Dissimulation électromagnétique d'une membrure d'antenne par un réseau de lignes de transmission

RESUME Dans le monde des telecommunications terrestres ou spatiales, les antennes sont presentes dans toutes les applications. Parmi les antennes les plus utilisees, on compte les antennes paraboliques que l’on peut voir sur les toits de certains bâtiments et qui se trouvent aussi dans l’espace. Ces antennes, constituees d’un reflecteur parabolique sont generalement alimentees par des sources primaires telles que des antennes de type cornet, ou des sous-reflecteurs. Ces elements sont maintenus devant le reflecteur principal a l’aide de supports ou membrures (« strut » en anglais) qui se situent dans le champ de rayonnement, et ils contribuent donc a une perte de signal. Pour remedier a ce probleme, differentes solutions ont ete proposees durant ces dernieres annees. Malheureusement, la plupart de ces solutions sont souvent complexes a realiser et elles fournissent des resultats assez limites en largeur de bande. Dans ce projet, nous explorons une nouvelle facon de concevoir une membrure d’antenne en y inserant un reseau de ligne de transmissions (RLT). Inserer un RLT dans un objet est une technique tres interessante pour masquer un objet des ondes electromagnetiques. Lorsqu’une onde electromagnetique arrive sur un objet (une membrure d’antenne par exemple) traverse par un RLT, elle est guidee a travers l’objet par les lignes de transmission comme si celui-ci etait invisible. Cette technique de masque electromagnetique (ME) a ete etudiee en [15] sur un objet quelconque et les resultats obtenus sont tres interessants pour l’appliquer sur une membrure d’antenne. Le RLT en question est constitue de paires de rubans metalliques tres minces qui passent a travers et non autour de l’objet. L’objet a masquer doit donc etre au prealable construit de facon a pouvoir y inserer les lignes de transmission. ----------ABSTRACT In the world of ground and spatial telecommunications, antennas are present in all applications. One of the most used antennas is the parabolic dish that can be seen on the roofs of some buildings and also on spacecrafts. Those antennas, constituted by a parabolic reflector are generally fed by primary sources such as horn antennas, or sub-reflectors. Those elements are maintained in front of the main reflector by struts which are placed in the radiation field, and they thus contribute to signal loss due to scattering. To fix this problem, several solutions have been proposed during the last years. However, most of these solutions are often complex to realize and they provide limited results in terms of frequency bandwidth. In this project, we explore a new way to design a strut by inserting a Transmission Line Network (TLN). Inserting a TLN appears as a very promising technique for cloaking an object from electromagnetic fields. When an electromagnetic wave impinges on an object (antenna strut for example) embedding a TLN, the wave is guided through the object by the transmission lines as if this one was invisible. This method of cloaking has been studied in [15] on an object of arbitrary shape and the results presented suggests that TLNs have potential for cloaking an antenna strut. The TLN is constituted of very thin microstrip lines that pass through the object and not around it. The object to cloak should however be built with the capability of embedding the TLN. The principal objective in this project is to decrease the loss of signal caused by the antenna struts by inserting a TLN. To reach this objective, we should first well understand the concept of cloaking using a TLN. In order to do so, we have reproduced the simulations done in [15] using the software HFSS® (High Frequency Structure Simulator) from Ansoft. After some few modifications of the TLN used in [15] for an application in strut cloaking, several simulations have been done and different results (reflection coefficient, transmission coefficient, RCS in transmission and RCS in reflection) were noted. In the beginning, we simulated a unit cell which could be considered as an element of an infinite periodic network of cells. The results being encouraging, we continued with a finite number of cells representing an antenna strut of parallelepiped shape crossed by a RLT. These simulations also demonstrated that the TLN transmits the incident waves very efficiently through the strut. We also studied the effect of the TLN on a rhombus antenna strut for comparison since this shape is currently considered the state-of-the-art in uncloaked antenna struts. It was found that the TLN brings better improvements in the case of the parallelepiped strut. Always for comparison, we have simulated a parallelepiped antenna strut crossed by a TLN and with dielectric strips on its sides for implementing a hard surface. By doing this, we are combining two approaches for the first time, i.e. cloaking using a TLN and cloaking using hard surfaces. The results obtained are better than the results obtained with the parallelepiped strut using a TLN but over a very limited frequency band. To validate the performance of the TLN in a more realistic antenna scenario, we simulated a hollow strut crossed by a TLN.