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Design and Feasibility Study of an Orthomode Transducer for the FAST Experiment

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Design and Feasibility Study of an Orthomode Transducer for the FAST Experiment Design and Feasibility Study of an Orthomode Transducer for the FAST Experiment Thesis submitted to The University of Manchester for the Degree of Masters of Astrophysics Jodrell Bank Centre for Astronomy. September 2012 Louis Smith School of Physics and Astronomy 2 Design and Feasibility Study of an Orthomode Transducer for the FAST Experiment Contents List of Figures 7 List of Tables 9 Abstract . 11 Declaration . 12 Copyright Statement . 13 Dedication . 14 1 Introduction 17 1.1 Overview . 17 1.2 Design Requirements . 18 1.3 Basic Theory . 19 1.3.1 Maxwell’s Equations . 19 1.3.2 Electromagnetic Spectrum . 19 1.3.3 Plane Waves and Polarization . 21 1.3.4 Linear Polarization . 21 1.3.5 Circular Polarization . 22 1.3.6 Isolation . 23 1.3.7 Cross Polarization . 23 1.3.8 Return Loss . 24 1.4 Waveguides . 24 1.4.1 Boundary Conditions . 27 1.4.2 Coordinate Systems and Dominant Modes . 28 Louis Smith 3 CONTENTS 1.4.3 Cut-off Frequency (Fc).......................... 30 1.4.4 Operating Bandwidth . 31 1.4.5 Binomial Rule . 32 1.4.6 Reflection and Scattering Parameters . 32 1.5 Computational Electromagnetism . 33 I Research and Development 39 2 Current state of OMT Research 41 2.1 Overview . 41 2.2 OMT Typologies . 44 2.3 Planar OMTs . 44 2.4 Waveguide OMTs . 48 2.4.1 B;ifot Classification, Class I . 50 2.4.2 B;ifot Classification, Class II . 51 2.4.3 B;ifot Classification, Class III . 56 2.5 Finline . 63 3 Detailed Development of a Turnstile Junction, appropriate for the FAST Experi- ment 67 3.1 The Turnstile Junction Waveguide OMT . 67 3.1.1 The Turnstile Splitting Junction . 68 3.1.2 Compact 90 degree Twist . 71 3.1.3 H and E plane Bends . 74 3.1.4 Y Junction Tapered Power Combiner . 76 II Design and Results 79 4 Simulated Disign and Results 81 4.1 Overview . 81 4.2 H-Plane Bend with E-Plane Stepped Impedance Transformer . 82 4.3 Turnstile Junction . 86 4 Design and Feasibility Study of an Orthomode Transducer for the FAST Experiment CONTENTS 4.4 Compact Ninety Degree Twist . 89 4.5 Y Junction Tapered Power Combiner . 92 4.6 Simulated FAST OMT Design . 95 5 Conclusions and Future Work 103 5.1 Conclusion . 103 III Appendices 105 Louis Smith 5 CONTENTS 6 Design and Feasibility Study of an Orthomode Transducer for the FAST Experiment List of Figures 1 Artist impression of the FAST project in situ . 15 1.1 Electromagnetic Spectrum, with the visible section having been expanded. 20 1.2 Cross section of a rectangular waveguide . 25 1.3 Waveguide flashlight analogy. 26 1.4 Magnetic fields propagating through a waveguide . 28 1.5 Propagation of EM field along waveguide at various frequencies . 29 1.6 2D Grid pattern examples for the FDTD method . 36 1.7 Grid pattern 2D and 3D examples for the FEM . 37 2.1 Schematic view of an OMT as a four-port device . 43 2.2 Reference diagram of a microstrip waveguide component. 46 2.3 Reference diagram of a basic microstrip OMT, where the microstrip is con- nected to the circular waveguide via the microstrips dielectric material. 47 2.4 A. High performance Class III waveguide based OMT with B. showing the incoming waves are coupled out within the inbuilt Turnstile Junction. 49 2.5 Class I OMT device. 50 2.6 Reference diagram of a class II OMT. 53 2.7 Reverse coupling class II structure showing both input and output modes . 56 2.8 Reference diagram of a class three turnstile junction. 57 2.9 Metallic scattering element it situ . 59 2.10 Reference sketch of a coaxial cable. 60 2.11 Reference sketch of a basic coaxial cable orthomode transducer. 61 2.12 Systematic diagram of a coaxial cable turnstile junction OMT. 62 Louis Smith 7 LIST OF FIGURES 2.13 Schematic view of the finline OMT. 64 2.14 A Finline OMT sketch with the resistive card illustrated at the base of the out- put port. 65 3.1 Turnstile Junction splitting a dually polarised TE11 mode. 69 3.2 Detailed view of a designed scattering element. 70 3.3 General configuration of the multi-section bow-tie steps twist. 73 3.4 Rectangular waveguide twist. 74 3.5 Rectangular waveguide bends. 75 3.6 Standardised simulated layout of a Y junction combining section as shown this consists if two incoming polarisations. 77 4.1 Design concept of E-plane transformer with bend . 82 4.2 Simulated 90 degree H-bend with an E-plane transition. 83 4.3 Return loss vs frequency graph for the reduced height H-bend transition. 85 4.4 FAST simulated OMT Turnstile Junction. 86 4.5 Simulated return loss characteristics for the independent Turnstile Junction. 88 4.6 Simulated Compact 90 degree twist. 89 4.7 Simulated return loss characteristics for the separate compact 90 degree twist. 91 4.8 Simulated Y junctions. 