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. [ABSTRACT FROM AUTHOR]