D.N. Zolotykh - Post-graduate Student, Department "Electrical Engineering and Electronics", Saratov State Technical University named after Gagarin Yu.A., Head of Sector of Department 13 SPC "Electronic Systems" of "RPE" Almaz ". E-mail: dzolotykh@mail.ru
B.K. Sivyakov - Dr.Sc. (Eng.), Professor, Head of Department "Electrical Engineering and Electronics", Saratov State Technical University named after Gagarin Yu.A.

Modern design of complex microwave devices is inextricably connected with the use of mathematical modeling and related software. In engineering practice problems of the calculation of certain specific characteristics of the various components and systems of microwave devices often appear. To solve these problems, we need to use mathematical conversions or calculations of certain models which allow us to determine the characteristics for less time. The paper proposes a method for calculating the coefficient of reflection of electromagnetic waves coefficient in the connection region of energy input/output and regular part of the slow wave structure of microwave devices. Its value is determined from the condition of the absence of reflections from the regular part (imitation semi-infinite periodic structure). The technique is based on the theory of quadripoles. Parameters of the structure under study describes the wave scattering matrix. Calculation of the coefficients of the scattering matrix is performed using HFSS - three-dimensional modeling program of microwave devices. To solve the problem is considered a cascade connection of two identical quadripoles. Each of the quadripoles is a structure which consist of an input device of microwave energy (waveguide with matching elements) and several periods of the regular part of the interaction space. These quadripoles are connect by the ports along the planes from the regular part of the interaction space. Thus, a new quadripole has type "input of energy - a periodic structure - energy input". The paper shows that if we know the scattering matrix of such a structure and the reflection coefficient in the middle of the interaction space, it is easy to determine the reflection coefficient by condition of absence of reflection on the regular part. Calculation of the reflection coefficient in the middle of the periodic structure is based on knowledge of the distribution of the electromagnetic field in this area and presenting it as a superposition of the incident and reflected waves. The proposed methodology has been tested in the calculation of the reflection coefficient in the coupling of the coaxial waveguide and spiral slow wave structure of X-band TWT. Reflection coefficient was calculated within the operating range and near the n-type frequency of the TWT. Were also obtained experimental dependence VSWR for two TWT that have good agreement with calculations.