Design and Synthesis of Dual-Polarized Full-Duplex Filtennas with Simple Feeding Structure and Isolation Enhancing Techniques

In this article, a design and synthesis method is proposed for dual-linearly polarized full-duplex filtennas based on multiple coupled dual-mode resonators. Techniques for enhancing port isolation are developed for simple side-fed structures, including external quality factor (Qe) equalization and first-cavity perturbation. ForQeequalization, the feeding ports are offset from the cavity midlines to equalize theQevalues of the degenerate modes, eliminating the frequency deviation of the port isolation. For first-cavity perturbation, the first dual-mode cavity is perturbed to mitigate the impact of source-load coupling on port isolation. Their working mechanisms are elaborated through theoretical analysis based on a 4-port coupling matrix model and an equivalent circuit. Then, a general synthesis method of 4-port coupling matrices to guide design is presented. High port isolation and Chebyshev filtering responses can be simultaneously achieved. To verify the proposed design concept, two full-duplex filtennas with different coupling topologies are designed. The experimental results are in good agreement with the calculated ones. For both of them, compact structures, filtering properties, and port isolation above 34 dB in their impedance bandwidths are achieved.