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In this study, the authors investigate compact half-wavelength slotline resonators and its applications to compact ultra-wideband (UWB) bandpass filter designs. The circuit models have been developed for the proposed UWB bandpass filters. By introducing additional slotlines and properly rearranging the filter geometry, the passband frequency response and upper stopband performance are improved. The basic filter and modified UWB filter have been designed, fabricated and validated to demonstrate the proposed design methodology. Their calculated, simulated and measured results show good agreement. The modified UWB filter has extremely compact size of 8.5 mm × 7 mm. Both simulated and measured results show that the fabricated modified UWB filter has an UWB passband with a fractional bandwidth of 117.8%.