Analysis of circular waveguide filter using enhanced FDTD

The finite difference time domain (FDTD) analysis of iris-coupled circular waveguide filters presents several challenges because they contain curved metal surfaces and electrically small slots. In this contribution it is shown that by enhancing the basic FDTD algorithm in three ways, filters of this type may be accurately characterized without the need for a highly dense mesh. The enhancements are: (i) an existing locally conformal mesh algorithm to model the curved metal surface, (ii) novel empirical correction factors to account for the singular field behaviour near the slots and their exact position relative to the FDTD cells and (iii) compensation for the numerical dispersion in the FDTD algorithm. The calculation of the correction factors and the implementation of the technique is discussed and the extent to which each of these three enhancements contributes to the reduction of numerical error is shown. In addition, it is shown that a novel combination (i) and (ii) leads to a robust and effective method of treating metal edges which are offset from the edges of the cells. The improvement obtained for a complete filter is demonstrated by comparison to measured results.