Coaxial connectors are one of the main contributors to passive intermodulation (PIM) in radio frequency systems. Such systems exhibit nonlinear characteristics, especially as the transmission power is increased. In this paper, microscopic surface analysis and macroscopic nonlinear models are introduced to provide explanations for PIM characteristics and to make more precise predictions for the third-order and fifth-order intermodulation products (IM3 and IM5) power resulting from n-type connectors. The equivalent circuit models are presented to provide insight into the effects of degradation at the microscopic level. Simulation calculations are performed to explore the resistance and capacitance in the circuit models and the relative contribution of various physical parameters. A full discussion of nonlinear current-voltage models coupled with an original method for solving polynomial coefficients is provided. A series of experiments based on two-tone testing is conducted to measure IM products with the input power from 30 to 44 dBm. The experimental results verify the predictions both in the un-degraded and the degraded conditions. The power-dependence of the PIM features and the effect of contact surface degradation on connectors' performance are observed from the measurements, which are consistent with the theoretical predictions.