Toward a low-cost portable reader for reflective-mode microwave sensors

Microwave sensors are typically characterized using a vector network analyzer (VNA) that measures the scattering parameters of the device under test (DUT) over a wide frequency band with great accuracy. However, these features of the VNA are not always required, and its bulky form factor and high cost can even be a limitation in many practical scenarios, such as microwave sensors, which are generally characterized by low cost, a small footprint, and often planar profiles. This article presents a novel, low-cost portable reader for reflective-mode microwave sensors. The proposed device is based on a hybrid 180° directional coupler that separates the input signal from the reflected one, a voltage-controlled oscillator (VCO) to generate the input signal, and a gain/phase detector board. The performance of the proposed system has been experimentally assessed for reference load cases, showing a good match between the simulated and measured results. The capability and usefulness of the proposed device are demonstrated through the implementation of a reflective-mode distance sensor where the input impedance of the one-port sensing element is the output variable of interest. The results obtained with the proposed reader well match the VNA measurements, with a mean error of about 8%. It is worth noting that the presented solution can be easily adopted as the measurement device by other reflective-mode sensors operating in the frequency band of the coupler. Overall, the proposed novel reader offers a new approach to measuring the output variable of one-port reflective-mode microwave sensors that is battery-powered, portable, versatile, accurate, and cost-effective compared with using a VNA.