Narrow-band Bandpass Filters with Improved Upper Stopband Using Open/Shorted Coupled Lines

A transversal narrow-band bandpass filter with improved stopband based on open/shorted coupled lines and transversal signal-interaction concepts are proposed in this paper. By utilizing the stopband transmission characteristic of the open/shorted coupled lines, three transmission zeros can be easily achieved to suppress the second harmonic for the bandpass filter. The transmission zeros near the passband can be adjusted conveniently by only changing the electrical length of the open/shorted stubs when the ratio of characteristic impedance is fixed. To verify the presented concepts, the prototype (epsilon(r) = 2.65, h = 0.508 mm, tan delta = 0.003) with 3-dB fractional bandwidth (FBW) 2.8% (1.76-1.81 GHz) is designed and fabricated. Good agreements are observed for the theoretical and measured results, indicating good in-band filtering performances and high selectivity.