The central theme of this dissertation is the study of passive nonlinearities in electromagnetic transmit/receive systems such as radar sensing and wireless communications systems. Passive intermodulation (PIM) is a form of nonlinear distortion that is caused by the electromagnetic nonlinearity of passive wireless system components. The vast majority of passive components are normally considered to have a linear electrical response under small excitation values. However, these passive components have trace levels of nonlinearity which cause a small amount of intermodulation distortion in high-power transmit/receive systems. This distortion can, in certain cases, fall into a transmitting device's receive band. This causes significant interference and limits the sensitivity and range of the system. The results of our study have given a better understanding of the causes and behavior of passive intermodulation in wireless systems and components, particularly coaxial connectors. Theoretical advances we have made toward this end include a point-source model of PIM sources, as well as a circuit-level model of the interaction between the passive nonlinearity and the surrounding linear impedances in a circuit. Both of these models achieve much higher agreement with experiment than has previously been attained and grant deep insights into the nature of the underlying nonlinear mechanisms that generate PIM distortion. On the more practical side, we have developed two new methods of mitigating PIM distortion: one in which the nonlinear distortion is cancelled by a specially engineered source of PIM distortion, and the other (specific for ferromagnetic nonlinearities) which relies on the imposition of an external magnetic field.
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