This article presents a simple method to design an ultra-wideband (UWB) bandpass filter (BPF) with a notched band. The structure of the filter is simple and is composed of a single half-wavelength resonator loaded with three sets of short-circuit step impedance stubs. An equivalent circuit model is presented to analyze the resonance characteristics. Compared with the traditional quarter-wavelength uniform impedance stub, the novelty of the short-circuit step impedance stub introduces two design parameters: the impedance ratio (K) and the electrical length ratio (α). Therefore, by adjusting these design parameters, the frequencies of the first two notched bands can be tuned widely, so a wide frequency band with a notched band can easily be achieved. With a K of 0.36 and an α of 0.6, the designed filter achieved an ultra-wideband bandpass response with a notched band. The UWB response had a passband range of 2.5 GHz–10.5 GHz and a notched band around 5.1 GHz with an attenuation of about 45 dB. The insertion loss in the entire passband was less than 1.26 dB, and the return loss was larger than 10 dB on average. The maximum group delay variation in the two passbands was less than 0.3 nS. The measurement results showed good agreement with the simulation results.

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