Study On MoM-UTD Method And Higher-Order MoM And Their Applications In Electromagnetics

The study on electromagnetic compatibility (EMC) of complex and electrically large system has been a hot topic in the world. To analyze complex and electrically large objects fast and accurately and calculate the isolation between antennas mounted on them, a new and more complete theory of isolation is proposed and the method of moments - uniform theory of diffraction (MoM-UTD) is applied combined with the technique of impedance matrix interpolation. Using the theory and method, the isolation between antennas on an airborne platform is calculated fast and accurately. The higher-order MoM is also studied for the analysis of electrically large objects. The hybrid higher-order MoM-UTD method is then proposed which can analyze larger scale objects with more accuracy and higher speed. The author's major contributions are as follows:1. After investigating the traditional definition and measurement method of isolation between antennas, the pitfalls of the definition and the unreliability of measured results are pointed out. On the basis of analyzing the different working states of the antennas, several new definitions are presented, which are random isolation, ideal isolation, and actual isolation between antennas. A simple and practical method of predicting isolation between antennas is proposed, which gives the maximum and minimum isolation values between antennas under working with matching networks as long as the input and output voltage standing wave ratios (VSWRs) are known. This new method solves the problem that the isolation between antennas under actual working state is difficult to determine without knowing the detailed matching networks.2. An airborne platform is modeled using flat plates, circular cylinders and cones. The hybrid MoM-UTD method is applied to the analysis of isolation between antennas mounted on an airborne platform. The MoM is used to analyze the wire antenna with pulse function as the base function and Dirac-delta function as the test function, which can easily give the properties of the antenna ports. UTD is used to analyze the affection of the complex and electrically large airborne platform to the antennas. With such treatments, the universal problem of insufficient computer memory is resolved when analyzing electrically large objects. As a result, practical engineering problems can be well solved.3. Ray tracing is the basis of UTD. In this thesis, all of the first-order rays and most of the second-order rays are determined analytically, which guarantees much more accurate results and faster calculation speed compared to old numerical searching method when doing ray tracing. 4. Much attention has been always paid to the calculation time when analyzing complex and electrically large objects. To reduce the time when calculating the frequency response of the isolation between antennas mounted on an airborne platform using the hybrid MoM-UTD method, the impedance matrix interpolation technique has been introduced. Only the impedance matrixes at several frequency points are calculated. The impedance matrixes at other frequency points are obtained using third-order spline interpolation to the real and imaginary parts of the impedance matrixes obtained before, respectively. The introduction of interpolation technique decreases the calculation amount greatly and reduces the calculation time, and as a result it makes the hybrid MoM-UTD method a fast and accurate method in analyzing the frequency response of the isolation between antennas mounted on an airborne platform.5. Modeling is crucial to the analysis using MoM because it determines the calculation accuracy. In this thesis, the quadrangles needed by higher-order MoM are extracted from the grid data generated by FEMAP so that the meshing can be done to objects of arbitrary shape.6. It usually takes a long time to fill the impedance matrix when using higher-order MoM. To solve this problem, the properties of the impedance matrix elements and the involved integral are investigated. Then it is proposed to take advantage of the symmetry of impedance matrix and the integral table to expedite the impedance matrix filling. The former can reduce the amount of impedance matrix elements by nearly a half. And the latter can avoid a lot of repeated calculations. These two methods are very effective in reducing the calculation amount and increasing the calculation speed.7. On the basis of the hybrid MoM-UTD method, the hybrid higher-order MoM-UTD method is proposed and studied. Preliminary success has been achieved.