Motivated by the demands of determining aircraft electromagnetic compatibility (EMC) performance using computational means, this thesis investigates a method of building small structures into Finite-Difference Time-Domain (FDTD) meshes larger than the structure size. The proposed modelling method is to build the characteristics of the small structure into the FDTD mesh; these are determined using an optimisation method on the fields penetrating the structure, which are obtained by detailed simulations or measurements.
Electric and magnetic polarisabilities are used to characterise the apertures. These polarisabilities are fitted by an optimiser, and a genetic algorithm (GA) is used as the optimisation method in this research program. The equivalent dipole moment to replace the aperture in the FDTD model is calculated from the polarisabilities obtained by the GA. This equivalent model shows good results in terms of both field intensity and phase. When applied on a single square aperture problem, the equivalent model shows field amplitude within 2 dB and phase within 10 degrees from that simulated using an FDTD detailed simulation.
A measurement system including field probes, a 3-D scanning frame, and the absorber box is built to provide validation and source data for the modelling work. A small dipole with a differential amplifier is used to measure the electric field. The measurement accuracy could be improved by further development of the measurement methods, such as encountering diffraction and noise.
A number of tests using both fine-grid simulated and measured field shown the model can produce good results in both fine- and coarse-grid mesh models, in both magnitude and phase.
