This paper introduces a novel high-performance millimeter-wave diplexer, engineered using a suspended microstrip line and rectangular waveguide, achieving operational bandwidths spanning DC to 40 and 70-110 GHz. The low-pass filter section features a tunable compact microstrip resonant cell based on elliptic filter technology, significantly enhancing transition band suppression. The ability to adjust transition bandwidth and suppression performance through modifications of open-circuit stubs exemplifies the filter's tunability. Distinct from traditional millimeter-wave diplexers, this design incorporates low insertion loss, excellent return loss, and high isolation while offering reconfigurability. This reconfigurability makes the diplexer adaptable for constructing various millimeter-wave applications, including mixers, providing a versatile platform for future technology integrations. The simulation results demonstrate that the diplexer achieves an insertion loss of less than 0.5 dB, a return loss greater than 15 dB, and isolation exceeding 40 dB. In addition, it has a fractional bandwidth exceeding 48.7%, making it suitable for advanced millimeter-wave communication systems and meeting the rigorous standards required for modern diplexer implementations.
