Analysis and Design of Compact Ridge Waveguide Bandpass Filter and Filtering Balun With Improved Upper Stopband Performance

This article proposes a controllable and quantizable coupling structure for ridge waveguide (WG) resonators, which is utilized to design miniatured bandpass filter (BPF) and filtering balun. It consists of ridge WGs and evanescent rectangular WGs with nonuniformed widths. The evanescent WGs between the ridge resonators can be equivalent to a T-shaped three-inductor network, and then its width is able to adjust the inductive coupling strength. As the width decreases, i.e., the shunt inductor is also decreased, the distance between the resonators is reduced so that the coupling strength between them can be maintained. Based on this, an inline filter is designed with a compact structure and two transmission zeros (TZs) are introduced. Based on the filter prototype, a double-ridge WG filtering balun is created by stacking two BPFs in a mirror structure. A metal plate in the middle is inserted to suppress two harmonics near the passband, thereby achieving a good out-of-band performance. For demonstration, a three-order BPF and filtering balun are fabricated and measured. Measured results indicate both of them have the advantages of low loss and compact size. Furthermore, the spurious-free frequency response of both designs is over three times the passband frequency.