Improving the Efficiency of NFC Systems Through Optimizing the Sintered Ferrite Sheet Thickness Selection

The reduction of embedded portable devices involves a magnetic field interference problem when it integrates near field communication (NFC) due to the presence of conductive surfaces, such as ground planes, batteries, or metallic enclosures. Flexible sintered ferrite sheets (FSFS) represent an interesting shielding solution to prevent electromagnetic interferences problems related to NFC, thanks to their ability to control the magnetic flux. The characterization of FSFS effectiveness is analyzed as a function of the sheet thickness in this contribution. This is performed with the aim of determining which is the optimum thickness value to retune an NFC antenna to its original operation frequency value (13.56 MHz) when it is affected by a near conductive surface. A finite element method simulation model is designed and corroborated with the experimental results to evaluate the performance of different FSFS thicknesses in terms of resonant frequency shift, magnetic field strength, and communication distance. An electromagnetic scanner is also used to determine the ability of the best FSFS thickness to shield an NFC transmitter antenna when a battery is set under it, emulating a usual problem. The results obtained show that the magnetic field strength is significantly attenuated and the communication distance is highly shorted. Therefore, besides selecting a material that provides high reflection and low losses at 13.56 MHz, thickness must be considered to ensure the greatest communication effectiveness. Consequently, the use of a wrong FSFS thickness could lead to shifting the resonance frequency to lower values than expected, detuning the communication.