Hybrid Stub-Loaded Resonators for Millimeter Wave Broadband Microstrip-to-Microstrip Vertical Transition in Multilayer LCP Substrate

In this paper, a broadband millimeter wave vertical transition using a three-layered liquid crystal polymer (LCP) substrate is presented. The transition includes microstrip transmission lines at the top and bottom layers, along with slotlines at the common ground plane. To enhance the bandwidth, a design with hybrid stub-loaded resonators (hybrid resonators) is proposed. These resonators consist of two types of lines - microstrip and slotline. By optimizing the position of the hybrid microstrip/slotline structure, a three-pole wideband vertical transition is achieved. The design is verified through fabrication and measurements, showing a frequency range from 49.6 to 73.9 GHz with insertion loss (IL) below −3dB and return loss (RL) better than −13dB.