Antenna Design for UHF RFID Tags-Review and a Practical Application
an overview of antenna design for passive radio frequency identification (RFID) tags is presented. We discuss various requirements of such designs, outline a generic design process including range measurement techniques and concentrate on one practical application: RFID tag for box tracking in warehouses. A loaded meander antenna design for this application is described and its various practical aspects such as sensitivity to fabrication process and box content are analyzed. Modeling and simulation results are also presented which are in good agreement with measurement data.
RADIO FREQUENCY identification (RFID) is a rapidly developing technology which uses RF signals for automatic identification of objects. Although the first paper on modulated backscatter (basic principle of passive RFID) was published in 1948 it took considerable amount of time before the technology advanced to current level . Now RFID finds many applications in various areas such as electronic toll collection, asset identification, retail item management, access control, animal tracking, and vehicle security . Several standards of RFID systems are currently in use (ISO, Class 0, Class 1, and Gen 2).
Globally, each country has its own frequency allocation for RFID. For example, RFID UHF bands are: 866–869 MHz in Europe, 902–928 MHz in North and South America, and 950–956 MHz in Japan and some Asian countries. A typical passive RFID transponder often called “tag” consists of an antenna and an application specific integrated circuit (ASIC) chip. RFID tags can be active (with batteries) or passive (batteryless). A passive back-scattered RFID system operates in the following way. A base station (reader) transmits a modulated signal with periods of unmodulated carrier, which is received by the tag antenna. The RF voltage developed on antenna terminals during unmodulated period is converted to dc. This voltage powers up the chip, which sends back the information by varying its front end complex RF input impedance. The impedance typically toggles between two different states, between conjugate match and some other impedance, effectively modulating the back-scattered signal. Fig. 1 illustrates a passive RFID system operation. Proper impedance match between the antenna and the chip is of paramount importance in RFID. Since new IC design and manufacturing is a big and costly venture, RFID tag antennas are designed for a specific ASIC available in the market. Adding an external matching network with lumped elements is usually prohibitive in RFID tags due to cost and fabrication issues. To overcome this situation, antenna can be directly matched to the ASIC which has complex impedance varying with the frequency and the input power applied to the chip. Several papers have been published on RFID antennas for both passive and active tags, including covered slot antenna design , circular patch antenna analysis , meander antenna optimization , planar inverted F-antenna, folded dipole antenna , etc. However, very few papers provided an overview of criteria for RFID tag antenna design and an analysis of practical application aspects. At the same time, there exist many papers on practical analysis and design of particular classes of antennas used for other applications
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