ANTENNAS FABRICATION FOR RFID UHF AND MICROWAVE PASSIVE TAGS

In this paper, the authors present several antenna structures for RFID passive tags with direct chip connection. In this study, the substrate is PET (Polyethylene terephthalate) with 80μm thickness, the conductive layer is copper with 10μm thickness and CST Microwave Studio is used for simulation. The antenna design process is described and related results are shown. The goal of this study is to achieve compact size, good matching impedance, and to increase the bandwidth and read range of the tag as much as possible. The prototypes were fabricated by sputtering technology with 1μm thickness, then thickened by electroplating technique and finally tested by hand-held reader for read range characterizations.


INTRODUCTION
RFID technology is a technology perfectly suited for our ubiquitous world.To get and retrieve information on an object, tags are very useful.And they are particularly useful in the case of logistics.At the moment, there is still a large use of barcodes, but the line-of-sight reading causes various inconvenient in real environment.Nowaday, RFID is more and more interesting and already used for some applications.This paper focuses on the antenna designs for RFID passive tag in UHF band and at 2.45 Ghz.In fact, at the Laboratory for Nanotechnology (LNT), a platform for RFID research was established in order to perform studies on passive RFID tags and to realize some RFID system prototype based on the antenna design and RFID tags fabricated in our facility.
Our designs are divided into two directions: (i) first to match with XRAG2 passive chip (for first design in the UHF band) and (ii) with Atmel ATA5590 chip (for second design at 2.45 Ghz and third design in dualband).Our target is to operate in the UHF band and at 2.45 Ghz, to have an omni-directional radiation pattern with sufficient gain, to extend the bandwidth and to increase the read range.
This paper is further organized as follows: Section II presents the antenna designs procedure using CST Microwave Studio for the antenna structures mounted with XRAG2 chip and Atmel ATA5590 chip.The fabrication process, the tag prototype, the result of the measurement and the read range are shown in Science & Technology Development, Vol 16, No.K1-2013 Trang 6 this section.And finally, the conclusions are given in Section III.

ANTENNA DESIGN PROCEDURE
The first attempt has been made for applications in UHF band using XRAG2 chip From Fig. 3b and 3c, it is found that the return loss at 2.45 Ghz is affected strongly by "a".But in UHF band, the effect of "a" is very small.In Fig. 3d and 3e, the return loss at 2.45 Ghz is also affected by "d", but the effect is not as strong as in case "a".Because of "a" is closer to chip than "d".And in UHF band, the effect of "d" is almost nothing.From Fig. 3f, 3g, 3h and 3i, it is obviously that the effects of "i" and "f" on the return loss at 2.45 Ghz are weaker.But in UHF band, it is found that the effects are stronger.With this design, "a" has a strongest effect on the return loss at 2.45 Ghz.Therefore, it can be used to adjustfor impedance matching at 2.45 Ghz first.And then, the others will be used to match impedance in UHF band.And its characteristics are shown in Fig. 4. The bandwidth of -10 dB is around 26 Mhz in UHF band and the return loss at near 2.45 Ghz is -10.7 dB, but it can be still accepted.The impedance matching is not good but it ensures that it is for both bands.The radiation pattern at 2.45 Ghz is rather distorted, but with the high gain of 3.6 dBi, it can be compensate for the bad return loss.And the radiation pattern in UHF band is good with the gain of 1.9 dBi.
The testing the first antenna with 500mW hand-held reader was performed, the maximum read range about 60 cm was obtained.

CONCLUSIONS
In this paper, the antenna design processes with direct chip connection have been presented.With all designs, we try to obtain the best impedance matching in UHF band (860-960 Mhz) and at 2.45 Ghz.Three antenna structures have been designed and fabricated with size can be fixed in an identification card.
The first design has large bandwidth cover all frequency in UHF band, this advantage make this antenna can be applied in many countries.
The second and third designs are prospective designs for applications at 2.45 Ghz and in dual-band.These tags can be fabricated easily and some of them can be tested by hand-held reader with the maximum read range can be as far as 60 cm.These tags are entirely possible to be realized in practice.In future, we hope to optimize these designs to obtain better characteristics.(1) Phòng thí nghiệm Công nghệ Nano, ĐHQG-HCM

Figure 1 .Figure 2 Figure 3 .
Figure 1.a) The first antenna and its, b) Return loss, c) Impedance smith chart, d) Radiation pattern.

Figure 4 .Figure 5
Figure 4. a) Return loss at 2.45 Ghz ; b) Return loss in UHF band ; c) Impedence smith chart ; d) Radiation pattern at 2.45 Ghz ; e) Radiation pattern in UHF band.

Figure 6 .
Figure 6.Measured input impedance of: a) the first antenna with various thicknesses; b) the second antenna with various thicknesses; c) the third antenna with various thicknesses.