Análise de antenas de aberturas circulares concêntricas alimentadas por guias de ondas circulares.

Abstract

In this thesis is presented an analyze of a planar antenna with circular concentric slots fed by coupling through iris. The fundamental mode of a circular waveguide is used to feed the iris. The planar structure of the antenna under study is suitable for aerospace applications due to its thin profile, low volume, low weight and the ability to operate in circular polarization. Furthermore, the antenna can be easly designed for applications in broadband and narrowband communications. Although the concentric slots antenna provide a thin profile, common to planar antennas, its analysis requires the use of techniques applied in the treatment of antennas in three dimensions. From this base, the equivalent circuit method is used in order to represent the electromagnetic problem via an electrical equivalent circuit using virtual and dual sources. The equivalent electric circuit is used to describe the behavior of fields within the structure. The great advantage of this method is to enable the use of circuit analysis techniques to analyse and solve the problem, It leads us to write the set equations of the problem to be solved. In the solution of problems of complex structures the integral equations require numerical methods to be solved. In this work the Method of Moment is used to solve the integral equations of the problem. Due to the complexity of the structure, to describe the field inside the iris and slots (pulse) subdomain basis functions were used in the MoM. The construction of the method of moments matrix is convenient for the application of the direct manipulation of admittance matrix method. This technique speeds up the optimization routine since it avoids unnecessary recalculation of matrix elements. The optimization routine used in this study is the Genetic Algorithm. The parameters used to construct the objective function in the GA are the position and the slots dimensions. The methodology used in the treatment of the structure was implemented in two computer programs written in Matlab®. The first implemented routine allows the analysis of transitions between two circular waveguides and it is used in the study of different types of feeding. The second implemented routine enables the analysis of circular concentric slots antenna and enables the study of the fields in the slots and the antenna radiation pattern. The simulation results for the transition between circular waveguides are compared with other published results. A good agreement between results is observed. Simulations of the proposed antenna in the microwave and milimeter frequency band were made. The simulated results were compared with results obtained using CST®. Measurements were performed in the frequency range of microwaves, betoween 9 GHz and 15 GHz. The obtained results for three cases presented good agreement. By existing limitations in the measured room and accuracy in the antenna building, a frequency shifts were observed. The appropriate choice of dimensions and number of slots in the structure allows the antenna to operate in narrow band or wide band. It is demonstrated that the number and dimensions of the slots have an important role in the antenna radiation pattern.