Compact printed monopole tag antennas for UHF RFID applications

Abstract

Due to the rapid growth and widespread usage of radio frequency identification (RFID) technology, design and characterization of RFID tags have become the focus of many research and development efforts. The RFID tag, which is affixed to the objects to be identified, is composed of an RFID chip and an antenna. The tag antenna plays an important role in the overall RFID system performance, as it directly impacts the size, cost, and detection range. Dipole antennas are commonly used in UHF RFID applications, because of their simplicity and omnidirectional radiation characteristics [1]-[3]. However, they are too big at UHF frequencies to be attached to small objects, thus, compact tag antenna designs need to be investigated. Another important consideration for RFID tag antenna design is matching. The chips that are used in RFID tags often have a highly capacitive input impedance. For maximum power transfer between the antenna and the chip, a conjugate impedance match should exist. To implement miniaturized low-cost RFID tags, it is not feasible to use external matching networks that contain lumped components. Therefore, the matching mechanism has to be integrated in the tag antenna layout. Implementation of an inductive coupling via a loop feed, is one of the direct matching techniques that is used for electrically small antennas [3]-[5]. The design of the tag antenna and the matching network becomes more challenging when the real part of the chips input impedance is very small.