Bandpass filter with wide stopband using loaded short-section of parallel-coupled lines

A compact third order bandpass filter (BPF) using dual and trimode resonators is proposed. To achieve compact size with wide and highly attenuated stopband with achievable fractional bandwidth (FBW), the proposed filter is designed using a dual-mode resonator in the form of a low-impedance feeding network and a trimode resonator formed using a miniaturized (one twelfth of a wavelength) parallel-coupled structure loaded with folded open-ended stubs of high/low-impedances and a semiannular resonator. To enable using easy to manufacture dimensions, the coupled structure of the resonator is designed to have moderate mode impedances. A complete design procedure for the filter is explained. Finally, a compact penta-mode BPF with 35% FBW with sharp upper and lower cutoffs is built and tested. The simulated and measured performance of the filter show less than 0.3 dB insertion loss in the passband and more than 32 dB attenuation across a wide stopband with sharp upper (107 dB/GHz) and lower cutoff (96 dB/GHz) skirts