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Abstract The conventional quarter/half‐wavelength common‐mode filter (CMF) cannot work well for a differential stripline when there is no ground via. A new and effective via‐free CMF design for the differential striplines is proposed in this study. An alternating high‐low impedance structure is developed to increase the coupling between the stripline and the CMF, thus achieving a good filtering effect. A cascaded CMF design is further studied to enhance the stopband without deterioration of the differential signals. Both simulation and measurement results demonstrate the effectiveness of the proposed high‐low impedance CMF. The significant advantage of the proposed CMF is its compactness and simplicity, which has almost no impact on the original layout of differential striplines.