Development of Low-Loss Bandpass Filters With Multiple Controllable Transmission Zeros by Using an Improved Connecting-Line Coupling Technique

This paper presents a novel connecting-line coupling technique for the design of compact and low-loss bandpass filters (BPFs) with multiple controllable transmission zeros (TZs). Traditional gap-coupling methods are highly sensitive to fabrication errors and have limited design flexibility. By contrast, the proposed approach enables precise control of coupling strength without modifying resonator geometry. Such control is achieved through independent adjustments of the lengths, widths, and positions of connecting lines. Furthermore, the proposed approach facilitates the generation of up to n TZs for an nth-order BPF, thus considerably improving the BPF’s frequency selectivity and stopband attenuation. To validate the effectiveness of this method, four filters are designed and fabricated: a second-order stripline BPF, a fourth-order microstrip BPF, a fifth-order stripline BPF, and a sixth-order wideband microstrip BPF. The second-order BPF is based on asymmetric connecting-line coupling, whereas the fourth-, fifth-, and sixth-order BPFs are based on symmetric connecting-line coupling. Excellent agreement between simulation and measurement results is achieved for these designs, with low insertion loss, high roll-off rates, and high tolerance to fabrication variations being noted. The proposed technique is suitable for filters ranging from narrowband to ultra-wideband filters and across low to millimeter-wave frequency bands, making it a highly versatile and practical solution for modern RF and microwave communication systems.