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