solid line) was 13.60 dBi at 2.78 GHz; the RHCP half power width was 29.61\({}^{\circ}\); and the LHCP side-lobe level was -20.84 dB. On the other hand, the realized gain in the LHCP boresight direction (red dotted line) was -9.23 dBi at 2.78 GHz on the \(\phi=90^{\circ}\) plane and the cross-polarization level was 22.83 dB. It can be observed that the antenna could receive the RHCP signals well and shields the LHCP signals. If an automatic rotating mechanism is added to the metamaterial plate, the antenna can easily realize the polarization change, obtain the multiplexing of the radiation aperture, and improve the utilization efficiency of the antenna. Overall, the experimental results agreed well with the theoretical values. Such theoretical and experimental results show that this kind of planar subwavelength-cavity metamaterial antenna is easy to be fabricated in the GHz region. Compared to the traditional high-gain circular polarization antennas, this metamaterial antenna has a low profile, is lightweight, and has a simple feed network. Thus, this kind of antenna can be used for many wireless communication types of equipment.