Analysis of Waveguide Discontinuities by Mode Matching
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Mode matching method (MMM), as a semi-analytic method, is considered to be faster calculation, higher accuracy, compared to a purely numerical analysis method. In the mode matching method (MMM) of waveguide discontinuity, the expression of mode function is solved according to the boundary conditions. Matching transverse field in the discontinuity surface, generalized scattering matrix can be derived.In the analysis of longitudinal variable cross-section issues, such as waveguide step and diaphragm, electromagnetic field in TE wave and TM wave on both sides of the discontinuity is expressed by mode function. Firstly the relationship between normalized voltage and normalized current can be derived according to the equation of transverse field continuity. Finally generalized scattering matrix can be obtained by the relation. In the analysis of rectangular and circular waveguide discontinuity structure, there are two circumstances which complicate the analysis. One is that coordinate system need to change, such as the rectangular and circular step, because the coordinate systems on both of step are different, Cartesian coordinate system must be converted to polar coordinate system and vice versa. The other is that the number of windows in step may increase, such as E-plane septum, three-port need to transform to two-port network by matrix block.Using mode matching method, the analysis of T-junction is different from the above. Firstly, assuming that one port excites and others are short, the self-admittance matrix and mutual-admittance matrix of the input port can be derived. Finally generalized scattering matrix can be solved by generalized normalized admittance matrix.By simulation of waveguide discontinuity, the curve results conform well and the calculation time is greatly shortened compared with that by the HFSS, which verifies the validity of this technique.