Research On L - Band Metal Photonic Crystal Mode Converter

Photonic crystals are periodic structures characterized by the existence of frequency bandgaps. The idea of employing photonic bandgap structures was first used in the optical frequency domain. However, with the development of knowledge about the photonic crystals, researchers find that photonic crystals are scalable to a wide range of frequencies, such as the microwave region. As a new kind of microwave photonic crystal, two-dimensional metallic photonic crystal in coaxial transmission line has been studied in this dissertation, and the research work covers its electromagnetic property and the practical application on the mode converter of high power microwave.The dissertation focuses on the analytical methods of the new kind of metallic photonic crystal, including the transfer matrix method, the circuit model analytical method and the CST simulation method. With the transfer matrix method, the equations of band structure and transmission property are obtained. After the analysis of the circuit model of the metallic photonic crystal loaded transmission line, the frequency characteristic of the model is studied. It is found that the first band gap starts from the zero frequency to a characteristic frequency (named as the first characteristic frequency). The first pass-band starts from the first characteristic frequency and ends at the second characteristic frequency. A finite metallic rod array structure is simulated by the commercial software CST Microwave Studio. The intrinsic relationships between the bandgaps and the diversified structure parameters are obtained, which would provide instructions for designing a novel structure of high power linear microwave radiation system applications.With the continuing demand of lighter, more complicated and higher power-handling capability HPM system in scientific research nowadays, the concept of metallic photonic crystal high power microwave mode converter is introduced in this dissertation. The idea of using the photonic crystals to achieve the TEM-TE11mode conversion is proposed which combies the dispersion characteristics of the two-dimensional metallic photonic crystals in coaxial transmission line with the requirement of mode converter in the high power microwave transmission system. The metallic photonic crystal mode converter is analyzed systematically and the prototype of the converter is designed. A metallic photonic crystal TEM-TE11mode converter with central frequency of1.31GHz is designed and simulated. It has a conversion efficiency of over99%at the central frequency. In the frequency range of1.27GHz-1.376GHz, the conversion efficiency exceeds90%with a bandwidth8.1%. Its output mode is a TE11circular waveguide mode. The simulation results also show that the power handling capability will be3.86GW.The cold test of the designed mode converter is carried out, and far-field measurement system is used to measure the mode patterns. The TEM-TE11mode converter is also tested with MILO (Magnetic Insulated Line Oscillator) HPM source. The measured radiation patterns of the mode converter agree well with the simulated results, which indicates that the simulated results are valid. The power density distribution of the converter reveals that it is capable of handling gigawatt level high power microwave.