Synthesis of High-Selectivity Ultra-Wideband Reflective/Absorptive Bandpass Filters With Unequal-Width Three-Coupled Line Sections

In this brief, the unequal-width three-coupled line section is adopted for the design of miniaturized high-selectivity ultra-wideband reflective and absorptive bandpass filters. These bandpass filters can achieve wider bandwidths and higher stopband rejection performances, benefited from the cascaded transmission lines and the short-circuited stubs. With the introduction of the unequal-width three-coupled line sections, extra transmission poles located at the cut-off frequency are generated, resulting in high skirt selectivity. Besides, a grounded resistor is introduced to eliminate out-of-band reflections. The design equations and the exact synthesis procedures of these bandpass filters are presented and explained. For demonstration, three bandpass filters operating at 3 GHz are designed, simulated, and measured, showing an excellent agreement between the measured and predicted results.