In the paper, a universal compensation method is presented to improve the imbalance of a trans-directional coupled-line (TRD-CL) based balun caused by the inevitable physical separation between the TRD-CLs. Using this method, the input mismatch and output imbalance can be effectively solved. Moreover, since the compensation is achieved by shortening the two TRD-CLs instead of adding additional stubs, size miniaturization is maintained. Design formulas are derived using the signal flowchart and even-odd mode analysis. A prototype operating at 1.6 GHz is also designed and measured to verify the proposed method.
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