Noninvasive Procedure for Measuring the Complex Permittivity of Resins, Catalysts, and Other Liquids Using a Partially Filled Rectangular Waveguide Structure

A waveguide based noninvasive procedure to measure the complex permittivity of epoxy resins and catalysts normally employed in the composite industry is presented. The proposed method combines an analytical approach with a numerical optimization technique, and is based on the injection of the liquid under test beforehand into a cuvette (disposable container) with rectangular cross section. The cuvette filled with the liquid specimen is positioned at the center of a WR-340 rectangular waveguide for measuring the scattering coefficient data in the S-band. The approximate value of the complex permittivity of the specimen is reconstructed from the measured scattering coefficient data using the newly proposed analytical closed-form relations. These analytical expressions are based on derivation of the transcendental equation for the multilayered partially filled waveguide structure using the transverse resonance method. In order to determine quite accurate values of dielectric parameters of the liquid specimen, the experimental setup is simulated with the help of a 3-D full-wave electromagnetic field simulator and accordingly theoretical values of the scattering coefficients are computed. The complex permittivity of the liquid specimen in this case is determined using an optimization algorithm, which minimizes the error between the theoretical and measured values of scattering coefficients. To avoid the local minima problem commonly encountered with optimization routines and to increase the convergence rate, the complex permittivity of the specimen reconstructed using the analytical approach is used as the initial guess. The applicability of the overall procedure is determined by measuring a number of liquid specimens used in the composite industry.