A hybridization approach to integrate simulation codes based on high and low frequency techniques is developed in this paper. This work allows the antenna design to be performed directly in the presence of the complex and large structures. Since the sizes of the complex structures can be extremely large electrically, and the antenna structure itself can be significantly complicated, such problems can not be resolved with a single technique alone. While low frequency techniques are generally applied for antenna design problems where small scale interactions are involved, high frequency techniques are adopted for the prediction of propagation effects inside the complex structures. The proposed hybridization approach provides a seamless integration of low and high frequency techniques that combines the advantages of both techniques in terms of accuracy and efficiency. Numerical example is presented to demonstrate the utilization of the proposed approach.