Improved Combline Bandpass Filter with Multiple Transmission Zeros

An improved combline bandpass filter with multiple transmission zeros is investigated in the study. To characterize the filter response, the characteristic equation of the proposed two-pole filter is derived and examined. In accordance with the derived characteristic equation, a lumped capacitor is attached between input and output ports so as to create more transmission zeros on the upper-stopband for providing the required attenuation for particular frequencies. Specifically, a multiple-notch two-pole combline filter is designed and fabricated with two transmission zeros around the passband for improved selectivity and another three on the upper-stopband for interference attenuation.