| In this dissertation, the finite element time domain (FETD) method is applied for the full wave analysis of complex microwave circuits and active nonlinear microwave circuits. Several numerical techniques based on FETD method have been developed. First, a hybrid time-domain technique combining finite difference and finite element methods for solving geometrically complicated structures is presented. The validity and accuracy of the hybrid code have been confirmed by comparing with both FDTD and measured results for a microstrip-fed ring patch antenna with a thin impedance-matching section. Second, the FETD method has also been extended to include the nonlinear and active microwave devices under its electromagnetic time domain analysis and versatile simulation capability. Employing the concept of Norton equivalent circuit, the device-wave interaction is characterized and incorporated into the FETD time-stepping algorithm. Consequently, investigation of highly nonlinear phenomena, such as intermodulation and injection locking, can be accomplished by utilizing a large-signal device circuit model. Examples include microwave amplifier and injection-locked oscillator. Theoretical results are verified by experimental results and found to be in good agreement. Third, a diakoptics technique based on FETD method is developed. This diakoptic approach enables the analysis of large microwave structures by modular computation of subdomains as well as the incorporation of nonlinear active devices. Cases demonstrated have shown that either memory requirement or computational effort can be reduced without loss of accuracy. Fourth, an isotropic perfectly matched layer (PML) is applied to the FETD method. Numerical results demonstrate the accuracy and future potential of such an absorber. Finally, a hybrid approach combining the finite element method and modal expansion technique is developed. In this approach, in order to treat the infinite uniform waveguide with the arbitrarily shaped cross section, the eigenmodes of the uniform waveguide are pre-calculated numerically and incorporated into the 3-D FEM formulation through the derived analytical relations. As a result, the analysis is verified to be accurate, versatile, and efficient through extensive comparison with the theoretical and measurement data in the available literature. |
