This thesis deals with numerical simulations of Maxwell's equations using the Finite-Difference Time-Domain (FDTD) method. The method is widely used in scattering, antenna and electromagnetic compatibility (EMC) applications.

A highly topical area in EMC analysis is Shielding Effectiveness (SE) simulations which is a measure of the (unintentional) coupling from outer electromagnetic sources into electrical components inside shielded electronic equipment. Even though the shielded enclosure and the components inside (e.g. printed circuit boards) can be resolved with FDTD, it is often impossible to resolve the apertures which cause the field penetration into the enclosure. The reason for this is that the apertures arc often much smaller than the resolution of the FDTD lattice and that the material properties at the aperture are often unknown (c.g gaskets, paint etc.). An important parameter in this context is the aperture transmission cross section which characterizes the electromagnetic properties of an aperture. Methods for extracting this parameter from FDTD simulations have been developed. Also, a semi-empirical method has been developed where the measured aperture transmission cross section can be parameterized and inserted in the numerical model as a point source representing the aperture.

An important parameter in radar applications is the Radar Cross Section (RCS) of a scattering object. FDTD is a suitable method for broadband simulations of the RCS. However, since only the volume in the vicinity of the object is included in the FDTD lattice, a near- to far-zone transformation must be applied for determining the far-field results. The transmitting source is also often in the far-zone, which implies that it can be approximated by a plane wave. A plane wave can be generated inside the FDTD lattice using the electromagnetic equivalence principle, or Huygens' sources. The thesis includes methods for improving both the near- to far-zone transformation and the plane wave excitation with respect to numerical dispersion. For Synthetic Aperture Radar (SAR) applications it is necessary to simulate the scattered field of objects placed on ground. The near- to far-zone transformation and the Huygens' sources have been extended to include a lossy dielectric homogeneous half-space representing the ground. A new timedomain version of the near- to far-zone transformation for this purpose has been developed which reduces the execution times significantly. The accuracy of plane wave excitation when a ground is present, has been improved by developing modified Fresnel coefficients consistent with the FDTD algorithm. The improvements provide the ability to simulate electromagnetic scattering of low amplitude using FDTD.