Analysis of an open-ended coaxial line sensor in layered dielectrics

The capacitance of an open-ended coaxial line immersed in a bilayered medium is evaluated both numerically and experimentally. Water layers of varying thickness backed by air and metal walls were assumed, and a quasi-static solution for the fringe-field capacitance was obtained using the method of moments. The numerical results compare well with experimental values obtained using an automated network analyzer. The two configurations, namely the metal and air wall, represent the upper and lower bound of the capacitance change from the homogeneous case, respectively. For a typical 6.4-mm (0.25-in)-diam sensor, the change in the sensor capacitance as compared with a homogeneous case is less than 1% for layers thicker than 6 mm. The results provide information on the response of the sensor when used to measure the properties of layered dielectrics.