Experimental and Modelling the Combined Measurements Technique for Microwave Absorbing Coatings Testing with Finite Difference Time Domain Method

Abstract

Because of the structure property of radar absorbing coatings, using of the existing methods for testing these materials is quite difficult. In this paper, a combined-measurement technique was proposed for broadband characterization of microwave absorbing coatings using flanged rectangular waveguide sensor. The technique combines both frequency sweep reflections measurement and two-layer method to provide two different test conditions of the needed reflection coefficients necessary to extract both complex permittivity and complex permeability over a given frequency range. The first set of reflection coefficients is obtained by directly placing the sensor alongside the coating material and performing broadband measurement using frequency sweep over the selected frequency points of frequency range. The same procedure is applied on a combination of known material followed by the test material to measure the second set of the respective reflection coefficients. Finite Difference Time Domain (FDTD) method was employed to numerically calculate the sensor reflection coefficient and analyse the influence of the sensor geometry and coating material thickness on measurement accuracy. The measurement results of both complex permittivity and permeability for various samples of microwave absorbing coatings were agreed well with reference data. Simple interpolation approximation was introduced in their extraction process from multiple reflection coefficient data since they are frequency dependent parameters . The proposed technique speeds up and simplifies coating materials testing, while it decreases the error due to repeatability of measurement.