[ES] Los sistemas de antenas de bajo perfil en la banda de ondas milimétrica han suscitado un creciente interés en los últimos años. Esta tesis presenta el diseño de antenas de bajo perfil basadas en lentes planas de microondas de doble polarización, con alta ganancia y con simetría en el diagrama de radiación. En concreto, la tesis se enfoca en lentes planas con dieléctricos artificiales, e iluminadas por bocinas de bajo perfil. Para mejorar esta iluminación, también se han diseñado alimentadores de bajo perfil con dieléctricos artificiales. En esta Tesis se realiza una revisión de diferentes métodos para el control de la constante de propagación en celdas periódicas de superficies selectivas en frecuencia (FSS), así como de los parámetros más importantes que rigen su comportamiento electromagnético. Las FSS multicapa son idóneas para el diseño de lentes planas de microondas ya que permiten controlar la fase del coeficiente de transmisión con pocas pérdidas, y son fáciles de diseñar y construir. Las lentes diseñadas con este tipo de FSS son estructuras planas, de bajo perfil y peso reducido. Esto se debe a la capacidad de las FSS de modificar la constante de propagación por medio de una estructura que no requiere contacto entre capas. Para poder iluminar las lentes apropiadamente, en esta tesis se analizan diversos tipos de alimentadores y se propone un alimentador de bajo perfil con gran pureza de polarización y alta eficiencia. Esto se consigue insertando corrugaciones radiales en las paredes de la bocina, con lo que se consigue excitar un modo híbrido cuya distribución de campo es prácticamente uniforme en toda la apertura. Dado el coste computacional del análisis de onda completa de las lentes planas de microondas, en esta tesis se propone el diseño de las lentes usando gráficas paramétricas de las celdas unidad de las FSS. Estos diseños se han fabricado y medido, dando unos resultados experimentales excelentes, tanto en adaptación como en radiación. Estos resultados demuestran la viabilidad de usar FSS multicapa para el diseño de lentes planas de microondas. / [EN] Low-profile antenna systems in the millimeter-wave band have experienced an increasing interest in recent years. This thesis presents the design of low-profile antennas based on microwave flat lenses with double polarization, with high gain, and symmetric radiation pattern. In particular, the thesis focuses on flat lenses with artificial dielectrics, and illuminated by low-profile feeders. To improve this illumination, low-profile feeders with artificial dielectrics have also been designed. In this thesis, a review of different methods for the control of the propagation constant in periodic cells of frequency selective surfaces (FSS), as well as the most important parameters that govern its electromagnetic behavior, is carried out. The multilayer FSS are ideal for the design of flat microwave lenses since they allow controlling the phase of the transmission coefficient with few losses and are easy to design and build. Lenses designed with this type of FSS are flat structures, low profile, and low weight. This is due to the ability of the FSS to modify the propagation constant through a structure that does not require contact between layers. In order to be able to illuminate the lenses properly, in this thesis different types of feeders are analyzed and a low-profile feeder with high polarization purity, and high efficiency is proposed. This is achieved by inserting radial corrugations into the walls of the horn, which makes it possible to excite a hybrid mode whose field distribution is practically uniform throughout the aperture. Given the computational cost of the full-wave analysis of microwave flat lenses, this thesis proposes the design of the lenses using parametric studies of the FSS unit cells. These designs have been manufactured and measured, giving excellent experimental results, both in matching and radiation. These results demonstrate the feasibility of using multilayer FSS for the design of microwave flat lenses. / [CA] Els sistemes d'antenes de baix perfil en la banda d'ones millimètriqueses han suscitat un creixent interés en els últims anys. Aquesta tesi presenta el disseny d'antenes de baix perfil basades en lents planes de microones de doble polarització, amb alt guany i amb simetria en el diagrama de radiació. En concret, la tesi s'enfoca en lents planes amb dielèctrics arti- ficials, i il luminades per botzines de baix perfil. Per a millorar aquesta il luminació, també s'han dissenyat alimentadors de baix perfil amb die- lèctrics artificials. En aquesta Tesi es realitza una revisió de diferents mètodes per al control de la constant de propagació en cel les periòdiques de superfícies selectives en freqüència (FSS), així com dels paràmetres més importants que regeixen el seu comportament electromagnètic. Les FSS multicapa són idònies per al disseny de lents planes de microones perquè permeten controlar la fase del coeficient de transmissió amb poques pèrdues, i són fàcils de dissenyar i construir. Les lents dissenyades amb aquesta mena de FSS són estructures planes, de baix perfil i pes reduït. Això es deu a la capacitat de les FSS de modificar la constant de propagació per mitjà d'una estructura que no requereix contacte entre capes. Per a poder il luminar el lent apropiadament, en aquesta tesi s'analitzen di- versos tipus d'alimentadors i es proposa un alimentador de baix perfil amb gran puresa