Dielectric measurements of tropical wood

The aim of this paper is to determine the dielectric characteristics of tropical wood species at microwaves frequency in order to use them as planar antenna substrate. The measurements have been done on dried wood in the frequency range 8.2-12.4 GHz The suitability of wood as a microwave printed antenna substrate will be determined by the values taken by the complex parameters "permittivity" epsilon and "permeability" mu and the loss tangent tan delta. The measurement methods were based on the waveguide method and the open resonator technique The first method gave us the reference values while the second one confirmed the previous measurements. Values obtained in the three principal axis of each species confirm the anisotropic nature of wood. These measurements will also confirm that the losses which are the main criteria of selecting wood as substrate grow with the density of the wood. (C) 2009 Elsevier Ltd. All rights reserved

3D analysis and optimization of microwave circuits by the method of moments

Francisco Jesus Nunez

The design and minaturization of microwave circuits involves the analysis and optimization of a large varity of structures, which, in general, are based on arbritrary objects embedded in also arbritary media. The most common situation is found when optimizing terminal antennas, whose main constraint is the antenna's volume, which must be fitted into a specified space, which in general is the phone or laptop case. This kind of antennas can also include stratified media, oblique metallizations, dielectric objects, waveguide components, in their definition making their analysis more difficult from the numeric point of view. The complete treatment of this kind of problems has been performed traditionally using Finite Differences or Finite Elements software, while it is more rare to find Method of Moments implementations for the solution of this kind of general problems. The Method of Moments has a certain number of advantages as the inclusion of radiation conditions or stratified media in the Green's function of the problem. Due to this it is possible to mesh only the surfaces or boundary conditions that are not included in the Green's function definition, obtaining, for instance, a more natural radiation boundary condition compared to other fullwave methods. The other big concern is the optimization of this kind of structures. In general the structures we are dealing with in this work can have very irregular shapes, in order to fit within the required volume constraints. The traditional optimizations techniques based on the gradient or conjugated gradient are not efficient when optimizing functions containning multiple minima and a large search space, which is the case in the optimization of terminal antennas. When miniaturizing terminal antennas or just a simple microstrip filter, the number of unknowns can be very large, and the different optimization variables can vary within wide limits in the search space. In this situation the Genetic Algorithms are ideal for seeking the global optimum solution of our problem. This work covers the analysis and optimization of microwave circuits, including antennas and filters in microstrip and waveguide technologies, paying more attention to the implementation of the Method of Moments (MoM) in conjugation with the Mixed Potential Integral Equation (MPIE) and Electric/Magnetic Field Integral Equation (EFIE/MFIE). The work has been split into 3 main goals: The analysis of 3 dimensional structures in stratified media, including arbritary shaped dielectric bodies using the Method of Moments, (MPIE,EFIE,MFIE). The analysis of shielded environments and more precisely the analysis of retangular waveguide cavities with the Method of Moments. The implementation of several optimization techniques, emcompassed in the frame of genetic algorithms, for the design and miniaturization of terminal antennas and microstrip filters. Some relevant examples linked to the efficiency and accuracy of the different methods and techiques explained in this work are included in each chapter, yielding to practical results suitable to be used in real life design problems. The main contributions of this work can be summarized as follows: In Chapter 2 we have developed the interpolation of 3D Green's function to the general solution of dielectric objects embedded in multilayered media. The interpolation method itself is preceeded by a spectral extraction of the quasi-static terms. This technique, very often applied in complex images techniques, has been applied to our specific Green's function problem yielding very good results in terms of accuracy and Green's function computation time. Another contribution, also emcompassed in Chapter 2, is the integration of the static part of the field dyadic G̿EM, performed by splitting the field dyadic into TM and TE components, allowing then an analytical integration of the dyadic terms in stratified media. In the frame of shielded environments in Chapter 3, the most important contribution is the extension of Ewald's acceleration technique to full electric and magnetic problems, allowing the acceleration of the field dyadic G̿EM and thus permit the extension of the MoM to the simulation of arbritary metallic shapes coupled to apertures through Ewald's approach. In Chapter 4 the most important contribution is the implementation of a Bayesian optimiser based on the estimation of probability made by dependencies trees. The dependency tree based method found in [1] and later in [2] has been adapted to the terminal antenna miniaturization yielding to excellent results in optimisation time and quality of the solution provided by the optimiser. Our implementation of the dependency tree method was found to be better than the traditional method used in this kind of problems. Chapter 4 contains also an implementation of the growing cells method presented in [3] for the specific problem of optimising pseudo periodic structures, like microwave filters, yielding to a powerful interpolating method which accelerates the optimisation process of microwaves devices whose fitness response is very time consuming.

EPFL2006

A simple & [and] accessible front-end satellite receiver for communications and e-learning in emerging countries

