Design and Fabrication of Miniaturized Fractal Antennas for Passive UHF RFID Tags 49 The measured return loss is (-17 dB) at a resonant frequency (889.62 MHz) compared with the simulated one of (-26 dB) at 900 MHz, and the bandwidth is (19.2 MHz) compared with the simulated bandwidth of (59 MHz). The disagreement between measured and simulated results of the fractal loop antenna is attributed to the fact that we lack sufficient information from the vendor of FR-4 material. This information would enable us to build accurate model for the dielectric material in the EM simulator, instead of working with single frequency point data. The radiation pattern for the fractal dipole antenna is measured in anechoic chamber as shown in Fig. 29 which is in good agreement with the simulated results. Fig. 29. Measured radiation pattern. 5. References Andrenko A. S., (2005). Conformal Fractal Loop Antennas for RFID Tag Applications, Proceedings of the IEEE International Conference on Applied Electromagnetics and Communications, ICECom. , Pages:1-6, Oct. 2008. Balanis C. A., (1997), Antenna Theory Analysis and Design, Jhon Whily, New York, (2nd Edition). Baliarda C. P., Romeu J., & Cardama A., M. (2000), The Koch monopole: A small fractal antenna. IEEE Trans. on Antennas and Propagation, Vol.48, (2000) page numbers (1773-1781). Curty J. P., Declerdq M., Dehollain C. & Joehl N. , (2007). Design and Optimization of Passive UHF RFID Systems, Springer, ISBN: 0-387-35274-0, New Jersey. Sabaawi A. M. A., Abdulla A. I., Sultan Q. H., (2010), Design a New Fractal Loop Antenna For UHF RFID Tags Based On a Proposed Fractal Curve, Proceedings of The 2 nd IEEE International Conference on Computer Technology and Development (ICCTD 2010) November 2-4, 2010, Cairo, Egypt. Sabaawi A. M. A., Quboa K. M., (2010). Sierpinski Gasket as Fractal Dipole Antennas for Passive UHF RFID Tags. Proceedings of The Mosharaka International Conference on Communications, Electronics, Propagation (MIC-CPE2010), 3-5 March 2010, Amman, Jordan. Advanced Radio Frequency Identification Design and Applications 50 Salama A. M. A., (2010). Antennas of RFID Tags, Radio Frequency Identification Fundamentals and Applications Design Methods and Solutions, Cristina Turcu (Ed.), ISBN: 978-953-7619-72-5, INTECH, Available from: http://sciyo.com/articles/show/title/antennas-of-rfid-tags. Salama A. M. A., Quboa K., (2008a). Fractal Dipoles As Meander Lines Antennas For Passive UHF RFID Tags, Proceedings of The IEEE Fifth International Multi-Conference on Systems, Signals and Devices (IEEE SSD'08) , Page: 128, Jordan, July 2008. Salama A. M. A., Quboa K., (2008b). A New Fractal Loop Antenna for Passive UHF RFID Tags Applications, Proceedings of the 3 rd IEEE International Conference on Information & Communication Technologies: from Theory to Applications (ICTTA'08) , Page: 477, Syria, April 2008, Damascus. Werner D. H. & Ganguly S. (2003). An Overview of Fractal Antenna Engineering Research. IEEE Antennas and Propagation Magazine, Vol.45, No.1, (Feb. 2003), page numbers (38-56). Werner D. H., Haupt R. L., Werner P. L., (1999). Fractal Antenna Engineering: The Theory and Design of Fractal Antenna Arrays. IEEE Antennas and Propagation Magazine, Vol.41, No.5, (1999) ,page numbers (37-58). 3 Design of RFID Coplanar Antenna with Stubs over Dipoles F. R. L e Silva and M. T. De Melo Universidade Federal de Pernambuco Brasil 1. Introduction Radio Frequency Identification system, initially projected for objects identification in large scale – a counterpart of the well-known barcode, has been expanding its horizons and has been used for the automation of several services such as tracking goods, credit card charging, supply chain controlling, and others. RFID systems consist on a Reader that interrogates an identification Tag and this, in turn, sends an identification code back to the Reader. Specifically, the passive RFID Tags take advantage of being free of batteries. It converts part of the incoming RF signal from the reader into power supply. Because of its versatility, lots of researchers have been investing on RFID, which, despite the 35 years old of the first patent, is still considered new and somewhat obscure. This chapter covers topics