A Method to Retrieve the Output Variables in Reflective-Mode Phase-Variation Sensors

In this paper, a method to retrieve the relevant information, i.e., the phase and magnitude of the reflection coefficient, in reflective-mode phase-variation sensors is reported. By using a pair of rat-race hybrid couplers conveniently connected, and with the ports appropriately loaded, including the one-port sensing load, the reflection coefficient of such load can be expressed in terms of the ratio between the output voltage waves at two of the ports of the coupler pair. By this means, vector network analysers (VNA), the typical (high-cost) instruments to gather the output variables at prototype level, can be replaced with low-cost voltage controlled oscillators (VCOs) and phase/amplitude detectors, more realistic in a real (operative) scenario. This method is applied in this work to obtain the output variable in a simple reflective-mode phase-variation permittivity sensor based on a 90° open-ended line. The results point out the potential of the approach.