Spectral properties of a rectangular waveguide section with a pair of two transversal ridges

Short metal ridges in a rectangular waveguide appeared to be promising units for different microwave filters providing complicated frequency response. In particular, a waveguide containing two rectangular even (or uneven) ridges was used [1] to provide high-quality resonant reflection of the incident mode. A pair of equal height posts in a rectangular waveguide was used lately [2] in order to design a bandpass filter generating an improved response with additional attenuation poles. A quasi-elliptic filter based on a pair equal height posts was reported recently [3]. Off-centered single post and slant single ridge were used successfully in a modern filter design as well [4, 5]. Nevertheless the physical nature of the phenomenon of an attenuation pole generation provided by all the mentioned resonant sections remained unrevealed. Here we use the spectral theory of open waveguide resonators [6-8] to solve this problem. Within the frames of the spectral theory a waveguide unit is considered as open waveguide resonator loaded by semi-infinite guides. Different resonant regimes are interpreted as a response on the excitation of the natural oscillations of the resonator. Similarly to [9], we expect that understanding the physical nature of the resonant phenomena makes their conditions and limitations more clear.