Research On Mode-Matching Method In Microwave Passive Component Design

With the rapid development of modern microwave techniques, the demand for high precision microwave passive components such as filters, diplexers becomes more and more active. Traditional equivalent circuit method can no longer keep up with the pace of microwave technique development, because components designed by this method have low precision and are found to be hard to adjust.Designers pay special importance on the mode matching method because its merits of high precision and fast simulation speed in microwave passive component design. Because the finite thickness of metal irises and the effect of higher order modes are rigorously taken into account, components designed by this method have very high precision. Besides, optimization based on modal analysis makes the structures of microwave components more reasonable. So the research on mode matching method is very important to improve the characteristics, shorten the design period and minimize the volume of microwave passive components.Generally speaking, microwave passive components are composed of a series of key blocks such as waveguide step discontinuity and T-junction, etc. These key blocks can be rigorously analyzed by mode matching method. The generalized scattering matrix of microwave passive components can be obtained by mode matching method and matrix cascading technique. Then, programming languages such as FORTRAN maybe used to simulate it on computer. The dimensions of the considered components under certain technique demands can be obtained by optimization procedure based on the mode matching method.The purpose of this paper is to provide a fast and precise design method of microwave passive components through simulation and optimization based on the mode matching method. The main works are as follows:1. Systematically analyzed a series of key blocks by mode matching method.Two-port network modal analysis in the uniform cylindrical system has some common characters. In this paper, firstly general expressions of the coupling matrix and scattering matrix are given. For certain discontinuities in rectangular waveguide, cylindrical waveguide, coaxial line and ridge waveguide, coupling matrix and scattering matrix can be directly obtained after the eigenfunction and power normalization coefficient are given. Among these, the analysis of ridge waveguide discontinuity is especially complicated due to the unknown cut-off wave number and the ridge waveguide eigenfunction in terms of series summarizing. Furthermore, rectangular three-port key blocks including waveguide bifurcation and T-junctions are also studied by mode matching method.2. Key block simulation programs had been made by FORTRAN based on the mode matching method.Programs for every key block are common. More complicated microwave passive component structures can be analyzed by cascading the needed key block scattering matrices by a cascading program. Component dimensions under certain technique demands can be obtained by optimization. Penalty function algorithm is used in this paper and the initial values are obtained by equivalent circuit method.3. A series of microwave passive components were designed and manufactured based on the programs described in 2.Microwave components involved in these paper including rectangular waveguide filters (both low pass and band pass), diplexers and couplers, cylindrical band pass filters, coaxial line low pass filters and broadband ridge waveguide filters. Simulation results agree well with measurements by HP scalar network analyzer. This not only proved the validity of the analysis and programming, but also shows the high precision of mode matching method.