On Mechanical-Thermal-Electrical Effect Of Passive Intermodulation In Microwave Devices

Passive intermodulation (PIM) is a tremendous challenge for the design of space high-power microwave devices. This paper systematically and deeply investigates the PIM problem caused by the mechanical-thermal-electrical coupling of microwave devices through theoretical derivations, numerical simulations and experimental verification. The main works and innovations can be concluded as follows.1. The nonlinear time-domain simulation methods of PIM were developed. The results of prediction of PIM frequencies and orders indicate that the odd-order passive intermodulation products (PIMP) increase sharply and the PIMP are more complex in the multi-carriers as the number of channels increases. Two time-domain full-wave simulation methods were developed to calculate the PIM power levels. One calculation method is the time domain physical optics (TDPO) method, which was used to analyze the PIM of planar, scattered caused by contact nonlinearities. This method introduced the correction term of nonlinear surface impedance and used Gaussian gradient function to change electric currents of contact areas smoothly. The PIM power level of each order of nonlinear contact areas was observed and the influence laws of various parameters on PIM were analyzed. The other method is the finite-difference time-domain method (FDTD), which was used to analyze the PIM of nonlinear lumped elements. According to the I-V model given by the power series of contact nonlinearities, the calculation formula of FDTD was derived to solve the time-domain nonlinear electric currents. The spectral characteristics of nonlinear currents obtained by fast Fourier transform (FFT) were generated to observe the PIM power levels. The effectiveness of the developed methods was verified by the experiments.2. The mechanical modeling and analysis methods of metal-metal contact were explored. The microscopic mechanism of metal-metal contact was analyzed and the contact stress distributions were derived on the basis of Hertz linear elastic theory. The relationships between the contact stress and nominal contact area of the smooth contact surface were established. The Greenwood-Williamson model was adopted to analyze the microscopic contact states of rough contact surfaces to reveal the relationship between the contact area and contact load of the asperity. The mechanical model of metal-metal contact was established and the relationships between the actual contact area and load were described in parametric form. The numerical examples were carried out to analyze the nonlinear relationship between the actual area and the load of metal-metal contact and to reveal their micromechanics relations.3. The mechanical-electrical coupling analysis method of metal-metal contact was then investigated by the means of the theoretical modeling, simulation analysis and experiment validation. An equivalent circuit model was established and the calculation formulas of electrical parameters are derived to reveal the strong coupling relationship between the electric parameters and contact load. The analysis methods of low frequency and high-frequency circuits were proposed on the basis of the equivalent circuit model. The designed experiment of single point contact was used to verify the correctness of the mechanical-electrical coupling analysis methods. In addition, three types of metal-metal dynamic contact were described as contact, semi-contact and out of contact. Based on the diodes’nonlinear resistance characteristics, the equivalent circuit model of dynamic contact was established. The nonlinear Ⅰ-Ⅴ characteristics and near-field PIMP of the wire mesh unit of ESA mesh antenna with the diameter of 12m were obtained. The far-field PIMP were then derived from the extrapolation of near-field PIMP using both the radar equation and FDTD methods. The obtained results are consistent with the test results of ESA. The reflex PIM test of the RF coaxial connectors was carried out using the Rosenberger analyzer. The experimental results show the pretensions have the seriously effects on the PIM of coaxial connectors and verify the effectiveness of the mechanical-electrical coupling laws of metal-metal contact. The work provides the reliable research methods and theoretical supports for PIM suppression of complex high-power microwave devices.4. The analysis method of thermal-electrical coupling PIM of microwave loads were finally studied. The fractional-order heat conduction equation was derived by combining the fractional differential method with heat conduction equations. The circuit model of the fractional-order heat conduction was established by the analogy of circuit and heat transfer patterns. The expression of each-order PIM of microwave loads caused by the thermal-electrical coupling was obtained by combining thermal resistance effect with the thermal-electrical coupling field. The three-order PIM of microwave terminal loads and microwave attenuators were calculated by the numerical simulations. The effects of temperature coefficients of resistance, thermal resistances and heat capacities on PIM were analyzed. According to the simulation results, the design method for the lower PIM of microwave loads caused by the thermal-electrical coupling was proposed, which provided a theoretical basis for lower PIM design of microwave devices with the thermal-electrical coupling. The effects of the temperature and frequency difference on PIM were studied by experimental test of microwave loads, which had an important engineering significance.