Error estimate for free-space dielectric measurement systems

In a free-space measurement system, the electrical constitutive parameters of a plane sheet of material are determined using incident, transmitted, and reflected beams of electromagnetic energy. The electrical parameters are almost always obtained from the measured transmission and reflection coefficients using a procedure that approximates each beam as a single plane wave whose wavefront is parallel to the surfaces of the sheet (the planewave approximation). A theoretical model is constructed to investigate, in a very general way, the limitations of the plane-wave approximation. Here, we only consider a procedure based on the power transmission coefficient; when the power reflection coefficient is used the results and conclusions are similar.