A tuning transformer for the automatic adjustment of resonant loop antennas in RFID systems

Radio frequency identification (RFID) systems allow contactless communication between a small portable tag and a reader device. In this paper, we address the problem of performance degradations due to the detuning of resonant reader antennas in the widely used 13.56 MHz frequency band. A novel design of a tuning transformer for the adjustment of the center frequency of resonant loop antennas is proposed. The ferrite core of the transformer is premagnetized using an additional DC winding. The HF and DC windings are decoupled by an iron core. Besides the tuning, the transformer may also be used for impedance matching and for balancing an unbalanced signal. The function of the device was proven by network measurements. Key Words: Loop antenna, resonance tuning, RFID, tunable inductance.