A Dual-Environment Active RFID Tag Antenna Mountable on Metallic Objects

A dual-environment active RFID tag antenna is proposed in this letter. A microstrip line fed at the middle portion is used to excite two operation modes under two different working environments. In free-space condition, the slot mode of the proposed antenna shows a 50-Ω load at the input port, while the patch mode shows a high impedance close to an open circuit. Therefore, only the slot contributes to radiation. In opposite, when the antenna is mounted on a conductive surface, the slot mode shows a high impedance at the input port, whereas the patch mode is matched and therefore radiates. A prototype of the proposed antenna is fabricated and tested. The measured impedance bandwidths are respectively 22.13% from 2.09 to 2.61 GHz and 4.17% from 2.35 to 2.45 GHz when the proposed antenna is respectively applied in free space and on a metallic surface. The measured peak gains and radiation efficiencies at 2.4 GHz in these two conditions are 4.13 dB and 93.2%, 9.21 dB and 95%, respectively. Good agreement between measurement and simulation demonstrates the feasibility of the dual-environment property.