| It has always been our goal to reduce the number of antennas on different platforms by combining multiple functions in the same antenna.In order to achieve this goal,the multifunction antennawith very wide bandwidth or multiple frequency bands is required to cover the frequencies of all desired functions.Meanwhile,the antennas of very wide bandwidth associated with proposed scientific systems,such as the square kilometer array(SKA)radio telescope that covering the frequency range of 100 MHz to 25 GHz,are also received increasingly attention as well.The Ultra-wideband(UWB)radio telescopes is one ofkey componentsin achieving the goals of ultra-sensitivity set by these next-generation projects.However,there are still several shortcomings of most existing ultra-wideband feeds,such as width variation in frequency and phase center instability.Most of the built ultra-wideband Eleven feed has the advantages of constant beamwidth,stable phase center,small size and light weight and so on.But the larger reflection coefficient is the natural defect of the Eleven antenna.Based on the first version of Eleven feed designed earlier,the design and its improvements of second version of Eleven feed is proposed in this thesis.The Eleven antenna cascades the folded dipoles in a logarithmic mannerin which way they can beoperated over a wide frequency band.The folded dipoles operating at each frequency have the same electrical dimensions,ensuring a constant beamwidth as well as a stable phase center.Moreover,the beamwidth and voltage standing wave ratio of the Eleven antenna can be adjusted accordingly by changing the inclination of the dielectric plate in which the folded dipole array is located,the size of the folded dipole,and the distance of adjacent folded dipoles.In order to improve the caliber efficiencyfurther,a circular Eleven feed with BOR(Bodies of Revolution)structure is proposed in this thesis,which is able to improve the aperture efficiency of the feed,and make the antenna structure more compact as well.While improving the efficiency of the feed aperture,the antenna structure is more compact also.The voltage standing wave ratio can be adjustedby changing the straight gap to the curve gap of the Eleven feed in the process of improvements,and good performanceis obtained at the same time.Furthermore,the resistance loading method is used,in which way its influence on the performance of Eleven feed can be found.In the design of the feed network,the feed connection structure is simplified based on the first version of the Eleven feed.Then,the flat-index gradient-type microstrip balun and Klopfenstein type gradient line plane T-type one-two splitter are designed.The difference of the power splitter between the presence and absence of metal columns and metal cavities are compared also.The proposed Eleven feed consists of two polarizations,each of which is consisting of two opposing folded dipole arrays,two ultra-wideband baluns and an ultrawideband splitter,and those two polarizations are orthogonal to each other in this scenario.Finally,the proposed antenna is fabricated and tested,the results show that it performs welland satisfies the requirements of design.The proposed Eleven antenna in this thesis still has problems that need further discussion.For example,the feed,as a reflective surface,should be as small as possible to reduce feed occlusion.Besides,under the demand of ensuring the performance,the size of the entire feed canbe changed significantly if the size of each folded dipole as well as the distance between adjacent folded dipoles reduced slightly.In this thesis,the two outlet ends of one power splitter are connected to the entrances of two baluns via coaxial lines.Therefore,the further discussion would focus on how to print the power splitter and the balun on the dielectric boarddirectly and easily,and then connected them together.These two methods are able to make the antenna structure much more compact conducive,which is beneficial to the application of the reflective surface feed. |
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