Compact 4×4 Dual‐Broadband Filtering Butler Matrix for 5G Applications

ABSTRACT This paper presents a novel dual‐band filtering branch‐line coupler (BLC) with integrated notch characteristics. The design is realized by loading open‐circuited stub‐coupled lines (OCS‐CLs) at each port of a conventional BLC to achieve bandpass filtering responses, while quarter‐wavelength open stubs are embedded to create notch functionality. Utilizing this proposed coupler alongside broadband 90° couplers and 45° phase shifters, a compact 4×4 dual‐band filtering Butler matrix is successfully constructed. Measurement results demonstrate that the proposed matrix simultaneously covers both the 5G NR n78 (3.3–3.8 GHz) and n79 (4.4–5.0 GHz) bands, while effectively suppressing inter‐band and out‐of‐band interference. Finally, the design is validated through a 1×4 antenna array, confirming its technical feasibility for achieving simultaneous dual‐band filtering and beam steering.