Microwave Linear Displacement Sensor With Notched Lowpass Filtering Response

This work presents a novel microwave linear displacement sensor that integrates a notched lowpass filtering response within a single structure. The design first employs a spoof surface plasmon polariton (SSPP) structure along a 50-ohm microstrip line to achieve a lowpass filtering passband. A square ring resonator (SRR) is then coupled underneath to introduce a sharp notch within this passband. The key innovation lies in utilizing the SRR as a movable sensing element. As the SRR translates along the transmission line, its distance to the input port changes, resulting in a highly linear and sensitive shift in the input reflection phase (∠S11). The proposed sensor demonstrates a high sensitivity of 13.4°/mm over a substantial displacement range of 80 mm. This work successfully merges filtering functionality with high-precision displacement sensing, offering a compact and promising solution for integrated RF sensing systems.