The installed radiation patterns of antennas on large platforms are of great interest when installing antennas. The computation of installed radiation patterns is challenging due to the large size of the platform. To reduce the effort of modeling the large platform, this work adopts the hybridization of the method of moments (MoM) and physical optics (PO) method. The multi-level fast multipole method is used to accelerate the solution of matrix equations, so that a large MoM region can be used to improve the accuracy of modeling the antenna and its surrounding structures. Another challenge of modeling antennas on large platforms comes from the complexity of the antennas. Furthermore, when an antenna is outsourced, detailed design information may be unavailable to those who install the antenna, which hinders direct modeling of antennas on large platforms. In this case, equivalent models of antennas are derived based on their uninstalled far field radiation patterns. Using the equivalent models, installed radiation patterns of antennas can be computed without knowing their detailed design information. By combining the hybrid method with the equivalent model, the installed radiation patterns are computed efficiently.