Dissertation Antenna design for future multi-standard and multi-frequency RFID systems ausgefu¨hrt zum Zwecke der Erlangung des akademischen Grades eines Doktors der technischen Wissenschaften unter der Leitung von Ao.Univ.Prof. Univ.Prof. Dipl.-Ing. Dr.techn. Arpad L. Scholtz Technische Universita¨t Wien Institut fu¨r Nachrichtentechnik und Hochfrequenztechnik (E389) zweiter Begutachter Prof. Dr.-Ing. Dr.-Ing. habil. Robert Weigel Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg Lehrstuhl fu¨r Technische Elektronik eingereicht an der Technischen Universita¨t Wien Fakult¨at fu¨r Elektrotechnik und Informationstechnik von Lukas W. Mayer, 9927042 Hahngasse 12/17 1090 Wien Wien, Juni 2009 Abstract In this work I investigate radio propagation and antennas used in radio frequency identification (RFID) systems operating in the high frequency (HF) band at 13.56MHz and in the ultra-high frequency (UHF) band between 860MHz and 960MHz. Where at 13.56MHz, the transmission between an interrogator (or reader) and a transponder (or tag) are well known and can be determined ana- lytically, modeling of the radio transmission in the UHF band is a far more chal- lenging task. Instead of modeling the UHF channel, I decided to pursue a rather empirical approach by performing and discussing measurements in RFID typical environments. The results show that the region where transponders are reliably recognized might be quite small. However, with some modifications to the scenario and with appropriate choice and placement of reader and tag antennas, it is shown how reliability can be significantly improved. In a further chapter I focus on the interaction between a transponder antenna and a transponder chip at UHF. In particular, the mechanism of backscattering that is utilized to achieve a data transmission from the transponder to the interrogator (the return-link) is theoretically analyzed. It is found that for a given transponder chip, there exists two distinct optimum antenna impedances, one maximizes the power available for the chip’s internal circuitry and one maximizes the signal that is reradiated towards the interrogator. To draw conclusions for the link budget of an RFID system, a numerical example is presented that relies on measured charac- teristics of state-of-the-art transponder chips. This new measurement method for thoroughly characterizing the input impedances and the sensitivity of transponder chips is also presented in this work. For exploiting the benefits of both the HF and the UHF band, a dual-band trans- ponder antenna was designed. The antenna consists of a shorted loop-slot antenna serving the UHF band and a printed spiral operating as an air coil in the HF band. Both structures were merged into one single-input dual-band antenna. Therefore, the UHF antenna structure was modified to additionally function as two turns of the HF coil. With this arrangement the performance at HF remains at the level of single-band antenna solutions. To determine the characteristics at UHF, a new and highly accurate measurement method was developed to characterize both input impedance and gain of electrically-small and autonomous antennas. By comparing i the dual-band antenna with a manufactured single-band antenna occupying the same amount of area, it was shown by measurement that the performance of the dual-banddesignatUHFisonlyimpairedbysomefewdecibel. In return-link limited UHF RFID systems, the read range is determined by the ca- pability of the interrogator’s radio frequency (RF) frontend to separate the strong transmitted signal leaking into the receiver from the weak backscattered signal reradiated by the transponder. To avoid costly carrier compensation circuitry in the RF frontend, I designed a microstrip patch antenna that uses a tunable di- rectional coupler to provide high transmit to receive separation. By continuously tuning this coupler, a separation of more than 52dB was achieved in time variant scenarios. ii Contents 1 Introduction 1 1.1 Primarily deployed RFID systems today . . . . . . . . . . . . . . . 1 1.2 Magnetic coupling in HF RFID systems . . . . . . . . . . . . . . . . 3 1.2.1 Antennas for HF systems . . . . . . . . . . . . . . . . . . . . 3 1.3 Radio transmission in UHF RFID systems . . . . . . . . . . . . . . 7 1.3.1 Measurements of UHF RFID channels . . . . . . . . . . . . 10 1.3.2 Reciprocity . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.3.3 Antennas for UHF RFID systems . . . . . . . . . . . . . . . 16 1.4 Radio transmission at UHF versus inductive coupling at HF . . . . 18 2 Antenna/chip interaction 21 2.1 Matching antenna and chip impedances . . . . . . . . . . . . . . . . 21 2.2 Scattering properties of antennas . . . . . . . . . . . . . . . . . . . 22 2.3 Backscatter modulation . . . . . . . . . . . . . . . . . . . . . . . . 24 2.4 Effects on a system shown with a numerical example . . . . . . . . 31 2.4.1 Link budget . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.4.2 Forward link . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.4.3 Return link . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.5 Consequences for an RFID system . . . . . . . . . . . . . . . . . . . 37 3 A dual-band HF/UHF transponder antenna for RFID tags 39 3.1 Dual-band transponder antenna requirements . . . . . . . . . . . . 40 3.1.1 Electrical definition . . . . . . . . . . . . . . . . . . . . . . . 40 3.1.2 Environmental definition . . . . . . . . . . . . . . . . . . . . 42 3.2 Antenna design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 3.2.1 HF antenna . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 3.2.2 UHF antenna . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.2.3 Combining the antennas . . . . . . . . . . . . . . . . . . . . 