Research On The Phased Array Feed Technology For The Radio Telescopes

Phased Array Feed (PAF), which is a small phased array antenna, playing a role asmulti-beam feed of reflector antenna. In radio astronomy applications, PAF provides thepossibility of observation with dozens or hundreds of closely overlapped beams, whichgreatly extents the Field-of-View (FoV) of the radio telescope and enhances the SurveySpeed (SS). Moreover, PAF also helps to mitigate the radio frequency interference (RFI)from human activities and reflector deformation due to gravity and other environmentalfactors. Thus it is called one of the key technologies for next generation of radiotelescopes. This paper reviews the development of PAF in China and abroad,systematically investigates the analysis and design techniques of PAF considering theproject requirement. The main contributions and innovations of this thesis are listed asfollows:1. The relationship between the PAF performance and the requirements fromradio astronomical observation is investigated. To pursue a radio telescope of highsensitivity, high resolution and fast sky survey, several multi-beam antennaconfigurations are studied and a conclusion that the reflector antennas with Focal PlaneArrays (FPA) are most suitable for radio astronomical observation is made. In thisscheme, two types of FPAs, the PAF and cluster feed are investigated and compared indetail. The former is more competitive in several crucial aspects, e. g. beam amount,beam pattern flexibility, the range and continuity of FoV and RFI mitigation, and bettermeets the observation requirements.2. The interaction between the feeds and the reflector systems is investigated. Theconfigurations of reflector system, the law to establish a reflector system aresummarized. Taking the “Five-hundred-meter Aperture Spherical radio Telescope”(FAST), the Shanghai65m radio telescope (SHAO65m), and the Chinese DishVerification Antenna (DVA-C) for Square Kilometer Array (SKA) middle frequencyarray antenna element for examples, the effect of reflector configuration and geometryon feed design is analyzed via calculating the field distribution on the focal planeexcited by incident plane wave. And it is concluded that the feed design is mostsensitive to the (equivalent) illumination angle of the reflector system, which decidesthe feed aperture and the requisite aperture field distribution. Furthermore, since PAF isusually employed in multi-beam applications, the effect of reflector configuration andgeometry, as well as reflector shaping, on beam scanning performance is studied andsome design parameter and principles are reached. 3. The PAF design method is established based on the focal field analysis. Theperformances of radios telescope with various types of PAF are investigated. For agiven reflector system and FoV requirement, the dimension of the feed array and themaximum element spacing can be estimated via focal field analysis and Nyquist Law.From antenna array aspect, the gain distinction between beams and FoV of FPA fedreflector antenna is investigated, the differences between rectangular and hexagonalarray arrangements are compared. And the relationship between the radio telescopeperformance and feed array configuration are concluded. A PAF which is capable ofsimultaneously synthesizing more than one hundred beams is designed for FAST (FASTPAF) and a smaller prototype is built for preliminary verification.4. In the design process of FAST PAF, The cavity-backed dipole is proposed tooperate as the feed element. It is fit to broaden the operating frequency bandwidth,improve the symmetry of pattern, and mitigate the mutual coupling between elements.After the comparison between cavity-backed dipoles of different cavity shapes,hexagonal cavity is adopted for hexagonal arrangement feed array. A single element wasfabricated first and then a19-element array was designed, manufactured, tested andverified. This type of antenna element can make best use of the array aperture and fit forhexagonal arranged PAF.5. The model for phased array fed radio telescope system performance estimationis established. Based on this model, the effect of beam-former on the performance ofradio telescope is analyzed. The characteristics of three common beam-formers: theconjugate field matching (CFM), the maximum directivity (max-directivity), and themaximum signal to noise ratio (max-SNR) are compared, and the connection betweenthe beam-former and corresponding beam gain, system noise temperature is studied.The composition and reduction method of radio telescope system noise temperature arepresented. As a major contributor to the system noise, the dismatch between arrayelements and LNAs is introduced, and the characteristics and feasibilities of severalimpedance matching methods are compared.6. Subject to the structure feature of FAST, the core ideology that using PAF withadaptive beam-forming to compensate for the feed cabin vibration, mitigate the beampattern deterioration and pointing accuracy decline is presented. The mathematic model,system framework, and calibration scheme are established, and the feasibility ispreliminarily verified in simulation.