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A transition from rectangular waveguide to unenclosed coplanar waveguide (CPW) is designed and tested at W -band. Full-wave finite-element analysis is applied to the design and optimization trends are shown for the critical parameters. A stepped-impedance filter suppresses a parasitic mode, which is generated as the circuit emerges from the waveguide block, and a rejection of -20 dB is achieved. A back-to-back structure is used to characterize the performance of the transition and an insertion loss of 1.6 dB from 75 to 100 GHz is measured. Removing line losses, a single transition insertion loss of 0.46 dB is obtained.