Slow Wave Rectangular Coaxial Transmission Line for Applications in Advanced Microwave Filters

This article presents a design methodology for a rectangular coaxial slow wave transmission line (TL) by integrating CNC with emerging additive manufacturing (AM) technology. First, an equivalent circuit model of the slow wave TL is established and subjected to detailed analysis. The structural configuration of the slow wave TL is derived through a comprehensive evaluation process by taking into consideration the constraints in manufacturability and frequency-dispersive performance. Then, the relationship between the structure and its performance is elucidated by correlating the circuit model with the numerical short-open-load (SOL) calibration technique in full-wave simulation. To demonstrate the superiority of the proposed slow wave TL, two distinct applications are explored, leveraging its effective per-unit-length guided-wave parameters, i.e., the attenuation constant and the phase constant. A slow wave TL is realized herein by exploiting the bandgap property from its attenuation constant. In addition, a transversal coaxial-line filter is designed by utilizing the slow wave characteristic associated with the phase constant. It is exhibited that the slow wave effect significantly reduces the required physical length of the odd-mode resonators, enabling the three paths within the transversal filter structure so as to achieve nearly identical electrical lengths with compact physical dimensions, thereby minimizing the overall circuit size. Finally, two prototype circuits are fabricated and measured. The good agreement between the simulated and measured results validates the proposed design methodology. In contrast to conventional approaches that treat coaxial resonators as identical units and focus primarily on interresonator coupling, this article emphasizes the inherent TL resonator properties of the coaxial resonator itself. It focuses on intrinsic resonator characteristics and presents a novel design method for coaxial filters.