Compact Integrated Antenna With Circulator for UHF RFID System

A passive UHF radio frequency identification (RFID) reader antenna with high transmit/receive isolation, which consists of two quadrifilar spiral antennas (QSAs) and the feeding networks, is proposed. The high isolation can be achieved by two concentric, circularly polarized antennas and 4-point feed circuits, which make the same circular polarization. The simulated and experimental results show that the transmit/receive isolation is more than 40 dB and the gains of two QSAs are 1.0 dBi for the inner QSA and 0.7 dBi for the outer QSA at the 910 MHz, respectively.

I. Introduction

Radio FREQUENCY IDENTIFICATION (RFID) systems have recently attracted much attention due to their potential capability in supply chain management and retail item management. In the passive RFID system, the reader transmits a continuous-wave (CW) RF signal to tags and receives the backscattered signal from tags, simultaneously. The transmitted CW signal may be directly coupled to the receiving part of the reader to degrade the receiving sensitivity. Therefore, the high isolation between the transmitting and receiving parts is very critical for the RFID performance [1].