This letter presents a detailed study of a microwave tomography system using 3-D finite-difference time-domain methods (FDTDs). The algorithm, which uses a subcell model in form of the thin-wire approximation to model wire antennas, has been validated numerically using published numerical electromagnetic code (NEC) data for dipole antennas, and experimentally by comparing with measurements obtained for a monopole antenna array. The agreement between calculated and measured performances of the monopoles is close. In order to understand the fabrication tolerance, a parametric study was performed with regards to position of ground plane and grid size. It is found that the ground plane plays an important role in the performance of the monopole antennas. The excellent agreement is very promising for future deployment of the algorithm in 3-D microwave tomography applications.