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A wideband vertical transition between co-planar waveguide (CPW) and parallel strip transmission line is presented. It functions as an unbalanced-to-balanced (balun) transformer as well. The transition is realized in back-to-back configurations for 50 Ω and 75 Ω characteristic impedance cases to verify their transition performances. The CPW-parallel strip-CPW transitions are found to have distinct resonant frequencies where the length of the parallel strips is equal to multiples of half wavelength, which is not characteristic of a single transition, but a result of two CPW-parallel strip transitions interacting with each other. Away from the resonant frequencies, the back-to-back configurations have low insertion loss over wide frequency band.