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RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...
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RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR ...

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  • 1. Research and Development of RFID Antennas at I2R 17 RESEARCH AND DEVELOPMENT OF RFID ANTENNAS AT INSTITUTE FOR INFOCOMM RESEARCH (I2R), SINGAPORE XIANMING QING, ZHI NING CHEN Institute for Infocomm Research 1 Fusionopolis Way , #21-01 Connexis, South Tower Singapore 138632 E-mail: qingxm@i2r.a-star.edu.sg, chenzn@i2r.a-star.edu.sg Abstract: Since 1995, there have been research and development (R&D) projects of antennas for radio frequency identification (RFID) applications at Institute for Infocomm Research (I2R), Singapore. This paper briefly reviews the R&D activities of antennas at I2R for a variety of RFID applications. First, the R&D facilities used for RFID antennas are briefed. After that, the designs of reader antennas, namely high performance loop antennas operating at high frequency (HF) band, broadband circularly polarized antennas operating at ultra high frequency (UHF), and microwave (MW) bands are elaborated. Then, small and efficient tag antennas are introduced. At last, the commercialization of antenna designs in industry applications is exemplified. Keywords: Circularly polarized antenna, dipole antenna, high frequency (HF), loop antenna, radio frequency identification (RFID), RFID antenna, reader antenna, tag antenna, ultra high frequency (UHF). 1. Introduction Radio Frequency Identification (RFID), which was developed around World War II, provides wireless identification and tracking capability that is more convenient than the use of bar codes and optical scanners [Want, 2006]. It therefore offers significantly improved supply chain management efficiency, in terms of lower labor cost, higher inventory speed and market intelligence. The use of RFID technology in tracking and access applications first appeared during 1980s. Currently, the RFID technology has been quickly developed because of its ability to track moving objects and low-cost implementation. Antenna is one of the key factors in RFID systems; the detection range / accuracy of a RFID system are directly dependent on the performance of reader / tag antennas. In addition, optimized antenna designs benefit the RFID systems with longer range, higher accuracy, lower antenna fabrication cost, as well as simpler system configuration and implementation
  • 2. 18 X. Qing & Z. N. Chen The Institute for Infocomm Research, Singapore (I2R - pronounced as i-squared-r, formerly known as Centre for Wireless Communications (CWC), a member of the Agency for Science, Technology and Research (A*STAR) family, Singapore) has accumulated great expertise in RFID technology since 1995, by designing and prototyping antennas and systems at different frequencies for a variety of applications. I2R has conducted R&D in the areas of system architecture, antenna design for readers and tags, highly efficient RF to DC power conversion and collision avoidance algorithms for multiple tag access. I2R has protected the inventions related to RFID technologies by filing patents to its credit and also has successfully commercialized some of them. R&D of RFID antennas is an important area which I2R focuses on. High performance HF reader antennas, broadband circularly polarized UHF / MW reader antennas, small and efficient tag antennas are the areas of expertise. Substantial team of 8 people including scientists, engineers with extensive know-how and experience in theoretical analysis, design, fabrication, and testing has been built up during years through the collaborating in RFID initiatives and projects. This paper gives an overview of the R&D activities of antennas at I2R for a variety of RFID applications. First, the R&D facilities used for RFID antennas are briefed. After that, the reader antennas, namely high performance loop antennas operating at high frequency (HF) band, broadband circularly polarized antennas operating at ultra high frequency (UHF), and microwave (MW) bands are elaborated. Then, designs of small and efficient tag antenna are introduced. At last, the applications of the antennas are exemplified. 2. Facilities for R&D of RFID antennas I2R has a range of facilities for RFID antenna R&D, including an anechoic chamber, an antenna measurement system, vector network analyzers, spectrum analyzers, probe station, a RFID testing-bed, and a variety of simulation software packages, which make the RFID antenna R&D very efficient. 2.1 Anechoic chamber: The anechoic chamber is essential for antenna R&D in radio frequency. The currently used anechoic chamber at I2R is a fully anechoic chamber with the dimensions of 10 m x 4 m x 4 m with an operating frequency range from 200 MHz to 40 GHz as shown in Figure 1. A newly built anechoic chamber with the size of 20 m x 12 m x 10 m and an operating frequency from 100 MHz to 100 GHz is expected to be completed in October 2008. 2.2 Orbit 3-D antenna measurement system: The Orbit far field antenna measurement system as shown in Figure 2 features the capability of 3-Dimensional antenna measurement and offers the capability of analyzing of the most important parameters such as gain, efficiency, beamwidth, sidelobe, and so on. In particular, the system has the capability of measuring circularly polarized antenna by spinning method, which is important for assessing the performance of the circularly polarized reader antenna at UHF and MW bands.
