Novas técnicas de otimização de casamento de impedância de antenas planares para sistemas de comunicação sem fio

Abstract

The work presented in this dissertation consists in the development of new techniques for the impedance matching optimization of printed antennas for wireless communication systems. The techniques proposed are based on the insertion of slots in the antenna feeding line or below it (in the ground plane), enabling to keep unchanged the patch geometry of the investigated microstrip antenna. The study was performed using a parametric analysis and enabled the development of three new impedance matching techniques. In the first, the insertion of the slot is carried out in the conducting strip of the microstrip line, being symmetrically placed. In the second, the slot is introduced into the ground plane, but it is inserted below and parallelly to the conducting strip of the microstrip line. In the third, the slot is also positioned in the ground plane, but is aligned parallel to the lower edge of the conducting patch, i.e. perpendicularly to the conducting strip of the microstrip line. The effect of the input impedance matching on the performance of rectangular patch microstrip antennas printed on fiberglass substrates (FR-4) was investigated. The antennas were fed by microstrip lines and designed for operation at 1.8 GHz, 2.45 GHz, and 3.5 GHz. Simulation and parametric analyses were performed using Ansoft Designer and Ansoft HFSS softwares, that implement the method of moments (MoM) and the finite element method (FEM), respectively. In addition, the wave concept iterative procedure (WCIP) was also used in the analysis, enabling a better comparison between the obtained simulated results. For validation purpose, several prototypes were fabricated and measured. The good agreement observed between simulated and measured results confirms the efficiency and accuracy of the new impedance matching techniques. The proposed techniques are easy to implement and allow considerable reductions in the reflection coefficient value, | S11 | (dB), indicating that the antenna efficiency was significantly increased. In the first case, for the slot inserted in the conducting strip of the microstrip, the value of | S11 | (dB) was reduced from -8.81 dB to -28.09 dB in the analysis for 2.45 GHz. In the second case, for the slot inserted in the ground plane, below and parallel to the conducting strip of the microstrip, the value of | S11 | (dB) was reduced from -7.62 dB to -37.17 dB in the analysis for 2.45 GHz. In the third case, for the slot inserted in the ground plane, parallel to the lower edge of the conducting patch, the value of | S11 | (dB) was reduced from -10.11 dB to -34.1 dB in the analysis for 1.8 GHz. The use of slots, as proposed in this work, enabled a good impedance matching between the antenna and the microstrip feeding line, avoiding the use of high impedance microstrip lines, abrupt transitions in the width of microstrip lines, changes in the shape of the antenna conducting patch, or increasing the antenna size.