Key Takeaways & Executive Findings
- •• Proposes a novel isomeric resonator configuration combining two ridge waveguide resonators and a half-wavelength resonant slot within a single cavity. • Embedding a tuning post in the ridge reduces the TE101 mode frequency by up to 45% without affecting the TE102 harmonic, enabling simultaneous miniaturization and wide stopband. • The HWRS introduces adjustable cross-coupling, allowing independent control of a transmission zero in the upper stopband via slot thickness. • Measured prototype achieves ~90% size reduction and superior upper stopband rejection relative to traditional ridge waveguide filters.
Abstract
This paper presents a miniaturized bandpass filter (BPF) with a wide upper stopband employing isomeric resonators inside a cavity. The filter consists of two ridge waveguide (WG) resonators coupled through a half-wavelength resonant slot (HWRS) resonator. A tuning post (Tup) embedded in the central region of the ridge significantly reduces the resonant frequency of the fundamental TE101 mode while leaving the first harmonic TE102 mode almost unchanged, thereby enabling both miniaturization and wide upper stopband rejection. The HWRS introduces cross-coupling that generates a transmission zero (TZ) in the upper stopband, with its thickness providing an extra degree of freedom for precise TZ positioning. Experimental results demonstrate approximately 90% size reduction and improved upper stopband rejection compared with a conventional ridge WG filter.
1. Introduction
Recently, the increasing demand for advanced telecommunication systems has spurred extensive research into bandpass filters (BPFs), with particular emphasis on miniaturization, reduction of insertion loss (IL), and enhancement of upper stopband rejection (Huang et al., 2021; Snyder et al., 2021; Lin et al., 2023; Zeng et al., 2023). The metal waveguide (WG) filter has been the subject of extensive research due to its high power capacity and low loss (Wong et al., 2021; Xiang et al., 2023). However, the substantial size of filter circuitry poses significant challenges to the miniaturization required in modern communication systems (Fang et al., 2022). Dielectric resonator (DR)-loaded cavities (Chen et al., 2016; Tomassoni et al., 2016; Widaa and Höft, 2023) and dielectric WGs (Xie et al., 2023; Qin et al., 2024; Tang et al., 2024; Xu et al., 2024) are widely employed to reduce dimensions. Nevertheless, spurious responses associated with either DR-loaded cavities or dielectric WGs persist, leading to insufficient suppression of the upper stopband and limiting their application in multi-standard wireless communication systems (Zhao et al., 2022).
Relatively speaking, ridge WG technology has attracted increasing attention because of its potential for size reduction and improved upper stopband performance (Fahmi et al., 2009; di Crestvolant and de Paolis, 2018; Chen et al., 2024; Zhang et al., 2025). Recently, a U-shaped ridge resonator was proposed to generate a transmission zero (TZ) either above or below the passband (Chaudhary and Ahmed, 2023). However, the upper stopband performance and overall size of the cavity BPF were not addressed. To reduce the filter length, the coupling structure between ridge WG resonators was modified to width-reduced rectangular WGs (Chen et al., 2025). This modification shortens the distance between ridge WG resonators and introduces TZs. Nevertheless, the filter length remains relatively large due to cascade coupling of the ridge WG resonators, and the improvement in upper stopband performance is limited. Moreover, the TZ in the upper stopband cannot be independently controlled because the coupling between the first and third resonators, K13, is determined once the coupling between the first and second resonators, K12, is established in traditional inline cavity BPFs.
In this paper, based on the E-field distributions of the first two modes in ridge WG resonators (TE101 and TE102 modes), a tuning post (Tup) is embedded in the central region of the ridge. By adjusting the Tup depth, the resonant frequency of the fundamental TE101 mode (f101) is significantly reduced, whereas the first harmonic TE102 mode remains almost unchanged, indicating that both miniaturization and a wide upper stopband can be achieved. Furthermore, a half-wavelength resonant slot (HWRS) resonator is introduced between two ridge WG resonators to construct a three-pole BPF. This configuration reduces the BPF length and introduces cross-coupling, thereby generating a TZ in the upper stopband. In addition, the thickness of the HWRS resonator provides an extra path to control the cross-coupling, enabling precise TZ positioning within a certain range. Measurement results indicate that the proposed filter achieves a size reduction of approximately 90% and wider upper stopband rejection compared with a filter based on a traditional ridge WG (Chen et al., 2025).
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Chengyang Zhang, Ying Xue, Qingyuan Lu, Jianxin Chen (2025). Miniaturized bandpass filter with a wide upper stopband using isomeric resonators in a cavity. Engineering Information Technology and Electronic Engineering. https://doi.org/10.1631/ENG_ITEE_2025_0023
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Frequently Asked Questions
What are the main advantages of the proposed bandpass filter?
The filter achieves significant miniaturization (~90% size reduction), wide upper stopband rejection, and a controllable transmission zero, while maintaining low insertion loss and high power capacity.
How does the tuning post affect the resonator modes?
The tuning post embedded in the central region of the ridge reduces the resonant frequency of the fundamental TE101 mode by up to 45%, while the first harmonic TE102 mode remains almost unchanged, thus enabling size reduction and wide stopband simultaneously.
How is the transmission zero generated in the upper stopband?
The transmission zero is generated by the cross-coupling introduced by the half-wavelength resonant slot (HWRS) placed between two ridge waveguide resonators, and its position can be independently controlled by adjusting the thickness of the HWRS.
What size reduction is achieved compared with traditional ridge waveguide filters?
The proposed filter achieves a size reduction of approximately 90% compared with a filter based on a traditional ridge waveguide design.
What is the role of the half-wavelength resonant slot (HWRS) in the filter design?
The HWRS acts as the middle resonator in a three-pole configuration, shortens the overall filter length, and provides an extra degree of freedom (its thickness) to control the cross-coupling strength, thereby enabling precise positioning of the transmission zero.
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