Analysis and design of post-wall waveguides for applications in SIW

Abstract

The post-wall waveguide (PWWG) is a microwave transmission line that combines integrability with printed circuit board technology with relatively low loss at millimeter-wave frequencies. The side walls of a PWWG do not consist of flat surfaces, as in a rectangular waveguide, but of a set of cylindrical posts. To characterize PWWGs electromagnetically we present a model based on a 2D formulation of Maxwell’s equations. A modal expansion of the fields around the cylindrical posts in the PWWG results in a matrix equation from which the scattered field induced by an excitation field can be determined. Using Lorentz’s reciprocity theorem, we derive integral equations that relate the electromagnetic fields to surface currents. The propagation constant can be determined by considering an infinitely-long PWWG. Since the series in the corresponding modal or moment equation does not converge for complex propagation constants, the calculation of losses requires an alternate model. Combining the modal formulation and the integral formulation we can adequately describe the electromagnetic behavior of a PWWG component by electric and magnetic surface currents at port planes. The transfer from port to port is calculated from this characterization and subsequently the scattering parameters of the PWWG component are determined. The second part of the thesis concerns the design, realization and measurement of PWWG structures. An excitation structure consisting of a slot-coupled grounded co-planar waveguide positioned in the top ground plane of the PWWG excites the dominant mode in the PWWG over almost its full bandwidth. As in the case of rectangular waveguides, straight sections of PWWG are the basis for the design of components such as bends and splitters. By combining components and by selectively adding and removing posts, complex components such as Butler matrices and filters are realized. To compare simulation and measurement results, sets of PWWG test components have been manufactured, including uniform lines, phase shifters, a bend, a T-splitter, couplers, and a 4x4 Butler matrix.