Properties of Building Blocks Comprising Strongly Interacting Posts and Their Consideration in Advanced Coaxial Filter Designs

TL;DR

This paper explores the electromagnetic properties of strongly coupled post-based building blocks for coaxial filters, emphasizing the importance of accurate modeling and layout considerations for advanced filter design.

Contribution

It introduces a new understanding of the electromagnetic resonances in strongly coupled post structures, challenging traditional equivalent circuit models and guiding improved filter optimization.

Findings

Equivalent circuits based on individual post resonances are unreliable for strongly coupled posts.

Layout-matched topologies can be optimized using full-wave simulations or measurements.

A 2nd-order in-line filter with a transmission zero was successfully designed using these principles.

Abstract

Building blocks containing strongly coupled posts offer new possibilities for advanced coaxial (comb-line) filter designs. Equivalent circuits based on the individual resonances of the posts cannot be used to reliably describe the behavior of these structures because of the strong coupling between the posts. Instead, sets of electromagnetic (EM) resonances that satisfy the boundary conditions are used. The resulting equivalent circuit is either a fully transversal circuit or contains locally transversal sub-circuits depending on the strength of the coupling between the cascaded blocks. The validity of similarity transformations that result in topologies with unusual strong coupling coefficients is questionable despite the fact that they yield the correct frequency response. Such coupling matrices obscure the physics of the problem and fail to predict the correct behavior of filtering structures. However, topologies that match the layout of the posts can be used to optimize the filter in connection with a full-wave solver or measurement. Examples of dual-post and triple-post units are used to illustrate the key findings. The basic knowledge of the real functionality of these special resonator configurations allows their consideration in advanced filter implementations by well-established classic design methods, without limitation by the design approach. This is demonstrated by an example of a 2-order in-line filter implementation providing one transmission zero by using the combination of single and transverse dual-post resonators. This fundamental understanding of the special properties provides the pre-requisite for a variety of novel filter solutions.