Modélisation des discontinuités d’un objectif radar par des points brillants

In order to study the scattering of a monochromatic electromagnetic wave by a complex radar target, specialists usually modelize the target by a discrete scatterers model. The majority of the scatterers are represented by specular points of reflexion on the surface of the target, but the radar cross section (rcs) is also a function of surface discontinuities. In this article, we develop a method which allows us to replace the discontinuities by groups of point contributors of which the positions and thercs are given. Eventhough thercs are generally functions which decrease as the frequency increases, which can at times be neglected, it can be shown there exists quite frequentlyrcs which are independent of the wavelength. General formulae are given. It is emphasized that the physical optic integral may be used to calculatercs eventhough the conditions of application are not correct. For this reason a number of coefficients including the polarisations are used. The formulae of these coefficients are given. As a result an evaluation of thercs may be simplified, and the same method may be used to study the specular regions and the surface discontinuities.     

Diffraction d’ondes électromagnétiques impulsionnelles par des obstacles métalliques

Transient scattering problem by perfectly conducting bodies is treated, directly in time domain, using the distribution theory. New integral equations for solving two dimensional scattering problems are established. Applications to metallic structures responses to electro-magnetic pulses (Emp)illumination are presented.     

Potentialités des séquences d’étalement chaotiques pour l’amélioration duTEEB d’un systèmeDS-CDMA asynchrone

The use of chaotic sequences as spectral spreading sequences in a Direct Sequence-Code Division Multiple Access is proposed in this paper. The simulation of the system is performed with the system simulator Omnisys. The chaotic spreading sequences are generated from a non-linear recursive equation. The Bit Error Rate of this system is compared with theBER of the same system using binary Gold sequences. The results show that chaotic sequences can improve performances in terms of allowable number of users.     

Une méthode ďétude des lignes de transmission dans un milieu non homogène multicouche en présence ďun champ électromagnétique externe

A new simple time-domain method for simulating the coupling between an electromagnetic plane wave and a planar transmission line in a multilayered inhomogeneous medium is proposed. The approach is based upon the modified telegrapher’s equations. The originality of this approach comes from the fact that the calculation of the distributed sources takes into account the electromagnetic field in each layer. The calculation of this field in the multilayered medium is performed by means of the physical optic concept. A finite difference time domain (FDTD) algorithm is used to solve the transmission line equations. The comparison between our results and other theoretical and measured results validates the proposed simulation approach. It has also been demonstrated that neglecting the effect of inhomogeneous medium results in erroneous simulations.     

Antennes en réseau à bas niveau de lobes secondaires par commutation de la loi d’alimentation. Optimisation par l’algorithme génétique

The paper presents an approach of synthesis from a new thinned microstrip antennas array with binary feeding law. The operating principle of this type of antennas array consists in establishing a number of elements supplied at the center of the array as well as the feeding of the extreme elements. The problem of synthesis with constraints is dealt with only by the research of the binary feeding law of the array eccentric elements, thus allowing to reduce to the maximum the sidelobes level. To solve this type of problem, the genetic algorithm is proposed.     

Mécanisme de l’évolution des champs électromagnétiques dans un simulateur de grandes dimensions. Projet d’un simulateur semi-rhombique

The purpose of this paper is to evaluate theoretically and experimentally the transient electric field in anemp simulator. Using a time domain analysis, the mechanism of the field deformation can be analysed and consequently modifications can be introduced in the conception of new simulators (for example: rhombic simulator).     

Correction de deux paquets d’erreurs par un code de Reed-Solomon

Algebraic decoding procedures can be greatly simplified in case the number of errors being corrected is small. The authors present here a decoder which is capable of correcting errors occuring on four bits packets. The considered code is a (15, 11) Reed-Solomon code over GF (16). A Meggitttype decoder has been selected. The hardware implementation consists of 49 ttl (ls) packages and operates for the present at a rate of 5 Mbit/s. The utilisation of this decoding scheme is considered for other Reed-Solomon codes.