Thesis (MSc)--University of Stellenbosch, 2006. / ENGLISH ABSTRACT: static density and magnetic structure functions of a melt of flexible polymers whose monomers possess bond-directed dipoles which interact with each other. In order to observe the effect of screening of the dipolar interaction on the structure functions we obtain results for cases with and without steric interactions and also for cases with and without Debye-H¨uckel screening of the dipole moments. / AFRIKAANSE OPSOMMING: Approximation” RPA) om die statiese digtheids- en magnetiese struktuurfunksies te bereken vir ’n smelt van hoogsbuigsame polimere, waarvan die monomere dipole langs die verbindings besit wat met mekaar in wisselwerking tree. Om die effek van afskerming op die dipolare wisselwerking en die struktuurfunksies te kan waarneem, bepaal ons resultate vir die gevalle met en sonder steriese wisselwerkings en ook vir gevalle met en sonder die Debye-H¨uckel afskerming van die dipoolmomente.

Leaky-waves have been a topic of increasing interest in the last years, with diverse practical applications in many different engineering fields. From periodic, FSS, EBG or even metamaterial leaky-wave based antennas to waveguide filters and higher efficiency energy guiding, they all share a common base structure: a travelling-wave propagating within a metal encapsulation, that can be open or closed, and altered by a planar metallization of periodic nature, from which the energy may radiate. Due to the fact that these antennas are usually electrically large and the periodic printed circuit requires a certain grade of complexity, 3D commercial software is prohibitively time consuming. Also, the homebrew methods developed up to this day are either not rigorous and accurate enough or unable to deal with complex periodic geometries. At this point, the evolution of leaky-wave antennas needs a solid, efficient and versatile tool where to base the future design research on. In this work a novel simulation tool for waveguide embedded leaky-wave antennas is presented. It is based on a full-wave Method of Moments applied to the spectral domain Green Functions for a rigorous modal analysis of the finite structure. The use of Subdomain basis functions allows the software to model complex periodic geometries, overcoming a main limitation, and the analytical nature of the method combined with its 2.5D approach, results in a significant computing time reduction. It is built on a modular coding philosophy and provided with a user-friendly graphical interface, and an intuitive working procedure, making the program not only fast and accurate, but also easy to use and extend to new geometries. Finally, it is remarkable the educational potential of this new analysis software, since it identifies higher order effects as bandgaps and multi-harmonic radiation from a complete and simple modal approach.

Higher-order basis functions are widely used to model currents and fields in numerical simulations of electromagnetics problems because of the greater accuracy and computational efficiency they can provide. Different problem formulations, such as method of moments (MoM) and the finite element method (FEM) require different constraints on basis functions for optimal performance, such as normal or tangential continuity between cells. In this thesis, a method of automatically generating bases that satisfy the desired basis constraints is applied to a MoM formulation for scattering problems using surface integral equations. Numerical results demonstrate the accuracy of this approach, and show good system matrix conditioning when compared to other higher-order bases.

