Circularly Polarized RFID Tag Antenna Design for Metallic Poles Using Characteristic Mode Analysis

This letter presents a novel circularly polarized ultra-high frequency radio frequency identification (902-928 MHz) tag antenna design for metallic poles using characteristic mode analysis (CMA). The traditional tags are usually mounted on the surface of the pole, which is a vulnerable position for tag mounting since it can be easily broken or anyone can easily remove the tag from the pole surface, whereas the proposed tag is placed on the chassis of the metallic pole with the metallic pole cut through the center of the tag. According to the CMA, the conical radiating mode will not be so effective in our application because it will provide the main beam in an upward direction, which made this tag difficult to read from the side direction. Therefore, the proposed tag exploits the broadside radiating mode as the optimal radiation mode to meet the application requirements such as main beam direction at 45 degrees to provide long read range. Moreover, a Wilkinson power divider is used to provide equal power with a 90 degrees phase difference to form the circularly polarized radiation. Furthermore, the proposed tag provides a read range of more than 7 m with and without the metallic pole in the center of the tag. The simulated and measured results show that the proposed tag design is a suitable candidate for tagging metallic poles such as traffic signal, guide boards, and street light poles, which is vital for the Internet-of-Things applications.