RFID Tag Antennas Mountable on Metallic Platforms

Abstract

Auto identification provides information without direct contacts and human intervention errors. Auto identification technology has become very popular in industries, such as the service industry, inventory control, distribution logistics, security systems, transportation and manufacturing process control. So far, the bar code technology leads the auto identification industry, but it has several limitations such as low storage capacity, required line-of-sight contact with the reader, and physical positioning of the scanned objects. Recently, the radio frequency identification (RFID) has been an attractive alternative identification technology to the barcode. The numerous potential applications of the RFID system make ubiquitous identification possible at frequency bands of 125 KHz (LF), 13.56 MHz (HF), and 860-960 MHz (UHF). The RFID system generally consists of two basic components: the reader and the tag, which communicate with each other by electromagnetic waves. The reader can be a read or a read/write device that uses an antenna to send an electromagnetic wave to wake up the tags. The tag is the data carrying device located on the object being identified. In general, the performance of the tag seriously affects the performance of the whole RFID system. The tag consists of the tag antenna and the microchip. Since good connection and power transmission between the tag antenna and the microchip directly impact on the RFID system performance, the tag antenna has to be designed considering its operating environments or platforms. As the use of RFID systems increases, manufacturers are pushing toward higher operating frequencies (UHF band) for long reading range, high reading speed, capable multiple accesses, anti-collision, and small antenna size compared to the LF or HF band RFID system. As the operating frequency of the RFID system becomes higher, the major part of the RFID system that mostly affects the ability to read the tag is the antenna. There are several possible antenna types which can be used for RFID tags in this frequency band. The dipole types of antennas such as folded dipoles and meandered dipoles are used in many applications since they can be printed on a very thin film. However, when they are mounted on the metallic objects, the antenna performance is seriously decreased because of the reactance variation on the antenna impedance. Particularly, the UHF band RFID system is a passive system where a tag does not contain its own power source. Therefore, the reader