| Radio astronomy,which was born in the 1930 s,is a subject that uses radio telescopes to detect celestial radiation to study astronomical problems.A typical radio telescope system consists of a reflector antenna,a feed horn,a polarization network,a cryogenic receiver and a digital back-end,etc.Tianma Radio Telescope(TMRT,also known as Shanghai 65-meter Radio Telescope),the largest steerable radio telescope in Asia,is located in Sheshan Town,Songjiang District of Shanghai.It was jointly funded by the Shanghai Science and Technology Commission,the Chinese Academy of Sciences(CAS)and the Chinese Lunar Exploration Program(CLEP).The main operating frequencies include eight bands from L-band to Q-band.In this paper,the first Q-band two-beam cryogenic receiver in China is developed,which is also the highest frequency band of TMRT.The key technical problems are solved consisting of high efficiency and compact broadband feed horns,low insertion loss and low axial ratio broadband polarization networks,and the design and integration of a Q-band two-beam receiver system.The main work of this dissertation is as follows.(1)A high efficiency and compact broadband feed horn has been developed.The idea and method of co-simulation and optimization of the reflector antenna and the feed horn are proposed.The seamless connection between electromagnetic field simulation and physical optics simulation is realized.The microwave optical path and the feed horn with the highest sensitivity are obtained.The far-field radiation pattern of the feed horn obtained by electromagnetic field simulation software is directly imported into reflector antenna for physical and optical simulation,and its objective function is optimized.The feed horn adopts a wave-shaped horn which has a sinusoidal profile and variable slot depth mode converter.The aperture of the horn is small and the phase center is stable,which solves the problem of the spatial distribution of two-beam feed horns in the focal plane.Platelet assembly technology is adopted in the manufacture,which solves the problem that the electrical performance of millimeter wave corrugated horn can only be guaranteed by one-time integral molding,and has the advantages of excellent performance and low manufacturing cost.Two Q-band feed horns have been developed for TMRT,which have good technical performance and verify the effectiveness of the design ideas and methods.(2)A broadband polarization network with low insertion loss and low axial ratio has been developed.The polarization network adopts the combination of a differential phase shifter and an orthogonal mode transducer(OMT),which solves the problem of excellent performance of working bandwidth of 35 GHz to 50 GHz,and has been successfully applied to Q-band twobeam cryogenic receiver.As one of the core devices of radio astronomy receiver,the polarization network is located between the feed horn and the monolithic microwave integrated circuit(MMIC)low noise amplifier(LNA),which is used to separate the received polarization signal.The difficulty lies in too many key indicators,including working bandwidth,insertion loss,circular polarization axial ratio,cross-polarization level,port isolation,return loss,etc.The differential phase shifter adopts a double-wall corrugated structure.Because millimeter wavelength is very small,manufacturing accuracy has always been the bottleneck.Compared with the traditional four-wall structure,the double-wall structure reduces the manufacturing difficulty,and adapts to the characteristics of the millimeter wave frequency band,but the design accuracy is greatly improved.Through design tolerance analysis and optimization simulation,not only the key dimensions that affect the specifications are found,but also the sensitivity to the key dimensions decreases with the increase of manufacturing tolerance,and it has a good phase shift characteristic and circular polarization axial ratio in the whole frequency band.The OMT adopts Boifort type with double ridges.Through the tolerance analysis,the problem of resonance points in the assembly of OMT sub-module is solved.Two sets of Qband differential phase shifters and OMTs have been developed.Their technical indicators all met the design requirements,which verifies the effectiveness of the technical scheme and design method.(3)China's first Q-band two-beam cryogenic receiver has been successfully developed.The whole cooling structure consisting of feed horns,polarization networks and MMIC LNAs is adopted,which solves the problem of extra 3-7 K noise introduced by the ambient feed horns of TMRT(L-band to Ku-band)low-frequency receivers,effectively reducing the noise temperature of the receiver and improving the sensitivity.The receiver works in the frequency range of 35 GHz to 50 GHz.The internal structure and technical specifications of the cryostat have been optimized.The feed horns,noise injection couplers,differential phase shifters,OMTs,and MMIC LNAs are supported by G-10 thermal insulating material,and are connected to the second stage platform of the cold head through cold belts with a working temperature of about20 K.The heat load problem caused by a large aperture microwave vacuum window has been solved.Multi-layer insulating material is installed between the feed horn and the microwave vacuum window as the infrared filter,and its working temperature range is close to 77 K.By optimizing the number of layers and the distance between the multi-layer insulating material,the noise contribution of the insulating material and the working temperature of the feed network can reach the final balance.Except for cryogenic LNAs,the development of all passive millimeter wave key components of the receiver has been completed,and the installation and debugging of the whole receiver on TMRT have also been accomplished.In addition,the development of TMRT L-band cryogenic receiver with the lowest frequency is also introduced.The broadband feed horn with high efficiency and compactness,the broadband polarization network with low insertion loss and low axial ratio and a Q-band two-beam cryogenic receiver developed in this paper have been used on TMRT for more than three years.Many single dish and very long baseline interferometry radio astronomical observation results were obtained. |
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