The research performed throughout this dissertation aims at implementing quantitative methods in order to assess economic and technological performances of firms, i.e. it tries to assess the impacts of the determinants of technological activity on the results of this activity. For this purpose, a representative sample of the most important R&D firms in the world is constituted. The micro-economic nature of the analysis, as well as its international dimension are two main features of this research at the empirical level. The second chapter illustrates the importance of R&D investments, patenting activities and other measures of technological activities performed by firms over the last 10 years. The third chapter describes the main features as well as the construction of the database. The raw data sample consists of comparable detailed micro-level data on 2676 large manufacturing firms from several countries. These firms have reported important R&D expenditures over the period 1980-1994. The fourth chapter explores the dynamic structure of the patent-R&D relationship by considering the number of patent applications as a function of present and lagged levels of R&D expenditures. R&D spillovers as well as technological and geographical opportunities are taken into account as additional determinants in order to explain patenting behaviours. The estimates are based on recently developed econometric techniques that deal with the discrete non-negative nature of the dependent patent variable as well as the simultaneity that can arise between the R&D decisions and patenting. The results show evidence of a rather contemporaneous impact of R&D activities on patenting. As far as R&D spillovers are concerned, these externalities have a significantly higher impact on patenting than own R&D. Furthermore, these effects appear to take more time, three years on average, to show up in patents. The fifth chapter explores the contribution of own stock of R&D capital to productivity performance of firms. To this end the usual productivity residual methodology is implemented. The empirical section presents a first set of results which replicate the analysis of previous studies and tries to assess the robustness of the findings with regard to the above issues. Then, further results, based on different sub samples of the data set, investigate to what extent the R&D contribution on productivity differs across firms of different industries and geographic areas or between small and large firms and low and high-tech firms. The last section explores more carefully the simultaneity issue. On the whole, the estimates indicate that R&D has a positive impact on productivity performances. Yet, this contribution is far from being homogeneous across the different dimensions of data or according to the various assumptions retained in the productivity model. The last empirical chapter goes deeper into the analysis of firms' productivity increases, by considering besides own R&D activities the impact of technological spillovers. The chapter begins by surveying the alternative ways proposed in the literature in order to asses the effect of R&D spillovers on productivity. The main findings reported by some studies at the micro level are then outlined. Then, the framework to formalize technological externalities and other technological determinants is exposed. This framework is based on a positioning of firms into a technological space using their patent distribution across technological fields. The question of whether the externalities generated by the technological and geographic neighbours are different on the recipient's productivity is also addressed by splitting the spillover variable into a local and national component. Then, alternative measures of technological proximity are examined. Some interesting observations emerge from the empirical results. First, the impact of spillovers on productivity increases is positive and much more important than the contribution of own R&D. Second, spillover effects are not the same according to whether they emanate from firms specialized in similar technological fields or firms more distant in the technological space. Finally, the magnitude and direction of these effects are radically different within and between the pillars of the Triad. While European firms do not appear to particularly benefit from both national and international sources of spillovers, US firms are mainly receptive to their national stock and Japanese firms take advantage from the international stock.

Le travail réalisé dans ce mémoire de thèse s'inscrit dans le cadre d'une expérience effectuée à l'institut Paul Scherrer (PSI) et dédiée à la mesure du moment dipolaire électrique du neutron (nEDM). Une sensibilité de 1027 e.cm est attendue à la fin 2013. La mesure est réalisée avec des neutrons ultra-froids (UCN) polarisés. Pour ce faire, de nouveaux systèmes de polarisation, de transport et d'inversion de spin ont été réalisés. Leur étude est présentée dans ce mémoire. Ils sont actuellement utilisés auprès de l'expérience. Grâce à des mesures de champs magnétiques effectuées sur le spectromètre, des simulations de champs réalisées avec les codes Radia et Maentouch, ainsi qu'à des simulations Monte-Carlo effectuées avec le code Geant4, l'efficacité du dispositif de polarisation a été calculée. L'efficacité mesurée en direct est de 88.5 0.3%, soit légèrement moins que la valeur attendue de 95%. Par ailleurs, la prise de données préliminaire effectuée en octobre 2011 a permis de déterminer certains des paramètres fondamentaux de l'expérience, tel les constantes de temps de remplissage, de stockage et de dépolarisation longitudinale du spectromètre. Ces paramètres sont encourageants pour la suite de l'expérience.

This thesis studies different aspects of firm decisions by using a dynamic model. I estimate a dynamic model of the firm based on the trade-off theory of capital structure that endogenizes investment, leverage, and payout decisions. For the estimation of the model I use Efficient Method of Moments (EMM), which allows me to recover the structural parameters that best replicate the characteristics of the data. I start analyzing the question of whether target leverage varies over time. While this is a central issue in finance, there is no consensus in the literature on this point. I propose an explanation that reconciles some of the seemingly contradictory empirical evidence. The dynamic model generates a target leverage that changes over time and consistently reproduces the results of Lemmon, Roberts, and Zender (2008). These findings suggest that the time-varying target leverage assumption of the big bulk of the previous literature is not incompatible with the evidence presented by Lemmon, Roberts, and Zender (2008). Then I study how corporate income tax payments vary with the corporate income tax rate. The dynamic model produces a bell-shaped relationship between tax revenue and the tax rate that is consistent with the notion of the Laffer curve. The dynamic model generates the maximum tax revenue for a tax rate between 36% and 41%. Finally, I investigate the impact of financial constraints on investment decisions by firms. Model results show that investment-cash flow sensitivity is higher for less financially constrained firms. This result is consistent with Kaplan and Zingales (1997). The dynamic model also rationalizes why large and mature firms have a positive and significant investment-cash flow sensitivity.

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