Medição das propriedades constitutivas eletromagnéticas de materiais na faixa de 1 a 6 GHz pelo método do espaço livre

Abstract

The technological demand in the last decades has brought the need for the development of new materials, as well as the importance of knowing the electromagnetic constitutive properties of those already employed in science and engineering. This entails a search for efficient ways to carry out simulations and measurements of the materials behaviour when they interact with electromagnetic waves. Currently many applications use spectrum bands in the region called microwave, in a search for highquality and precision applications. When working in this spectral region it is important to take into account effects that, due to high frequencies, become significant, but that could simply be disregarded in low frequency applications. In order to evaluate a system in the range of units or tens of GHz, it is common to use vector network analyzers (VNA), which are devices with high processing power and several builtin functions. They basically measure the relations (known as S-Parameters) betweenthe transmitted and received signals in each of their ports. In a materials characterization application, the main idea is to use these signals measured by the VNA as input for an algorithm that converts them to the permittivity and permeability of the material under test. There are different ways of designing such a system, depending on the interest in case. The Free-Space methods are naturally suited to applications involving radiation absorbing materials (RAM), but are also important in applications that do not allow that the sample of the material being measured undergoes major interventions, in other words, they have the advantage of little sample preparation, in contrast to other methods thatrequire a precise sample preparation. This work uses the electromagnetic theory (like microwave circuits, antennas theoryand signal processing) to develop a Free-Space measuring system, which permits the evaluation of the electromagnetic constitutive properties of different materials. A Free-Space environment has noises due to various reasons. Then there are some necessary steps related to the calibration of the measurement system to take the influences of only the sample itself. A filtering process, called Time-Domain Gating, wasimplemented in order to get cleaned S-Parameters. The assembled vertical setup was designed to permit the measurement of at andhard slab samples. Teflon (PTFE) and Perspex (PMMA) samples were tested. Taking into account that the Transmission and Reflection methods (and specially the Free-Space method) have limitations while measuring low loss tangent materials, like the ones tested, good results are obtained for the real part of the electromagnetic constitutive properties.