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A wideband balun designed by exact synthesis is presented in this letter. The designed balun has a symmetrical four-port structure with one port open-ended. It consists of a wideband impedance transformer and a broadband phase inverter, which is realized by connecting bottom and top layer parallel strip through two via holes. To design the wideband impedance transformer, an S-plane highpass prototype has been exactly synthesized with third-order Chebyshev response. The proposed design procedure is very flexible, and it can be easily scaled for other frequency bands. A wideband parallel strip balun operating from 0.72 to 2.05 GHz is fabricated and tested to verify the design concepts. The measured results match well with the design theory.