  • 3. Research and Development of RFID Antennas at I2R 19 Figure 1. Anechoic chamber. Figure 2. Orbit 3-D antenna measurement system. 2.3 RFID testing-bed: The in-house built testing-bed shown in Figure 3 includes a turntable, an automatic slider, and HF / UHF RFID readers. The multi-purpose testing-bed can be used to quantify the detection range of the reader antennas, the field distribution of the antennas at low / high frequencies, and the performance of the tag, and so on. Figure 3. RFID testing-bed at I2R.
  • 4. 20 X. Qing & Z. N. Chen 2.4 Casecade probe station: The probe station has a modular design that offers a broad range of configuration possibilities for precision electrical measurements for variety devices. It is capable of characterizing the impedance of micro chip and tag antenna, which is vital for UHF / MW RFID tag design. 2.5 Network analyzer / Spectrum analyzer: I2R has a range of vector network analyzers / spectrum analyzers which are suitable for RFID antenna R&D, such as Agilent 8753E (300 KHz to 6 GHz), Agilent N1957B (45 MHz to 50 GHz), and Agilent 8595E (9 KHz to 6.5 GHz) etc. 2.6 Simulation software: Electromagnetic simulators are necessary for antenna design and optimization, it shortens antenna design circle and makes the antenna design more efficient. I2R has a variety of software packages including HFSS, XFDTD, IE3D, ADS, FIDELITY, WIPL, and ENSEMBLE, which are suitable for various antenna analysis and design. 3. RFID reader antennas Reader antennas play an important role in RFID applications. An optimized reader antenna benefits RFID systems with longer detection range, higher detection accuracy, simpler system configuration and implementation, and lower cost. As the operating frequency of RFID systems ranges from low frequency (LF, ≤ 400 KHz) to microwave up to 24.125 GHz [Finkenzeller, 2003], a variety of antennas have been reported for this purpose [Foster, 1999; Kossel et al, 1999; Salonen and Sydanheimo, 2002; Texas Instruments, 2003; Raumonen et al, 2004; Choo et al, 2005; Kim et al, 2006; Erhan et al, 2007; Tseng et al, 2007; Ukkonen et al, 2007]. Loop antennas have been widely used in LF and HF RFID systems, wherein the main consideration of antenna design is to achieve required field coverage. For antennas operating at UHF and MW bands, the reader antennas are preferred to be circularly polarized, broadband, and high gain. In addition, common factors such as reliability, size, weight, and cost must always be considered in all RFID antenna designs. 3.1 HF multi-loop antenna (patent pending) [Qing et al, 2007] The patent pending multi-loop antenna shown in Figure 4 was developed at I2R for HF RFID smart shelf system applications. It comprises two elements which are composed of a plurality of loops. The elements are separated at a certain distance. By selecting the shape, size and location of the loops, the magnetic field produced by the loops can compensate for each other to achieve uniform field distribution in an interrogation region located around the antenna. The loop elements are configured to make the current direction either clockwise or anti-clockwise, and the current direction with respect to each adjacent loop to be in phase opposition. A tuning / matching network is used to tune the antenna to 13.56MHz and match to 50 Ω. Good impedance matching and uniform field distribution are achieved as shown in Figure 5 and 6, respectively. The main challenge of the antenna design is to generate uniform magnetic field distribution above the antenna of large area.
  • 5. Research and Development of RFID Antennas at I2R 21 Figure 4. Multi-loop HF antenna with casing. Figure 5. Measured return loss. Figure 6. Measured field distribution.