92 4.9 Simulated return loss characteristics for the personalised Y junction. 94 4.10 Simulated Planar waveguide based OMT for the FAST experiment. 96 4.11 FAST planar OMT systematic diagram. 97 4.12 FAST OMT Simulated return losses for the first port S11. 99 4.13 FAST OMT Simulated Isolation measurements for the second ports S 21. 100 4.14 FAST OMT Simulated Cross Polarisation measurements for the second ports S 21. ........................................101 8 Design and Feasibility Study of an Orthomode Transducer for the FAST Experiment List of Tables 1.1 OMT Requirements according to the FAST documentation . 19 5.1 OMT Requirements vs simulated results according to the FAST documentation 104 Louis Smith 9 LIST OF TABLES 10 Design and Feasibility Study of an Orthomode Transducer for the FAST Experiment The University of Manchester An Orthomode Transducer (OMT) is a waveguide structure capable of discriminating two in- dependent signals of an orthogonal dominant mode incident upon its common port and to the output port in its fundamental single mode form, therefore enabling the wave to be separated in a passive manner. It is essential that this function is achieved whilst maintaining excellent matching between all electrical ports and a high cross polarisation discrimination and isolation to maintain the required independent signals. The purpose of this thesis is to produce a feasibility study into the production of an Orthomode Transducer for the FAST experiment; china’s contribution to the SKA, a global cosmological collaboration which will operate with approximately a 34 percent bandwidth across the L-band (0.95-1.45Ghz). With this in mind, a class three planar turnstile based OMT was investigated, designed and then simulated using Ansofts HFSS 13, a commercially available package. The method presented in the final design was formed through the use of a H-plane bend with an integrated reduced height transition, a compact ninety degree twist, turnstile junction and a Y junction recombining section all individually simulated during the course of this project. This resulted in a highly, compact of total volume of 212mm3 uniquely planar low loss component; average return losses -28dB, isolation of -49dB and cross polarization of -55dB. All having been recorded across the chosen scaled W-band of 75-115GHz, with a view for this simulated design to be manufactured and analysed using the University of Manchester’s VNA operating at this frequency. These measurements represent a sufficient improvement on both the specified thresholds stated in the FAST literature as well as other waveguide designs of similar size. September 2012. Louis Smith 11 Declaration I do hereby declare that no portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning. 12 Design and Feasibility Study of an Orthomode Transducer for the FAST Experiment Copyright Statement (i) Copyright in text of this thesis rests with the Author. Copies (by any process) either in full, or of extracts, may be made only in accordance with instructions given by the Author and lodged in the John Rylands University Library of Manchester. Details may be obtained from the Librarian. This page must form part of any such copies made. Further copies (by any process) of copies made in accordance with such instructions may not be made without the permission (in writing) of the Author. (ii) The ownership of any intellectual property rights which may be described in this thesis is vested in The University of Manchester, subject to any prior agreement to the contrary, and may not be made available for use by third parties without the written permission of the University, which will prescribe the terms and conditions of any such agreement. (iii) Further information on the conditions under which disclosures and exploitation may take place is available from the Head of School of Physics and Astronomy. Louis Smith 13 Dedication To me, my family, friends and supervisor without whom none of this would have been possible 14 Design and Feasibility Study of an Orthomode Transducer for the FAST Experiment Figure 1: Artist impression of the FAST project in situ Louis Smith 15 16 Design and Feasibility Study of an Orthomode Transducer for the FAST Experiment 1 Introduction 1.1 Overview An Orthomode Transducer (OMT) is a passive device which has the task of separating orthog- onal polarizations. It thus allows the use of two frequency channels simultaneously, enhancing a systems capacity, whilst improving its versatility by permitting the structure to operate over an improved bandwidth. The intention of this masters thesis is to research, design and simulate the performance of an Orthomode Transducer (OMT) operating with approximately a 34 percent bandwidth centred at 1.45 GHz (the L-frequency band) using current waveguide technology.
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