Because the possibility of array element failure during operation cannot be ruled out , selfrecoverable antenna array systems have recently been receiving considerable attention in modern radar technology. Thus, an approach to maintaining the radi
This work studies the three-dimensional (3D) identification performance of UHF RFID systems with commodity hardware. Detailed three-dimensional propagation modeling is developed, with ray-tracing that allows examination of tag- as well as reader-ante
This paper explains our proposed wireless security and monitoring system using array antenna, referred to as array sensor. The proposed system exploits an antenna array on the receiver side and decomposes received signals into eigenvectors and eigenv
This paper presents new study and design of microstrip phased array antennas used for extending the coverage area of RFID applications (ISO 18000-4 for 2.45GHz). The main idea is to modifying the antenna radiation pattern in order to extend the cover
Switched array antenna with six microstrip patch elements was designed and characterized for UHF RFID application. A hexagonal-shaped RF switching circuit board was developed to select one of the array elements to read RFID tags surrounding the RFID
This paper addresses the issue of channel estimation (CE) for multiple antennas. Multi-antenna systems at the transmitter and/or receiver can improve the receivers performance in the presence of fading. Comparing the space-time coding (STC) schemes a
Coverage area is an important performance metric for RFID systems, especially those used for inventory management. As such, there are a range of methods being developed to try to increase the coverage area of RFID systems without requiring additional
This article presents simulation results for characterizing an 11 × 11 antenna array using phaseless planar near-field technique. The outcomes of this analysis show that with the help of the well-known iterative Fourier transform and with minimal a p
A circularly polarized antenna with low cost foam substrate and film is proposed to obtain wide bandwidth and high performance. The proposed antenna is composed of stacked corner-truncated patches with sequential rotation feed. Having above 20% retur
A dual-corner-fed square patch is proposed to realize the dual-polarization operation. Its analysis results are compared with those of a dual-edge-fed patch. A new single-layer hybrid array of these elements has been designed, fabricated and measured
A construction, calculation relationships and directive characteristics of travelling wave tunable array with central excitation are studied. It is shown that by variation of the corner angle of this array the constant direction of main lobe maximum
Eva Rajo-Iglesias is the editor of this month's Wireless Corner.
A multiple null algorithm that can adjust the number, width and depth of multiple nulls in accordance with the environment requirements is proposed for the improvement of the interference cancellation capability of two-dimensional array antennas. The
Summary form only given, as follows. Radio-frequency identification (RFID) technology provides an effective tool for managing traceability along food supply chains. This is because it allows automatic digital registration of data, and therefore reduc
A novel analytically based synthesis technique for the design of sparse antenna arrays is presented. The proposed technique permits the synthesis of linear arrays with arbitrarily shaped patterns and it is applied here to the design of an Earth-cover
This paper presents concept of an ultrawide band penta-beam antenna array with stable coverage area. The proposed antenna array produces five narrow beams (HPBW
This paper presents an array antenna for WIFI/bluetooth access point application. The antenna is made of a four element array. The operation bandwidth is from 2400 to 2480 MHz. The array is excited by a feeding network made of tapered pair-wire trans
The downlink capacity and coverage of mobile WiMAX system with array antenna is evaluated based on given interference analysis with multi-cell deployment. The link budget model with array antenna is also given. The results show that the capacity and
Optically-fed distributed antenna system (DAS) technology is combined with passive ultra high frequency (UHF) radio frequency identification (RFID). It is shown that RFID signals can be carried on directly modulated radio over fiber links without imp
Abstract form only given. The Cospas (Cosmicheskaya Sistyema Poiska Avariynich Sudov)-Sarsat Search-and-Rescue (SAR) satellite system provides distress alert and location data to assist rescue operations at sea, in the air, or on land. This paper sum
The increase in small, distributed facilities expected in the near future has raised the necessity of having low-cost, transportable radar systems. In turn, this requires the design of high-gain, low-cost, planar, robust, and easy-to-assemble antenna
Nattali (Tuli) Herscovici
