Application of FDTD for analysis of waveguide transition

The transitions between rectangular and circular waveguides are very useful components especially for some complicated microwave systems. In general, trial and error methods are used to design these types of transitions. Some numerical techniques in frequency domain have previously been applied to waveguide transition structures, but the procedure is cumbersome and depends on expensive analytical calculations. The FDTD is claimed to be a versatile technique for analysis of passive microwave structures. The purpose of the paper is to confirm this claim by applying FDTD to several complex waveguide transitions which are not amiable to many frequency domain techniques such as the mode matching method. It is demonstrated that this algorithm is versatile and can be used to analyze complicated waveguide transitions with a high degree of accuracy. Analysis of such structures can be quite complicated in the frequency domain. For example, the plane wave functions are used to represent the fields inside a rectangular waveguide, while the cylindrical wave functions are used for the cylindrical waveguide. As a result, the frequency domain analysis of a rectangular to circular waveguide transition requires careful considerations for proper numerical convergence. Such a problem is successfully analyzed in the time domain by application of the FDTD algorithm to the structure geometry.