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Design and Characterization of Meander Line Slow Wave Structures for W-Band Space Traveling Wave Tubes

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Design and Characterization of Meander Line Slow Wave Structures for W-Band Space Traveling Wave Tubes DESIGN AND CHARACTERIZATION OF MEANDER LINE SLOW WAVE STRUCTURES FOR W-BAND SPACE TRAVELING WAVE TUBES Juan M. Socuéllamos A thesis submitted to Lancaster University for the degree of Doctor of Philosophy Lancaster, United Kingdom April 2021 Declaration I declare that this thesis is my own work and has not been submitted in substantially the same form for the award of a higher degree elsewhere. Acknowledgement I would like to express my deepest gratitude to my supervisors at Lancaster Univer- sity, Prof. Claudio Paoloni and Dr. Rosa Letizia, for their expert guidance, constant support and understanding during the research and writing of this thesis. I would also like to thank my workmates there, specially Rupa Basu and Laxma R. Billa, for their friendship, wise advice and making this research more enjoyable. I am profoundly grateful to my supervisor at the European Space Agency, Mr. Roberto Dionisio, for supporting this research, giving me the opportunity to work with him and sharing his deep knowledge and valuable experience with me. Not to mention the excellent colleagues that I had the opportunity to meet at ESTEC and made my time there truly unforgettable. Last but not least, my wholeheartedly appreciation to my family and friends for their love, support and encouragement that made possible a successful completion of the PhD. Abstract Satellite communication systems are nowadays facing the growing demand of dif- ferent services such as internet, streaming or teleconferencing that consume a lot of resources and exhaust the available links. Increasing the frequency band of op- eration is becoming ever-more necessary to cope with the challenges of the future communication services. The exploitation of W-band frequencies (71-86 GHz) may offer many benefits for satellite communications. The wide band channel and high data rate permits to transmit and receive more information at higher velocities. The reduced size of the components decreases the weight of payload and the mission cost. To overcome the huge atmospheric attenuation at W-band, traveling wave tubes (TWTs) are the only solution to provide enough power to feed the link between satellites and ground stations. The amplification in a TWT is based on the interaction between an electromagnetic wave and an electron beam in such a way that, under specific circumstances, the electron beam is able to transfer part of its energy to the wave and amplify the signal. The main component of a TWT is the slow wave structure (SWS), whose role consists of slowing down the radiofrequency signal to a phase velocity comparable to that of the electrons in the electron beam in order to get amplification. Microwave TWTs are mostly based on helix SWSs. Going up in frequency to millimetre waves implies a reduction of the wavelength and, consequently, of the dimensions of the SWS. The helix is particularly affected by this reduction, making its manufacture unfeasible at W-band. New SWSs have been then recently investigated as an alternative to the helix. One of them is the meander line SWS which, in its standard form, consists of a dielectric substrate where a serpentine-shaped metallization has been patterned. Compared to other SWSs at W-band, meander lines are potentially capable of offering very good performance while lowering the beam voltage and enhancing the interaction impedance, providing higher efficiency and reducing the size and weight of the TWT. This is particularly important for space applications, where saving power and reducing the mass on spacecraft has direct implications for the final system and launch costs per satellite. Given the simplicity and straightforward fabrication of the meander line, this kind of SWSs are also adequate for low-cost and high volume production. Thanks to its many benefits, the meander line is seen as a very vii viii promising SWS for a new generation of low-cost, lightweight and compact TWTs for the establishment of future cost-effective satellite communication networks. This thesis will dedicate the first chapter (Ch.1) to a brief summary of the history and evolution of the TWT and its role as power amplifier for satellite communications and, in particular, at W-band. The working principle and main components of the TWT will be described as well as the requirements for space applications. Special emphasis will be put on the SWS and the different options available at W-band. The second chapter (Ch.2) will focus on the description of the characteristics and properties of the meander line SWS and will go through an extensive litera- ture review to become familiarized with this kind of SWS and the different coupling transitions