Measurements of the permittivity and permeability of thin radar absorber materials in waveguides

The reflection-transmission (RT) method offers many advantages for measuring the properties of various types of materials. However, for very thin and lossy sheets, there are some practical problems. To overcome some of these problems, we proposed the use of a two-layer structure for the measurement of radar absorber materials (see Williams, T. et al., IEEE Antennas and Propagation Int. Symp. and USNC/URSI Meeting, 2001). To overcome the problem of test sample placement in the waveguide, the sample is deposited on a block of acrylic whose dimensions fit tightly into the waveguide. The acrylic block not only maintains the sample in place, but its length can be optimized to limit the uncertainty of measurements. We now present an analysis of the two-layer method, experimental results and numerical analysis of errors associated with imperfections in the deposition of thin layers (thickness, air gaps). The two-layer measurements are viable and relatively accurate. Comprehensive uncertainty analyses allow for optimal selection of the thickness (and properties) of the material supporting the test sample, evaluation of the effect of imperfections in the test sample dimensions, and a quick check of the correctness of the permittivity and permeability measured (thus ensuring that the results converged properly).