de polarització i alta eficiència. Això s'aconsegueix inserint co- rrugacions radials en les parets de la botzina, amb el que s'aconsegueix excitar una mode híbrid amb una distribució de camp pràcticament uni- forme en tota l'obertura. Donat el cost computacional de l'anàlisi d'ona completa de les lents planes de microones, en aquesta tesi es proposa el disseny de les lents usant gràfi- ques paramètriques de les cel les unitat de les FSS. Aquests dissenys s'han fabricat i mesurat, donant uns resultats experimentals excel lents, tant en adaptació com en radiació. Aquests resultats demostren la viabilitat d'usar FSS multicapa per al disseny de lents planes de microones. / Moy Li, HC. (2020). Lentes de Microondas de bajo perfil en la banda de ondas milimétricas [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/149563

Made available in DSpace on 2014-12-17T14:55:19Z (GMT). No. of bitstreams: 1 GustavoAC_TESE.pdf: 3178455 bytes, checksum: bdea1ce583a318f3a35fb4f3221877a8 (MD5) Previous issue date: 2014-04-28 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The use of flexible materials for the development of planar circuits is one of the most desired and studied characteristics, lately, by researchers. This happens because the flexibility of the substrate can provide previously impracticable applications, due to the rigidity of the substrates normally used that makes it difficult to fit into the circuits in irregular surfaces. The constant interest in recent years for more lighter devices, increasingly more compacts, flexible and with low cost, led to a new line of research of great interest from both academic and technological views, that is the study and development of textile substrates that can be applied in the development of planar circuits, for applications in the areas of security, biomedical and telecommunications. This paper proposes the development of planar circuits, such as antennas , frequency selective surfaces (FSS) and planar filters, using textile (cotton ticking, jeans and brim santista) as the dielectric substrate and the Pure Copper Polyester Taffeta Fabric, a textile of pure copper, highly conductive, lightweight and flexible, commercially sold as a conductive material. The electrical characteristics of textiles (electric permittivity and loss tangent) were characterized using the transmission line method (rectangular waveguide) and compared with those found in the literature. The structures were analyzed using commercial software Ansoft Designer and Ansoft HFSS, both from the company Ansys and for comparison we used the Iterative Method of Waves (WCIP). For the purpose of validation were built and measured several prototypes of antennas, planar filters and FSS, being possible to confirm an excellent agreement between simulated and measured results / A utiliza??o de materiais flex?veis para o desenvolvimento de circuitos planares ? uma das caracter?sticas mais desejadas e estudadas, ultimamente, pelos pesquisadores, pois essa maleabilidade do substrato proporciona aplica??es antes imposs?veis, devido ? rigidez dos substratos normalmente utilizados o que dificultava a adequa??o dos circuitos em superf?cies irregulares. O constante interesse nos ?ltimos anos por dispositivos mais leves, cada vez mais compactos, flex?veis e com custo reduzido, levou a uma nova linha de pesquisa de grande interesse tanto do ponto de vista acad?mico quanto tecnol?gico que ? o estudo e desenvolvimento de substratos t?xteis que possam ser aplicados no desenvolvimento de circuitos planares, para aplica??es nas ?reas de seguran?a, biom?dica e telecomunica??es. Este trabalho prop?e o desenvolvimento de circuitos planares, tais como antenas, superf?cies seletivas de frequ?ncia (FSS) e filtros planares, utilizando tecidos (lona, jeans e brim santista) como substrato diel?trico e o tecido Pure Copper Polyester Taffeta Fabric, um tecido de cobre puro, altamente condutivo, leve e flex?vel, comercialmente vendido como material condutivo. As caracter?sticas el?tricas dos tecidos (permissividade el?trica e tangente de perda) foram determinadas utilizando o m?todo de linha de transmiss?o e comparadas com os encontrados na literatura. As estruturas foram analisadas utilizando os softwares comerciais Ansoft Designer, Ansoft HFSS ambos da empresa Ansys e para efeito de compara??o foi utilizado o M?todo Iterativo das Ondas (WCIP). Para efeito de valida??o foram constru?dos e medidos v?rios prot?tipos de antenas, FSS e filtros planares sendo poss?vel constatar uma excelente concord?ncia entre os resultados simulados e medidos

Frequency selective surfaces (FSSs) provide a variety of spatial filtering functions, such as band-pass or band-stop properties in a radome or other multilayer structure. This filtering is typically achieved through closely-spaced periodic arrangements of metallic shapes on top of a dielectric substrate (or within a stack of dielectric materials). In most cases, the unit cell size, its shape, the substrate parameters, and the inter-element spacing collectively impact the response of the FSS. Expanding this design space to include reconfigurable FSSs provides opportunities for applications requiring frequency agility and/or other properties. Tuning can also enable operation over a potentially wider range of frequencies and can in some cases be used as a loading mechanism or quasi-ground plane. Many technologies have been considered for this type of agility (RF MEMS, PIN