Research On Wideband Feed Technology And Multi-feed Application For Radio Telescopes

Among the branches of astronomy,radio astronomy is significant in that it deals with the largest portion of the electromagnetic spectrum.The radio astrnomy signals are extremely weak when observed,because most astronomical objects are very remote from the earth.To explore such signals,radio telescopes are commonly used in radio astronomy observations and have a superb performance in microwave receiving systems.Benefiting from the high sensitivity,radio telescopes are also applied in the filed of aviation and navigation.A morden radio telescope contains the reflector antenna,the feed antenna,the receiver and the digital processing unit.Among them,the feed has a large influence on the radio telescope system performance and its operating frequency range directly determines the observing frequency band.Nowadays,to facilitate the radio astronomy researches,there is a strong willing to expand the observing frequency range of the radio telescope system.Therefore,this dissertation gives a research on the wideaband feed techonolgy and the application of multi-feed system.The main work of this dissertation is as follows:(1)A quadruple-ridged flared horn(QRFH)with the highest operating frequency is proposed.A four-petal structure is proposed for high-frequency QRFHs.The four-petal structure can be easily assembled and provides a high stablility.Some related key parameters in the QRFH structure are analyzed and discussed.The four-petal structure helps to solve the problem of designing a QRFH with stable beamwidth at high frequencies.Allowing a one-piece connection between the thin ridge and a quarter of the conical horn in the structure ensures the high-frequency QRFH performance.A numerical optimization approach is porposed to design the QRFH effectively.Functions evaluating the feed performance of the QRFH in the radio telescope are proposed and extensive codes have been written in MATLAB to automatically communicate with the electromagnetic solver CST Microwave Studio.Measured results show that the high-frequency QRFH covers the whole frequency range from the X band to the Q band and meets requirements of a radio telescope system in reflection coefficients,isolation and patterns.The system performance of the high-frequency QRFH is evaluated in a SKA radio telescope system.The feed achieves a good aperture efficiency and receiving sensitivity.This high-frequency QRFH can be a good demonstration for high-frequency wideband applications in the future radio telescope systems.(2)A fast optimization approach is proposed for the multi-mode feed horn.A K band and a Q band multi-mode feed horns are proposed by using the approach.The relations between the feed performance and the geometry of a multi-mode feed horn have been discussed.In the approach,analysis programs based on mode maching technique have been written in MATLAB to give a rapid analysis of the modes propagations and radiation patterns of the multi-mode feed horn.The analysis programs achieve good efficiency and provide a possibility to introduce a global optimization algorithm in a large sample space.A method is proposed in the approach to coding and decoding multi-mode feed horns and facilitates the implementation of the genetic algorithm.Proper evaluation functions are proposed to give a rapid optimization of the the multi-mode feed horn structure.The measurements of the K band and Q band feed horns agree well with the analyzed results.Both feeds achieve good aperture efficiencies in the multi-feed application.(3)A K/Q dual-feed multi-frequency radio astronomy cryogenic receiver is proposed and measured.The dewar of the cryogenic receiver is well designed and has a compact size.K band and Q band septum polarizers are proposed and provide circular polarizations in the cryogenic receiver.A small LO module is also developed in the system to provide LO signals with low a phase noise.The measurement of the sample receiver shows a good result in the receiver noise temperature.The K/Q dual-frequency cryogenic receiver can be used in the multi-feed application for the space VLBI radio telescope array.By focusing on the feed techonolgy,the research work can be divided into two aspects in this dissertation.The first aspect is the research on the wideband feed techonolgy for the next generation telescope systems.The performance and structure of the quadruple-ridged flared horn(QRFH)are studied and discussed.A QRFH with the highest operating frequency is developed and measured.The high-frequency QRFH is evaluated in the SKA radio telescope and achieves good results in the system performance.The second aspect is the research on the the multi-feed application in space VLBI radio telescope array.The multi-feed application is used as a transitional technology before the real implementation of the next generation radio astronomy ultra-wideband receiving systems and gives a rapid frequency extension based upon modern radio astronomy receiving techonlogies.The K band and Q band feed horns are proposed and evaluated in the research.A K/Q dual-feed multi-frequency radio astronomy cryogenic receiver is developed and achieves good results in measurements.