  • 6. 22 X. Qing & Z. N. Chen 3.2 HF / UHF integrated antenna (patent pending) The patent pending HF / UHF integrated antenna shown in Figure 7 comprises two loop elements which are corresponding to HF near-filed and UHF far-filed RFID applications. Figure 8 illustrates the operating mechanism of the antenna. A bigger loop element with a matching circuit at the input generates magnetic fields for HF near-field RFID applications, the HF tags in the interrogation region will be detected by inductive coupling. A slotted loop radiator at the bottom is for far-field RFID applications at UHF. It generates bi- directional circular polarization. The UHF tags in the interrogation region will be detected by electromagnetic waves. Two antenna elements are integrated by merging the ground plane of the slotted loop antenna element into the HF loop antenna element. By selecting the shape, size and location of the loop and slotted loop antenna element, the antenna can be used for point-of-sale (POS) retail applications, where HF / UHF tags may be mixed. The Figure 7. HF / UHF integrated antennas with casing. Figure 8. Operating mechanism of the HF / UHF integrated antenna
  • 7. Research and Development of RFID Antennas at I2R 23 co-design of the two elements reduces the impact of the mutual coupling between two parts and thus the performance of the antenna operating at two modes is maintained well. Figure 9 shows the measured return loss and field response of the HF / UHF integrated antenna at 13.56MHz. Good impedance matching is achieved by using a simple matching network. The unloaded Q factor of the loop antenna is up to 80. Figure 9. Measured return loss and field response of the HF / UHF integrated antenna at 13.56MHz. Figure 10 exhibits the results of the HF / UHF integrated antenna over the frequency of 880 MHz to 960 MHz. The measured return loss is less than -15 dB over the range of 902–928 MHz. The maximum gain (4.5dBic) is obtained at +z direction (θ = 0°, ϕ = 0°), and 3.5 dBic gain is achieved at –z direction (θ =180°, ϕ=0°). Good axial ratio is observed for both direction, the axial ratio is less than 1 dB at +z direction (θ = 0°, ϕ = 0°) and 2dB at –z direction (θ = 180°, ϕ = 0°). (a)
  • 8. 24 X. Qing & Z. N. Chen (b) Figure 10. Measured results of the HF / UHF integrated antenna at UHF band; (a) return loss, (b) gain and axial ratio. 3.3 Printing HF antenna [Cai et al, 2007] Printing technology has been proposed for RFID electronics and tag antennas since 2002 [Cichos et al, 2002; RFID Journal, 2003; Koptioug et al, 2003]. Compared to conventional antennas made by copper / aluminium wires / plate or etched metal trace onto dielectric substrate, the printing antennas are created by printing the low-cost conductive material such as silver ink on a cheap substrate such as PCB, paper, plastic film, cloth and so on. The significant cost reduction in material and fabrication of a printing antenna offers great advantage and more freedom for cost effective antenna design. Printing technology has been used in industry to configure ultra-low-cost RFID tag antennas for years. However, the applications of printing technology for RFID reader Figure 12. Printing HF RFID antenna prototype.
  • 9. Research and Development of RFID Antennas at I2R 25 antennas have yet been reported. The antenna prototype shown in Figure 12 is believed the world first reported printing RFID reader antenna. The antenna prototype was fabricated by Singapore Institute of Manufacturing Technology (SIMTech). The conductive silver ink was printed on a 20-mils FR4 substrate by Micro-tec Screen Printer. Some portions of the loop are specially processed to withstand heat due to the soldering of the coaxial line and lumped components. A few resistors and capacitors are used to tune the antenna to the required resonant frequency and Q factor. A “T” matching method is used to match the antenna to 50 Ω. The antenna is fed by using a coaxial cable which is split into two wires (screen & core); these wires are soldered to two matching stubs, respectively. Required impedance matching can be achieved by adjusting the length and width of the stubs. Figure 13. Field response of the printing antenna and copper antenna. Figure 13 compares the measured field response of the printing antenna and a copper antenna with identical antenna configuration. The response is obtained 10 cm away the antenna. Similar responses are observed except that the copper antenna offers a larger peak value. The field strength of printing antenna is 2 dB lower than that of the copper antenna, which is reasonable because the lower conductivity of conductive silver ink material causes more ohmic losses. The research into the printing antenna is ongoing as there are some problems to be solved before using it in practical RFID applications. 3.4 Single loop HF antenna [Cai et al, 2007] A variety of cost-effective single loop antennas have been developed at I2R for various applications. As shown in Figure 14, the single loop antenna with dimensions of L x M is tuned / matched at 13.56 MHz by using a simple matching circuit with few capacitors, which features low cost for material, fabrication and implementation. The main design consideration of such antennas is to achieve required field distribution over the specified region with desirable field intensity.
