Open-Ended-Line Reflective-Mode Phase-Variation Sensors for Dielectric Constant Measurements

This paper reports a detailed analysis of reflective-mode phase-variation sensors based on open-ended microstrip lines. These sensors are useful for measuring dielectric constants or other variables related to it (e.g., material or liquid composition). For that purpose, the so-called material under test (MUT) should be placed on top of the open-ended line, the sensing region. A change in the dielectric constant of the MUT modifies the electrical length and the characteristic impedance of the sensing line, thereby varying the phase of the reflection coefficient, the output variable. The analysis provides the optimum conditions for sensitivity optimization. It is concluded that either high-impedance 90° or low-impedance 180° sensing lines are needed in order to obtain a strong dependence of the phase of the reflection coefficient with the dielectric constant of the MUT. Such conclusions are validated by electromagnetic simulations and experiments.