including the system surveying and the working basics of the RFID, especially the physical air interface between the RFID tags (the mobile part) and the so-called Interrogators, which are fixed part of the network. This chapter focuses on the project of 2.45 GHz planar antennas, with a gain higher than the commercial ones, in such a way that, when these brand new antennas are used in RFID tags, they increase the system efficiency. More coverage area can be achieved with these higher gain antennas, as well as lower power requirements of the Interrogators. Most of the necessary theory topics to project this antenna are shown. As well as the theory, measured and simulated results are presented such as: input impedance, frequency response, radiation pattern and gain, which could certainly be the starting point for future works. With respect to academic research over RFID, it is increasing year after year. The number of publications in important periodicals is increasing in recent years. This happens due to its great applicability in many areas like, health, commerce, safety, etc. In recent years, it is becoming one of the most attractive areas in wireless applications. Figure 1 presents the number of publications about RFID from 2003 to 2009 in the IEEE (Institute of Electric and Electronics Engineers). As one can see, there is a considerable increase in recent years. This Figure shows only the most relevant publications according to the algorithm of the IEEE research in a sample space of 100 publications. In reality, the number of publications is in the order of tens of hundreds. In general the RFID system publications can achieve different focus. These can be about development of antenna, chips identification, software control, etc. As usual, in all engineer systems, there is something to improve. The system still is a bit expensive, as an Interrogator may cost U$ 2,000.00. Another point is behind intersystem and intra-system interference, as Advanced Radio Frequency Identification Design and Applications 52 it operates in the ISM bands (Industrial Scientific and Medical), free bands. Many others systems, operating in that band, can interfere with RFID systems. Fig. 1. RFID Publication in the IEEE. It is included publications over performance evaluation, development of news tools, new hardwares, etc. Publicações sobre antenas para RFID no IEEE* 0 5 10 15 20 25 30 35 40 45 2002 2003 2004 2005 2006 2007 2008 2009 Ano N° de publicações Fig. 2. Number of publications specifically for RFID antennas in the IEEE, in a sample space of 100 publications. Publicaçõe s sobre RFID no IEEE** 0 5 10 15 20 25 30 35 40 45 2003 2004 2005 2006 2007 2008 2009 Ano N° de publicações year N o of p ublication RFID Publications in the IEEE year N o of p ublications RFID antennas Publication in the IEEE Design of RFID Coplanar Antenna with Stubs over Dipoles 53 For a matter of power saving, design of high gain antenna can be necessary in the case of longer distance reading. On the other hand, some specific radiation patterns are suitable for grouped tags, avoiding the interfering effects. Besides, some Interrogators antenna arrays, can optimize the system power consume and/or optimize the number and position of the Interrogators, decreasing both the cost of implementation and the maintenance. It is clear that there is no any unique solution for whole problems, and perhaps, a particular solution for a particular problem. Figure 2 also shows an increase in the number of publications specifically for RFID antennas from 2002 to 2009 in the IEEE. These are only publications in the IEEE, there are other important periodicals, conferences, meeting, symposiums, etc. about RFID all over the world. Certainly, in this research area there is much work to do about optimization and cost reduction. As the antenna design is one of the most important parts of RFID system development, it becomes necessary to see some basic concepts, analysis, and characterization of antennas used in RFID applications. 