Microstrip-line millimetre-wave bandpass filter using interdigital coupled-line

An interdigital microstrip coupled-line bandpass filter is proposed for millimetre-wave application. A quadruple-mode microstrip-line resonator is designed to constitute the passband. The interdigital coupled-line sections are designed as the input/output network of the resonator. Meanwhile, sharp selectivity of the passband is achieved. As design examples, the in-band and out-of-band performances of a filter prototype using low-cost single-layer microstrip-lines has been designed and experimentally examined. The measured results show that the filter achieves a passband insertion loss of 1.07 dB at 40 GHz, the lower- and upper-stopband rejections are larger than 18 dB and 3 dB fractional bandwidth is 20%.