Characterization of embroidered dipole-type RFID tag antennas
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Radio Frequency Identification (RFID) is a technology which is used for automatic identification of objects. A typical RFID system consists of a stationary radio-scanner unit, called reader, and a movable transponder, called tag, which is attached to an object. The tags include an antenna and a microchip with internal read/write memory. Tag antenna plays an important role in the overall RFID system performance factors, such as the read range, and the compatibility with tagged objects. This thesis focuses on garment-integrated embroidered tags which can be used for the means of human monitoring and identification. The embroidered tag antennas are sewed on fabric using conductive threads and computer aided sewing machine. Modeling of embroidered tag antennas is not a straightforward task, because embroidered antennas do not have a distinct conductivity, which could be used in the simulation model of them. In fact, conductivity of sewed flat conductive layer depends on the selection of the conductive thread, the thread and stitch density and the sewing pattern. The aim of this thesis has been to investigate the effect of these factors on the conductivity, and evaluate conductivity values for the embroidered dipole-type RFID tag antennas. In this project, T-matched dipoles have been sewed on cotton with two different sewing patterns and also with many different stitch densities. The effect of the geometry of the antenna is also investigated by sewing and measuring straight simple dipoles with both sewing patterns. The achieved read range values of the sewed tag antennas have been up to 7.5 m. In this thesis it is proved that each sewing pattern has its own conductivity and con-ductivity of a sewing pattern improves if the pattern consists of sewed lines along the direction of current flow. It is not necessary to sew the antenna with a high thread and stitch density. We can achieve high conductivities even from very sparsely sewed an-tennas, using less conductive threads and spending considerably less time on sewing. The evaluated conductivities and the presented simulation model of the sewed dipoles in this project can be used in future for optimization of the sewed antennas to operate in the vicinity of body.