Transmission-Line Schematic for Harmonic-Controlled Bandpass Filters

This article presents a rigorous synthesis method of a transmission-line filter whose second passband can be placed at any multiple of the center frequency, in theory. Two coupled-line schematics for harmonic-controlled bandpass filters are derived from fundamental LC filter prototypes. Their design parameters are provided in closed-form expressions so filter dimensions can be acquired rigorously without relying on EM-simulation-based coupling tests. Three filters were designed and fabricated for verification purposes, and their measurement results exhibited excellent agreement with the theory.