New microstrip bandpass filters with increased upper rejection band

A novel class of microstrip bandpass filters with increased upper rejection band is proposed. Coupling properties between the microstrip resonators and feed lines are analyzed via voltage-wave functions. Suppressions of second and third harmonics can be achieved by proper selection of coupling regions. Using two half-wavelength resonators with and without centrally-loaded resistors, two experimental filters, one for second and one for third harmonic suppressions, are designed, fabricated and measured. The simulated and measured results show that the upper rejection bands of the two filters are in fact increased up to nearly thirdand fifth-order harmonics, respectively, with the rejection level below -20 dB.