Miniaturized diplexer with wide-stopband based on half-mode substrate integrated waveguide

Abstract

A miniaturized diplexer with a wide-stopband based on half-mode substrate integrated waveguide (HMSIW) is proposed. The diplexer combines a dual-mode resonator (DMR) with single-mode resonators (SMRs). The employment of HMSIW technology breaks through the limitations of SMRs on miniaturization, while effectively addressing the limitation on wide-stopband performance that is typically encountered with the TE202 mode in the SMRs. A second-order prototype, centered at 10.34 GHz and 13.90 GHz, has been designed and fabricated, using a TE101/TE301 DMR. The measured out-of-band rejection is better than 20 dB with a frequency of up to 2.16f1 (f1 is the center frequency of channel 1). Meanwhile, the size of the diplexer is reduced to 1.363λg² (λg is the guided wavelength in the dielectric substrate at f1).

1. Introduction

As a special type of filter, diplexers separate the transmission and reception channels of a common antenna. Due to the significant impact on wireless communication systems, the trend in diplexer design focuses on wide-stopband performance, high isolation, low insertion loss, and miniaturization (Bavandpour et al., 2021; Roshani et al., 2023). Substrate integrated waveguide (SIW) technology has become an effective solution for diplexer designs as it provides high Q and electromagnetic interference shielding while significantly reducing the size.

In a conventional design, T-junctions are used in diplexers to separate two passbands (Sirci et al., 2015). Many studies aim to minimize the circuit size by eliminating T-junctions. A compact diplexer has been proposed (Cheng et al., 2013), using multiple DMR connections, which cannot control the bandwidth of two channels separately. Another approach integrates DMR and SMRs within a diplexer to independently control the bandwidth of each passband, establishing a classic topology (Zhou et al., 2018a). However, this topology still incorporates multiple SMRs, which occupy a significant amount of space and hinder further miniaturization. Alternative miniaturization methods have been proposed, such as multi-layer stacking technology (Iqbal et al., 2019), HMSIW (Zhou and Wu, 2021), and folded SIW (Sieganschin et al., 2021). Nevertheless, both multi-layer technology and folded SIW increase the design complexity and manufacturing costs.

The stopband characteristic is also one of the key performances of diplexers, as the wide-stopband can effectively reduce the external or internal signal interference. However, designing compact SIW diplexers with a wide-stopband remains a challenge. Building on the topology that combines DMR and SMRs, various techniques have been implemented to achieve wide-stopband SIW diplexers without T-junctions, such as the harmonic staggered technique (Zhou et al., 2020), orthogonal transmission (Xie et al., 2020), and center-coupled windows (Ma et al., 2023). However, in these studies, since the TE202 mode in the SMRs is not completely suppressed, the boundary of the stopband is limited to around 2f1.

In this study, a compact and wide-stopband SIW diplexer is proposed, based on the HMSIW technology. Using half-mode substrate integrated rectangular cavities (HMSIRCs) as DMR and SMRs overcomes the limitations of SMRs on the miniaturization and wide-stopband performance. The TE202 mode is effectively eliminated from SMRs as the specific even modes in HMSIRC are not excited due to magnetic walls in the symmetry plane (Lai et al., 2009). The TE102 mode is suppressed, elevating TE301 as the first unsuppressed mode in SMRs. Therefore, the diplexer can obtain a wider stopband only through second-order resonance. The usage of HMSIRCs, coupled with a reduced number of resonators, further minimizes the circuit size. A second-order prototype based on TE101/TE301 DMR is designed and fabricated, covering a wide range of frequency ratios. Measurements show that the prototype exhibits excellent stopband performance with a small size.