Ultra-Compact Surface-Mountable Air-Filled Coaxial Filter for 5G Applications

This paper presents a compact coaxial filter for 5G applications, designed to match the performance and compactness of dielectric-filled waveguides. The design incorporates an 11th-order Chebyshev function with three lower-band transmission zeros and one upper-band zero. By introducing three additional complex reflection zeros, all couplings are positive, simplifying the manufacturing and assembly process. The filter features an ultra-compact re-entrant coaxial resonator and a simple “Folded” topology with dangling structures at the input and output ports. A microstrip feed allows for surface-mount integration. Both simulation and experimental results validate the design's performance, addressing challenges in miniaturization, transmission zeros, and manufacturing complexity.