Novel Low-Loss Millimeter-Wave Transition From Waveguide-to-Microstrip Line Suitable for MMIC Integration and Packaging

This letter presents a unique low-loss transition from microstrip to full height rectangular waveguide at W-band. This microstrip transition can be made as a part of the mm-wave monolithic microwave integrated circuit (MMIC) of arbitrary size, and thus, avoid the use of bond wires at the high-frequency port of the MMIC circuit. As a result, the MMIC can be coupled directly to the waveguide. The working principle of the transition is based on electromagnetic coupling, where the coupling between the microstrip mode and the TE10 waveguide mode is achieved via a resonant cavity. A perfect magnetic conductor (PMC) surface is placed over the cavity to facilitate the smooth coupling of electromagnetic energy from the microstrip line to the cavity and then from the cavity to the waveguide. The PMC surface also suppresses the unwanted waveguide mode coupling to the oversized MMIC substrate. The measured back-to-back transition works over the frequency band of 80-114 GHz (relative BW of 35%) with minimum return loss of 13.5 dB. The total insertion loss of the manufactured prototype is found to be varying from 0.54 to 0.803 dB, which implies a single transition loss of less than 0.27-0.4015 dB in W-band.