Most simulations and performance comparisons of mobile ad hoc network routing protocols are based on a simplistic and idealistic physical layer model. In real applications, there are different kinds of noise or interference that impact the signal power. We use a shadowing propagation model in our simulation evaluation of two on-demand routing protocols: AODV and DSR. Because of signal power fluctuation, active routes will break, which causes significant throughput degradation and longer packet delay. In this paper, we analyze the impact of a shadowing model on the performance of these two routing protocols. Then we set a new signal power threshold during the route discovery process so that only those links with strong enough signal power will be chosen; we also reduce some control messages for DSR. The simulation results show significant increases in packet delivery ratio and decreases in packet latency for both protocols.