Modeling of Passive Intermodulation Induced by Concentrated Current Density in Bent Coaxial Connectors

This work first reports a radio frequency (RF) tee coaxial connector, which is three-port reciprocal as a linear system but nonreciprocal as a nonlinear system. Initial experimental observations show that when the bend path of a tee connector is involved, stronger nonlinearity is found, manifested in the higher amplitude third-order intermodulation (IM3) products. According to the experimental result, the bend nonlinearity is introduced to provide explanations for extra passive intermodulation (PIM) generation in the bend of connectors resulting from nonlinear resistance and inductance caused by concentrated current density in the bend. A comprehensive analysis for bend nonlinearity was carried out using an equivalent circuit model and a finite element analysis (FEA) model, along with experimental verification. Accounting for the reflections and the extra PIM sources in the bend, an equivalent circuit model of bent connectors was developed. Considering the concentrated current density in the bend, the modified mechanism of nonlinear resistance and inductance with three modifications was investigated and an FEA model of bend was developed to evaluate the bend nonlinearity. Varying geometric parameters of the coaxial connector bend structure were simulated. It was shown that this model can be used to theoretically predict the PIM product levels for a wide variety of bent coaxial connectors. In addition, a series of PIM tests were conducted with two groups of customized bent coaxial connectors with different curvature radius, radius, and angle of bend. The measured results agree well with the predicted results, verifying the proposed model of bend nonlinearity.