Silver Ink Patch Antenna for Passive RFID

Metallic objects strongly degrade the performance of a conventional radio frequency identification (RFID) tag antenna by affecting its impedance and radiation pattern. When a general dipole-type antenna is mounted near a metallic object, current will be induced on both the antenna surface and the metallic surface due to the RFID reader’s radiation. The metallic surface acts as an image part of the original antenna, which has a negative influence on the tag antenna’s scattered field and makes the tag unreadable for normal ranges [1]. In order to enable an RFID tag to work near metallic objects, special attention must be paid to the structural design of the antenna. In general, there are two approaches to design an RFID tag antenna for metallic objects. One way is to utilize the metallic object itself in order to construct the antenna, such as in [2], where an RFID tag is formed by cutting a slot into metallic foil. The other approach is to design a general tag antenna which is able to be mounted on any metallic surface. The microstrip antenna (MSA) possesses this unique advantage. Several proposed RFID tag antennas for metallic objects, such as planar inverted-F antenna (PIFA), U slot inverted-F tag antenna [3-4] are examples of this type. The MSA has a low profile planar configuration and can be made to be conformal to the host surface. With a conducting ground plane, a microstrip patch antenna can be designed to operate in the vicinity of metal. These advantages make them preferable for numerous military applications and with compact design they are also used in personal mobile communication. As an electronic identification technology to compete with rock-bottom pricing barcodes, the cost involved is one of the main issues holding back the widespread adoption of RFID. A traditional MSA has generally been constructed using good conducting materials, such as copper or aluminium and low-loss substrate in order to achieve relatively high radiation efficiency, but the high cost involved has proved to be a significant limitation to its application in the RFID field. In order to reduce the cost of an MSA, some low price material, such as foam, has been used as the substrate [5]. However, no cheaper alternative solution has been proposed for the conducting parts. Recently the concept of printing antennas utilizing commercial printing techniques has received a great deal of attention due to the requirement for low cost, high volume RFID tags. In the printing technique, the ink’s colour pigment is replaced by small silver particles, making the printed inktraces electrically conductive and the use of this technique can significantly reduce the cost of the tag antenna. Although the printed trace is not as conductive as copper, conventionally simple RFID tag antennas printed using silver based ink have proved to be very effective [6]. Unlike a conventional dipole-type antenna, the MSA is a structure with two conducting layers and thus is more sensitive to the ohmic loss. In this paper, the performance of silver ink printed microstrip patch antenna for passive RFID application is investigated. Two compact patch structures working at 869MHz are presented where the conducting parts of the patch antenna are printed with silver ink utilizing the flexographic printing machine and the cardboard is adopted as the substrate. The performance parameters, such as impedance and the read range of these two printed patch antennas are measured with no material behind the structures and when backed up by a finite metallic plate. Their performance is evaluated by means of comparisons with the same antenna structures made of copper. Proceedings of ISAP2007, Niigata, Japan