High Selectivity Broadband Bandpass Filter With Dual-Mode Folded-T-Type Resonator

Abstract

Abstract-In this letter, a compact and high selectivity broadband bandpass filter (BPF) is proposed using the dual-mode folded-T-type resonator and the short stub loaded parallel-coupling feed structure. The resonator can generate one even-mode and one odd-mode in the desired band. Two resonant frequencies can be adjusted easily to satisfy the bandwidth of the BPF. A parallel-coupling feed structure with a cross coupling has been applied to generate two transmission zeros in the lower and upper stopband. Furthermore, the loaded short stub can create two transmission zeros near the upper cut-off frequency and in the upper stopband. Simultaneity, the transmission zero in the lower stopband moves towards the cut-off frequency. One filter prototype with the fractional bandwidth 57% is fabricated for experimental verification of the predicted results. The size for the resonator is only 0.156λ g ×0.303λ g in which λ g is the guided wavelength of 50 Ω microstrip at the center frequency.