Multilayer Dual-Band Bandpass Filter Using Microstrip-to-Slotline Transitions and Transversal Signal-Interference Microstrip Lines

This paper reports a type of multilayer dual-band bandpass filter (BPF). It consists of two back-to-back-cascaded microstrip-to-slotline transitions connected by a transversal signal-interference microstrip close loop. Due to the employed transversal in-parallel-connected asymmetrical microstrip lines, a second-order dual-band BPF with three inter-band transmission zeros (TZs) can be attained, where the two created passbands are symmetrical regarding the design frequency. The operational principle and design rules of the devised dual-band BPF are discussed through the presented theoretical frequency responses. For experimental-validation purposes, a two-layer second-order dual-band microstrip BPF prototype is designed, fabricated, and characterized. The two measured passbands are centered at 1.1173 GHz and 2.8965 GHz with minimum in-band power-insertion-loss levels of 0.25 dB and 0.84 dB, and 3-dB fractional bandwidths of 40.41% and 15.36%, respectively.