Circularly polarized antenna with gain enhancement via a single-layer Near-Zero-Index metasurface superstrate

Abstract

This paper presents a dual-fed square patch antenna loaded with a single-layer near-zero-index metasurface (NZIM) superstrate for circularly polarized operation at 5.8 GHz. The radiating patch is excited by two orthogonal quarter-wave microstrip feed lines, allowing the TM10 and TM01 modes to be generated simultaneously and used for circular polarization. The proposed superstrate consists of a 9 × 9 array of nested split-ring resonator elements and is placed 26 mm above the patch, which is close to one half of the free-space wavelength at the operating frequency. The effective-parameter retrieval of the unit cell confirms a near-zero refractive index around 5.8 GHz, supporting wave collimation and improving the antenna directivity. Compared with the reference antenna without the metasurface, the proposed structure increases the simulated peak gain from 6.1 dBi to 10.2 dBi and improves the radiation efficiency from 69% to 95%. In addition, the axial ratio decreases from approximately 12 dB to less than 3 dB near resonance, confirming the improvement of circular polarization. These results show that the proposed single-layer NZIM superstrate can be an effective approach for compact antennas requiring higher gain, better radiation efficiency, and more stable polarization performance in 5G, satellite, and IoT applications.