Broadband Vialess Microstrip-to-Microstrip Vertical Transition in Multilayer Liquid Crystal Polymer Substrate for W-Band Applications

In this paper, a W-band broadband vialess microstrip (MS)-to-MS vertical transition based on coplanar waveguide (CPW) multimode resonators (MMRs) on a four-layer liquid crystal polymer (LCP) substrate has been proposed. In this four-layer structure, the CPW MMRs are located in the second layer, the top layer T-type MS and the third layer MS are combined to form the feeding structure and excite the resonant modes. The proposed CPW MMRs can achieve multimode excitation by shifting the location of the feeding points, and then mutual coupling is to form a broadband vialess vertical transition. In order to further improve the transmission performance of this vialess vertical transition in W-band, the offset distance between two feeding points of the CPW MMRs is optimized by HFSS, thus three resonant modes are introduced within the frequency range from 70.38 to 100.03 GHz. To verify this design, a three-pole broadband vertical transition fabricated on a four-layer LCP substrate is measured. The measured results indicate that a broadband structure ranging from 75.66 to 97.71 GHz can be obtained with a minimum in-band insertion loss (IL) of 1.8 dB and a return loss (RL) of above 10 dB. Therefore, the superiority of the proposed CPW MMRs in the realization of broadband vialess vertical transition is effectively verified.