Study Of Compact Coaxial Circular Waveguide Mode Converters

High power microwave technology is widely used in military and civil fields,and has important commercial value and strategic significance.Waveguide mode converters play an important role in high power microwave system,which can convert and control the microwave modes generated by high power microwave sources,thus achieve the purpose of efficient transmission and directional radiation of microwave signal.In this thesis,theoretical analyses and experimental studies of the circular waveguide TM02-TE11 mode converter and the circular waveguide TE11-HE11 mode converter are carried out around the problems of conversion and radiation of circular waveguide microwave modes.A compact coaxial linearly polarized circular waveguide TM02-TE11 mode converter is proposed and studied,its mode conversion structure is mainly composed of a metallic cylinder and a metallic baffle loaded in a circular waveguide.Based on the theoretical analysis,the linearly polarized mode converter is simulated and experimentally studied in detail.Simulation results show that the purity of output linearly polarized circular waveguide TE11 mode of the linearly polarized mode converter exceeds 99.6% for an operating frequency of 9.4 GHz and the power capability is as high as 1.66 GW,the conversion efficiency is more than 90% from 9.23 to 9.83 GHz band.A good agreement between measurements and simulations is observed.Using the concept of a polystyrene dielectric slab reasonable loaded in the output circular waveguide of the linearly polarized mode converter,a compact coaxial circularly polarized circular waveguide TM02-TE11 mode converter is designed and implemented.Simulation results show that conversion efficiency of the circularly polarized mode converter is 94.6% and axial ratio is 1.06 at 9.4 GHz.Over the frequency range of 9.06 to 9.53 GHz and 8.80 to 9.64 GHz,the conversion efficiency exceeds 90% and 80% respectively,and the axial ratio s are both less than 1.20.A compact coaxial circular waveguide TE11-HE11 mode converter is proposed and studied based on the thoroughly analysis of the propagation characteristics of microwave in the circular waveguide loaded with radial double-layer dielectric,its mode conversion structure is composed of inner dielectric of a polystyrene with relative permittivity of 2.5 and cladding dielectric of a polystyrene rigid foam with relative permittivity of 1.06.Simulation results indicate that the mode converter can output high purity circular waveguide HE11 mode from 8.2 to 10.2 GHz band.By connecting a terminal opening circular waveguide or a small flare angle conical horn at the output of this mode converter,two circular waveguide TE11-HE11 mode transducing antennas are designed and realized respectively.Simulation results indicate that the far-field radiation patterns of the E-plane and the H-plane of the two mode transducing antennas have a high equalization characteristic within a certain beam width.Measured results reveal that the standing wave ratios of these two mode transducing antennas are both less than 1.05 from 9.0 to 9.8 GHz band and the experimental results of gain are in good agreement with the simulation results.