# Subwavelength-Cavity High-Gain Circularly Polarized Antenna with Planar Metamaterials
## 1 Introduction  
Antennas play an important role in wireless communication systems. They generally include linearly polarized and circularly polarized antennas. Compared to linearly polarized antennas, circularly polarized antennas possess several unique advantages in the harsh electromagnetic environment [1; 2; 3; 4; 5]. For example, circularly polarized antennas have a stronger anti-interference ability and can receive radio signals of arbitrary polarizations including linearly and circularly polarized radio signals. Therefore, they have been widely used in satellite communication, early warning monitoring, radio observation, etc. [3; 4; 5; 6]. To realize long-distance transmission and high-precision radio observation, an antenna must radiate and receive electromagnetic waves in one or more specific directions. The demand for high-gain antennas has increased significantly [7; 8; 9; 10]. Otherwise, the miniaturization of antennas has also been an important topic in engineering many types of high-quality modern equipment, including unmanned aerial vehicles, communication satellites, and advanced spaceborne radio detectors. Therefore, designing high-gain circularly polarized antennas has become an essential research topic. The traditional high-gain circularly polarized antennas include parabolic antennas, horn antennas, waveguide slot antennas, lens antennas, etc. [11; 12; 13; 14]. Even though these antennas provide high-gain circular polarization, most of them suffer from disadvantages, such as large size, difficult assembly, and poor flexibility. In recent years, micro-strip antennas have been widely studied because of their small size, low profile, easy integration, and good flexibility [15; 16]. Array micro-strip antennas are also widely used to obtain ahigh-gain circular polarization [17; 18; 19; 20; 21]. However, designing an array antenna is a complex process because of the complicated feeding networks that are required to generate both a circularly polarized behavior and a high gain; in fact, such networks lead to low efficiency in high-gain antennas. Therefore, their application is restricted to only certain fields. Metamaterials have unique electromagnetic properties compared to natural materials [22; 23; 24; 25]; therefore, they have many important applications, including sensing and diagnostics, biomedicines, telecommunications, absorbers, measurements, etc. [26; 27; 28; 29; 30; 31; 32; 33; 34; 35]. Therefore, metamaterial antennas are more advantageous than their conventional counterparts [24; 25; 36; 37]. Metamaterials include double- and single-negative metamaterials (SNMs) [36; 38]. SNMs include epsilon-negative metamaterials (ENMs) with negative permittivities and mu-negative metamaterials (MNMs) with negative permeabilities. Perfect ENMs and MNMs can be used as electric and magnetic mirrors, respectively. The reflection phase of perfect ENMs is \(\pi\), while that of perfect MNMs is 0. A cavity metamaterial antenna, which is generally composed of a partially reflective surface (PRS), a resonant cavity, and a reflector, can be used to realize high-gain performance. Metals are a type of natural ENMs, the reflection phase of a smooth metal surface is close to \(\pi\), and the minimum thickness of a cavity antenna composed of a smooth metal surface is half the resonant wavelength [39; 40]. However, when made of MNMs, which are also called artificial magnetic conductors, the thickness of a cavity antenna can be equal to or less than 1/4 of the resonant wavelength [41; 42; 43; 44]. Thus, new ultra-thin-cavity MNM antennas are attracting significant attention. Based on analysis and research on cavity antennas [40; 44; 45; 46], a high-gain circularly polarized antenna is presented in this paper. We have designed a subwavelength planar MNM antenna with an asymmetric microstructure such that the PRS composed of the MNM units has dual characteristics of a negative permeability response and circular polarization selection.