RFID 螺旋偶极振子天线设计

for RFID Transponder of UHF Band

Kihun Chang, Sang-il Kwak, and Young Joong Yoon

Dept. of Electrical and Electronic Eng., Yonsei Univ., Seoul, Korea Phone: +822-2123-8505, Fax: +822-2123-2876, E-mail: zangief@yonsei.ac.kr

Abstract?In this paper, the small-sized spiral dipole antenna for UHF RFID transponder is proposed. For the expected application of the UHF RFID systems, such as a supply chain integration, industrial automation, track and trace, and so on, the small and efficient printed antenna is required. To reduce the antenna size and to minimize the efficiency lowering due to small size, the proposed antenna is designed to have the spiral structure rather than the meander structure. In this structure, the current cancellation on the antenna is much less than in the meander structures, and the total current induces an identical magnetic dipole, which makes the omni-directional radiation pattern. The proposed antenna is designed for UHF RFID systems of Korea, to have the operating frequency bandwidth of 908 MHz ~ 913 MHz for VSWR < 2. The antenna has the physical size of 15.5 mm by 14.8 mm, which is about lambda over 16 in electrical length. This antenna is fabricated on the PET dielectric, to be utilized as an antenna of the transponder for the application of the UHF RFID systems.

ACKNOWLEDGEMEMT

This research was supported by the MIC (Ministry of Information and Communication), Korea, under the ITRC (Information Technology Research Center) support program supervised by the IITA (Institute of Information Technology Assessment).

I. INTRODUCTION

RFID (Radio Frequency Identification) has become very popular as a wireless identification technology without either contact or a line of sight between the transponder and the reader, to lead a ubiquitous computing environment in place of the existing barcode system with low speed and small storage.

RFID systems are composed of three basic components: transponder affixed to the objects to be identified, reader located where objects are to be identified, and host systems to separate and utilize the data collected by the readers. Transponders, which are the data carrying components of the system, consist of a microchip attached to an antenna, all of which is contained within some packaging to protect the antenna and microchip. Passive UHF RFID transponders harvest all of their operating power from the reader’s communication signal, and they communicate to the reader by modulating their backscatter. [1-4]

RFID systems are divided according to the operating frequency. Generally, the low frequency RFID systems are small in readable range and reliable, however, the high frequency RFID systems have a large readable range and a poor reliability. Thus, in view of that, the UHF RFID systems are most significant at the applications such as a supply chain integration, industrial automation, track and trace, and so on. [5]

The transponder is required to be small sized and flexible

0-7803-9433-X/05/$20.00 ?2005 IEEE APMC2005 Proceedings

to easily contact any products for use of the supply chain. As previously explained, the transponder consists of an antenna and a tag microchip, thus the size of the transponder is definitely dependent on the antenna size. Thus the antenna for the transponder should be small and light. Furthermore, it should be inexpensive for the mass production, and have an omni-directional radiation pattern to be detected independently of the transponder position or direction.

Therefore, in this paper, the small-sized printed spiral dipole antenna is proposed for UHF RFID transponder. The antenna characteristics included in the operating mechanism, the radiation, and the readable ranges are investigated and examined. Based on these characteristics, the antenna is proposed as a practical use for the UHF RFID systems.

II. ANTENNA DESIGN

The configuration of the small-sized spiral dipole antenna for the UHF RFID transponders proposed in this paper is shown in Fig. 1. The proposed antenna has a printed structure with a square spiral shape. Total length of the spiral is half wavelength same as the resonant length of the dipole antenna. This is designed by bending the arms of the dipole.

In this case, since the current direction is clockwise or count clockwise according to the phase, as shown in Fig. 2, like a small loop antenna, the total current leads an identical magnetic dipole which makes the omni-directional radiation pattern with an axis normal to the transponder plane. The most of the existing antenna with the meander line for the UHF RFID transponder have the current cancellation on adjacent meandered sections which reduces the effective length of the radiator, thus these often have poor radiation, e.g. low antenna efficiency. On the other hand, the winding of the spiral dipole antenna means that the total current contributes to yield an identical magnetic dipole and that the current cancellation is much less. This gives better but still not sufficient radiation resistance for antenna. [6]

Fig. 1 Configuration of the small-sized spiral dipole antenna

for UHF RFID transponder

Fig. 2 Current distribution of the proposed antenna

The proposed antenna has the T-matching circuit, which gives the freedom of design by transform the input impedance of the antenna to 50 ohm or chip impedance.

Since the total length of the antenna is electrically half wavelength of the resonant frequency, the current distribution on the radiator is not uniform likely an ideal