  • 10. 26 X. Qing & Z. N. Chen Figure 15 illustrates the field distribution of a loop antenna with the dimensions of 490 mm x 490 mm. Uniform distribution is achieved over the region above the antenna. Figure 14. Cost effective HF loop antenna. Figure 15. 3-D field distribution of the loop antenna (z = 10 mm). 3.5 UHF probe-fed broadband circularly polarized stacked patch antenna [Chung et al, 2007] Since the frequencies for UHF RFID applications vary world widely, they cover the 840.5– 844.5 MHz / 917–922 MHz bands in China, the 864–868 MHz band in Europe, the 902–928 MHz band in north America, and the 952–955 MHz band in Japan, and so on [EPC, 2007].
  • 11. Research and Development of RFID Antennas at I2R 27 Therefore, a broadband reader antenna which features desirable performance across the entire UHF RFID band from 840 to 960 MHz (13.3%) would be preferable for system configuration and implementation, and cost reduction. I2R has developed a variety of broadband circularly polarized antennas for RFID applications since 1998. Figure 16 shows the configuration of a probe-fed circularly polarized stacked patch antenna. The antenna is composed of a branch line hybrid coupler which is etched on an FR4 substrate (εr = 4.4, tanδ = 0.02, thickness = 0.8128 mm), and three radiators which are all made of brass. The primary radiator (Patch 1, 150 mm x 150 mm, 0.5 mm thick) is fed by two feeding probes connected to the output ports of the hybrid coupler respectively. The feed-points are positioned symmetrically with the square patches with a distance d of 18.5 mm away from the edge of the primary patch. The feeding probes (a) (b) Figure 16. Geometry of the probe-fed circularly polarized stacked patch antenna; (a) snapshot of the antenna prototype, (b) schematic top view and section view.
  • 12. 28 X. Qing & Z. N. Chen have the height of 10 mm and the diameter of 2.2 mm. To further improve the bandwidth, two more 0.5 mm thick square brass patches (138 mm x 138 mm, 130 mm x 130 mm) are stacked over the primary patch with separation of h2 = h3 = 5 mm. The antenna achieves desirable performance over the frequencies of 820–980 MHz (17.7%) with the gain of more than 6.5 dBic, axial ratio of less than 3.0 dB and return loss of less than -15 dB. It is capable of covering the entire UHF band. Some results of the antenna are shown in Figure 17. (a) (b) Figure 17. Measured gain and axial ratio of the probe-fed stacked patch antenna; (a) gain, (b) axial ratio.
  • 13. Research and Development of RFID Antennas at I2R 29 3.6 UHF single-fed circularly polarized patch antenna (patent pending) Apart from the hybrid coupler based circularly polarized antenna, the circularly polarized antennas with different feeding techniques have been investigated and developed at I2R. The patent pending single-fed circularly polarized patch antenna comprises a truncated patch and a spiral microstrip feeding line with overall size of 250 mm x 250 mm x 35 mm. The antenna is configured by two metal plates without any PCB. It features cost effective because of very simple configuration, high mechanical tolerance and good manufacturability. (a) (b) Figure 18. Measured gain and axial ratio of the single-fed patch antenna; (a) gain, (b) axial ratio.
  • 14. 30 X. Qing & Z. N. Chen The antenna exhibits good performance in terms of the measured return loss of less than -10 dB, gain of more than 8 dBic, axial ratio of less than 2 dB, and the 3 dB axial ratio beamwidth of lager than 60o. over the entire UHF RFID frequency of 840–960 MHz. Some results are shown in Figure 18 and 19, respectively. Figure 19. Measured radiation patterns of the single-fed circularly polarized patch antenna. 3.7 UHF sequentially-fed circularly polarized truncated stacked patch antenna (patent pending) Besides the single-fed patch antenna, a broadband sequentially-fed circularly polarized patch antenna was recently developed for UHF RFID applications. The antenna is composed of two corner-truncated patches and a microstrip feedline suspended over a finite-size ground plane. The main radiating patch is fed by four sequentially positioned probes from
  • 15. Research and Development of RFID Antennas at I2R 31 the feedline. The probes provide the excitation phase difference of 0°, 90°, 180°, and 270°, respectively. The 90° phase difference between the probes is attained through the λg/4 length of the feedline. The patches are corner-truncated to create additional degenerating modes necessary for further widening the axial ratio bandwidth. The proposed antenna has achieved desirable performance over the entire UHF RFID frequencies with compact size. In addition, the developed antenna which is a type of suspended antenna without use of any expensive dielectric substrate, features low cost of material and fabrication. The measured results of the antenna are exhibited in Figure 20 and 21. It is seen that the return loss is less than -15 dB, gain more than 8.3 dBic, axial ratio less than 3 dB, and 3-dB axial ratio beamwidth larger than 75° over a bandwidth of 16.4%. (a) (b) Figure 20. Measured gain and axial ratio of the sequentially-fed stacked antenna; (a) gain, (b) axial ratio.