between the meander line and a rectangular waveguide. The main mi- crofabrication techniques needed for manufacturing meander line SWSs will be also briefly explained. The thesis will continue with the analysis and design of novel planar (Ch.3) and three-dimensional (Ch.4) meander line SWSs. From a thorough analysis of the most optimum material of the substrate, configuration of the metallization and geometry of the electron beam, two new planar meander line SWSs will be proposed to interact with a sheet electron beam and improve the performance of conventional meander lines. Other two novel three-dimensional meander line SWSs will be discussed for more efficient and higher-output-power TWTs using a cylindrical electron beam. Phase velocity and interaction impedance are two of the most important parame- ters for characterizing TWTs. Ch.5 will present an experimental method to compute the phase velocity based on the measurement of the phase delay of the same meander line SWS with two different lengths as well as a novel theoretical model to experimen- tally determine the interaction impedance from measurements of the same perturbed and unperturbed meander line SWS. The thesis will conclude with the experimental validation of the theoretical models developed for measuring the phase velocity and the interaction impedance from the characterization of four different planar meander line SWSs that have been tested at Ka-band (Ch.6). Contents List of Tables xiii List of Figures xv 1 The traveling wave tube for satellite communications1 1.1 Wireless communications..........................1 1.2 Satellite communications...........................3 1.3 Satellite communications at W-band....................4 1.4 Traveling wave tubes.............................6 1.4.1 Beam-wave interaction........................7 1.4.2 Electron beam dynamics....................... 10 1.4.3 Main slow wave structure parameters............... 13 1.5 Space traveling wave tubes......................... 16 1.6 Traveling wave tubes and slow wave structures at W-band....... 17 1.7 Summary.................................... 21 2 The meander line as slow wave structure for traveling wave tubes 23 2.1 Meander line slow wave structures..................... 23 2.1.1 Dispersion............................... 24 2.1.2 Interaction impedance........................ 26 2.1.3 S-parameters............................. 26 2.1.4 Meander line topologies....................... 27 2.1.5 Phase velocity tapering....................... 33 2.1.6 Double-substrate and 3D meander lines.............. 34 2.1.7 Modifications on the substrate................... 38 2.1.8 Attenuators on meander lines.................... 40 2.2 Meander line - Waveguide transitions................... 41 2.3 Microfabrication techniques......................... 43 2.3.1 Sputtering............................... 44 2.3.2 Lithography.............................. 44 2.3.3 Electroplating............................. 45 2.3.4 Etching................................. 46 2.3.5 LIGA (Lithography, Electroplating and Molding)........ 46 ix x Contents 2.4 Summary.................................... 46 3 Analysis and design of novel planar meander line slow wave structures 49 3.1 Effect of the dimensions and material of the substrate.......... 49 3.2 Effect of the dimensions of the metallization................ 53 3.3 Beam geometry analysis........................... 56 3.3.1 Cylindrical beam........................... 57 3.3.2 Sheet beam.............................. 60 3.4 Designs at W-band: Standard meander line (SML)............ 64 3.5 Designs at W-band: Standard meander line with round corners (SMLR) 66 3.6 Designs at W-band: New meander line 1 (NML1)............ 67 3.7 Designs at W-band: New meander line 2 (NML2)............ 68 3.8 W-band housing................................ 69 3.9 Results and comparison of SML, SMLR, NML1 and NML2....... 71 3.10 Designs at Ka-band: Soft substrate..................... 74 3.11 Designs at Ka-band: Alumina substrate.................. 77 3.12 Ka-band housing............................... 80 3.13 Other meander line structures........................ 81 3.14 Summary.................................... 84 4 Analysis and design of novel three-dimensional meander line slow wave structures 87 4.1 Pillared meander line (PML)......................... 88 4.2 3D meander line (3DML)........................... 90 4.3 Cylindrical beam interaction analysis................... 93 4.4 Fabrication................................... 95 4.5 Summary.................................... 96 5 Development of theoretical models for the experimental determination of phase velocity and interaction impedance of meander line slow wave struc- tures 97 5.1 Calculation of the phase velocity...................... 98 5.2 Determination of the interaction impedance................ 98 5.2.1 Derivation of the coefficients Gjk;z, Gjk;x and Gjk;y
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