Olivier Videme Bossou

For the universities of the emergent countries, and those of Cameroon in particular, the fight to reach a durable development requires to know the needed scientific and technical information for their development, how to get it and to spread it out among partners in the country. Obviously, the complete answer to these preoccupations lies well beyond the framework of this project, but a projection, partial but highly significant, can nevertheless be obtained if one concentrates one's efforts on a specific technological field : in the present study, we shall treat of antennas for telecommunications systems. This subject is highly suitable as example and potential incubator for future achievements. The ultimate goal is to design and carry out the fabrication of a system of satellite communications adapted to the financial resources and to the development needs of the universities of the South. First of all, we will review the state of the art in satellite communication systems, both for already operating ones and also for new systems, still under development. Then we will determine which system of satellite telecommunications has a receiver and communication fees with affordable costs : our choice will be made on Worldspace satellite system, which was actually designed for developing countries. We will also review the status of planar antenna and low noise amplifier technology, and then explicit the basic challenge involved in our thesis project. One of the aims of this project is to use local wood as dielectric in SSFIP type antennas. Before using a material as dielectric in an antenna design, it must be characterized. Before carrying the characterization of local wood species, we will begin by reviewing some commonly used dielectric measurement techniques. Then we will carry out a series of measurements to characterize the species of wood growing in the Cameroonian forest. At the end of the series of measurements and according to the dielectric losses observed, we will select the species of wood best suited to make a good substrate for a microstrip antenna. Next, we will design, manufacture and test low cost antennas. These antennas must satisfy not only technical criteria compatible with those of the telecommunications system chosen but, even most importantly, it must meet socioeconomic criteria : the antennas must be manufactured with low cost and locally available materials, making use of simple technological processes. Once we have found the wood species that exhibits the lowest dielectric losses, we will simulate, and then manufacture the SSFIP type antenna with wood as substrate and aluminium as conducting element. Then we will measure its characteristics and check whether they are compatible with those of a Worldspace receiver. The signal received from the satellite by the antenna is very weak. It will be important to strengthen the received signal with a low noise amplifier (LNA) before connecting the antenna to the receiver. Thus the antenna and the LNA constitute an active antenna and form what we call here the radiofrequency (RF) front-end. We will design, manufacture and measure the LNA characteristics. A simple manufacturing process must be used to realize these antennas, because later on they must be manufactured locally in Cameroon : this is a determining challenge to ensure the economic independence, which is essential in our thesis project. Afterwards we will join the LNA to the planar antenna and connect the active antenna obtained in this manner to a commercial Worldspace receiver. Finally, we will perform receiving tests of the Worldspace radio station with the system designed. In parallel, our project includes a second goal, which presents an equivalent significance. We will use existing Matlab codes to develop an online course to teach antenna theory. In addition, at the end of the project and when the Cameroonian local manufacture would have started, we will put all algorithms used for the antenna design on the web site in a multi-media applet, together with a complete documentation of all the manufacturing technologies used. The courses that already exist in Lausanne and Yaounde, relating to the description of other types of antennas will be added, thus leading to a complete teachware that would ensure the durability and the dissemination of the results obtained, and would provide an invaluable tool for teaching and training.

EPFL2007

Near-field imaging

Libo Yu

Bloch surface waves (BSWs) are surface electromagnetic waves excited at the interface between a truncated periodic dielectric multilayer and a surrounding medium. This particular solution to the Maxwell equations in stratified media has been known since the late 1970s. However, few studies have been conducted till now on the manipulation and control of BSW propagation over 2D surfaces of photonic structures. There are several advantages provided by BSW propagation on almost flat surfaces. Due to the use of dielectric materials, losses are very low, thus allowing the propagation of BSWs over long distances. Another advantage in using BSWs is the possibility of operating within a broad range of wavelengths by properly designing a suitable multilayered structure. Furthermore, since the maximum intensity associated with BSWs can be tuned on the surface, a strong field intensity increased by several orders of magnitude can be achieved and can thereby enhance the field close to the structure's surface. This tunable localized field confinement is particularly attractive in fluorescence biosensing. In the following, we will use an exemplary BSW-sustaining multilayer as a general platform suitable for manipulating and controlling BSW propagation on its surface in the spectral range of telecom wavelengths. Several 2D optical photonic devices can be implemented starting from an ultra-thin (~ λ /15) polymer layer spun on the multilayer, which can be subsequently shaped as desired. The presence of the polymer modifies the local effective refractive index, enabling a direct manipulation of the BSWs. We experimentally and theoretically demonstrate that BSWs can be diffracted, focused, coupled and made resonating by locally shaping the geometries of 2D photonic devices—such as lenses, prisms, gratings, bended waveguides and waveguide couplers—on the multilayer. One of the main advantages is that these 2D photonic devices can have arbitrary shapes, which are difficult to obtain in 3D. The strong point of the platform concept is that a thin-film multilayer enables standard-wafer-scale production. The top surface can be modified to customize a 2D micro-system by, e.g., e-beam writing, optical lithography, stamping or other replication techniques. Since a BSW can be considered as a 2D wave and it is bounded at the surface of the multilayer, near-field imaging is one of the preferred tools to directly monitor and characterize the near field produced on the structured multilayered surface mentioned above. A multi-heterodyne scanning near-field optical microscope (MH-SNOM) developed in our lab (EPFL-OPT) makes near-field characterization possible. The operation wavelength is in the near infrared range (1460 nm–1580 nm). A polarization-retrieval method is developed in this thesis to measure and characterize the near-field polarization response for arbitrary polarization- sensitive photonic nano-devices, such as photonic crystal microcavities, waveguides, thin films, nanoparticles, spiral gratings and other near-field polarization-sensitive imaging applications. Polarization retrieval is easily accessible in far-field microscopy, but is challenging for the near field. This is because when a SNOM probe interacts with the near field and scatters the signal to the far field, the near-field polarization may be considerably altered. In this thesis, the developed method uses an isotropic region in the vicinity of the nanostructure as a calibration area, whose known polarization properties provide a global criterion to calibrate and quantify the polarization distortion induced by the detection system. It is then applied on the field measured above the structure of interest to compensate and reconstruct its complex field response. We experimentally show the effectiveness of the method on a silicon form- birefringent grating (FBG) with significant polarization diversity. Three spatial dimensional near-field measurements are in agreement with theoretical predictions obtained with rigorous coupled-wave analysis (RCWA). Pseudo-far- field (~10λ away from the surface) measurements are performed to obtain the effective refractive index of the FBG, emphasizing the validity of the proposed method. This reconstruction algorithm makes the MH-SNOM a powerful tool to analyze concurrently the polarization-dependent near-field optical response of any nanostructures with subwavelength resolution as long as a calibration area is available in close proximity.

EPFL2013