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A new type of lowpass filter with an ultra‐wide stopband, sharp transition and compact size is proposed. To reduce the circuit size, a bended transmission line is adopted in the stepped impedance resonators. To achieve a sharp roll‐off, a pendulum‐shaped resonator is implemented in the filter. The transition band is from 1.30 to 1.45 GHz with −3 and −20 dB, respectively. The stopband with an attenuation level better than 20 dB is from 1.45 up to 23.4 GHz, hence an ultra‐wide stopband with 18th‐harmonic suppression is achieved. Furthermore, the proposed filter exhibits an extremely small size of 0.09λ_g × 0.11λ_g where λ_g is the guided wavelength at the cutoff frequency.