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Calculation and optimization of powerful coaxial-waveguide probe-type transitions using multiphysical models

DOI 10.18127/j00338486-201812-14


A.S. Kondrashov – Ph.D.(Eng.), Head of Department - Deputy Main Product Engineer, JSC «Russian Space Systems» (Moscow)
V.P. Meschanov – Honored Scientist of RF, Dr.Sc.(Eng.), Professor, Director of NIKA-Microwave, Ltd (Saratov)
N.F. Popova – Ph.D.(Eng.), Head of Department, NIKA-Microwave, Ltd (Saratov)
V.M. Rozhkov – Ph.D.(Eng.), Head of Research Division, JSC «Russian Space Systems» (Moscow)
Ya.V. Turkin – Senior Research Scientist, NIKA-Microwave, Ltd (Saratov)

In this work we design high power coaxial to waveguide transition using mathematical modeling. Electromagnetic characteristics of the transition device are obtained through numerical optimization algorithm. We use simplified electromagnetic numerical model to accelerate the optimization procedure. Central conductor of coaxial line is shorted with the highest step of the impedance transformer to avoid the possibility of the corona discharge in the air gap. The height of the highest transformer step was fixed during numerical optimization process.
We choose coordinate search algorithm as the main method of numerical optimization of the coaxial-to-waveguide transition geometry. Coordinate search algorithm includes the numerical approximation of the gradient of goal function and reduces its overall evaluation number. The calculation of microwave heating of coaxial-to-waveguide transition was done using the multyphysical model of coupled electromagnetic and heat transfer phenomena. Multiphysical model consists of Helmholtz wave equation and heat equation coupled trough resistive losses on the surface of waveguide channel walls and losses in the volume of coaxial line insulator. The dependence of complex dielectric permittivity is taken into account using empirical exponential formula for polymer insulator in the frequency range of 3 to 4 GHz. Multiphysical model completed with the boundary conditions to take into account surface impedance of waveguide channel, contact impedance in the place of connection between probe and impedance transformer and convective heat exchange between the coaxial-to-waveguide transition and environment.
The threshold input power of 61×10 mm coaxial-to-waveguide transition is calculated using multyphysical model in the frequency range of 3.419 to 4.919 GHz. For the critical temperature of the thermal breakdown we use melting temperature of PTFE polymer in coaxial channel. Thermal breakdown threshold power is about ~360 W which is acceptable for aerospace applications.

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