A narrowband bandpass filter with multiple transmission zeros based on parallel coupled lines structure

Purpose This paper aims to present the theoretical design of a narrowband bandpass filter (BPF) using a multiple parallel-coupled line structure. The proposed filter delivers robust narrowband performance, generates multiple transmission zeros (TZs) and achieves wide-stopband rejection, thereby effectively addressing the inadequate frequency selectivity typically associated with conventional filters. Design/methodology/approach The proposed BPF uses a cascaded coupling structure. By combining uniformly parallel-coupled lines and nonhomogeneously parallel-coupled lines, this design overcomes the low quality factor (Q) of planar microstrip circuits and achieves narrowband performance. Resonant mode characteristics are analyzed using the ABCD matrix method. Owing to multipath cancelation and the virtual short mechanism, multiple TZs can be excited and controlled. Findings For demonstration, the circuit is designed, fabricated and measured. Measurement results show that the center frequency (f0) of the BPF is 2.044 GHz with a 3-dB fractional bandwidth (fbw) of 1.7%, demonstrating excellent frequency selectivity. The insertion loss (IL) is better than 1.7 dB within the passband, the out-of-band rejection exceeds 20 dB and the covered frequency range reaches 3 f0. Originality/value A novel filter topology is proposed that achieves narrowband high selectivity, multiple transmission zeros, low insertion loss and ultra-compact size and exhibits broad application prospects in fields such as satellite communication and radar detection.