ANALYSIS OF AN OPEN-ENDED COAXIAL PROBE WITH LIFT-OFF FOR NONDESTRUCTIVE TESTING

The open-ended coaxial probe with lift-off is studied using a full-wave analysis, and an uncertainty analysis is presented. The field equations for the following terminations are worked gut: (1) the sample extends to oo in the positive axial direction, (2) the sample is backed by a well-characterized material, and (3) the sample is backed by a short-circuit termination. The equations are valid for both dielectric and magnetic materials. The model allows the study of the open-ended coaxial probe as a nondestructive testing tool. The analysis allows a study of the effects of air gaps on probe measurements. The reflection coefficient and phase are studied as a function of lift-off, coaxial line size, permittivity, permeability, and frequency. Numerical results indicate the probe is very sensitive to lift-off, For medium to high permittivity values and electrically small probes, gaps on the order df fractions of a millimeter strongly influence the reflection coefficient. In order for the field to penetrate through the air gap, larger size coaxial line or higher frequencies need to he used. A comparison of the theory to experiment is presented. The results are in close agreement. A differential uncertainty analysis is also included.