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A new semi-fractal technique is applied to hairpin microstrip lowpass filters to realise compact size and sharp roll-off. To achieve wide stopband suppression, both a semi-fractal hairpin resonator and a U-shaped resonator are employed in the filter. A demonstration filter with 3 dB cutoff frequency at 1.51 GHz has been designed, fabricated and measured. The results show that a roll-off of 217.6 dB/GHz with a relative stopband bandwidth of 159.7% (referred to a suppression degree of 32 dB) can be obtained while achieving a high figure-of-merit of 44 832.