A subcell model is presented for including thin material sheets in the finite-difference time-domain method. The subcell model removes the normal restriction which sets the spatial grid increment at least as small as the smallest physical feature in the solution space. Removing this restriction leads to greatly reduced storage requirements and a corresponding reduction in the number of time steps needed. The subcell model is verified by comparison with the exact results for the loss and phase shift for a parallel plate waveguide loaded with a thin material sheet. Specifically, thin conducting as well as thin dielectric sheets are investigated for both TEM and TM/sub 1/ time-harmonic excitations of the waveguide. The FDTD results are in very good agreement with the exact results. Finally, the subcell model is used in the analysis of a practical problem-a resistively loaded monopole antenna formed from a thin-walled conducting tube. The FDTD results are compared with accurate measurements for this antenna, and, again, the two are in very good agreement. >