Integrated Wideband 2-D and 3-D Transitions for Millimeter-Wave RF Front-Ends

This letter reports on broadband 3-D and 2-D transitions on flexible organic liquid crystal polymer (LCP) substrate with return loss below 15 dB for frequencies up to 110 GHz. The presented novel 3-D coplanar waveguide–coplanar waveguide–microstrip (CPW–CPW–MSTRIP) transition features an insertion loss (IL) of 0.45 dB for a 3.3-mm total (longitudinal) length of transition, while the novel 3-D CPW–CPW transition has an IL of 0.25 dB for a 2.8-mm (longitudinal) length of transition. These transitions require strategically placed vias and tapering of the CPW ground planes in order to suppress radiation loss and optimize the performance over a very broad frequency range. This letter also includes a 90$^{\circ}$CPW bend that shows a return loss lower than 15 dB up to 100 GHz and an insertion loss of 0.75 dB for a 6.35-mm total length of the transition. All these transitions are simple to realize and are compatible with low-cost substrate fabrication guidelines allowing for the easy integration of ICs in 3-D modules, especially in compact automotive radar applications and beam-steering wideband antenna arrays. An example of the integration of the proposed 3-D transitions with a practical antenna array is presented, and experiments verify the very good performance of the integrated topology up to 100 GHz.