Tunable absorptive bandpass filter with two transmission zeros based on image parameter method

A tunable absorptive bandpass filter with two transmission zeros is presented. A lowpass absorptive filter prototype consisting of a matching section and a m-drive filter section is established based on image parameter method, and the filter elements are easily derived through the image impedance with the required return loss and transmission zeros. Moreover, the two transmission zeros are controlled by adjusting the parameters of the m-drive filter section. A distributed tunable absorptive bandpass filter is realized based on the proposed lowpass filter prototype. The fabricated filter shows a return loss of 21-25 dB and an insertion loss of 2.1-2.8 dB at the center frequency within the tuning range of 1.35-1.58 GHz.