Design of millimeter-wave bandpass filters based on improved asymmetric parallel-coupled microstrip line structure for direct detection system of CubeSat radiometer

In this paper, two improved asymmetric parallel-coupled microstrip bandpass filters with center frequencies of 89 GHz and 150 GHz were designed and implemented. The improvement of the circuit structure is based on the rotation of several asymmetric parallel-coupled microstrip line segments, which achieves a great area reduction compared to the conventional structure. The improved bandpass filters were designed and optimized based on the conventional structure. The measured results show that the return loss of the W-band bandpass filter is better than 16 dB and that of the D-band bandpass filter is better than 15 dB within the passband. The insertion loss is less than 2.3 dB and 2.5 dB within the passband for W-band and D-band bandpass filters, respectively. These results verify the feasibility of the improved asymmetric parallel-coupled microstrip line bandpass filter as a broadband pre-filter for the direct detection system of a CubeSat radiometer.