ROTATIONALLY SYMMETRIC COUPLED-LINES BANDPASS FILTER WITH TWO TRANSMISSION ZEROS

A rotationally symmetric short-circuited stub-loaded structure is proposed to design a microstrip-line bandpass fllter with two transmission zeros near the lower and upper cut-ofi frequency edges of operating millimeter-wave bands. Furthermore, interdigital coupled-lines and additional resonators are integrated into the proposed rotationally symmetric bandpass fllter to improve the out-of-band rejection. As design examples, the in-band and out-of-band performances of two fllter prototypes using single layer microstrip-lines are designed and experimentally examined. The measured results show that the fllter without any interdigital coupled- lines achieves a passband insertion loss of 0.97dB at 40GHz and out-of- band rejection of larger than 23dB, while the fllter with the interdigital coupled-lines realizes the suppressions in lower and upper-bands of, respectively, larger than 30dB and 19dB for the overall insertion loss of 2.1dB at 40GHz.