The Closed-Form Solution of Frequency Shift for an HF RFID Coil Antenna in Metallic Environments

The performance of a high frequency radio frequency identification (RFID) coil antenna is strongly dependent on its operating environments. The presence of the metallic environments can shift up the resonant frequency of the antenna and change its field intensity. This paper presents a novel equivalent model based on image theory to study quantitatively the proximity effects of metallic environments on the antenna. From the proposed model an equivalent circuit has been extracted, and it is used to derive the frequency shift formulas of the antenna based on the theory of magnetic resonant coupling. Then the closed-form solution of operating frequency for the antenna in metallic environments can be obtained in terms of the geometric parameters of coil antenna as well as the distance between the coil antenna and the metal plate using the proposed formulas. To confirm the validity of the proposed model, simulations have been carried out and compared with the measurement results. The results show that the proposed model can predict the operating frequency of the antenna in metallic environments and the distribution of magnetic field accurately and rapidly. The closed-form solution of frequency shift derived from this paper can be helpful to engineers to enhance the RFID antennas performance by alleviating the effects of metallic environments, which is pivotal for the Internet of Things technology.