Robust antenna concept for RFID tag applications

The paper proposes an approach for immunizing the input impedance of a tag-antenna against disturbances by the proximity effect of external objects. The idea is to drive two symmetric closely-spaced antennas with out-of-phase excitations, resulting in a differential input impedance. When an object is placed in the close proximity of the antenna system, the coupling with the two symmetric antennas will be comparable over a wide range of angles around the array broadside. In such a coupling scenario the differential input impedance is maintained compared to its free-space value.