Investigations Of Novel Coaxial Mode Converters And Mode-Transducing Antennas For High-Power Microwave

Many high-power microwave (HPM) sources, such as virtual cathode oscillators (vircators), relativistic backward wave oscillators (BWOs), and magnetically insulated transmission-line oscillators (MILOs), generate azimuthally symmetric output modes, including TM₀₁ circular waveguide mode and TEM coaxial waveguide mode. If radiated directly or used to drive conventional antennas, these modes will produce a kind of doughnut-shaped radiation pattern, with a boresight null. Usually, mode converters, such as the dual bent TM₀₁-TE₁₁ mode converter and the Vlasov radiator, are considered to transform the TM₀₁ mode into TE₁₁ mode. Unfortunately, their input and output ports are not aligned on the same axis, which affects the compactness of the whole HPM system. Mode-transducing antennas, such as the COBRAs, have also been explored, but they are still not suitable for designing compact HPM radiation systems, since their aperture efficiencies are typically low. In this dissertation, a novel kind of mode converter that transforms the coaxial TEM (or circular TM₀₁) mode into the TE₁₁ circular waveguide mode, called as coaxial plate-inserted mode converter (CoPIMC), is proposed. It has the virtues of high conversion efficiency, co-axis with system, compactness, and easy fabrication. The mode converter is studied systematically with theoretical analyses, numerical simulations, and experimental tests. Furthermore, mode-transducing antennas, composed of a shortened CoPIMC and a compact coaxial horn, are also proposed and investigated. Additionally, in order to radiate the TE₁₁ circular waveguide mode from the mode converter, a compact dielectric lens-horn antenna is also studied. The detailed contents and innovational work include the followings:1. A novel mode converter is proposed and studied systematically.In this dissertation, a novel concept to realize a TEM-TE₁₁ mode conversion with inserting several metal plates into a coaxial waveguide is proposed, and the prototype of a TEM—TE₁₁ mode converter is designed. This converter is analyzed systematically, including the mode conversion processes, the reflection characteristics, and the frequency features. Many valuable conclusions are achieved. Moreover, a TM₀₁—TE₁₁ mode converter with the central frequency of 3.6 GHz is designed and simulated. It has a high conversion efficiency of over 99% at 3.6 GHz, and its bandwidth exceeds 15% with the conversion efficiency higher than 90%. The measured results are in good agreement with the simulated ones. It is also tested in the vircator HPM source. The whole device (including the vircator and the mode converter) works well and outputs a TE₁₁ circular waveguide mode with a peak power of over 600 MW.2. In order to radiate the TE₁₁ circular waveguide mode directionally, a compact dielectric lens-horn antenna is developed.It is shown that a short conical horn with a dielectric lens loaded in the aperture is suitable for designing a compact HPM antenna for the TE₁₁ circular waveguide mode. The ray tracing method is employed in calculating the radiation patterns of the lens-horns. Three types of dielectric lens, in concrete, hyperbolical lens, elliptical lens, and plano-convex lens, are studied and compared. The results show that a conical horn with a plano-convex lens can produce higher gain and lower reflection than other two types of lens-horn with the same dimension. Therefore, a plano-convex lens-horn is designed, simulated, and tested. Over a wideband frequency range of 3 to 6 GHz, its aperture efficiency is greater than 75% and its reflection is less than -17dB. At 4.0 GHz, the length and the aperture area of the antenna is only about 47% and 63%, respectively, of a conventional conical horn that has the same gain. The designed lens-horn, together with the above-mentioned TM₀₁-TE₁₁ converter forming a HPM radiation system, is experimentally studied in our laboratory. The results accord with the predicated ones.3. A new kind of mode-transducing antenna is presented and investigated systematically.In order to get a more compact radiation system, a coaxial conical horn is introduced to work together with a shortened CoPIMC. In such a case, the converter is shortened for it outputs coaxial TE₁₁ mode (compared with that of outputting circular TE₁₁ mode). Here, the shortened converter and the coaxial conical horn are interdependent, so they are regarded together as a mode- transducing antenna. The coaxial horn is studied with finite-element methods (FEM). The results indicate that this type of horn is characterized by high aperture efficiency, short axial length, wide bandwidth, and low sidelobes. Moreover, a mode-transducing antenna at 1.76 GHz is designed with a gain of 17.6dBi and an aperture efficiency of 77%. The measured results agree with the simulated ones. The HPM experiments of the antenna are also performed with an L-band MELO, outputting pencil beam in boresight with peak-power over 3 GW. In addition, an improved mode-transducing antenna suited for high frequencies and oversized waveguides is developed.4. Novel circularly polarized coaxial plate-inserted mode converters (CP-CoPIMCs) and circularly polarized mode-transducing antennas are investigated.In this part, a novel CP-CoPIMC that converts coaxial TEM mode to CP coaxial TE₁₁ mode is proposed and investigated. Furthermore, two kinds of CP mode-transducing antennas centered at 4 GHz are developed, one is slightly larger but easy to design, and the other is more compact. The sizes of the first one are of 38.5 cm in length and 32.0 cm in aperture diameter with a gain of 19.3 dBi and an aperture efficiency of 47.4%. Whereas, the corresponding dimensions of the compact one are 33.0 cm and 30.8 cm, respectively, with a gain of 19.8 dBi and an aperture efficiency of 57.7%. The compacter one is also tested in lower power and high power cases, respectively. The measurements are in agreement with the simulations. Moreover, a CP-CoPIMC that converts the coaxial TEM mode to the CP circular TE₁₁ mode with high conversion efficiency is also presented. The calculated results show that it has a high conversion efficiency of 99% at 4.05GHz with an axial ratio of 1.01. Over the frequency range of 3.80 to 4.35 GHz, the conversion efficiency exceeds 90% with a bandwidth of about 13.6% and an axial ratio of less than 1.40.