Research on the Phenomenon of Mode Separation Based on Characteristic Mode Analysis and Its Application in the Design of Metasurface Antennas

Abstract

During the process of characteristic mode analysis on metasurfaces, an intriguing phenomenon is observed: when the unit cells exhibit 90-degree rotational symmetry, the two fundamental modes of the metasurface nearly completely overlap in the frequency domain. However, when the structure of the unit cells lacks 90-degree rotational symmetry, the two fundamental modes become separated. This phenomenon can be termed modal separation. Research reveals that the cause of modal separation is the alteration of the modal current paths for the two fundamental modes. This discovery provides a novel perspective for designing broadband metasurface antennas. Based on this theory, a broadband circularly polarized metasurface antenna is designed. To successfully excite the two modes that are significantly separated in the frequency domain, a stepped coplanar waveguide (CPW)-feed radiator is devised. The proposed metasurface antenna is fabricated and measured, with dimensions of 0.42λ_L × 0.42λ_L × 0.045λ_L (λ_L is the wavelength in free space at the lowest operating frequency). The measured results show that the antenna has an impedance bandwidth of 4.2–8.3 GHz (65.6%), a 3 dB axial ratio bandwidth of 5.2–6.8 GHz (26.7%), and a peak gain of 5.83 dBi.