Universidad Politécnica de Madrid Escuela Técnica Superior de Ingenieros de Telecomunicación TRABAJO FIN DE GRADO Ku band waveguide diplexer design for satellite communication Implementation by additive manufacturing and experimental characterization MADRID, 2015 IRENE ORTIZ DE SARACHO PANTOJA Trabajo Fin de Grado Ku band waveguide diplexer design for satellite communica- tion. Implementation by additive manufacturing and experi- mental characterization Autor Irene Ortiz de Saracho Pantoja Tutor José Ramón Montejo Garai Departamento Señales, Sistemas y Radiocomunicaciones Tribunal Presidente: D. Juan Enrique Page de la Vega Vocal: D. Javier Gismero Menoyo Secretario: D. José Ramón Montejo Garai Suplente: D. Mariano Barba Gea Fecha de lectura: Calificación: Universidad Politécnica de Madrid Escuela Técnica Superior de Ingenieros de Telecomunicación TRABAJO FIN DE GRADO Ku band waveguide diplexer design for satellite communication Implementation by additive manufacturing and experimental characterization MADRID, 2015 IRENE ORTIZ DE SARACHO PANTOJA Abstract A great amount of telecommunication services such as television distribution or navigation systems are based on satellite communication. As it occurs in other spatial applications, there are some key resources which are severely limited on board spacecrafts, as mass or volume. In this sense, one of the most important passive devices, which allows a better use of such resources, is the diplexer of the feed antenna system. This device enables the use of one single antenna for both transmission and reception channels, resulting in an optimization of the above resources. The main goal of this work is to design a diplexer fulfilling real satellite-communication specifications. This device consists in two filtering structures joined by a three-port junction. In addition, the use of waveguide technology is imperative, due to the high power level handled. The diplexer design is accomplished by dividing the structure in separate parts, in order to make the process feasible and efficient. Firstly, different filter configurations are developed – high-, low- and band- pass responses –, even though only two of them will be diplexed. When tackling their initial design, a theoretic synthesis is performed through the use of circuit models. The filters are subsequently optimized by using full-wave CAD techniques, particularly mode matching. At this point it is essential to analyze the structures and their symmetry in order to determine which modes are actually propagating, to reduce computational effort. Finally, FEM method is used to verify the results previously obtained. Once the filter design is concluded, the three-port-junction dimensions are calculated. Eventually, the whole diplexer is optimized to fit the electric specifications. Furthermore, this work presents a brand-new added value: the physical implementation and experimental characterization of both the diplexer and the filters. This possibility, unfeasible until now because of its high cost, derives from the development of additive manufacturing techniques. The prototypes are printed in plastic (PLA) by means of a low-cost 3D-printer, and afterwards metallized. This technology entails two different limitations: the precision of the geometric dimensions (±0.2 mm) and the conductivity of the metallic paint which covers the walls of the waveguide. A comparison between simulated and measured values is included in this work, as well as an analysis of the experimental results. In summary, this work expounds a real engineering process: the problem of designing a device which satisfies real specifications, the limitations caused by a manufacturing process, the eventual experimental characterization and the inference of conclusions. Keywords: diplexer, waveguide filter, Ku band, satellite communication, additive manufacturing Resumen Gran cantidad de servicios de telecomunicación tales como la distribución de televisión o los sistemas de navegación están basados en comunicaciones por satélite. Del mismo modo que ocurre en otras aplicaciones espaciales, existe una serie de recursos clave severamente limitados, tales como la masa o el volumen. En este sentido, uno de los dispositivos pasivos más importantes es el diplexor del sistema de alimentación de la antena. Este dispositivo permite el uso de una única antena tanto para transmitir como para recibir, con la consiguiente optimización de recursos que eso supone. El objetivo principal de este trabajo es diseñar un diplexor que cumpla especificaciones reales de comu- nicaciones por satélite. El dispositivo consiste en dos estructuras filtrantes unidas por una bifurcación de tres puertas. Además, es imprescindible utilizar tecnología de guía de onda para su implementación debido a los altos niveles de