RFID Antennas – Possibilities and Limitations

Abstract

Marking items for remote identification not only places high demands on the heart of the RFID tags, the Application Specific Circuit (ASIC), but also on the tag antennas. For long reading distances, efficient antennas are crucial and their efficiency is directly proportional to the maximum reading distance of both semi-active and passive tags. This chapter is an in-depth investigation of the requirements for the antenna part of UHF RFID tags, with focus on antenna design, characterization and optimization from the perspectives of both costs involved and technical constraints. The main focus is devoted to antennas that could be manufactured using more or less standard manufacturing techniques available in the packaging industry. The chapter also presents some new ideas on how to utilize the antenna structure itself as a sensor for measuring different physical properties during the logistic chain. The chapter starts by describing the most general requirements for a functional UHF antenna, explaining why designs for RFID tag antennas cannot be taken directly from traditional antennas designed for other applications since RFID chips input impedances differ significantly from traditional input impedances such as 50 Ω and 75 Ω. Ordinary antennas, not made for RFID, are usually also designed to work at a particular location or at locations where the surroundings of the antenna are well known. This is not generally the case for RFID applications, where the antenna is largest part of the tag and where the near field region of the antenna will be defined by the object and material that the RFID tag is attached to. For cost and practical reasons, tag antennas should be twodimensional and if they are applied to paper or soft plastics they should be non-sensitive to bending, particularly as they might be placed over an edge of an object. An antenna’s resonant length is also directly dependent on its surrounding medium. If the antenna is designed to be placed in free space it will change its properties if placed next to a dielectric medium, even if this medium is electrically lossless. There is thus a demand for some kind of general tag antennas and guidelines for designing and optimizing tag antennas when their final location is known in advance. The type of antenna properties and designs to be avoided are similarly given as well as which types should be used when, for example, putting antennas onto flexible, dielectric and metallic materials. Specific solutions presented also include how to enhance the tags’ commercial value by incorporating insignias such as company logos in the antenna design. Antennas used in traditional RFID tags are mainly made out of copper or aluminum. In order to keep the antenna prices to a minimum and facilitate very large production it is desirable to fabricate the tag antennas using commercially available web shaping techniques