AN78010 - NXP.com

Philips Semiconductors Rev. 1.0 November 200213.56 MHz RFID Proximity Antennas mifare ® (14443A)Now, as shown in Figure 13 a ferrite plane (µ R =40) is positioned in between the metal plane and the antennacoil itself. The field strength very near to the ferrite increases, but this increasing of the magnitude is notcombined with an increasing of the operating distance. This is marked once again with the H MIN valueaccording to ISO 14443.dField strengthcolor map7.5 cmminimum field strengthaccording to ISO 14443Hmin=1.5 A/mferrite planemetal plane6 420Figure 13: Ferrite shielded field distribution of a circular antenneThese simulations show how the use of ferrite reduces the generated eddy currents in a metal plane. Theferrite generates an additional field component and the effect for the design of the antenna is a fixeddetuning of the antenna itself.Figure 14 gives recommendations how to dimension the ferrite to find the optimum dimensions betweenferrite plane and metal plane. To calculate the optimum dimensions of the ferrite plane and the optimumdistance and overlapping is very hard and not recommended. Application specific tests have to be made tofind the best ferrite dimensions.18 PUBLIC