Inline Rectangular Micro-Coaxial Line Bandpass Filters With Novel Mixed-Coupling Structure for Ka-Band SatCom Application

Abstract

This article presents inline Ka-band rectangular micro-coaxial line (R $\mu $ CL) bandpass filters (BPFs) with mixed-coupling, fabricated using the micro-structure electrochemical fabrication (MEFAB) process. A novel mixed-coupling structure based on a pair of fully coupled grounded half-wavelength stepped-impedance resonators (GHSIRs) is proposed for designing R $\mu $ CL BPFs, which is particularly advantageous for the MEFAB process. By controlling the middle section of the structure between two coupling GHSIRs, the mixed-coupling polarity can be flexibly tuned to be either electric-dominant or magnetic-dominant, producing transmission zeros (TZs) at the lower or upper stopband, respectively. The TZ generation and control of the two proposed mixed-coupling structures are analyzed in detail and verified experimentally with two second-order R $\mu $ CL BPF designs. To verify the scalability of this design method, two fourth-order and one seventh-order R $\mu $ CL BPFs operating in the same 30–31 GHz range are further designed based on the proposed structures, and then fabricated and measured. In addition, to mitigate probe-to-probe coupling that prevents accurate out-of-band suppression characterization in on-chip probe measurement, a low-cost surface-mount technology (SMT) measurement method using isolation blocks (IsoBlks) is presented. Despite the narrowband operation, these three mixed-coupling R $\mu $ CL BPFs still feature low insertion loss (IL), controllable TZs, compact size, and a quasi-planar structure, showing promising potential for Ka-band SatCom applications.