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Analysis of an open-ended coaxial probe with lift-off for nondestructive testing

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4 Author(s)
J. Baker-Jarvis ; Div. of Electromagn. Fields, Nat. Inst. of Stand. & Technol., Boulder, CO, USA ; M. D. Janezic ; P. D. Domich ; R. G. Geyer

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 out: (1) the sample extends to ∞ 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 that the probe is very sensitive to lift-off. For medium to high permittivity values and electrically small probes, gaps on the order of 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 be used. A comparison of the theory to experiment is presented. The results are in close agreement. A differential uncertainty analysis is also included

Published in:

IEEE Transactions on Instrumentation and Measurement  (Volume:43 ,  Issue: 5 )