  • 16. 32 X. Qing & Z. N. Chen Figure 21. Measured radiation patterns of the sequentially-fed stacked patch antenna. 3.8 2.45 GHz aperture coupled circularly polarized patch antenna [Qing and Yang, 2004] An aperture coupled feed technique has been adopted to make the antenna more compact in this design because the feed structure is fabricated on a separate substrate under the radiating patch. Besides, this feed technique has more merits such as: (1) the feed circuit is isolated from the radiating element by the ground plane which prevents spurious radiation; (2) active devices can be easily fabricated in the feed substrate for system size reduction. The configuration of the antenna is shown in Figure 22. The branch line coupler is etched on one side of the lower substrate which is 20 mils FR4 (εr = 4.4, tanδ = 0.02), the impedance of the two wider parallel strips is 35.4Ω, and all other lines 50Ω. A 50-Ω resistor
  • 17. Research and Development of RFID Antennas at I2R 33 is connected to the port 2, which is used as a load to absorb the reflecting power from the unmatched antenna feed ports. Two rectangular slots are cut on a ground plane; the slots are perpendicular to feed strip of the branch line coupler separately. The radiator is a square patch (48 mm x 48 mm), and etched on one side of the upper substrate. A foam with εr ≈ 1.05 and thickness of 5 mm is used to support the upper radiator for high gain and broad bandwidth. The measured return loss is less than -12 dB from 2.35 GHz to 2.55 GHz. The gain is more than 9.0 dBic, and the axial ratio less than 1.0 dB over the frequency range. Wide angle circular polarization radiation has been achieved with the beamwidth for 3 dB axial ration of larger than 90°. The measured results are tabulated in Table 1. Figure 22. Configuration of the aperture coupled patch antenna for 2.45 GHz RFID application. Table 1. Measured results of the 2.45 GHz aperture coupled circularly polarized antenna. F(GHz) 2.35 2.40 2.45 2.50 2.55 Return Loss (dB) -15 -18 -15 -13.2 -12.1 Gain (dBic) 0.08 9.62 10.00 9.33 9.21 Axial ration (dB) 0.27 0.56 0.09 0.90 0.97
  • 18. 34 X. Qing & Z. N. Chen 4. RFID tag antennas Similar to the RFID reader antennas, the selection of type of RFID tag antennas is much dependant on the operating frequency. Loop antenna (coil) is the most used type of tag antennas at LF and HF bands. In UHF and MW RFID applications, a variety of antennas have been reported to be used as tag antennas, including meander line antennas [Marrocco et al, 2002; Marrocco et al, 2003], folded dipole antennas [Li et al, 2004], loop antennas [Andrenko, 2005; Cole and Ranasinghe, 2006], slot antennas [Chen and Hsu, 2004; Padhi et al, 2004], inverted F antennas [Ukkonen et al, 2004], planar inverted-F antennas [Hirvonen et al, 2004; Choi et al, 2005], slotted planar inverted-F antenna [Kwon and Lee, 2005], patch antenna [Ukkonen et al, 2004] and so on. Tag antenna design is much more dependent on specific applications since the performance of the antenna is degraded when the tags are attached to various objects with unpredictable property. The antenna can be optimized by considering the practical application scenario. R&D of tag antenna is the area I2R has focused on. We have developed various tag antennas for different applications at the frequency bands of 13.56 MHz, 433 MHz, 860 MHz, 915 MHz and 2.45 GHz. Some of the prototypes are illustrated in Figure 23. For brevity, only the folded dipole antenna and the dual-port antenna will be discussed in this paper. Figure 23. Tag antennas developed at I2R. 4.1 Folded dipole antenna [Qing and Yang, 2004] When RFID operating frequency rises into the UHF / MW region, the tag antenna must be carefully designed to match the application specific integrated circuit (ASIC). This must be made to maximize the transfer of power between the antenna and the ASIC. It is especially important in a passive RFID system where the ASIC’s power supply is only from the interrogating radio wave. Over years, antennas have often been developed to match to 50Ω or 75Ω. However, in RFID applications, the input impedance of the ASIC is an arbitrary value rather than
