Ultra-Compact Tunable Filtering Rat-Race Coupler Based on Half-Mode SIW Evanescent-Mode Cavity Resonators

Abstract

In this paper, for the first time, an ultra-compact frequency-tunable filtering rat-race coupler based on half-mode evanescent-mode (EVA) cavity resonators is presented. Employing EVA cavity resonators in the coupler design yields attractive characteristics, such as a wide tuning range and a wide spurious-free range, while also adding filtering functionality to the device. The presented coupler is synthesized as a two pole bandpass filter using the coupling matrix method. Then, the coupling coefficients are mapped to a coupler topology. Design, simulations, and experimental validation are presented. The implemented rat-race coupler has a very compact size [ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.14\lambda _{g}\times 0.14\lambda _{g} $ </tex-math></inline-formula> ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{g}$ </tex-math></inline-formula> at 885 MHz)] achieving a 92% miniaturization in the structure footprint compared to the conventional planar one. The coupler has an 86% tuning range ranging between 0.885–1.65 GHz. Throughout the tuning range, the coupler shows an equal power splitting ratio and filtering profile with a measured insertion loss and 3-dB fractional bandwidth between 5 and 4.2 dB and 6.8% and 8%, respectively. The measured isolation is better than 30 dB. In addition, the coupler has a measured spurious-free range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10.2f_{o}$ </tex-math></inline-formula> (at the lowest frequency) to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5.45f_{o}$ </tex-math></inline-formula> (at the highest frequency).