Sensitivity analysis of microwave non destructive diagnostic procedures based on the use of capacitive sensors

This paper deals with the use of capacitive sensors in Non Destructive Techniques (NDT) at microwave frequencies, with the aim of analyzing the measuring conditions which allow to improve the detection sensitivity. Particular reference was made to the case where the capacitive sensor is used in connection with a Vectorial Network Analyzer (VNA). Detection sensitivity was first theoretically studied with reference to the following measured variables: input impedance, its real and imaginary part, absolute value and phase of the complex reflection coefficient. The influence of the measuring reference plane for such variable was discussed and the choice of its position was proved to be of great importance in order to maximize the measuring sensitivity. The experimental results confirmed the criteria derived from the theorical analysis to improve the measuring procedure.