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A novel microstrip bandpass filter with ultra-broad rejection band is proposed and constructed by using the enhanced aperture-backed stepped impedance resonator (SIR) and high-impedance transformer. In this SIR, the aperture is formed on the ground underneath the narrow strip conductor in the center so as to raise its characteristic impedance much higher than that at two sides. As a result, the 1st spurious resonance is significantly raised to the frequency that exceeds the four times of its dominant resonant frequency. Instead of the weakly coupling capacitive gaps used between two adjacent SIRs, the aperture-backed λ/4 transformers with enhanced impedance are utilized at the input/output ports as the strongly coupling elements to effectively widen our concerned pass bandwidth with reduced insertion loss. After investigation on the resonance performance of this SIR and the coupling behavior of the transformer-based coupling element, a three-stage bandpass filter is designed, fabricated and measured to exhibit in reality its highly broadened rejection band.