An open-ended co-axial line has been used as an electromagnetic sensor or probe for various industrial and scientific applications. These applications are based on the principle that the characteristics of the echo signal produced by the co-axial opening depend upon the sample material terminating the probe. Thus, if the aperture admittance (or reflection coefficient) characteristics can be precisely formulated, then the electrical parameters of the sample can be found. Several admittance models have been reported during the last two decades. A comparative study of these models is reported in this review. Furthermore, the inverse process of calculating the complex permittivity of materials is summarized.
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