Lincoln Laboratory - Group 61 Massachusetts Institute of Technology 244 Wood Street Lexington, MA 02420-9108 USA Tel: +1 (781) 981-0801 Fax: +1 (928)832-4025 Skype/AOL: tuli01 E-mail: [email protected]
Department of Electrical and Computer Engineering University of New Mexico Albuquerque, NM 87131-1356 USA Tel: +1 (505) 277 6580 Fa~+1 (505)2771439 E-mail: [email protected]
RFID Coverage Extension Using Microstrip-Patch Antenna Array Mehmet Abbak and ibrahim Tekin Electronics Engineering, Sabanci University 34956 Istanbul, Turkey Tel: ++90(216) 483 9534; Fax: ++90(216) 483 9550; E-mail: [email protected]
Abstract In this paper, a UHF-band 2 x 2 microstrip phased-array antenna is designed and implemented to extend the coverage of an RFID reader system. The phased-array antenna has four microstrip-patch antennas, three Wilkinson power dividers, and a transmission-line phase shifter. These are printed on a dielectric substrate with a dielectric constant of 4.5. The array has dimensions of 34 cm x 45 cm, operating at a frequency of 867 MHz, as specified in RFID Gen2 protocol European standards. The phased-array antenna has a measured directivity of 12.1 dB, and the main-beam direction can be steered to angles of ± 40°, with a HPBW of 90°. The phased-array antenna is used as the receiving antenna in a commercial reader system. Experimental results indicate that the coverage of the RFID system with the phased-array antenna is superior to the coverage with a conventional broader-beamwidth microstrip-patch antenna. The proposed system can also be used for a wireless positioning system. Keywords: RFID; identification; microstrip antennas; UHF antennas; array antennas
ments. This paper proposes a phased-array antenna system, with the goal of extending the coverage range of an RFID system.
FID systems are now deployed in our daily lives, and have Rstarted to improve the quality of daily life by making things easier and faster, across the board. However, due to the small size of tags and, hence, the size of the antenna's aperture, long-range operation and coverage is troublesome for RPID systems. In order to extend the coverage area of an RPID system, one may inlplement many readers and antennas with small reading ranges to cover the monitoring area. Alternatively, one may use a high-gain phased-array antenna system to obtain an extended reading range of the RPID reader, for a smaller number of total reader deploy-
For passive RPID systems in the EPCglobal Classl Gen2 RPID standard working at UHF, the working range of the RPID system is limited compared to that of active systems . RPID systems for general-purpose applications use antennas with wide beamwidths and, hence, small gains, to receive and transmit the RPID signals. Consequently, to overcome the short range limitations of RPID systems - both due to passive tags and wide beamwidths - a novel phased-array antenna system with higher gain can be used for beam fonning, to increase directivity and, hence, to increase range.
IEEE Antennas and Propagation Magazine, Vol. 51, No.1, February 2009
The operating range of an RFID system is based on tag parameters, such as the tag antenna's gain and radar cross section, the distances between readers, the operating frequency, the transmitted power from the reader to the tag, and the gain of the reader's antenna. The number of receiving and transmitting ports is another factor affecting the operation range of the system. A new approach to RFID systems is a multi-static system design, with its significantly better sensitivity to weak tag backscatter signals, and its superior RF coverage area. To demonstrate the advantage of this approach - in which two antennas can both receive and transmitradiated power levels were plotted. For a certain signal level, the bistatic approach offers larger coverage, as shown in Figure 1. Various approaches to increasing the range of UHF passive RFID systems have been discussed in the literature. Increasing the sensitivity of the RFID reader, which can work with weaker signals received from the tag; reducing power consumption; and increasing power efficiency on the tag circuit are methods of increasing the operating range . Other improvement suggestions for the design of the RFID tag's antenna and chip concurrently to decrease the tum-on voltage of the tag's chip for increased reading-range operation were given in . Furthermore, the theory of a diversity system, which could decrease the required power level for the same bit-error rate, and could therefore increase the operating range, was investigated in . In addition, the operating range of a hand-held RFID reader for different types of patch antennas has been investigated . This showed that the gain of the antenna is a fundamental factor ofRFID system range. However, the most applicable way for increasing the reading range of a UHF RFID system is to increase the gain of the reader's antenna, since there is a relaxed size limitation, unlike that of the RFID tag.