2. Important concepts As predicted by Friis (Balanis, 1982) in (1), the reading range r is a function of the following parameters: wavelength in the free space λ, EIRP power P t ·G t , tag antenna gain G r and the minimum required power for activating the RFIC chip P r (Karthaus & Fischer, 2003). RFIC operating with 16.7μW minimum power level (Karthaus & Fischer, 2003) and indoor Reader EIRP of 27dBm, gain improvements on the tag antenna could increase the reading range of the system. Figure 3 shows the system reading range as a function of the antenna gain. According to (Karthaus & Fischer, 2003), (Finkenzeller), passive RFIC tags have generally negative input reactance and may have low input resistance. The impedance of the RFIC and the antenna must be matched each other (Finkenzeller). 4 ttr r PGG r P λ π ⋅ ⋅ = ⋅ (1) Fig. 3. Reading range versus tag antenna gain. Advanced Radio Frequency Identification Design and Applications 54 3. Tag antenna design Let us see the design step by step. It consists of two λ/2 folded dipole array fed by λ/4 Transmission Line (TL) sections. Each folded dipole works like a load for a λ/4 transmission line (TL). As described in (de Melo et al., 1999), two loaded λ/4 TL are connected at the position A-A´. This yields to array of two planar dipoles. The transmission lines TL, as shown in Figure 4, of length λ/4 works like an impedance transformer for the required input impedance at the feeding points A-A’. From Figure 5, one can see the load in the shape of a planar folded dipole. Fig. 4. Loaded CPS transmission lines. Fig. 5. Load in the shape of a dipole. The transmission lines are connected together at the terminals A-A’, as shown in Figure 4. Arrays of radiating elements produce higher gain than isolated elements (Balanis, 1982). This fact allows this antenna to be useful when farther reading ranges are required. Because its symmetry related to the central plane, only half the antenna is analyzed and the results are further corrected in order to represent the whole antenna. With the dimensions described in Table 1 (Condition 1), the input impedance of one dipole can be calculated using quasi-static equations of conformal mapping (Lampe, 1985), (Nguyen, 2001), (de Melo et al., 1999) and such impedance is referred to as Z dipole . It is the load impedance for the transmission line. In practice it is not simple to obtain the dipole impedance, taking into account the real values of the geometrical parameters. The known usual expressions are suitable for ideal conditions and do not take into account some parameters, like width D, shown in the Figure 6. Another example is the gap G created in one of the strips for the signal feeding. Besides, the lower strip becomes smaller, comparing with the upper one. However, the expressions, published by (Lampe, 1985) still may be used to have an idea of the dipole behavior with variation of line width, space between strips, etc. To obtain the dipole impedance di p ole d d ZR j X = + some simulations were carried out using the full wave simulator CST, varying the dipole geometric parameters. Design of RFID Coplanar Antenna with Stubs over Dipoles 55 Fig. 6. Dimensions and parameters of the coplanar strip folded dipole. Figures 7(a) and 7(b) present the real and imaginary part of the input impedance as a function of W1, respectively. Figures 8(a) and 8(b) present the real and imaginary part of the input impedance as a function of W2, respectively. Following the same idea, Figures 9 and 10 present the input impedance variations with S and D dimensions, respectively. Fig. 7. Input impedance as a function of W1. (a) is the real part and (b) is the imaginary part. Fig. 8. Input impedance as a function of W2. (a) is the real part and (b) is the imaginary part. W1(mm) ( a ) W1(mm) ( b ) W2(mm) ( a ) W2(mm) ( b ) Advanced Radio Frequency Identification Design and Applications 56 Fig. 9. Input impedance as a function of s. (a) is the real part and (b) is the imaginary part. Fig. 10. Input impedance as a function of D. (a) is the real part and (b) is the imaginary part. The half-antenna input impedance at the plane A-A’ (Figure 4) is given by the usual equation for transmission lines (Chang, 1992): ( ) () dipole 0 in 0 0dipole ZZtanhγL ZZ ZZ tanhγL + = + (2) where γ is the propagation constant of the wave, L is the transmission line section length and Z 0 is the characteristic impedance of the transmission line. The value