## 1 Introduction  
Circularly polarized (CP) antennas, owing to their unique capabilities of generating and receiving signals at any polarization angle and effectively mitigating the effects of multipath interference, have found extensive applications in various wireless communication systems including satellite communications, global navigation satellite systems (GNSS) [1, 2], radio frequency identification (RFID) [3, 4], and 5G sub-6 GHz networks [5, 6, 7]. However, traditional CP antennas [8, 9, 10] frequently encounter intrinsic drawbacks, including low gain, a narrow axial ratio bandwidth (ARBW), and a high profile, which significantly constrain their performance and applicability in modern communication systems requiring high data rates and wide operational bandwidths. In recent years, advancements in antenna technology have led to the exploration of innovative approaches to overcome these limitations. The metasurface (MTS) has emerged as a promising solution for enhancing the radiation performance of antennas, including bandwidth optimization [11, 12, 13], gain amplification [14, 15, 16], and polariza-tion conversion [17, 18, 19]. For instance, ref. [20] proposes a low-profile antenna with a stacked structure. The MTS comprises a 3 \(\times\) 3 patch array placed above a custom-designed driving element. This configuration can excite multiple resonances in the MTS, leading to an enhanced ARBW of 10.2% and a gain of 8.1 dBic. However, there is an air gap between the MTS and the radiating patch in this design, which reduces the mechanical stability of the antenna structure. Moreover, its relatively narrow ARBW limits its application scenarios. Additionally, slot-fed antennas have garnered attention for their compact size, low profile, and ease of integration [21; 22; 23], but they too face challenges in achieving wideband CP radiation with high gain. For instance, ref. [24] presents a co-polarized CP grid slot antenna. It uses inverted Z-shaped slots to excite rectangular-grid slot patches on the upper and lower planes, generating co-polarized CP with a broader ARBW of 20.1%. Moreover, characteristic mode analysis (CMA) has emerged as a powerful tool for antenna designers, enabling a deeper understanding of the inherent electromagnetic behavior of antenna structures. By identifying and manipulating the characteristic modes of an antenna, designers can optimize its radiation characteristics, including bandwidth, gain, and polarization without requiring complex feeding networks or additional components [25; 26]. In [27], by utilizing CMA, a 3 \(\times\) 4 MTS structure was analyzed to identify the useful modes supported by this structure. However, it only identified the effective modes of the MTS without suppressing the interfering modes. Moreover, the feeding structure was too simple to fully unlock the potential of the antenna's CP radiation. Consequently, it only achieved an ARBW of 19.42% and a gain ranging from 4.5 to 8 dBic within the CP operating band. In this paper, a wideband CP antenna with high gain is introduced. Firstly, by employing CMA, two modes with orthogonal modal currents are selected as the desired modes. Then, parasitic patches and mode-suppressing patches are designed, respectively, to enhance the desired radiation modes and suppress the unwanted ones. Subsequently, a feeding network is designed to ensure efficient excitation of the selected orthogonal modes and the overall performance of the antenna. Through extensive simulations and parameter tuning, we optimize the dimensions and positions of the transmission lines and coupling slots to achieve a good impedance match and stable circular polarization over the wide frequency range. In addition, we conducted full-wave simulations using HFSS and demonstrated the effectiveness of CMA through a comparison of current density. Then, an antenna prototype is fabricated through Printed Circuit Board printing. Finally, its performance is experimentally verified through measurements, with the test results showing a good match with the simulation outcomes.