is called the free-space loss factor. Wavelength
and distance are variables in this factor, and cannot be changed for a certain application. The transmitted power, PT , is also maximized to a certain point according to the regulations on UHF RFID systems. The other remaining factors are the gains of the antennas of the reader and the tag. With respect to the monostatic approach, two antennas are used on the reader side where they transmit and receive signals on separate antennas. When the reader has two antennas for separate receiving and transmitting purposes, we can conceptualize the unit as a radar system. This is because transmission from the tag to the reader in a UHF RFID system is that of simple scattering. From this view, the power received at the receiver reader in Figure 2 can be written as in Equation (2), where ( j is the radar cross section, a measure of an object's ability to reflect electromagnetic waves: (2)
A minimum received signal level is specified for the maximum possible operating range. In other words, there is a minimum PR for which the system can operate. For a fixed RR ,mm. to increase
For wireless-sensor network applications, the range-extension capability can be used, and this will decrease the number of nodes in the network. Alternatively, for the same communication distance, the proposed system will require less transmitted power, since a higher-gain phased-array system is deployed. One further advantage of using a phased-array antenna system is that the directional information is already available at the reader. This information can be used to find the position of the tags: wireless positioning. In this paper, the design of the microstrip-patch antenna array - including all of the components, such as Wilkinson power dividers, phase shifters, and antenna elements - will be specified. Moreover, phased-array simulations and measurements will be presented. The field measurements, which are taken using a commercial RFID system, will be provided to show that the coverage of the RFID system is actually extended.
Figure 1. Monostatic and bistatic reader coverage compared.
2. Range Extension of the RFID System Using a Phased-Array Antenna System The reading range of a passive tag is limited by its ability to provide sufficient voltage and power at the antenna to power the tag's integrated circuit. In a basic sense, to extend the range of a UHF passive RFID system, received power should be increased. As the famous Friis transmission equation states in Equation (1), received power is based on the transmitted power, the wavelength, the distance, and the gains of the antennas on both the transmitting and receiving sides: 186
Figure 2. A bistatic RFID system. IEEE Antennas and Propagation Magazine, Vol. 51, No.1, February 2009
the range ofRFID operation (Rl and/or R2 ), either the receiving or
transmitting antenna gains, Gl or G2 can be increased. In our approach, one of the antennas of the bistatic reader will be replaced by the phased-array antenna, with a more directive beam with higher gain. Increased antenna gain will increase the radial range. However, due to the narrower beamwidth, the angular coverage will be decreased. In our technique, the phased array can be steered to two different directions so that the angular coverage is not affected. Instead, it will be extended.
IOJA Power Divider
One might argue that effective radiated power (ERP) will be increased by using a high-gain antenna. However, if the average power is calculated, it will be the same as for the fixed-beam lessdirective antenna, because the beam will be steered back and forth between these two states and will decrease the average power. In normal operation, the beam will be shifting between the two states for a predetermined amount of time. The operating range of passive UHF RFID systems can also be affected by interference. The most common types of interference would be caused to the tag and to the reader by multiple readers in dense reader environments, such as warehouses and manufacturing facilities. Signals transmitted from distant readers can be strong enough to obstruct the accurate decoding of the signals back-scattered from tags. How the interference affects the operating range of passive UHF RFID systems through the SIR (signal-to-interference ratio) was explained in . Another useful aspect of a directional antenna system can thus be the reduction of the SIR in multi-reader environments. This is because the gain of the readers' antennas will be much more less at the undesired angles, compared to conventional antennas that need to cover the same area.
Figure 4. A diagram of the antenna array. Patch Antenna Return Loss S11
, , I I
1 I , I I I I I I I II I'
3. Implementation and Results
A schematic view of the proposed antenna array is shown in Figure 4. The phased array consists of four (2 x 2) patch antenna elements, Wilkinson power dividers, and phase shifters enabled by SPDT switches. By using transmission-line-based phase shifters, the main beam of the array can be steered to two main directions, shown in Figure 3 as State 1 and State 2. The main purpose of steering the main beam of the array is to extend the coverage while increasing the gain of the antenna. A typical radiation pattern of a microstrip-patch antenna is shown in Figure 3 as radiating into the
850 900 freq (MHz)
Figure 5. The return loss of a patch antenna.
..3dB beamwidth: 70,68
._ ...._.. _. co polar '" cross polar
Radiation pattem of single patch antenna
X axis Figure 3. Extending the coverage area. IEEE Antennas and Propagation Magazine, Vol. 51, No.1, February 2009
Figure 6. The measured co- and cross-polarization (H-plane) radiation patterns. 187
Figure 10. The power divider.
