Compact dual-band narrowband bandpass filter with small frequency ratio

A compact but very simple dual-band bandpass filter (BPF) based on just five pairs of coupled lines is designed in this paper. It can be examined using the even- and odd-mode method because of its symmetry structure. Six transmissions zeros (TZs) can be obtained and calculated by using input impedance analysis. For demonstration, a dual-band narrowband BPF example with center frequencies at 0.94 and 1.12 GHz is fabricated and measured. The 3-dB fractional bandwidths of two passbands are 2% (0.93-0.95 GHz) and 3.4% (1.1-1.14 GHz), whose frequency ratio of upper-band to lower-band is very small. The measurement results agree with the simulation results, confirming the design process.

Automated summary

This paper presents a compact and simple dual-band bandpass filter (BPF) design using only five pairs of coupled lines. The even- and odd-mode method is applied due to its symmetry structure. Six transmission zeros (TZs) are obtained and calculated through input impedance analysis. A dual-band narrowband BPF example is fabricated and measured, with center frequencies at 0.94 and 1.12 GHz. The measured results agree with the simulation results, validating the design process.