In this paper, a hybrid technique combining the FDTD method and Physical Optics (PO) approximation is presented as a tool for analysis of planar antennas radiating in the presence of electrically large conductive structures. The surface equivalence theorem is used to interface the FDTD and PO subdomains and establish equivalent current sources. To increase computational efficiency of the proposed approach, the spatial interpolation technique of the electromagnetic field components on the Huygens' surface is employed. Numerical results are given to illustrate the efficiency of the hybrid technique.