Simulation is a common technique for the evaluation of new approaches and protocols in networked systems and provides many benefits. However, it is also well known that the relevance of the simulation results for real-world applications depends on the various models which are used within the simulation, e.g., for the characteristics of the radio communication. In this paper, we introduce the Extended Multipath Raytracing Model, an extension to the ray-tracing radio medium available in Cooja, to improve the modelling of wireless links in simulated Wireless Sensor Networks. Our extension allows the simulation of environmental influences onto links on a per node basis, allowing the analysis of various effects observed in experiments in a virtual environment. Furthermore, the packet-based modelling of transmission errors is extended to provide the simulation of bit errors, allowing new usage scenarios, like the simulation of error detection and Forward Error Correction codes in Cooja.