diodes, etc.). This includes the recent use of microfluidics and dispersions of nanoparticles, or fluids with controllable dielectrics, which have entered the design space of numerous other EM applications including stub-tuners, antennas, and filters. In this work they provide a material based approach to reconfiguring an FSS. An FSS based on a four-arm spiral with tunable band-stop characteristics is presented in this work. A thin colloidal dispersion above each element provides this tuning capability. The radial expansion and contraction of this dispersion, as well as the variable permittivity of the dispersion, are used to load each element individually. This design incorporates thin fluidic channels within a PDMS layer below the substrate leading to individual unit cells that provide a closed pressure-driven subsystem that contains the dispersion. With the capability to individually control each cell, groups of cells can be locally altered (individually or in groups) to create gratings and other electromagnetically agile features across the surface or within the volume of a radome or other covering. Simulations and measurements of an S-band tunable design using colloidal Barium Strontium Titanate dispersed Silicone oil are provided to demonstrate the capability to adjust the stop-band characteristics of the FSS across the S-band.

This study focused on the behaviours of adult video game players in the context of positive and negative effects of video games, to accurately differentiate video game addicts from highly engaged and non-addicted players. To accomplish this, we adopted the Problematic Video Game Playing Test (PVGT) to measure the components of addiction and Flow Short Scale (FSS) to measure high-level engagement. This is a concept which has been lost in the previous studies, setting the current study apart from other studies which were primarily concerned with investigating the negative impact of video games on its players. To get the data needed for this study, we conducted an online survey with a 40-item questionnaire which included demographic information of the respondents, gaming experience and behavioural components of flow and addiction. We were able to attract 80 adult video game players to participate in the study. Our findings showed that 60% of these 80 adult video game players were not addicted, 34% were highly engaged while 6% of the players were addicted. These findings helped us to infer that not all highly engaged video game players are addicted. Furthermore, most of the addicted players were players who have been playing video games for a long time.

An increase in the co-operativity in the motion of particles and a growth of a suitably defined dynamical correlation length seem to be generic features exhibited by all liquids upon supercooling. These features have been observed both in experiments and in numerical simulations of glass-forming liquids. Specially designed NMR experiments have estimated that the rough magnitude of this correlation length is of the order of a few nanometers near the glass transition. Simulations also predict that there are regions in the system which are more liquid-like than other regions. A complete theoretical understanding of this behaviour is not available at present. In recent calculations, Berthier, Biroli and coworkers [1, 2] extended the simple mode coupling theory (MCT) to incorporate the effects of dynamic heterogeneity and predicted the existence of a growing dynamical correlation length associated with the cooperativity of the dynamics. MCT also predicts a power law divergence of different dynamical quantities at the mode coupling temperature and at temperatures somewhat higher than the mode coupling temperature, these predictions are found to be consistent with experimental and simulation results. The system size dependence of these quantities should exhibit finite size scaling (FSS) similar to that observed near a continuous phase transition in the temperature range where they show power law growth. Hence we have used the method of finite size scaling in the context of the dynamics of supercooled liquids. In chapter 2, we present the results of extensive molecular dynamics simulations of a model glass forming liquid and extract a dynamical correlation length ξ associated with dynamic heterogeneity by performing a detailed finite size scaling analysis of a four-point dynamic susceptibility χ4(t) [3] and the associated Binder cumulant. We find that although these quantities show the “normal” finite size scaling behaviour expected for a system with a growing correlation length, the relaxation time τ does not. Thus glassy dynamics can not be fully understood in terms of “standard” critical phenomena. Inspired by the success of the empirical Adam-Gibbs relation [4] which relates dynamics with the configurational entropy, we have calculated the configurational entropy for different system sizes and temperatures to explain the nontrivial scaling behaviour of the relaxation time. We find that the behaviour of the relaxation time τ can be explained in terms of the Adam-Gibbs relation [4] for all temperatures and system sizes. This observation raises serious questions about the validity of the mode coupling theory which does not include the effects of the potential energy (or free energy) landscape on the dynamics. On the other hand, in the “random first order transition” theory (RFOT), introduced by Wolynes and coworkers [5], the configurational entropy plays a central role in determining the dynamics. So we also tried to explain our simulation results in terms of RFOT. However, this interpretation has the drawback that the value of one of the exponents of this theory extracted from our numerical results does not satisfy an expected physical bound, and there is no clear explanation for the obtained values of other exponents. Thus we find puzzling values for the exponents relevant to the applicability of RFOT, which are in need of explanation. This can be due to the fact that RFOT focuses only near the glass transition, while all our simulation results are for temperatures far above the glass transition temperature (actually, above the mode coupling temperature). Interestingly, results similar to ours were obtained in a recent analysis [6] of experimental data near the laboratory glass transition, on a large class of glass-forming materials. Thus right now we do not have any theory which can explain our simulation data consistently from all perspectives. There have been some attempts to extend the RFOT analysis to temperatures above the mode coupling temperature [7, 8] and to estimate a length scale associated with the configurational entropy at such temperatures. We compare our results with the predictions arising from these analyses. In chapter 3, we present simulation results that suggest that finite size scaling analysis is probably the only feasible method for obtaining reliable estimates of the dynamical correlation length for supercooled liquids. As mentioned before, although there exists a growing correlation length, the behaviour of all measured quantities (specifically, the relaxation time) is not in accordance with the behaviour expected in “standard” critical phenomena. So one might suspect the results for the correlation length extracted from the scaling analysis. To find out whether the results obtained by doing finite size scaling are correct, we have done simulations of very large system sizes for the same model glass forming liquid. In earlier studies, the correlation length has been extracted from the wave vector dependence of the dynamic susceptibility in the limit of zero wave vector, but to estimate the correlation length with reasonable accuracy one needs data in the small wave vector range. This implies that one needs to simulate very large systems. But as far as we know, in all previous studies typical system sizes of the order of 10, 000 particles have been used to do this analysis. In this chapter we show by comparing results for systems of 28, 000 and 350, 000 particles that these previous estimates are not reliable. We also show that one needs to simulate systems with at least a million particles to estimate the correlation length correctly near the mode coupling temperature and this size increases with decreasing temperature. We compare the correlation length obtained by analyzing the wave vector dependence of the dynamic susceptibility for a 350, 000particle system with the results obtained from the finite size scaling analysis. We were only able to compare the results in the high temperature range due to obvious reasons. However the agreement in the high temperature range shows that the finite size scaling analysis is robust and also establishes the fact that finite size scaling is the only practical method to extract reliable correlation lengths in supercooled liquids. In chapter 4, we present a free energy landscape analysis of dynamic heterogeneity for a monodisperse hard sphere system. The importance of the potential energy landscape for particles interacting with soft potentials is well known in the glass community from the work of Sastry et al. [9] and others, but the hard sphere system which does not have any well defined potential energy landscape also exhibits similar slow dynamics in the high density limit. Thus it is not clear how to treat the hard sphere systems within the same energy landscape formalism. Dasgupta et al. [10, 11, 12, 13, 14, 15] showed that one can explain the slow dynamics of these hard core systems in term of a free energy landscape picture. They and other researchers showed that these system have many aperiodic local minima in its free energy landscape, with free energy lower than that of the liquid. Using the Ramkrishnan-Yussouff free energy functional, we have performed multi parameter variational minimizations to map out the detailed density distribution of glassy free energy minima. We found that the distribution of the widths of local density peaks at glassy minima is spatially heterogeneous. By performing hard sphere event driven molecular dynamics simulation, we show that there exists strong correlation between these density inhomogeneity and the local Debye-Waller factor which provides a measure of the dynamic heterogeneity observed in simulations. This result unifies the system of hard core particles with the other soft core particles in terms of a landscapebased description of dynamic heterogeneity. In chapter 5, we extend the same free energy analysis to a polydisperse system and show that there is a critical polydispersity beyond which the crystal state is not stable and glassy states are thermodynamically stable. We also found a reentrant behaviour in the liquid-solid phase transition within this free-energy based formalism. These results are in qualitative agreement with experimental observations for colloidal systems.