A Small All-Corners-Truncated Circularly Polarized Microstrip Patch Antenna on Textile Substrate for Wearable Passive UHF RFID Tags

We present a wearable passive UHF RFID tag based on a circularly polarized (CP) patch antenna on a textile substrate. The antenna miniaturization is based on applying a combination of the cross- and L-shaped slots in the radiator. In conjunction, the right-hand circular polarization is achieved by asymmetrically truncating all four corners of the square-shaped radiator. Despite using a regular low-permittivity textile as the antenna substrate, we downsized the antenna to a 5 cm × 5 cm footprint with the thickness of 4 mm, which is equal to 0.1525 λ × 0.1525 λ × 0.0091 λ, where λ is the free space wavelength at 915 MHz. In the numerical modeling and optimization of the antenna, we used a simplified cuboid-shaped and anatomical human body models. In addition to simulated conventional antenna performance indicators, we introduce spatial coverage as a new parameter for assessing the detection reliability of UHF RFID tags. Finally, we measured a manufactured tag worn in four different configurations on the body. The measured axial ratio value was approximately 2 dB in all cases and the tag provided a high attainable read range of around 5.8 meters for a right-hand CP reader emitting 3.28 W EIRP.