Investigation On Mode Conversion In Over-Moded State And Beam Steering Lens Antenna For High Power Microwave Applications

As Pf² is an important parameter to judge the effectiveness of high-power microwave?HPM?,it is foreseeable that with the rapid development of the HPM technology,higher peak power,higher frequency working band will be the development trend.Due to the inherent limitations of the HPM generation and the HPM source process structures,various HPM sources have their power limitations.In order to overcome these limitations,researchers are trying to explore the microwave power combination technology.Spatial power combination technology is one of the efficient ways to improve the equivalent radiation power.Therefore,the single-source radiation system in this technology is one of the key components to be investigated,especially in high frequency bands.In order to minimize the influences of high order modes,the size of the device is usually small which may cause the low power handling capability.However,the device with large size may take the influences of high order modes into account.Additionally,high power handling capability beam steering antenna is also a requirement in HPM spatial power combination technology.In this dissertation,the key devices of HPM spatial power combination and its key technologies are studied,which includes high power handling capability mode converter,phase shifter and beam steering lens antenna.The key technologies mainly include the technical method of realizing the high power handling capability mode converter in high frequency bands,the mixed mode proportions and the phase relations measurement method in circular waveguide system,the technical method of realizing the high power handling capability phase shifter,and the beam steering lens antenna design method.The paper mainly carries out the theoretical analyses,numerical simulations and measurements,especially including the following parts:1.Over-moded TEM-circularly polarized TE₁₁ mode converterA compact over-moded n-arm TEM-circular polarized TE₁₁ mode converter with high power handling capability in high frequency band is proposed.Through theoretical analyses,it is concluded that the n arm converter?n=3,4……?can convert the TEM mode into circularly polarized TE₁₁ mode and circularly polarized TE_?n-1?1 mode,simultaneously.In order to obtain the pure circularly polarized TE₁₁ mode,it is necessary to optimize the size of output waveguide to cutoff the transmission of TE_?n-1?1mode.This method can make the mode converter working in over-moded,which can increase the power capability of mode converter.According to the above conclusions,a converter with 20 arms is established.The input and output waveguide dimensions meet the over-moded conditions.It is found by simulation that it can convert TEM mode into a circularly polarized TE₁₁ mode efficiently and the power capacity is estimated to be above 2 GW according to its maximum electric field strength.In order to verify the theoretical analyses and simulation results,we have fabricated the converter and carried out cold test and hot test.Cold test results show that the reflection of the mode converter is less than-24 dB at the center frequency and less than-20 dB in the frequency range of 9.225-9.500 GHz,which means that the reflection of this mode converter is little and can be neglected.The radiation patterns agree well with the simulation patterns.The axial ratios are greater than 1.05 and less than 1.15 in the frequency range 9.225-9.525GHz.Hot test results show that the converter can realize the power of 1 GW and the pulse width of more than 80 ns microwave mode conversion.Besides,the converter can be designed and work in higher frequency band with enough power handling capability,such as a Ku band converter was designed and the power handling capability is not the limitation of the HPM system.2.Over-moded TEM mode phase shifterThe over-moded TEM mode phase shifter is composed of two identical n-arm TEM-circularly polarized TE₁₁ mode converters.Keep one converter stationary and rotate another converter angularly can realize the phase adjustment of the TEM mode microwave,and the phase adjustment is equal to the relative rotation angle.In this dissertation,the principle of the phase shifter is analyzed and as a validation,a TEM mode phase shifter was established and simulated at the central frequency of 9.375 GHz,the results show that the TEM mode transmission efficiency is more than 99%when the phase shifted for the range of 0-360°,and the power handling capability is more than 2GW,which satisfy the requirements of HPM in high frequency bands.The properties of the working bandwidth were also monitored,and the results show that the S₂₁magnitude is still larger than 98%and the phase can be adjusted accurately in the frequency range of 9.1-9.6 GHz.Additionally,the influence on the working properties is analyzed by simulation when there are some fabricating errors.When the length error is-0.05??35??0.05 mm between the adjacent arms,the simulation results show that the S₂₁ magnitude is still larger than 98.8%and the phase can be adjusted accurately.In other words,this range of errors cannot affect the TEM phase shifter significantly.Moreover,the TEM phase shifter can be designed and work in higher frequency band with enough power handling capability,and the power handling capability will not be the limitation of the HPM system.3.Mixed modes measurement by electrical probeThe most widely used waveguides are circular waveguide systems including hollow circular waveguide and coaxial circular waveguide in the current HPM area.However,it is difficult to accurately measure the mixed mode proportions and the phase relations in circular waveguide systems,as the polarizations of non-rotational symmetric modes are arbitrary.In comparison with the measurements on circular waveguide system,there have been many reports on the measurements of the mixed modes in rectangular waveguides,as the polarizations of rectangular waveguide modes are simple,which are horizontal and vertical.In this dissertation,the mixed mode proportions and phase relations in circular waveguide systems are investigated by means of multi-point measurement on the internal electric field of the circular waveguide and calculating the equations which related to mode compositions and probe positions.In order to verify the method,an irregular structure and absorbing material were established in a coaxial waveguide,the mode proportions and phase relations in input and output waveguides can be deduced from the measured data by electrical probes,which match well with the results of directly calculated by CST.The mixed mode measurement setup about the TEM-circularly polarized TE₁₁ mode converter was established and used to measure the mixed mode proportions and phase relations of the TEM-circularly polarized TE₁₁ mode converter by this method,and the results show that the output mode is a circularly polarized TE₁₁ mode,other high order modes are very small,which can be neglected.The measurement results are in accordance with the simulation results.The rest of the power is dissipated by the converter with several transition waveguides,which is about 0.6 dB.At last,based on the measurement method and the properties of HPM,the measurement of high power mixed modes was designed.4.Beam steering lens antenna for HPM applicationsIn this dissertation,we propose a beam steering lens antenna by mechanically rotating two gradient-refraction lenses.An all-metal beam steering lens antenna consisting of numerous all-metal transmission elements was designed and investigated.Transmission network theory and simulation were combined to design the element,and the simulation results show that 360°phase adjustment with high transmission efficiency can be achieved by rotating the micro-structure on the element.The power handling capability of the element is estimated as 3.1 MW,and the power handling capability of a lens antenna with a dimension of 1 m×1 m is is about 5.6 GW.A lens consists of 121 elements was established based on the element.In simulation,the all-metal beam steering lens antenna can also realize the radiation beam steering in any direction in the range of conical angle of 60°in the upper hemisphere.In addition,as a validation design,for simplicity,we have also fabricated and tested an all-metal beam steering lens antenna,which can realize beam steering in any direction in the range of conical angle of 40°in the upper hemisphere.The measured results showed that the reflection are less than-15 dB in the frequency range of 9.275-9.475 GHz,and less than-20 dB at the frequency of 9.375 GHz.Moreover,the steering angle of these situations matched well with the theoretical and simulation results,which show that the lens has the application potentials in beam steering antenna for HPM applications.