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Abstract This article presents a closed‐form synthesis method for wideband bandpass filters with suppressed harmonics, where stepped‐impedance resonators (SIRs), scaled lines, and vertically installed planar (VIP) structures are introduced to improve the in‐band and out‐of‐band performances. By combining SIRs and scaled lines, the rejection level of the filter is greatly improved and a more compact size is achieved. And the VIP structures help to bring wide passband as well as suppressed harmonics. Then, a 3rd‐order filter is designed to verify the validity of the proposed method and topology. The simulated and measured results demonstrate that the proposed design has a wide passband (FBW = 52.9%), low insertion loss (<0.4 dB), and suppressed harmonics with the level of 20 dB up to 7.1f0.