A Novel Coupling Structure for Inline Realization of Cross-Coupled Rectangular Waveguide Filters

Muhammad Latif,Farooq Mukhtar,Giuseppe Macchiarella

doi:10.1109/access.2020.3000847

Abstract

This paper presents a new cross-coupling structure for introducing transmission zeros in waveguide filters with inline configuration. The cross-coupling is realized by an internal bypass metallic loop structure (wire), short-circuited to the filter top plate and passing through the waveguide cavities. Positive and negative cross-couplings are realized by only varying the length of the structure suitably. Full-wave electromagnetic (EM) simulations of the coupling structure are performed for extracting the coupling coefficient sign and value ( ${k}$ ). The relationship of the values of ${k}$ with the coupling structure dimensions is presented graphically. The loading effect of coupling structure on the resonance frequency of the waveguide cavities is also discussed, as well as the possibility of post-manufacturing tunability. The feasibility of the proposed coupling structure is demonstrated by the design of three test filters. Two filters verify the in-line design of a triplet configuration using the proposed coupling wire to introduce a transmission zero below or above the passband. These filters have been also manufactured and measured for an experimental validation of the new structure. The third filter demonstrates the use of the wire loop to create a negative coupling in a cross-coupled configuration. Power handling capability of this configuration has been also thoroughly analyzed by full-wave EM simulation.

Highlights

In recent years, the continuous growth of the frequency channels request has resulted in increasingly stringent requirements on the selectivity of microwave filters used in communication systems
The introduction of transmission zeros (TZ) in the response of rectangular waveguide filters is typically realized by using folded configuration to allow the cross-couplings implementation [2]–[4]
We introduce in this work a new coupling structure, allowing the implementation of cross-couplings in rectangular waveguide filters with in-line topology

Summary

INTRODUCTION

The continuous growth of the frequency channels request has resulted in increasingly stringent requirements on the selectivity of microwave filters used in communication systems. The cross-coupling between non-adjacent cavities is realized by an internal bypass loop structure formed by a metal strip/wire with opposite ends short-circuited to the top plate of the filter. From a circuit point of view (Fig. 3), this coupling can be represented by a positive or negative series reactance (X13), connecting two equivalent shunt resonators (representing the coupled waveguide cavities). The eigenmode frequencies are always sorted in ascending order and the proper selection of feven and fodd may require considering the actual field distribution of the computed modes For this reason, before facing the evaluation of the coupling coefficient for the proposed coupling loop, we will discuss how to identify the sign of k. The loading effect can be compensated by slightly increasing the length of coupled resonators

FILTER DESIGN EXAMPLES

ANOTHER APPLICATION OF THE NOVEL COUPLING LOOP

CONCLUSION