Compact UWB bandpass filter using ring open stub loaded multiple-mode resonator

A novel ultra-wideband (UWB) bandpass filter with a compact size, improved upper-stopband performance and reduced radiation loss has been studied and designed using a multiple-mode resonator (MMR). The MMR is formed by attaching three pairs of ring open stubs in shunt to a high impedance microstrip line. By simply adjusting the radius of the circles of the ring stubs, the resonant modes of the MMR can be roughly allocated within the 3.1-10.6 GHz UWB band while suppressing the spurious harmonics in the upper-stopband. Meanwhile, the insertion loss is higher than 20.0 dB in the upper-stopband from 11.7 to 33.5 GHz in simulation. Finally, the filter is fabricated and measured. The EM-simulated and measured results are presented and excellent agreement between them is obtained.