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Miniature broadband 1:1 baluns for differential mode antennas are exhibited with an asymmetric broadside coupled line in an inhomogeneous medium. To design characteristics of a balun for both the in-band and the rejection band, c- and π-mode parameters for the coupled lines are extracted by both simulation and measurement. The rejection band of the broadband balun is shown to be caused by the c-mode transmission. With the extracted parameters, optimal line dimensions are obtained for 50 Ω single-mode input and differential-mode output port impedances. Measured fractional bandwidth is as wide as 178% for the 0.2-mm-wide broadside coupled line in a 1-mm-thick multi-layer FR-4 substrate.