A new design for two-pole transmission line bandpass filters using a closed-loop resonator and coupled lines

This paper presents a new design method for two-pole transmission line bandpass filters (BPFs) using a closed-loop resonator and coupled lines. The concept of this method is to adjust the length of the closed-loop resonator for J_l2 inverter realization and attenuation poles at desired frequencies. To analyze the filter structure, we start with an equivalent π-network for the closed-loop resonator that can be transformed into J-inverter-based network. The lengths of the closed-loop resonator and coupled lines are determined from the resonance condition and the relationship between J-inverters and resonant susceptance. To demonstrate the practicality of this new method we designed and fabricated a two-pole bandpass filter. The measured and simulated results of the filter show an excellent agreement with each other.