Study On Bent Circular Waveguides TM_0.1-TE₁₁ Mode Converter

Many high power microwave sources produce cylindrically symmetric TM₀₁ mode which is not convenient for directional radiation, so it is important to study a mode converter from TM₀₁ mode to TE₁₁ mode which is good for beamed radiation. In this paper, a double-bent circular waveguide TM₀₁-TE₁₁ mode converter is improved and minimized. At the same time, a combined bent circular waveguide TM₀₁-TE₁₁ mode converter is investigated.In part one, the mode converter is optimized. At first, the mode converter is improved, keeping the axes parallel of both the output and the input ports, so that the radiation direction of output microwave is still along axis. Then, it is analyzed theoretically considering the needed two modes, and the formula of TE₁₁ mode power conversion efficiency is given. Its structure parameters are thoroughly analyzed by theory and simulation and some important conclusions are obtained. Finally, its structure is minimized by decreasing the inner radius of waveguide, according to the conclusion that the length of axis of mode converter is in proportion to the beat wave wavelength. The optimized results are shown as follows: the inner diameter of bent circular waveguide is 7.0cm, the axial distance between the above two ports is 8.03cm, and the axial length is 15.11cm, The overall power conversion efficiency exceeds 90% in the frequency range from 3.41 to 3.74GHz, with its value up to 95.6% at 3.60GHz, the power capacity exceeds 2.00GW.In part two, a combined mode converter is investigated, which consists of two bent circular waveguides with a straight circular waveguide between them. It is useful in some experiments which the axial distance between the two ports need to be enlarged, and this combined structure can also be used to further reduce the overall dimension of the TM₀₁-TE₁₁ mode converter. Firstly, keeping the axes parallel of the output and input ports, its structure is analyzed theoretically. Secondly, a practical combined TM₀₁-TE₁₁ mode converter is investigated, its power conversion efficiency is calculated by theory and simulation, the results from both methods are in good agreement. With the considering of higher-order modes coupling and the power attenuation of the straight circular copper waveguide, the overall power conversion efficiency is 93.8% at 3.75GHz when the inner diameter is 9.0cm and the axial distance between the above two ports is up to 10.57m. Finally, the optimization design of TM₀₁-TE₁₁ mode converter is carried out by thecombined structure, the results are shown as follows: the inner diameter of bent circular waveguide is 7.0cm, the axial distance between the above two ports is 8.00cm, and the axial length is 14.52cm, the overall power conversion efficiency exceeds 90% in the frequency range from 3.41 to 3.79 GHz, with its value up to 96.0% at 3.60GHz, the power capacity exceeds 2.10GW.