Research On The Broadband High Efficiency Feed For Radio Telescope And Its Application

As a research tool for pulsar observation,interstellar molecular spectral line research and other astronomical activities,radio telescopes have a growing demand for the performance of reflector antennas in its systems.Under the background of the construction requirements for the 110-meter large aperture fully steerable radio telescope in Qi Tai county(Qi Tai telescope,QTT),this dissertation aims to realize the wideband,high aperture efficiency and high-quality factor G/T_sys value of the reflector antennas.Around the wideband high performance horn feeding technology,and focusing on the feed designs of symmetrical main focus and standard Gregorian reflector systems,this dissertation covers five aspects that are the optimum radiation characteristics of feeds,dielectric loaded quad-ridged horn antenna,transmission line balun,variable flare angle multi-mode horn antenna and orthogonal mode transducer.The main work and research results are summarized as follows:1.The optimum radiation characteristics of the feeds for symmetric main focus and standard Gregory systems are studied in depth.By means of focal plane field matching method and geometrical optics analysis of the reflector antenna,the aperture electric field distribution and far-field radiation pattern characteristics of the ideal feeds for the two kinds of reflector systems above are theoretically and respectively deduced and calculated.Under the construction plan of the QTT,the far-field radiation pattern of the feed for 100%irradiation efficiency of symmetrical main focus reflector antenna is analyzed.Several far-field radiation patterns of the feed are listed,which can lead high irradiation efficiency and low sidelobe level of the reflector antenna,and then the design principles of the feed for the 20 cm-band are given.The aperture electric field expression of the feed is derived,matching well with the focal plane field of the standard Gregory antenna.Moreover,the feed design principles for the Q-band is given with the help of the examples derived by quasi optical analysis method.The obtained radiation characteristics of the ideal feeds provide research direction and theoretical basis for the related designs.2.A dielectric loaded quad-ridged horn antenna with the characteristic of nearly constant beam width across an ultra-wide frequency band is designed.The restriction relationship between the structural parameters of the quad-ridged horn antenna and the in-band performance of reflection coefficient and far-field radiation pattern is established.The dielectric influence on the radiation characteristics is studied.The multi-layer dielectric rod loading strategy with decreasing relative dielectric constants from inside to outside is adopted,and finally the antenna design with relatively stable beam width across ultra wideband is realized.The measured results of the three-layered dielectric loaded quad-ridged horn antenna are in good agreement with the simulated ones,and its stable beam width characteristic verifies the suitability for the antenna to be used as the feed for the 20 cm-band reflector system of the QTT.The four-layered dielectric loaded horn antenna has been effectively improved in the aspects of reflection coefficient,equalization of E-plane and H-plane of the far-field radiation pattern and stability of the equivalent phase center.3.An ultra-wideband transmission line balun based on a novel inversion structure is proposed.In the design of the double-side parallel strip line(DSPSL)transmission line balun realized by conductor exchange method,an ultra-wideband inverter based on90-degree electric field rotating transition structure is presented to improve the phase imbalance and amplitude imbalance characteristics between two balanced output ports of the balun.The working principle of the novel phase inverter is explained.The restriction relationship between the structural parameters and the performance of reflection coefficient and isolation is analyzed,and the design method of the transmission line balun is put forwarded.The high consistency between the measurements and the simulations of the prototype shows that the balun design can support the high performance differential feeding of the feed for the 20 cm-band system.4.A broadband variable flare angle multi-mode horn antenna with good equalization of E-plane and H-plane of the radiation pattern is developed.Aiming at the feed design with rotationally axisymmetric radiation pattern,the contribution of each aperture mode to the total radiation field of the conical horn antenna is analyzed,and the appropriate range of spatial factor is selected to reduce the approximation error caused by the"3-mode synthesis method".Under specified edge taper,the total mode ratio coefficient of the aperture field for realizing rotationally axisymmetric radiation pattern is solved.The relationship between the structure parameters of each part of the feed and its corresponding mode ratio coefficient is established,and the optimized Q-band design is obtained.5.A miniaturized finline orthogonal mode transducer(OMT)based on multi-section impedance transformation structure is proposed,and a high-performance Turnstile OMT is developed by modular design method.In order to meet the requirement of miniaturization of the feed group,the design method of applying the Chebyshev impedance distribution to the double ridged or stepped waveguide multi-section impedance transformer of finline OMT is presented,which efficiently realizes the impedance matching characteristic within the limited space.In order to meet the demand of high performance of the feed group,the modular design method is adopted to optimize the low reflection components of the turnstile OMT,and then the final high-performance design is obtained by assembly.The measurements of the two prototypes show that the finline OMT has the characteristics of low reflection,high isolation and compact size in the Q band,while the turnstile OMT has the characteristics of low reflection,low insertion loss and high isolation in this frequency band.6.The performance of the dielectric loaded quad-ridged horn antenna and the variable flare angle multi-mode horn antenna loaded in the corresponding reflector systems is analyzed.The main focus reflector antenna and the standard Gregorian antenna of the QTT achieve the performance of high aperture efficiency,high G/T_sys value and low sidelobe level in each working band when the two antennas above are used as the feeds.Based on the broadband matching and radiation pattern control technology of the feed,this dissertation aims to realize the reflector antenna designs with high aperture efficiency and high G/T_sys value,and makes efforts for the development of the next generation of receiving system for the high efficiency and sensitivity of radio astronomical receiving system.