Compact UHF RFID MIMO Tag Antenna for Platform Tolerant Applications

A compact UHF RFID MIMO tag antenna is proposed for platform tolerant applications. The antenna structure is made up of two bent-loop feeding networks between two substrates, a square patch and a folded radiator. Due to the folded structure and 3D cross slot, the antenna is reduced to a volume of 35 mm×35 mm×6 mm (0.1λ×0.1λ×0.02λ). In order to enhance the isolation between two ports, the feeding networks placed orthogonally are introduced to effectively excite the orthogonal modes of the outer radiator. With proximity coupling feed besides the loop matching network, the conjugate matching between the antenna and the chip is realized. Simulation results show that the -3 dB impedance bandwidth of the proposed antenna in free space, mounted on water and mounted on metal are 96 MHz (902-998 MHz), 93 MHz (900-993 MHz) and 92 MHz (905-997 MHz) respectively. It means the band of 902-928 MHz can be covered by the tag antenna in various platforms. The isolation is greater than 30 dB in three platforms and two ports have mutually orthogonal radiation characteristics, satisfying the polarization diversity of the MIMO antenna.