Miniaturized Tag Antennas with Artiflcial Magnetic Conductor for UHF RFID On-body Applications

The paper presented a compact dipole tag antenna with a miniaturized artiflcial magnetic conductor for UHF RFID on-body applications. Two kinds of artiflcial magnetic con- ductor are designed and studied in order to insulate the in∞uence from the human body. The miniaturized AMC structure is then placed on the button of a folded dipole tag antenna con- nected with a Monaz ® 4 microchip. The tag with the AMC is designed and simulated by the simulator HFSS. The design structure is also constructed to verify the simulation results. The study flnds that the tag with the AMC can increase antenna gain and reading range. When the tag is put close to a human body, the foldeddipole tag can achieve about 6m reading distance. 1. INTRODUCTION UHF RFID technique is popular in the logistics, inventory management or bioengineering. A UHF tag does not work well when it is put close to a human body or held in hand. The input impedance, radiation pattern, and realized gain of the tag antenna are in∞uenced because of the body-proximity efiects. The in∞uences are caused by absorption, difiraction, and scattering of electromagnetic waves due to the lossy and conducting human body. The body-proximity efiects must be considered when a tag is held or put on the body. Some literatures have studied the efiect of the human body on the designed tags (1,2). Recently, a few scholars proposed artiflcial magnetic conductors (AMCs) to insulate the efiect of the human body for wearable device or RFID applications (3,4). Ta etal. proposed a two- dimensional square metal patch as the unit cell of the artiflcial magnetic conductor (AMC) (5). Then, four T-type slots are symmetrically inserted into each side of the metal patch to miniaturize the AMC size and lower the flrst resonant frequency. The bandwidth is limited although it is a low proflle AMC structure. Recently, Hadarig etal. presented a kind of AMC using capacitive interdigital structure. The AMC structure exhibits proper bandwidth and higher angular stability under oblique incidence (6). Using an artiflcial magnetic conductor (AMC) in the backplane of the tag actually helps to reduce the in∞uence of a human body and increase reading range. However, the tag with an AMC is still large and bulky for on-body applications. In this presentation, two kinds of AMC structure were studied for on-body applications, one is a square metal patch with T-type slots structure (5) and the other is four rectangular metal pads with interdigital capacitive structure (6). Considering the minimum operating bandwidth and antenna sizes, we compared the performance between the two AMC structures after simulation and chose the minimum-size AMC structure. Then a miniaturized dipole tag antenna was designed and placed on the above of the AMC structure to insulate the in∞uence from the human body. The designed tag on the AMC substrate is then placed close to a four-layer stratifled elliptical cylinder human model to study the performance of the designed tags. In Section 2, we designed and compared two AMC structures. Section 3 is devoted to designing a folded-dipole tag antenna with an AMC. In Section 4, the tag with the miniaturized AMC is pasted on a human body. The gain and reading range of the folded-dipole with and without AMC are compared. Finally, Section 5 draws some conclusions.