Inkjet Printed UHF RFID Humidity Sensor for Metallic Surface Assets

In this paper, we propose a low-cost, label-type, ultrahigh frequency (UHF) radio-frequency identification (RFID) antenna for tagging metals and humidity sensing on metallic surfaces. The antenna consists of a feeding loop integrated with two orthogonal slots. This feed configuration provides an impedance match both on metal and air by creating a resonant current distribution due to the transverse slots technique. As a result, the tag exhibits a good omnidirectional radiation pattern in both air and above a metal plate. Additionally, this tag antenna was fabricated on a 0.74 mm thick paper substrate, which enables it to function as a humidity sensor. This design feature contributes to a read range of 2.5 m above a 200 × 200 mm metal plate in a dry state. Notably, the read range reduces to 1 m in a wet state. Therefore, this tag presents a low-cost solution for both tagging and humidity sensing on metallic surfaces, offering increased versatility and functionality for asset management in various industrial and infrastructure applications.