Network simulation tools are frequently used to analyze performance of MANET protocols and applications. They commonly offer only simple radio propagation models that neglect obstacles of a propagation environment. In this paper, we integrate a more accurate radio propagation model into a simulation tool. The model is based on ray tracing and considers geographic data of the simulation area. We prove that the usage of a more precise propagation model changes simulated connection topologies considerably. Consequently, we obtain different performance evaluation results. To our best knowledge, no other study of MANETs has been performed so far with such a detailed radio propagation model. Hence, this paper also gives new insights on the realistic performance of MANETs in outdoor environments.