Design of miniaturized ultra-wide stopband lowpass-bandpass diplexer using hexagon-shaped resonators

This paper presents a lowpass-bandpass diplexer with ultra-wide stopband and low insertion loss using hexagon-shaped resonators. The proposed diplexer consists of a bandpass (BPF) and a lowpass filter (LPF), representing the core concept of the proposed design method that aims to concurrently design BPF and LPF. In this proposed design method, the influence of the LPF filter on the BPF's design has been identified through coupling matrix analysis for the first time. Initially, an LPF is designed based on three coupled hexagon-shaped elliptical resonators. Subsequently, a novel model for BPF design, utilizing coupled high-impedance lines, has been introduced. Following this, the BPF model is developed using coupling matrix analysis while considering the impact of LPF resonators. The LPF have a 1.32 GHz cut-off frequency and ultra-wide stopband up to 17.42 GHz. The BPF consisted of four resonators and the hexagon-shaped structure is used instead of low impedance lines. The utilization of hexagon-shaped resonators serves the purpose of enhancing the precision of the coupling effect, aligning with the proposed coupling matrix analysis. Additionally, hexagon-shaped resonators exhibit a greater capacitive effect, leading to a reduction in insertion loss within the passband when compared to rectangular-shaped resonators. The BPF has narrow passband with center frequency of is 2.25 GHz and 0.31 GHz bandwidth. The measured insertion losses of LPF and BPF are < 0.75 dB and 0.81 dB, respectively in 60% of passbands.