Capacitive Obstacle Realizing Multiple Transmission Zeros for In-Line Rectangular Waveguide Filters

This paper presents a capacitive obstacle for rectangular waveguide filters capable of realizing multiple transmission zeros (TZs). By combining a capacitive window with multiple thin stubs, a filter designer has the ability to control the total coupling implemented by the obstacle, and the frequency location of the TZs generated. The resulting structure is compact, and can be employed at the input/output and intermediate stages of direct-coupled-resonator filters. This letter includes a parametric analysis of the obstacle and practical design guidelines. To validate its practical application, an in-line five-pole filter realizing three TZs has been successfully realized.