I. Introduction
With the rapid development of communication technology, high power, wide bandwidth, and multicarrier technology are becoming the inevitable trend. However, passive intermodulation (PIM) in high-power passive microwave components cause one kind of serious electromagnetic compatibility problems on the design, implementation, and stable operation of the communication system. The PIM product (PIMP) may fall into the receiving frequency band which will result in electromagnetic interference [1]– [4]. The generation mechanism of PIM is complex. There are many influencing factors, in which the contact nonlinearity and material nonlinearity are the two main mechanisms of PIM [5], [6]. The contact nonlinearity mainly refers to the electrical nonlinear effects caused by the loose contact between two metal–metal (MM) components [7]–[10], in which, the surface roughness is the fundamental factor, and the input power level and contact force are the exciting factors.