Realization of a Low-Cost Displacement Sensor on PCB With Two-Metal-Layer Coplanar Waveguide Loaded by an EBG Structure

There are some difficulties in the realization of a microwave displacement sensor with conventional CPW loaded by electromagnetic bandgap structure on RF laminates with a regular printed circuit board manufacturing process. These difficulties have made this process impossible in practice. In this paper, for the first time, this kind of sensor has been implemented using a two-metal-layer (TML) CPW as a transmission line to overcome these difficulties. The proposed sensor consists of a TML CPW line loaded by two periodic-air-hole structures at both sides of the transmission line. Sliding the periodic structure at one side of the transmission line makes a change in the maximum insertion loss at the fundamental central frequency of the bandgap proportional to the length of the displacement. Analysis, finite-element method simulation, and fabrication have been carried out, and an average sensitivity of about 9 dB/mm and a dynamic range of 3 mm have been achieved for the 21 unit cells sensor with a period of 6 mm at 14.24 GHz. Furthermore, some practical issues, such as influence of the air gap between the metal traces and the movable structured dielectric on the sensor behavior and robustness of the sensor against the temperature variations, are practically investigated.