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Analysis and Measurement of Horn Antennas for CMB Experiments

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Analysis and Measurement of Horn Antennas for CMB Experiments Analysis and Measurement of Horn Antennas for CMB Experiments Ian Mc Auley (M.Sc. B.Sc.) A thesis submitted for the Degree of Doctor of Philosophy Maynooth University Department of Experimental Physics, Maynooth University, National University of Ireland Maynooth, Maynooth, Co. Kildare, Ireland. October 2015 Head of Department Professor J.A. Murphy Research Supervisor Professor J.A. Murphy Abstract In this thesis the author's work on the computational modelling and the experimental measurement of millimetre and sub-millimetre wave horn antennas for Cosmic Microwave Background (CMB) experiments is presented. This computational work particularly concerns the analysis of the multimode channels of the High Frequency Instrument (HFI) of the European Space Agency (ESA) Planck satellite using mode matching techniques to model their farfield beam patterns. To undertake this analysis the existing in-house software was upgraded to address issues associated with the stability of the simulations and to introduce additional functionality through the application of Single Value Decomposition in order to recover the true hybrid eigenfields for complex corrugated waveguide and horn structures. The farfield beam patterns of the two highest frequency channels of HFI (857 GHz and 545 GHz) were computed at a large number of spot frequencies across their operational bands in order to extract the broadband beams. The attributes of the multimode nature of these channels are discussed including the number of propagating modes as a function of frequency. A detailed analysis of the possible effects of manufacturing tolerances of the long corrugated triple horn structures on the farfield beam patterns of the 857 GHz horn antennas is described in the context of the higher than expected sidelobe levels detected in some of the 857 GHz channels during flight. Additionally the pre-flight measurements of the flight horns and qualification horn are analysed in detail which verifies the multimode nature of the horns. This computational work is complemented by a novel approach to the measurement of millimetre-wave antennas using digital holographic techniques particularly with the location of their phase centres in mind. The measurement at 100 GHz of a horn antenna specially designed for future CMB polarisation experiments is presented. Finally some additional applications of millimetre-wave holography are discussed. i Acknowledgements I would like to express my gratitude to my supervisor Prof. J.A. Murphy for giving me the opportunity to undertake the PhD. I would also like to thank him for his much appreciated support, advice, and expert help throughout this work. I would also like to thank all the staff of the Experimental Physics Department without whom this would not have been possible. Undertaking the mammoth task that is a research PhD on a part-time basis while working full-time would have been impossible with the support of my colleagues. In particular I would like to thank my fellow technical staff Derek Gleeson, John Kelly, Marie Galligan, David Watson, and Pat Seery. Also thanks to Gráinne Roche and Dr. Niall McKeith. My thanks to the members of the department’s Terahertz Optics Groups, especially Dr. Créidhe O'Sullivan, Dr. Neil Trappe, and Dr. Marcin Gradziel. I also wish to thank Daniel Wilson whose work on Planck proved extremely useful to my own, and for being such a good sport when United regularly thrashed Villa. I would like to thank Darragh McCarthy for his help with experimental measurements of his prototype horn antenna. Thanks also to Dr. Colm Bracken for helping to generate the Winston cone geometry files. I wish to thank my fellow postgraduates, particularly those who were also part of the Terahertz Optics Groups. I must mention Stephen Scully whose insane antics helps keep the rest of us sane. Thanks to my sister Lisa for proof reading Chapter 1 and for the full breakfast fries every Saturday. Finally, I would like to thank the Human Resources Office for providing me with support for fees though staff development and training schemes. ii Contents Abstract ........................................................................................................................... i Acknowledgements ........................................................................................................ ii Chapter 1 Introduction ....................................................................................................1 1.1 Cosmology and the Cosmic Microwave Background ...........................................2 1.2 Early CMB Research.............................................................................................3 1.3 CMB Anisotropies ................................................................................................5 1.3.1 CMB E-mode and B-mode Polarisation ........................................................8 1.4 CMB Experiments ...............................................................................................10 1.5 Submillimetre Foreground ..................................................................................14 1.6 An overview of the Planck Satellite ...................................................................17 1.7 The Planck Telescope .........................................................................................19 1.8 The LFI and HFI Instruments .............................................................................23 1.9 Thesis Outline .....................................................................................................27 Chapter 2 Modelling Single Mode and Multimode Corrugated Horn Antennas ..........30 2.1 Introduction .........................................................................................................30 2.2 Cylindrical waveguides modes ...........................................................................31 2.3 Mode-matching in Circular Waveguides ............................................................35 2.3.1 Mode-matching for cylindrical waveguides.................................................42 2.4 Computing Aperture Beam Patterns ...................................................................45 2.5 Computing Farfield Beam Patterns .....................................................................49 2.6 The SCATTER Software ....................................................................................53 2.7 Application of Single Value Decomposition in SCATTER to Determine Hybrid Modes ........................................................................................................................62 2.8 Alternative Approach using the Surface Impedance Model for Corrugated Structures ..................................................................................................................68 2.9 Conclusion ..........................................................................................................74 iii Chapter 3 The Planck Multimode Channels .................................................................75 3.1 Introduction .........................................................................................................75 3.2 Beam Pattern Simulations of the 857 GHz Channel ...........................................78 3.3 Numerical Issues with Running SCATTER for the Broadband Beam Predictions ...................................................................................................................................85 3.3.1 Numerical Instabilities .................................................................................85 3.3.2 SCATTER Geometry File Anomalies .........................................................95 3.4 Simulation Results Across the Band ...................................................................98 3.5 Planck 545 GHz Channel ..................................................................................107 3.6 Planck 353 GHz Channel ..................................................................................113 3.7 Modelling the Detector in the Cavity ................................................................114 3.8 Conclusion ........................................................................................................125 Chapter 4 The Effects of Manufacturing Tolerances on the Planck Multi-Mode Horn Antennas ......................................................................................................................127 4.1 Introduction .......................................................................................................127 4.2 The Effects of Filled Corrugations on the Planck horn.....................................137 4.3 The Effects of Partially Filled Corrugations on the Planck horn ......................147 4.4 The Effects of Missing Fins on the Planck horn ...............................................151 4.5 Flight Horns ......................................................................................................152 4.6 Modelling Sidelobe Spillover ...........................................................................158 4.7 Conclusions .......................................................................................................165 Chapter 5 Analysis of Pre-Launch Test Campaign Data for Planck HFI Multimode Channels ......................................................................................................................169 5.1 The Cardiff Pre-Launch Test Campaign ...........................................................169
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