# Subwavelength-Cavity High-Gain Circularly Polarized Antenna with Planar Metamaterials
## 2 Simulations and Experimental Measurements
It can be seen that the real part of \(\mu_{\textit{eff}}\) was negative, while that of \(\varepsilon_{\textit{eff}}\) was positive over 2.68 GHz, and the former was minimal when near 2.80 GHz. These results indicate that the structure behaved like an MNM at 2.80 GHz. Using Equations (5) and (6), the values of the electric field strength for different polarizations were calculated, and the \(AR\) of the electromagnetic waves that radiated from the cavity could be obtained using Equation (7). The simulated boresight \(AR\) of the MNM-PRS cavity is plotted as a function of the frequency in Figure 4d. The \(AR\) of the antenna gradually decreases when increasing the frequency from 2.70 GHz to 2.80 GHz and increases with the frequency changing from 2.80 GHz to 2.90 GHz. The minimum \(AR\) was 1.02 at 2.80 GHz. Therefore, using this microstructure, a circularly polarized electromagnetic wave could be obtained from a linearly polarized electromagnetic wave, as shown in \[a_{1}=e^{-j\varphi}:x-pol \tag{1}\]3 This structure can achieve low transmittance at a frequency of 2.80 GHz and can partially transmit resonant electromagnetic waves. Therefore, if the structure is periodically arranged and separated by a distance and combined with a metal plate, a resonance-enhanced FP cavity structure can be formed. If a linearly polarized radiation source is placed inside the cavity, the new antenna will have a high performance of radiation enhancement and circular polarization conversion. Using the proposed MNM-PRS unit cell, n \(\times\) n perfect metallic meshes and square patches with a pair of truncated corners were patterned on the top and bottom surfaces of the substrate to form an MNM-PRS layer. We studied a subwavelength cavity antenna using MNM metamaterials with antenna dimensions of 240 \(\times\) 240 \(\times\) 21 mm\({}^{3}\). The thickness of the subwavelength air-filled cavity was 18 mm, which was approximately one-sixth of the operating wavelength of 107.1 mm. The radiation source was a simple, linearly polarized patch antenna.. We could perform a full-scale modeling of the antenna in the microwaves and RF/Optical Antenna module, and all material electromagnetic parameters came from the material library of the software. Appropriate parameter settings could ensure the accuracy of simulation results. The simulated boresight \(AR\)s of the proposed antenna were plotted as a function of frequency in Figure 6a. The minimum \(AR\) was nearly 2.8 GHz, and this result agrees well with that observed in Figure 4d. At frequencies greater than 2.80 GHz, the \(AR\) of the antenna decreases. However, at frequencies below 2.80 GHz, the \(AR\) of the antenna increases. The metamaterial antenna exhibits the best circularly polarized properties at 2.80 GHz. The reflection coefficient of the proposed antenna is presented in Figure 6b, wherein the minimum value is -22.8 dB at 2.80 GHz. The antenna has a narrow working band at about 2.80 GHz, which proves that the antenna has narrow-band filtering characteristics. The reflection coefficient of the antenna exhibits narrowband characteristics and can achieve good antenna radiation at 2.80 GHz. This radiation frequency is the same as the circularly polarized boresight \(AR\)s.  
\[a_{1}=e^{-j\varphi}:x-pol \tag{1}\]4 (**a**) Simulated MNM-PRS reflection and transmission coefficients; (**b**) simulated reflection and transmission phases; (**c**) effective permeability (\(\mu_{\textit{eff}}\)) and permittivity (\(\epsilon_{\textit{eff}}\)) of the MNM-PRS unit cell, where the red and black lines denote the real parts of \(\mu_{\textit{eff}}\) and \(\epsilon_{\textit{eff}}\), respectively, and the magenta and blue lines denote the imaginary parts of \(\mu\)eff and \(\epsilon_{\textit{eff}}\), respectively; the green dashed line denotes zero effective permittivity or permeability; (**d**) simulated boresight axial ratios (\(AR\)s) for the proposed structure. \[a_{1}=e^{-j\varphi}:x-pol \tag{1}\]5 Circular polarization behavior of the MNM-PRS.