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Two robust millimeter-wave microstrip-to-waveguide transitions are proposed in this paper. The central operating frequency is 77 GHz, for collision-avoidance radar applications. The microstrip line has been first transformed into an air-filled rectangular waveguide whose height is equal to the substrate thickness. Then, a conventional multisection lambda/4 transformer has been used in the first transition, and a patch antenna in the second transition, to transform to the feeding WR12 waveguide. Experimental results for the first transition show an insertion loss less than 2 dB and a return loss better than 15 dB in a bandwidth of 3 GHz for the back-to-back transition.