Multisection transmission line with random characteristic impedance

A probabilistic model of a multisection transmission line with a random characteristic impedance is developed. The average value of the voltage at the end of the random line is estimated from an equivalent homogeneous line, which depends on the mean values of the characteristic impedance and admittance distributions     

Determination of conductivity profiles by time-domain reflectometry

A new numerical method for the reconstitution of the inhomogeneous media conductivity is described. The conducting medium is nonmagnetic, and its relative permittivity equals one. Its frequency-independent conductivity varies normally to its interface. The medium is normally illuminated by a transverse electromagnetic (TEM) plane wave of causal time dependence. The reconstruction process is a direct approach to the inverse problem. It is based on a space time discretization of an exact integral equation. The conductivity profile is determined step-by-step without approximations other than numerical. Some examples are given to illustrate the most interesting properties of the method; special attention is given to the simulation of experimental errors.     

Spherical near-field facility for microwave coupling assessments inthe 100 MHz-6 GHz frequency range

This paper describes a spherical near-field facility set up at the Centre d'Etudes de Gramat (CEG) to perform low-power microwave coupling assessments. Specifically, this facility has been designed to determine the coupling cross section of a complex object. However, it can also be used for the characterization of any radiating system in electromagnetic compatibility (EMC) or high-power microwave (HPM) environments. The near-field approach is shown to be complementary and to offer increased flexibility when compared to other more conventional measurement techniques. More particularly, the use of a probe array in the upper part of the frequency band significantly speeds up the near-field measurement process. Consequently, broadband and multiparameter acquisitions can be performed within acceptable duration. The examples given in this paper provide a broad illustration of the capabilities of near-field techniques for EMC and HPM applications     

Étude numérique des antennes épaisses par l’équation intégrale d’Albert et Synge

The Albert and Synge integral equation allows to determine the current on a rotationnally symmetric conducting antenna of arbitrary meridian shape in the emission mode. This equation is deduced from the use of generalized boundary conditions and therefore possesses a regular kernel. This regularity is a cause of a numerical ill-conditionning of the linear systems deduced from such equations by use of moments methods. An iterative algorithm of inversion due to Le Foll allows to overcome the difficulties linked with this illconditionning. Computations have been made on various structures (spheres, cylinders with or without sharp edges, ...) and their validity has been checked from results already available or by comparison with experimental ones. Therefore such integral approach appears particularly interesting in the case of thick structures, the longitudinal or transversal dimensions of which may attain a few wavelengths. Such structures are useful in many domains. The development of wide bandwidth electromagnetic sensors, the coupling of active or passive devices from the millimeter waves up to the infrared are two important exemples.     

Active microwave imaging of inhomogeneous bodies

A numerical method and experimental technique for microwave imaging of inhomogenous bodies is presented. This method is based on the interpretation of the diffraction phenomena and leads to tomographic reconstruction of the body under investigation. Various numerical examples are given on spatial impulse response, recognition of dielectric rods, inhomogeneous bodies, and simulated human arm. Different experimental results on dielectric rods and isolated animal organs are also given.     

Far-field accuracy investigation using modulated scattering technique for fast near-field measurements

Near field measurement techniques in conjunction with near-field to far-field transformation algorithms are widely used today. Two of the most important concerns are, firstly, the degree of accuracy achieved, and secondly, the measurement duration. Although high degrees of accuracy can be obtained, the time required to scan completely the near field of an antenna using the classical near-field measurement techniques is rather long. The modulated scattering technique would offer a means to reduce this time by a factor of 10 to 100 while maintaining a reasonable degree of accuracy. Using this technique, however, one introduces further sources of inaccuracy such as the mutual coupling between the elements of the array used to probe the test antenna, and the further limitation of the available measurement dynamic range. In this paper, these two sources of inaccuracy inherent in this technique or other techniques which use a similar set-up, are explored. Multiple reflections between the test antenna and the probe array are ignored. A parabolic reflector is chosen as the test antenna, and an array of dipoles is chosen as the probe antenna in the numerical simulation.     

Evaluation des cadences de mesure de champs proches au moyen de sondes modulées

The problems involved in the direct far-field measurements of large antennas have led to the development of the near-field measurement technique. According to this method, the far-field pattern of the antenna is calculated from the near-field measurements close to the antenna. The only inconvenience in this technique is the slow rate of measurements. This slowness is due to the mechanical displacement of the measuring probe or the test antenna. The modulated scattering technique is a method to reduce the measurement time while preserving acceptable levels of accuracy. This article is mainly concerned with estimating the possible measurement rates in typical configurations.     

Absolute measurement at 1 THz of the optical coupling of a F.I.R. conical antenna with a Josephson detector

We present the first absolute measurement of the optical coupling of a FIR antenna with a Josephson detector. We have studied a wide band FIR coupling system made of a conical antenna (discone) and a focussing paraboloid mirror. The calculated efficiency is (6.8±0.3)10^?2 for the optimum choice of parameters at 1 THz. The best measurements using a HCN laser at 891 GHz are in agreement while the average achieved coupling is (2.3±0.4)10^?2 reflecting the experimental difficulties.     

Formation d'un Réseau Bidimensionnel dans un Cristal Liquide Nématique Soumis à un Champ Électrique Alternatif

Lorsqu'un champ électrique alternatif est appliqé perpendiculairement á une couche de MBBA, on observe sous certaines conditions l'établissement d'un réseau bidimensionnel. Les étapes successives du processus de formation sont décrites et un modèle qualitatif expliquant ce processus est présenté. La stabilité du réseau est discutée en fonction des différents paramétres expérimentaux.