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Printed Electronics World
Posted on October 25, 2004 by  & 

Printed RFID tag antennas versus chip

Frequency Mismatch

Companies developing printed transistors for RFID tags are focusing on lower frequency RFID systems, which match the current performance of the thin film transistor technologies. For example, organic semiconductors perform less well than silicon, because typically they require higher voltages, their mobility is poor and they have a large feature size because of the print resolution. This limits the speed at which they can operate and therefore the frequency. PolyIC and Philips have demonstrated sample RFID tags working at low frequency - 125KHz band - but PolyIC and OrganicID, among others, claim they will be able to achieve 13.56MHz within two years.
However, at these RFID frequencies the tag and reader typically communicate using induction - which requires a highly conductive coil for the tag antenna for best range, and it is usually etched copper or aluminum. This means the antenna cannot be printed using similar inks easily, which would be the most elegant and cost effective solution.
Ironically, at higher RFID system frequencies, such as UHF and 2.45GHz, a more resistive antenna is acceptable, because the shape of the antenna becomes more important. These antennas are already being printed today, typically with a silver based conductive ink. Cure at room temperature is sufficient. However, current printed electronic thing film circuits will struggle to achieve frequencies of UHF over the next ten years, although some say this could be possible eventually.
There is one more technology that could be exploited and provides a good match between the printed chip and printed antenna - capacitive coupling - which works at low frequencies, such as 125KHz band. This uses a printed antenna which can be very resistive. The technology "BiStatix" was developed by Motorola many years ago and was licensed by several companies, such as Dai Nippon Printing and Toppan Forms, but over the past few years R&D on this has been scaled back as silicon RFID moves to higher frequencies such as UHF, based on mandates from retailers, airports, the military etc. However, in April this year Power Paper have licensed the technology and announced they eventually intend to offer a completed printed tag working at 125KHz - antenna and chip - once others successfully develop the printed circuits for the "chip".
Learn more by hearing from some of these these companies at Printed Electronics 2004, 6-8 December, New Orleans.
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