potencia manejados. El diseño del diplexor se lleva a cabo dividiendo la estructura en diversas partes, con el objetivo de que todo el proceso sea factible y eficiente. En primer lugar, se han desarrollado filtros con diferentes respuestas – paso alto, paso bajo y paso banda – aunque únicamente dos de ellos formarán el diplexor. Al afrontar su diseño inicial, se lleva a cabo un proceso de síntesis teórica utilizando modelos circuitales. A continuación, los filtros se optimizan con técnicas de diseño asistido por ordenador (CAD) full-wave, en concreto mode matching. En este punto es esencial analizar las estructuras y su simetría para determinar qué modos electromagnéticos se están propagando realmente por los dispositivos, para así reducir el esfuerzo computacional asociado. Por último, se utiliza el Método de los Elementos Finitos (FEM) para verificar los resultados previamente obtenidos. Una vez que el diseño de los filtros está terminado, se calculan las dimensiones correspondientes a la bifurcación. Finalmente, el diplexor al completo se somete a un proceso de optimización para cumplir las especificaciones eléctricas requeridas. Además, este trabajo presenta un novedoso valor añadido: la implementación física y la caracterización experimental tanto del diplexor como de los filtros por separado. Esta posibilidad, impracticable hasta ahora debido a su elevado coste, se deriva del desarrollo de las técnicas de manufacturación aditiva. Los prototipos se imprimen en plástico (PLA) utilizando una impresora 3D de bajo coste y posteriormente se metalizan. El uso de esta tecnología conlleva dos limitaciones: la precisión de las dimensiones geométricas (±0.2 mm) y la conductividad de la pintura metálica que recubre las paredes internas de las guías de onda. En este trabajo se incluye una comparación entre los valores medidos y simulados, así como un análisis de los resultados experimentales. En resumen, este trabajo presenta un proceso real de ingeniería: el problema de diseñar un dispositivo que satisfaga especificaciones reales, las limitaciones causadas por el proceso de fabricación, la posterior caracterización experimental y la obtención de conclusiones. Palabras clave: diplexor, filtro en guía de onda, banda Ku, comunicaciones por satélite, manufac- turación aditiva Para M., por cuidar de mis neuronas. Para J.R., por intentar que salieran cestos con los mimbres que había. Contents 1 Introduction and design specifications 2 2 Waveguides as transmission lines 4 2.1 Propagation in rectangular waveguides . .4 2.2 Attenuation in rectangular waveguides . .6 2.3 Waveguide implementation of filtering structures . .7 2.3.1 Low-pass filters . .7 2.3.2 High-pass filters . .8 2.3.3 Band-pass filters . .9 3 Modal analysis as full-wave design technique 10 3.1 Symmetry and its implications . 11 4 Low-pass filter 13 5 High-pass filter 15 6 Band-pass filters 17 6.1 Synthesis process . 18 6.1.1 Chebychev filters and low-pass response . 18 6.1.2 J-inverters and band-pass transformation . 19 6.1.3 Slope parameter and distributed resonators . 21 6.2 Physical dimensions . 22 6.3 Optimization and final response . 24 6.3.1 TX filter . 24 6.3.2 RX filter . 25 7 Diplexer 26 7.1 Main challenges when designing a diplexer . 27 7.2 Junction design and final optimization . 29 7.3 Double bend . 32 7.4 Diplexer . 34 8 Manufacture and losses 35 8.1 Additive Manufacturing (AM) . 35 8.2 Low-cost 3D printing and RepRap . 36 8.3 Challenges and limitations of 3D printing . 36 8.4 Metallization process . 38 8.5 Conduction losses . 39 9 Measurement and analysis of results 41 9.1 Low-pass filter . 42 9.2 High-pass filter . 43 9.3 Band-pass reception filter . 45 9.4 Diplexer . 47 10 Conclusions 48 11 Future work 50 12 Contributions 50 References 51 1 1 Introduction and design specifications Satellite communication is one of the most important space applications. It encompasses many services used by millions of people every day, such as television broadcast or navigation systems. It also includes less-known services, for instance tele-medicine. Besides, the importance of mobile satellite systems to connect remote areas with the rest of the world must not be forgotten. This variety of applications makes satellite communication a very mature and developed field, with certain peculiarities. Design specifications of the passive microwave devices on board spacecrafts are practically unique, since there is a great amount of satellites, each of them working in different orbits and frequencies. Satellite frequencies range from 3 to 30 GHz (Super High Frequency). However, this wide spectrum is divided into sub bands, each of them devoted to certain applications. Power levels handled also depend on the different purposes. Besides, the architecture of each satellite launched into orbit presents