A Study On Finite Flange Open-ended Coaxial Probe And Its Application To Electromagnetic Properties Characterization Of Curved Surfaces High Loss Coating Materials

The open-ended coaxial probe technique has been extensively studied due to its potential for materials characterization. Since the initial investigation in early 1980s, most of the research work done with respect to open-ended coaxial probe technique was, however, limited to measure complex permittivity of dielectric materials with planar surface. For testing materials with curved surfaces, using of open-ended coaxial probe remains to be uncovered after a long period of time debating in the reach society. The problem lies in the air gaps between the probe with its finite dimension of the flange and the material under test by which the flange size could lead much uncertainty to the measured data. Therefore, analysis the influences of these curvature surfaces on measurement accuracy and develop a new configuration of coaxial probe are necessary since the characterization of materials with curved surfaces such as microwave absorbing coatings on metallic structures gave received considerable interest in practice.In this thesis, utilization and development of finite flange open-ended coaxial probe for electromagnetic properties (both complex permittivity and permeability) are studied. The finite-difference time-domain (FDTD) technique, that is very applicable to the boundary problem, is employed for modeling the probe loaded with the materials under test. The influence of probe flange diameter on measurement accuracy of materials with planar surface is first analyzed. The results have shown that a flange size varies, in general, with the EM-parameters of the sample under test, sample thickness and the operating frequency.We found that a flange dimension of (D/2-b) larger than (1-1.5) m can be considered for the practical purpose to be an infinite.Then we focus on curved surface materialsmeasurement technique. The first step is related to an uncertainty analysis to specify the errors created due to the gaps between the standard open-ended coaxial probe and samples of concave surface is more serious on the measurement than that between the probe and samples of convex surface. Therefore, use of standard coaxial probe for concave surface materials measurement is highly not suitable even for large radii, while for convex surface materials there is no serious problem except for small radii.The second aspect is related to a new configuration of open-ended coaxial probe proposed for concave surface materials testing. The inner conductor or the standard coaxial probe is extended throughout the air gap between the probe and surface of material under test. A ring patch is embedded at the terminal of the conductor in contact with the material under test. The performance of the modified coaxial probe is studied as a function of flange diameter, sample radii and patch dimensions. The FDTD modeling and experimental results indicate that a patch with appropriate dimensions plays an important rule on improving reflection coefficient measurement accuracy. The modified coaxial probe has been used to measure electromagnetic properties of several microwave-absorbing materials coated on concave surface prototype boxes of different radii. The deduced * and * values are relatively in good agreement with the published data. It has been demonstrated that the technique is suitable for nondestrutive testing of curved surfaces coating materials and other cases where it would be difficult to fit the material under test with standard coaxial probe.