## 1 Introduction  
For both transmitters and receivers, CP antennas offer superior signal transmission and reception sensitivity compared to linearly polarized antennas. They eliminate the need for polarization alignment and mitigate issues such as polarization mismatch, multipath effects, and channel crosstalk during signal propagation, thus reducing losses caused by polarization mismatch. These advantages make CP antennas widely applicable in various systems, including capsule endoscopy systems, satellites, radio frequency identification (RFID) systems, and wireless local area networks (WLANs) [1]. In the RF front-end, the antenna and the filter are two critical components. In conventional designs, they are designed separately and then cascaded, which inevitably introduces insertion loss. Consequently, the integrated design of antennas and filters, resulting in filtering antennas possessing both radiation characteristics and filtering functionality, has become a major research direction. Therefore, filtering antennas with CP characteristics hold significant importance and offer broad application prospects [2; 3], making them a current research focus. Among various approaches to developing high-performance CP filter antennas, the most direct and common method uses a filter with a 90\({}^{\circ}\) phase shift capability as the CP feeding network [4; 5; 6; 7; 8; 9; 10; 11; 12]. Metasurfaces have been used to achieve CP filtering in [13; 14], while Ref. [15] combined a CP polarizer with a linearly polarized filtering antenna. However, both methods tend to increase antenna size and structural complexity. Microstrip patch antennas, being easy to integrate and structurally simple, are often the preferred choice for designing CP filtering antennas. Techniques such as introducing parasitic elements, etching slots, loading shorting elements on the CP patch antenna, or loading open-circuited stubs on the feed line can introduce radiation nulls on both sides of the antenna's passband, achieving good bandpass filtering characteristics. For example, Ref. [16] introduced parasitic elements to generate opposing currents with the main radiating patch at specific frequencies to achieve filtering performance. Refs. [17; 18] created slots on the metallic patch to induce current cancellation, thereby introducing radiation nulls. Ref. [19] achieves radiation nulls by introducing symmetrical rectangular parasitic elements on both sides of the main radiating patch and microstrip feed line, where proper coupling induces out-of-phase currents between the parasitic elements and the main patch; however, this method does not contribute to broadening the antenna's circular polarization (CP) bandwidth. In [20], symmetrical slots etched on the ground plane are excited by the microstrip feed line to generate one CP operating frequency along with a radiation null. Since the slots are excited to facilitate CP generation, the back lobe radiation at this CP frequency becomes relatively large, necessitating an additional large reflective ground plane, which increases the antenna's profile height. Ref. [21] realized good bandpass filtering characteristics by loading shorting patches and open-circuited stubs at the radiating slots. Ref. [22] implemented CP filtering performance by loading open-circuited stubs on the microstrip feed line. Beyond these methods, Ref. [23] used capacitive coupling between two orthogonally placed resonators to achieve second-order bandpass filtering. However, although these circularly polarized filtering patch antennas achieve bandpass filtering performance, they generally exhibit relatively narrow axial ratio bandwidths. Ref. [24] shifts the CP operating frequency toward the lower band by introducing counterclockwise-oriented metal strips on the left side of the four arms of a crossed dipole, yet this approach does not enhance the CP operating bandwidth.