  • 19. Research and Development of RFID Antennas at I2R 35 50Ω or 75Ω. For maximum power transfer, the input impedance of the tag antenna must be conjugate matched to the output impedance of the ASIC. It is a challenge to design the antenna that will have arbitrary input impedance with the constraints such as size, cost and so on. We designed a folded dipole antenna as shown in Figure 24, the input impedance of the antenna can be adjusted easily by choosing suitable geometry parameters. The open-end folded structure provides great freedom for impedance adjustment, especially for imaginary part, Xin, which is very import for conjugate impedance design. Figure 24. Configuration and coordinate system of the folded dipole antenna. The impedance of the folded dipole antenna is mainly determined by parameters L1, L2, and L3. The effects of these parameters on antenna impedance are shown in Figure 25. The bigger antenna with larger L1 and L2 results in larger antenna impedance, both Rin and Xin increase with L1, L2 becoming larger. The effect of L3 on antenna impedance is uniquely, the real part, Rin, is little affected, while the imaginary part, Xin, shows a significant variation against L3, which offers great convenience for antenna impedance adjustment.
  • 20. 36 X. Qing & Z. N. Chen (a) (b) (c) Figure 25. Input impedance of the folded dipole antenna against geometrical parameters at 2.45 GHz; (a) L1 (L2 = 2 mm, L3 = 10.5 mm, w = 0.5 mm); (b) L2 (L1 = 40 mm, L3 = 10.5 mm, w = 0.5 mm), (c)L3 (L1 = 40 mm, L2 = 2 mm, w = 0.5 mm).
  • 21. Research and Development of RFID Antennas at I2R 37 4.2 Dual-port loop antenna [Chia et al, 2004] The RFID tag is known to be assigned three tasks. These are to rectify AC power derived form the carrier wave, to modulate the backscattered response to the initial carrier wave and to demodulate the control signal from the reader. Normally, a tag has an antenna. If all of these functions are to be carried out using the same antenna and by a shared port of the antenna, it will be very difficult to achieve optimal performance as different parameters are required for each of the functions to perform optimally. Improved performance of a RFID tag can be achieved by using dual-port ASIC, where one port is for the modulator and demodulator, and the other solely for the rectifier. Thus, either two antennas or a single antenna with multiple ports is needed for such dual-port tag configuration. One of such dual-port tag antenna configuration is shown in Figure 26. The perimeter of the loop antenna is one wavelength. The two ports are positioned in the middle of the adjacent sides, such a compact configuration makes the ports locate at the points where the current null exists, which offers good isolation between the two ports and keeps the performance of the loop antenna unaffected. Figure 26. Configuration of the dual-port loop antenna. 5. Application examples 5.1 RFID smart shelf RFID smart shelf system has received much attention because of increasing demands for large-scale item-level management such as grocery products in the retail supply chain, large volumes of books in libraries, bottles in pharmaceutical industry, and important
  • 22. 38 X. Qing & Z. N. Chen documentation in offices [Online available]. In library applications, the RFID smart shelf system enables librarians to take the stock of the books in real time within minutes. Library users will no longer be frustrated in their search for a misplaced or lost book that the current library database system designates as available in the library. The smart shelf system will provide the users with the exact locations of available books. The library can collect the statistics on the popularity of the books based on the frequency a reader browses them, helping in decision-making about additional copy acquisitions. Two HF RFID smart shelf prototypes were developed by I2R for library applications. Figure 27 shows the smart shelf prototype using the patent pending multi-loop antenna, where the antennas are horizontally placed on the shelf rack. Only one antenna is required to cover the whole tier which is 900 mm long and 280 mm wide. The RFID smart shelf prototype using the antennas achieves high detection accuracy as shown in Table 2. Although the detection accuracy degrades as the height of the tag increases, the smart shelf using horizontal placement antenna features merits such as: (a) only one antenna is needed to cover each tier, which greatly reduce the number of antennas and multiplexers; (b) modification to the shelf is hardly required, thereby reducing system implementation cost; (c) less interference between the antennas in the shelves, which is vital for achieving high detection accuracy. Figure 27. HF RFID smart shelf prototype using horizontally placed multi-loop antennas. Table 2 Detection accuracy of the RFID smart shelf using the multi-loop antenna. No. Of Books Height of tag above Defected books Detection accuracy (%) the antenna (mm) 25 40 100.0 40 37 40 100.0 50 38 95.0