of Z 0 is also calculated by quasi-static conformal mapping equations. Figure 11 shows a coplanar folded dipole design. This structure is more suitable for matching with only the real part of the input impedance. Figures 12(a) and 12(b) present the real and imaginary part of the input impedance as a function of the length of the stub l, respectively. The imaginary part goes from negative to positive values as the length l increases from 0(mm) to 20(mm). For a fixed value of l, Figures 12(a) and 12(b) can be used for impedance match between the antenna and the chip or between the antenna and the network analyzer. Note that the input impedance also can change with the spacing g, the width k and the distance H. s(mm) ( a ) s(mm) ( b ) D(mm) ( a ) D(mm) ( b ) Design of RFID Coplanar Antenna with Stubs over Dipoles 57 feed terminal Fig. 11. Dimensions and parameters of the coplanar strip folded dipole. Fig. 12. Input impedance as a function of l. (a) is the real part and (b) is the imaginary part. Fig. 13. Antenna layout. The stubs are placed over the dipoles. l(mm) ( a ) l(mm) ( b ) Advanced Radio Frequency Identification Design and Applications 58 Dimensions Condition 1 Condition 2 h 53 mm 53 mm w 4 mm 4 mm a 3 mm 3 mm d1 8.5 mm 8.5 mm d2 8.5 mm 8.5 mm L 26.5 mm 26.5 mm T 38.5 mm 38.5 mm A 0 mm 14 mm Table 1. Dimensions of the antenna Note that all dimensions have the same value for condition 1 and 2, except for A . The A = 0 mm means no stubs. For all dimensions described in Table 1 - condition 1, the input impedance of half the antenna is Z 100 j100Ω in = + . Because its symmetry, the impedance of whole antenna at the plane A-A´ is to be Z in Z ant 2 = . In other words, Z 50 j50Ω ant =+ . The imaginary part of Z ant can be significantly decreased by placing planar stubs over the dipoles. On the other hand, the real part of Z ant is slightly altered. Those facts are important when purely real impedance is needed. That is the case when stubs of length A = 14mm are added to the dipoles (Table 1 - Condition 2). At that length, the above described impedance becomes Z49Ω ant = and the imaginary part is no longer seen. 4. Fabrication measurement and simulation The antenna described in the previous section was simulated with a full wave EM software. The fabricated antenna with stubs over the dipoles is shown in Figure 14. It was implemented on a RT6002 substrate of thickness 1.5mm, relative dielectric permittivity ε r = 2.94 and loss tangent δ = 0.0012. Simulations were taken in the 1.5 – 3 GHz range. Calculations of input impedance were taken at 2.45GHz, which is the central frequency of the free 2.4GHz part of the spectrum. Figure 15, 16 and 17 show the comparison between simulated and measured results and good agreement can be noticed. Figure 18 and 19 show the radiation pattern and the gain of this proposed antenna at 2.45GHz. The antenna lies in the plane θ = 90° and has its printed strips at the right-hand side. The maximum gain is increased over the direction perpendicular to the antenna plane. Still from Fig. 7, one sees that the highest simulated gain reaches 5.97dB over an isotropic radiator. The measured gain reaches 5.6dB, which is very close to the simulated one. These values are at least twice higher than the gain of an ordinary dipole (Finkenzeller). Simulated results show how the stub length can modify the Z dipole and the antenna input impedance Z ant , as a consequence. Thus, it is possible to choose some suitable stub length for the desired antenna input impedance. For example, for A = 14 mm, one finds the simulated antenna input impedance of Z50j7Ω ant =+ . It is very close to that one of 49 Ω , expected in the section before. On the other hand, the measured value of the new antenna is ant Z48j7Ω=+ . [...]