A Generalized Approach for Measuring the Dielectric Properties of Lossy Composite Materials

A generalized technique is presented to measure the dielectric properties of lossy composite materials. The overall method is based on measuring the spectral domain reflection and transmission coefficients of the material-under-test (MUT) using a Vector Network Analyzer (VNA), and then finding the dielectric properties of MUT from the measured scattering data using a proposed reconstruction algorithm. The effect of noise on the reconstruction is examined, and it is observed that even with 5% relative error in the scattering data, the proposed algorithm produces a stable inversion. INTRODUCTION The use of microwaves for the curing of composite materials has got much attention in recent years. The microwave processing provides the uniform, rapid and selective heating of reinforced materials, which can generally not be achieved with the conventional autoclave curing [1]. A fully automated system for the microwave processing of composite materials has also recently been developed and tested to provide a uniform temperature for even very large structures [2]. However, till now the complex nature of interaction of electromagnetic waves with composite materials has not been fully understood. It is also not fully clear that how the dielectric properties of these reinforced materials may change with different parameters such as the cure cycle, the composite thickness, and the proportion of different fibers and resins. The determination of dielectric properties of composite materials at different stage of curing may provide some information about the absorption and reflection characteristics of these materials. The methods presently available to measure the dielectric properties of materials can be broadly classified into two major categories. The first category requires the material to be placed in some cavity, and the dielectric property of the material is determined from the changes in the resonant frequency and the quality factor of the cavity. This method some time provides an accurate estimate, but this is a resonant method and it requires the sample to be machined very precisely to be fitted inside the cavity. The second category of method is generally broadband, which requires the sample to be placed either in the transmission line or in free-space. The present paper describes a technique for the measurement of dielectric properties of lossy composite materials, which comes under the second category. The proposed method is quite general in the sense that it can be applied for the dielectric samples, which are either placed in transmission lines (such as waveguide), or in free space, and can also be extended for multi-layer structures.