45 3.2.4 Antenna tuning . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.2.5 Dual-band antenna with HF coil on the inside . . . . . . . . 49 3.3 Antenna simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 3.3.1 Full model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.3.2 Simplified planar model . . . . . . . . . . . . . . . . . . . . 51 3.4 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 iii 3.4.1 HF measurements . . . . . . . . . . . . . . . . . . . . . . . . 53 3.4.2 UHF impedance measurements . . . . . . . . . . . . . . . . 54 3.4.3 UHF gain measurements . . . . . . . . . . . . . . . . . . . . 56 3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 4 An interrogator antenna with high transmit to receive separation 67 4.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.2 Antenna principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.3 Tunable hybrid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 4.3.1 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 4.3.2 Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . 75 4.4 Square patch antenna . . . . . . . . . . . . . . . . . . . . . . . . . . 76 4.4.1 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 4.5 Automatic tuning hardware . . . . . . . . . . . . . . . . . . . . . . 79 4.6 Performance testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 4.7 Considerations for automatic tuning . . . . . . . . . . . . . . . . . . 82 4.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 5 Gain and impedance measurement method for transponder antennas 85 5.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 5.2 Directional pattern and antenna efficiency measurement . . . . . . . 87 5.3 Rotation unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 5.4 Small battery driven oscillator . . . . . . . . . . . . . . . . . . . . . 89 5.5 Antenna impedance measurement . . . . . . . . . . . . . . . . . . . 92 5.6 Test measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 5.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 6 Impedance measurement method for transponder chips 99 6.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 6.2 Chip test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 6.3 Chip test fixture . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 6.4 Wake-up sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 6.5 Impedance measurement . . . . . . . . . . . . . . . . . . . . . . . . 107 6.6 Power sweep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 6.7 Efficiency of backscattering . . . . . . . . . . . . . . . . . . . . . . . 109 6.8 Comparison of the results with manufacturer’s data sheets . . . . . 112 6.9 Parameter sweeps . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 6.10 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 7 Conclusion 115 iv 1 Introduction This work relates to practical antennas for the economically most relevant radio frequency identification systems that are in use or under development today. RFID is a method that allows to remotely retrieve, store, and manipulate data contained in a transponder unit that is permanently attached to an object. This introduction focuses on these systems and explains their mode of operation as well as the phys- ical basics that can be exploited to achieve energy and data transmission between an interrogating device (or reader) and a passive transponder (or tag). Antenna requirements for RFID systems are rendered and today’s most frequently used an- tenna solutions are explained. To obtain the characteristics of the radio channel, some results of a measurement campaign that was conducted with RFID typical antennas in prevalent scenarios are presented. The chapter concludes with a com- parison of two systems, one of them utilizes magnetic coupling at 13.56MHz and the other one uses radio transmission in a frequency band between 860MHz and 960MHz. Original publications related to this chapter L. W. Mayer, M. Wrulich, and S. Caban, “Measurements and channel modelling for short range indoor UHF applications,” in Proc. The First European Conference on Antennas and Propagation, (Nice, France), Oct. 2006. L. W. Mayer and A. L. Scholtz, “Antennas and radio transmission in RFID sys- tems.” Talk: EEEfCOM - Workshop, Ulm, Germany, May 2008. L.W.MayerandA.L.Scholtz,“AntennenundFreiraumausbreitungfu¨rUHFRFID Systeme.” Talk: Tutorial on ”Radio Frequency Identification Systems (RFID)”, Forschungszentrum Telekommunikation Wien, Austria, Oct. 2007. 1.1 Primarily deployed RFID systems today Object identification for managing a supply chain is probably the most relevant field for RFID technology. This task requires cheap, compact, reliable, and often 1 1 Introduction long-lasting transponder units that contain a non-volatile random access memory for storing and updating data. Storing data directly with the object saves costs and allows to quickly process or pass on the object without the need for an inquiry at a remote database in real-time. Low-cost transponder units are passive, which means that they do not have a dedicated power source, but draw the energy needed for operation from the electromagnetic field that is produced by an interrogating device. In recent years, two system concepts have evolved that utilize the electromagnetic field for data and energy transmission between the transponders and the interroga- tors. Apart from an affinity in the principle operation these two systems operate at very different carrier frequencies and thus require separate consideration, especially concerningtheantennasandRF1 frontends. Thesesystemsare RFID systems that operate in the HF2 band