... Hilbert-curve slot and tuning pad For circular polarization analysis, the current distribution and electric field are exhibited The inductive and broadband characteristics of frequency responses and directivity feature of radiation patterns and polarization are studied and presented  64 Advanced Radio Frequency Identification Design and Applications 2 Antenna configuration and basis 2.1 UFH RFID meander-line.. .Design of RFID Coplanar Antenna with Stubs over Dipoles Impedance - real part (Ohms) Fig 14 The printed antenna measured simulated frequency (GHz) Impedance - imaginary part (Ohms) Fig 15 Simulated and measured real part of the impedance measured simulated frequency (GHz) Fig 16 Simulated and measured imaginary part of the impedance 59 60 Advanced Radio Frequency Identification Design and Applications. .. – Analysis and Design, 2nd ed., New York, NY, USA: John Wiley & Sons, Chapter 2, p 88 U Karthaus and M Fischer (2003) “Fully Integrated Passive UHF RFID Transponder IC With 16.7μW Minimum RF Input Power”, IEEE Journal of Solid State Circuits, vol 38, no 10, pp 1602-1608, Oct 62 Advanced Radio Frequency Identification Design and Applications K Finkenzeller, RFID Handbook: Fundamentals and Applications. .. line and the right Hilbert-curve slot, the elliptic polarization will be obtained Thus, the circular polarization can be observed along a certain direction 68 Advanced Radio Frequency Identification Design and Applications Fig 7 Current distributions Fig 8 Electric fields 2.5 Applications The maximum activation distance of the tag for the given frequency is given [ 14] –[15] by d max = c EIRPR 4 f... reduction is notable when the SR is more than 0 .40 [2] 66 Advanced Radio Frequency Identification Design and Applications (a) (b) (c) Fig 4 Self-complementary antenna configurations, (a) log-period (b) spiral (c) circular disk A typical circular polarization dipole cross-pair usually consist of two individuals with horizontal and vertical locations, and a two-phase signal with 90° difference Fig 7... Bing-Hao Zeng1 and Dau-Chyrh Chang2 1Ching 2Oriental Yun University Chung-Li City, Taoyuan County, Taiwan Institute of Technology Pan-Chiao City, Taipei County, Taiwan R.O.C 1 Introduction Recently there has been a rapidly growing interest in RFID systems and its applications Operating frequencies including 125 KHz–1 34 KHz and 140 KHz– 148 .5 KHz LF band, 13.56 MHz HF band and 868 MHz–960 MHz UHF band were... proposed antenna with UHF-bands of 900 MHz are measured and simulated shown in Fig 9 The simulated and measured results of frequency responses are in agreement In measurement, while the return loss is smaller than -10dB, the frequency responses cover both Europe 865.6–867.6 MHz band and USA 902–928 MHz band, ranging from 820 to 935 MHz (bandwidth = 115 MHz) For applications, the frequency responses are... applied to various supply chains 43 3 MHz band was decided for active reader and 2 .45 GHz band was applied for WiFi reader Besides the reader antennas, the requirements of tag antennas are necessary for applications In which, due to the benefit of long read range and low cost, the UHF tag will be used as the system of distribution and logistics around the world [1–13], [29 41 ] Meander line antennas were commonly... H-shaped meandered-slot antennas with the performance of broadband and conjugate impedance matching were developed for on-body applications [ 14] , [15] On the other hand, the self-complementary dipoles were introduced for the performance of wideband, high impedance and balun [16]–[23] The Hilbert-curve, proposed by Hilbert and introduced by Peano [ 24] , was known as the space-filling curves The structure of... length and number of line segments with 2nd, 3rd and 4th iterations, this dimension are 1 .46 5, 1.6 94 and 1.8 34 These values point to the fact the geometry has fractional dimension As the dimension approaches 2, the curve almost fills a space In other words, for large iteration orders, the total length of the line segments tends to be extremely large This could be a significant advantage in lower frequency . Jordan. Advanced Radio Frequency Identification Design and Applications 50 Salama A. M. A., (2010). Antennas of RFID Tags, Radio Frequency Identification Fundamentals and Applications Design. 1602-1608, Oct. Advanced Radio Frequency Identification Design and Applications 62 K. Finkenzeller, RFID Handbook: Fundamentals and Applications in Contactless Smart Cards and Identification, . radiation patterns and polarization are studied and presented. Advanced Radio Frequency Identification Design and Applications 64 2. Antenna configuration and basis 2.1 UFH RFID meander-line antenna