Performance of RFID Tag Antennas Printed on Flexible Substrates

Radio Frequency Identification is becoming more and more popular. It has found application in many various fields, like person identification or supply chain management. Nowadays unit price of RFID tag is too high comparing to bar codes. It causes that widespreading of RFID techniques is still restrained. The decrease in RFID tag’s unit price is possible with printed electronics which allows to produce low cost devices on large scale. In this study, screen printed UHF RFID antennas were investigated. A semi-automatic screen printer with polyester mesh screen (68 T/cm) was applied. As conductive materials silver pastes were used. They were printed on photo paper and foils to provide flexibility of produced RFID antennas. After printing process antennas were cured in 120°C for 15 min in exception of a paste with silver nanopowder which was dried in 120°C for 15 min and then cured in 300°C for 1 h. To evaluate properties of printed layers their thickness and resistance were measured using contact profilometer and 4-probe method, relatively. Microwave properties of printed antennas were measured. In this order network analyzer was applied which allows to measure reflection coefficient of performed antenna. Then, it was possible to evaluate an influence of used substrate and conductive materials on the received results. Tested UHF antennas were simulated in CST Microwave Studio. Reflection coefficient was taken into consideration. 1. Basics Recently a rapid progress has been made in the field of printed RFID tags. Elaborated investigations are focused especially on printed RFID antennas which work in HF (13,56 MHz) [1, 2] or UHF (860 – 960 MHz) [3, 4] frequency range. To print them, pastes based on silver particles, carbon nanotubes or conductive polymer PEDOT:PSS are used. These materials are deposited on flexible and low-cost substrates like paper and foil using various printing techniques (for example flexography, screen printing, inkjet) [5]. Printing of RFID antennas is the first step to produce completely printed tag. It should have comparable parameters with traditional tags whose antennas are made from copper or alumina. Performed investigations show that it is possible to achieve. However, an efficiency of such antennas is lower comparing to its equivalents made from copper or alumina [6]. This paper presents the results of investigations on printed UHF RFID tag antennas. Two pastes based on silver flakes or silver nanopowder were applied. It makes possible to compare properties of layer printed with pastes containing silver particles in two different form. As substrates photo paper and foil were used. After printing process thickness and resistance of printed layer were measured using a contact profilometer and digital multimeter, relatively. To evaluate microwave properties of conducted antennas their reflection coeffiecient was measured using network analyzer. In order to compare obtained results a simulation of tested antenna was performed in CST Microwave Studio. Reflection coefficient was considered. 2. Experimental In this section, material and antenna pattern used in performed investigations are described.