Abstract:

The importance of Radio Frequency Identification Systems (RFID Systems) is rapidly growing due to their benefits like cost reduction and simplicity. RFID Systems working at 13.56 MHz consist in principle of two components, the Reader and the Tag which are magnetically coupled to each other. The Reader generates a strong magnetic field at the frequency of 13.56 MHz. If a Tag approaches the reading area of the Reader, the magnetic field penetrates the Tag antenna and induces a voltage which is then rectified and used as supply voltage for the integrated circuit inside the Tag. The power supply of the Tag as well as the communication between Reader and Tag is performed by using magnetic coupling by means of coil antennas. It is obvious that the quality of the power transfer has a great influence on the system performance. For a maximum reading range an additional capacitor is used for tuning both coil antennas to a resonance frequency of nearly 13.56 MHz. Due to the sharp resonance curve of the circuit engineers need high accuracy modelling tools for the design of coil antennas.

In this paper the development of formulas for the precise design of coil antennas is described. First an equivalent circuit is presented which fits the measured impedances very well. The development of the formulas is based on analytical calculations, field simulations and reference measurements. The error of these formulas, which take the skin and the proximity-effect into account, is less than 5 percent for a big range of geometry parameters and a broad frequency range.

 

Dr.-Ing. Bernd Geck

Institut für Hochfrequenztechnik und Funksysteme

Appelstraße 9A

30167 Hannover

Tel +49 511 762 5163 Fax +49 511 762 4010

geck@hft.uni-hannover.de