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Abstract
An initial and an improved bandpass filters incorporated with quasi-lumped impedance inverters are proposed in this work. The quasi-lumped impedance inverter is a K-inverter which is implemented by metallic vias to ground and two sections of adjacent microstrip transmission lines. The initial filter is mainly based on parallel quarter-wavelength (λ/4) short-circuit transmission line sections, while the improved filter partly represents the characteristics of substrate integrated waveguide filter (SIW). The designed center frequency of the two filters is 10 GHz, and the fractional bandwidths of the initial and improved filter are 3.3% and 5.2%, respectively. The measured results are consistent with the simulated ones, which show that the improved filter has outstanding lower insertion loss, better stopband features and no spurious response at the second harmonic frequency.