Dual‐Band Circularly Polarised Filtering Antenna Based on the Characteristic Mode Analysis

This paper presents a novel dual‐band circularly polarised (CP) filtering antenna designed using electromagnetic coupling theory. First, characteristic mode analysis (CMA) is employed to verify that introducing different types of coupling between two adjacent patches significantly modifies the resonant frequencies of the inherent modes. Consequently, by applying specific coupling configurations between the patches, a desired modal distribution is achieved, enabling dual‐band CP filtering performance across the two operational frequency bands. Subsequently, the analysis of the magnetic field distributions of the characteristic modes leads to the selection of slot‐coupled feeding as the optimal excitation method to effectively activate the desired modes and achieve the intended functionality. Finally, to further enhance the filtering response without degrading the CP performance within the passbands, slots and open‐circuit stubs are integrated into the feedline. This modification offers additional control over out‐of‐band suppression and improves selectivity. Following fabrication and measurement, the proposed antenna exhibits dual‐band impedance bandwidths of 3.3–3.67 GHz (10.6%) and 4.74–5.28 GHz (10.7%). The corresponding axial ratio (AR) bandwidths are 1.4% (3.47–3.52 GHz) and 1.2% (4.86–4.92 GHz), respectively. These results validate the effectiveness of the synergistic integration between the guided patch coupling mechanism and the parasitic feedline elements. The proposed design approach enables the realisation of the multifunctional antenna structure, achieving dual‐band operation, CP and intrinsic filtering characteristics within a single structure.