Rotationally symmetric microstrip-line millimetre-wave bandpass filter with two transmission zeros

A rotationally symmetric short-circuited stub-loaded structure is proposed. This structure is used to design a dual-mode microstrip-line bandpass filter. Moreover, two transmission zeros are generated near the lower and upper cutoff frequency edges of operating millimetre-wave bands. As design examples, the in-band and out-of-band performances of one filter prototype using single layer microstrip-lines have been designed and experimentally examined. The measured results show that the filter achieves a passband insertion loss of 0.97 dB at 40 GHz and lower- and upper-stopband rejection of larger than 24 dB.