Jia-Sheng Hong

This article introduces a modified fan-shaped folded waveguide resonator (FWGR) that exhibits a lower fundamental frequency, improved unloaded quality factors ( $Q_{\mathrm {u}}$ ), and a unique field distribution. Notably, the field distribution enables mixed coupling without requiring any additional structures, which is essential for realizing in-line frequency-variant coupling (FVC) filters with transmission zeros (TZs). Based on the resonator, two coupling schemes, slot coupling and gap coupling, are proposed for constructing higher-order in-line FVC filters. Comprehensive analysis, including the extraction procedure for determining the FVC coefficient and the corresponding TZs, is also presented. Three third-order in-line filters with the same in-band responses are first designed using slot coupling to demonstrate superior control of TZs, where each filter generates two independently controlled TZs due to asymmetrical structures. A fourth-order filter with three TZs is implemented based on symmetrical slots coupling further. To explore additional design possibilities, a novel kind of coupling topology integrating slots and gap couplings is proposed. One four-pole and one six-pole in-line FVC filters with a similar volume are developed for validation. In general, all developed filters in the work can achieve compact volumes and $N$ -1 TZs with simple physical structures. For experimental validation, demonstration, fourth- and sixth-order filters were selected for fabrication using 3-D metal printing technology. All prototypes were printed well, so the test results were consistent with the simulated results.

This article employs a modified dual-mode folded waveguide resonator (DM-FWGR), folded waveguide, and air slots to design a fourth-order dual-band filter for the first time. Two types of coupling windows were used to flexibly tune the internal couplings, while metal cylinders in the first and last resonators enabled independent adjustment of the external coupling. A“back-to-back” topology was adopted to match the coupling characteristics. Owing to the advantages of the DM-FWGR, the proposed filter features a compact size, high- $Q_{\mathrm {u}}$ , and excellent stopband performance with four transmission zeros (TZs). For verification, the filter was fabricated using 3-D metal printing, and the measured results agreed well with simulations.

This letter presents a high-gain compact quadruplex filtering antenna with a small frequency ratio (FR) based on a two-layer substrate integrated waveguide (SIW) structure. The lower-layer feeding section consists of two isosceles right-triangle SIW cavities sharing a common hypotenuse, where independent excitation of the TE₁₀₂ and TE₂₀₁ modes in each cavity enables four isolated channels. The upper-layer radiation section is composed of a hexagonal SIW cavity with a centered square aperture, incorporating a square via-wall and two nested triangular patches. The SIW slot supports two orthogonally polarized modes for the lower bands, while the triangular patches dominate the two higher bands. Perturbation posts and coupling posts enable independent quad-band frequency tuning through modal decoupling. A prototype operating at 3.33, 3.59, 3.84, and 4.03 GHz demonstrates high gains of 6.91, 6.72, 6.22, and 5.3 dBi with channel isolation exceeding 18 dB, validating its suitability for compact multi-band wireless front ends.