at 13.56MHz and • RFIDsystemsthatoperateintheUHF3 bandbetween860MHzand960MHz. • Both systems are available at a quite advanced stage today. Some different stan- dards treating the air interface have been released for HF [1, 2] and UHF [3, 4]. The latter have very recently been consolidated and published by the ISO4 as “Amendment 1” to the existing ISO18000-6 standard [5]. However, there is great potentialinoptimizingthefunctionalityandreliabilityofRFIDsystemsbeyondthe standardized—especially in the UHF domain. Many techniques like beam-forming, active carrier cancelation, advanced RF frontend designs, and extensive use of digi- talsignalprocessinghardwaretomanagee.g.collisionsofmultipletranspondersare not yet provided by most commercially available RFID systems. In 2004, Infineon Technologies Austria AG started the research project Comprehensive Transponder System (CTS) to determine whether a system can be set up that is able to switch between the HF and the UHF band, thus exploiting the benefits of both frequency bands. In the CTS project I was assigned to investigate the radio transmission in both the HF and the UHF band and to do research on practical antennas that enable dual-band operation. At the interrogator side a dual-band antenna design was not considered because separate HF and UHF antennas can be used. Designing dual-band antennas for RFID transponders on the other hand is very challeng- ing. The characteristics of the transponder antenna have a direct impact on the system performance that can hardly be regained by relaxing other system con- straints. The transponder antenna thus has to compete with the performance of 1RF: radio frequency 2The high frequency (HF) band ranges from 3MHz to 30MHz. 3The ultra-high frequency (UHF) band ranges from 300MHz to 3GHz. 4ISO: International Organization for Standardization 2 1.2 Magnetic coupling in HF RFID systems single band variants and is subject to the same tight constraints regarding size and costs. 1.2 Magnetic coupling in HF RFID systems At 13.56MHz, magnetic coupling is used to transfer energy between two coils, one is the interrogator coil and one is the transponder coil. The interrogator coil is loaded with an alternating current and produces a dominantly magnetic field. The transponder coil that is pervaded by this field produces an induced voltage at its output. Figure 1.1 shows the principle of the transformer-like coupling be- tween the two windings. The magnetic field around the interrogator coil can be calculated by adding the contributions of a segmented version of the current car- rying conductors [6, 7] and it can be shown that the field strength H decays in- versely proportional with the distance r from the coil taken to the power of three ( H 1/r3). | | ∼ This loosely-coupled transformer-like arrangement can be used to transfer energy from the interrogator to the transponder. The power transmission loss between the interrogator coil input and the transponder coil output is related to the mutual inductance [8, 9, 10, 11] and decays with the transmission distance to the power of six (P 1/r6). This shows that a long read range can not be achieved with two Tag ∼ electrically-small coils at a frequency of 13.56MHz. As a representative example the power transmission coefficient h 2 = P /P (1.1) HF RX,Tag TX,Reader | | between two circular windings that are aligned coaxially in free space is given by Equation 1.2 [12]. A description of the symbols is given in Table 1.1. In Section 1.4 a plot of an exemplary HF transmission coefficient versus distance will be shown (Figure 1.6). 1 (µ ωπN N a2b2)2 h 2 = 0 1 2 (1.2) | HF| 4R R (r2 +a2)3 1 2 1.2.1 Antennas for HF systems For low-cost applications in the HF band, wound and printed coils are used at the interrogator and the transponder, respectively. Since there is almost no power radiated because the coils are very small compared to the free space wavelength, the power transmission loss mainly depends on the characteristics of the windings and their position and orientation in space [8, 13]. 3 id283122406 pdfMachine by Broadgun Software - a great PDF writer! - a great PDF creator! - http://www.pdfmachine.com http://www.broadgun.com 1 Introduction Z < qs $ i‚sI‚„ Figure 1.1: Principle of coupling between reader and tag at 13.56MHz. HF interrogator coils To power a transponder the interrogator coil has to provide a minimum required magnetic field strength H at the transponder’s position. Practical and efficient Min interrogator coils achieve that with a minimum number of turns N that conduct a minimum current I. When optimizing the coil radius a of a circular coil that provides H at the transponder’s position with respect to a minimum number of Min ampere-turns NI, an optimum coil radius can be determined. According to [12], the optimum coil radius a that requires the minimum number of ampere-turns Opt for a particular read range r is a = √2 r. The read range of an HF system Opt · is thus closely related to the size of the interrogator antenna. Coils with a radius smaller than a will either require more turns, or more current. Both measures Opt lead to higher losses in the coil and—more importantly—to a higher magnetic field strength in the coil center. This relation implicates that the compact long- range reader for an HF RFID system is not feasible—at least not when the limits for the emitted magnetic field strength enacted by local authorities have to be met. Efficient interrogator coils usually consist of a wire or a band made from a well conducting material like copper or silvered metal. Depending on the application the coil might fit in a hand-held reader device, surround a conveyor belt, or might 4