IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009 1275 A Universal UHF RFID Reader Antenna Zhi Ning Chen, Fellow, IEEE, Xianming Qing, Member, IEEE, and Hang Leong Chung Abstract—A broadband circularly polarized patch antenna is proposed for universal ultra-high-frequency (UHF) RF identifica- tion (RFID) applications. The antenna is composed of two corner- truncated patches and a suspended microstrip line with open-cir- cuited termination. The main patch is fed by four probes which are sequentially connected to the suspended microstrip feed line. The measurement shows that the antenna achieves a return loss of 15 dB, gain of 8.3 dBic, axial ratio (AR) of 3 dB, and 3-dB AR beamwidth of 75 over the UHF band of 818–964 MHz or 16.4%. Therefore, the proposed antenna is universal for UHF RFID appli- cations worldwide at the UHF band of 840–960 MHz. In addition, a parametric study is conducted to facilitate the design and opti- mization processes for engineers. Index Terms—Axial ratio (AR), broadband antenna, circularly polarized (CP), RF identification (RFID), sequential feed, ultra high frequency (UHF). I. INTRODUCTION R F IDENTIFICATION (RFID), which was developed around World War II, is a technology that provides wireless identification and tracking capability. In recent years, RFID technology has been rapidly developed and applied to many service industries, distribution logistics, manufacturing companies, and goods flow systems [1], [2]. In an ultra-high-frequency (UHF) RFID system, the reader emits signals through reader antennas. When an RFID tag com- prising an antenna and an application-specific integrated circuit (ASIC) is located in the reading zone of the reader antenna, the tag is activated and interrogated for its content information by the reader. The querying signal from the reader must have enough power to activate the tag ASIC to perform data pro- cessing, and transmit back a modulated string over a required reading distance. Since the RFID tags are always arbitrarily ori- ented in practical usage and the tag antennas are normally lin- early polarized, circularly polarized (CP) reader antennas have been used in UHF RFID systems for ensuring the reliability of communications between readers and tags [3], [4]. Globally, each country has its own frequency alloca- tion for UHF RFID applications, e.g., 840.5–844.5 and 920.5–924.5 MHz in China, 866–869 MHz in Europe, 902–928-MHz band in North and South of America, 866–869 and 920–925 MHz in Singapore, and 952–955 MHz in Japan, and so on, so that the UHF RFID frequency ranges from 840.5 to 955 MHz (a fractional bandwidth of 12.75%) [5]. Therefore, Manuscript received May 24, 2008; revised November 18, 2008. First pub- lished March 27, 2009; current version published May 06, 2009. The authors are with the Institute for Infocomm Research, Singapore 138632 (e-mail: chenzn@i2r.a-star.edu.sg; qingxm@i2r.a-star.edu.sg; changleo@dso. org.sg). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMTT.2009.2017290 a universal reader antenna with desired performance across the entire UHF RFID band would be beneficial for RFID system configuration and implementation, as well as cost reduction. In this paper, we propose a sequentially fed stacked CP patch antenna for UHF RFID applications. The antenna comprises two suspended truncated patches and a suspended microstrip line. The main patch is sequentially fed by four probes which are con- nected to the microstrip line. A parasitic patch is positioned right above the main patch for enhancing the bandwidth. The corners of the patches are truncated to enhance the axial ratio (AR) per- formance. The proposed antenna is designed to cover the UHF RFID band of 840–960 MHz with acceptable performance in terms of gain, AR, and impedance matching. Meanwhile, the antenna configuration is simple and easy for fabrication. The remainder of this paper is organized as follows. Section II describes the geometry of the proposed antenna. The measured results, analysis, and discussion are presented in Section III. Section IV demonstrates the results of parametric study. The validation of the proposed antenna in RFID system applications is exhibited in Section V. Finally, a conclusion is drawn in Sec- tion VI. II. ANTENNA CONFIGURATION CP antennas can be realized when two orthogonal modes of equal amplitude are excited with a 90 phase difference [6]. In general, the feeding structures of CP antennas can be categorized into single and hybrid feeds. A single feed of a CP antenna has the advantages of simple structure, easy manufacture, and small size in arrays. However, the single-fed single-patch CP antenna in its simple form has inherently narrow AR and impedance bandwidths of 1%–2% [7]. To improve the bandwidth, a variety of CP antennas have been studied, wherein the bandwidth of AR, impedance matching, and gain have been enhanced, e.g., by modifying the radiator shape, designing feeding structures, and optimizing antenna or array configurations [8]–[17]. Usually, a CP antenna with the hybrid feed features a wide AR bandwidth, but suffers a complicated structure, expansive manufacture, and increased antenna size. Fig. 1 shows the configuration of the proposed antenna. The antenna comprises four layers of conductor, which include two suspended radiating patches, a suspended microstrip feed line, and a finite-size ground plane. Air substrate is used in this con- figuration to achieve higher gain, broader bandwidth, and lower cost. The microstrip feed line of a width of 24 mm is suspended above the ground plane (250 mm 250 mm) at a height of (5 mm). One end of the feed line is connected to an RF input, while the other one is open circuited, which simplifies the an- tenna structure. The main radiating patch of 156 mm 156 mm and with a truncation of 24.5 mm at two diagonal corners is placed above the feed line at spacing of mm. The 0018-9480/$25.00 © 2009 IEEE Authorized licensed use limited to: ASTAR. Downloaded on May 19, 2009 at 03:54 from IEEE Xplore. Restrictions apply.