Absorbing boundary conditions (ABCs) are needed to surround an antenna and terminate the finite difference time domain spatial domain. The proximity effects of ABCs which are close to the antenna are investigated, and it is shown that specifying the physical distance between an antenna and the ABCs is preferable to specifying the number of cells. Distances of about 0.5λ have been shown to produce good radiation patterns at all observation angles, but larger distances are necessary for accurate prediction of antenna gain. The use of the Berenger perfectly matched layer (PML) allows the amount of free space surrounding a structure to be reduced but incurs a computational penalty depending on the thickness of the PML. ¿ American Geophysical Union 1996<