Investigation Of Compact L-band Circular Waveguide TM₀₁-TE₁₁ Mode Converters

Many conventional high-power microwave sources generate azimuthally symmetric output modes whose radiation patterns are doughnut-shaped, which is boring for the applications of high-power microwave (HPM). Therefore, the mode converter is usually necessary and very important. Some of the high-power microwave sources require a mode converter that has a relatively wideband because their output frequency may be tunable in a certain range. The mode converters having both the properties of wide-band and no bended waveguide are rare. In this thesis, we are motivated to investigate this kind of mode converter for high-power microwave radiation system applications, and we design a compact L band wideband circular TM01 to TE11 mode converter. Secondly, a novel slow-wave structures mode converter is investigated for the first time.Firstly, a compact L band wideband circular TM01 to TE11 mode converter is designed by using FEM simulation software in which the quasi-rectangular waveguide and coaxial waveguide are employed. Because of the limit of computer memory, the mode converter is divided into 3 parts to avoid using a large amount of memory for running FEM codes and data storage. After designing each part, we integrate them together and optimize the parameters. At last we get aΦ16cm×55.2cm model. Its input and output ports are aligned on the same axis, which is significant to the design of the compact microwave system. The quasi-rectangular waveguide is employed instead of rectangular waveguide to reduce the discontinuities. In the frequency range of 1.63~2.22GHz, the conversion efficiency exceeds 90% with a bandwidth over 30%. The reflection is less than -11dB in the work frequency range when the mode converter is fed by the TM01 mode at the entrance of the device. The simulation results also show that the power handing capability is about 1GW for continuous microwaves in the frequency range of interested. If the E-field of breakdown in vacuum (10-4 Torr) is 0.6MV/cm for a pulsed microwave, the power handing capability will be more than 1GW.Secondly, a novel mode converter——slow-wave structures mode converter——is proposed in this thesis. A plate separates the cylinder waveguide into two 180o sector waveguides, and in one of them the phase velocity of microwaves is smaller than c because slow-wave structures are inserted. When the circular TM01 mode feeds the input port, the sector TE11 mode will be excitated at the front edge of the plate. If the phase difference of the TE11 modes in the two sector waveguide isπ, the two TE11 modes will re-couple into the cylinder TE11 mode at the rear edge of the plate. Finally, the circular TM01 mode is converted into TE11 mode. Numerical simulation verified our theory, and aΦ15.4cm×40.8cm model is explored. The simulations reveal that this converter has a maximum conversion efficiency of 96% at 1.8GHz and the reflection rate is less than 0.03.Finally, we give an engineering design of the compact L band wideband circular TM01 to TE11 mode converter working in the frequency range of 1.63~2.22GHz, and the conversion efficiency exceeds 90% with a bandwidth over 30%.