  • 23. Research and Development of RFID Antennas at I2R 39 Figure 28 exhibits the HF RFID smart shelf system implemented in Singapore National Library, where the patent pending HF / UHF integrated antenna was vertically mounted in the shelves. The vertically placed antennas provide higher detection accuracy as compared to the horizontal version because the vertically placed tags in the books are in the best orientation to get maximum magnetic flux, especially when the height of the tag is larger. The disadvantages of the vertically placed configuration include: (a) at least two antennas are needed to cover one tier, therefore system complexity and cost is increased; (b) mounting antennas on the shelf rack needs some mechanical modification to the book shelf, hence increasing system implementation cost; as well as (c) interference between the antennas in adjacent tiers / shelves is severe. A modified antenna configuration was proposed and adopted in the system, which restrains the interferences between the antennas effectively [Qing and Chen, 2007]. The tier based detection accuracy is up to 97.5%. Figure 28. HF RFID smart shelf system in Singapore National Library. 5.2 RFID smart table The RFID smart table can determine the position and existence of each specific tagged item on it with time and thus is an efficient method of tabulating price, monitoring customers’ habits and stock-taking. For instance, a RFID smart table can be used in a restaurant, especially buffet style, for automatic billing and tracking the favorite dishes for customers. Figure 29 shows the on-site testing of an HF smart table prototype which was developed for automatic billing in restaurant, where the single loop antenna with dimensions of 490 mm x 490 mm was used. Upon requirements, the antenna can be mounted on the table or embedded in the table. In the testing, an Omron HF reader (VS 702S) with 4W output was used, and Phillips SLI tags were affixed to the bottom of the plastic plates. The height of the plate is 15 mm. The maximum detection range is up to 150 mm above the antenna, namely 10 stacked plates can be detected. A total of 90 plates can be detected one time by the smart table prototype with a detection accuracy of 100%.
  • 24. 40 X. Qing & Z. N. Chen Figure 29. On-site testing of the HF RFID smart table prototype. 5.3 Portable reader at 2.45 GHz In 2003, a 2.45 GHz portable RFID reader was developed at I2R. Then, the technology of the reader was transferred to a Japanese company. Figure 30 shows the photo of the reader with the casing. The aperture coupled circularly polarized patch antenna was successfully used as the reader antenna, which features desirable performance of high gain and low axial ratio with compact size and low cost. Figure 30. 2.45 GHz RFID reader. 5.4 Dual-port tag at 2.45 GHz The dual-port tag consists of a dual-port chip and two orthogonal folded antennas as shown in Figure 31. The main design considerations of the antenna are: (a) with the limited size of the tag, the antennas must be carefully positioned to restrain the multi-coupling effect
  • 25. Research and Development of RFID Antennas at I2R 41 and (b) the package of the tag shows significant effect on antenna performance, especially the impedance of the antennas. The antennas must be co-designed with the package by accounting for the property of the package material. (a) (b) Figure 31. Dual-port tag at 2.45 GHz; (a) antenna configuration, (b) tags with package. 6. Future research and development activities R&D of RFID antennas at I2R are still going on. The RFID antenna related work are foreseen in the following areas: • Antennas for HF / UHF RFID smart shelf • Printing reader antennas with ultra low cost • Compact antennas for portable UHF readers • Beam steering reader antennas • Study of environment effect on RFID antennas • Customized reader / tag antenna development ACKNOWLEDGEMENTS The authors would like to acknowledge their colleagues: Miss Ailian Cai, Mr. Hang Leong Chung, Dr Wee Kian Toh, Dr Ning Yang, Dr Michael Yan Wah Chia, and Mr. Chip Hong Ang from Institute for Infocomm Research, and Mr. Boon Keng Lok from Singapore Institute of Manufacturing Technology, for their contributions to the work in the paper.
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