Figure 7. The measured co- and cross-polarization (E-plane) radiation patterns.
Wilkinson Power Divider Or--..,.--....,....----..,.-----,.--.,.......-~---"T'"""---r-----.---.
-5 -10 -15 ~ -20
-25 -30 -35
Figure 8. The phase shifter.
1,000 1,100 1,200 1,300 1,400 1,500 freq (MHz)
Figure 11. The S parameters for the power divider.
Figure 9. The S parameters for the phase shifter. 188
Figure 12. The array antenna. IEEE Antennas and Propagation Magazine, Vol. 51, No.1, February 2009
half space. Also shown in the figure is a more-directive beam of a phased-array antenna with two different pointing directions.
Hp'ane (State 1)
The antenna feeding network was designed to steer the beam in two directions in the H plane (±400). This design necessitated a phase difference of 120° between the antenna sets (1, 2) and (3, 4) for 0.4l spacing between the antennas, as shown in Figure 4. There was no phase shift between antennas 1 and 2, and between antennas 3 and 4. The spacing in the x direction was set to 0.3l, and the spacing in the y direction was set to 0.4l, to obtain optimum gain and mutual coupling. For extended simulations, the ADS Momentum 2.5D electromagnetics software was used. Each component in the antenna-array system - microstrip patch antenna, phase shifter, and Wilkinson power divider - was first designed using ADS Momentum, and then implemented. It was finally measured using an Agilent 8270ES S-Parameter Network Analyzer. For the design of the microstrip-patch antenna, various geometrical parameters and material parameters had to be determined. The initial values were resolved with the use of the equations for a microstrip-patch antenna from Balanis . The PCB (printed-circuit-board) etching technique for the microstrippatch antenna was fabricated with a substrate material having a relative electric permittivity of 4.55 (Gy ) and a thickness of 1.52 nun. The measured input return loss of the patch antenna is given in Figure 5. The 10 dB return-loss bandwidth was about 15 MHz, and the antenna radiated at 867 MHz with a return loss of 22 dB. The radiation pattern of the patch antenna was measured in a compact test range. The broadside direction pattern had better than a 15 dB cross polarization in the E and H planes. The 3 dB bandwidth was 70° in the H plane, and 80° in the E plane, with a directivity of 7.5 dB, as shown in Figures 6 and 7. For implementation of the phase shifter, the delay arm had to provide an additional 120° phase difference with respect to the reference ann, because the 240° phase difference necessary between the left and right arm required ±120° phase differences. The phase shifter as implemented FR4 ubstrate is shown in Figure 8. The measurement resul are given 'n Figure 9. An insertion loss of 1.1 dB and a return loss of 50 dB were measured for a 120° phase difference (Figure 9). A MAlCOM high-power SPDT switch chip was used to switch between the two branches of the
H plane (Stale 2) ~
Figure 14. The measured co- and cross-polarizations (H-plane) for two states of the phased-array antenna.
Return Loss (S11)
transmission line. As the last block of the array system, the Wilkinson power-divider circuit was realized and measured, as shown in Figures 10 and 11. According to the measurement results, the power was delivered equally with an insertion loss of 0.1 dB (812 ~ 813 ~ -3.1 dB), alongside an isolation of 40 dB at 867 MHz. (Figure 11). Finally, the total array feeding network, composed of power dividers, phase shifter, and patch-antenna elements, was appropriately designed, minimizing line losses and mutual coupling while maximizing array gain and efficiency [7, 9].
-20 -25 -30
Figure 13. 811 for the array antenna. IEEE Antennas and Propagation Magazine, Vol. 51, No.1, February 2009
The final layout of the RFID array antenna is shown in Figure 12, where NH9450 substrate (Gy = 4.5, tan l5 = 0.002 at 2 GHz with 1.52 nun thickness) was used. Figure 13 depicts the overall response of the phased-array antenna for two different positions of the switches. A return loss of 30 dB was obtained at the resonance frequency of 867 MHz for two different main-beam positions. A bandwidth covering the regulated frequencies (865.6 MHz to 867.6 MHz) of the EPC Gen2 standard was obtained. The meas189
ured radiation patterns of the array for two different states of the switches are shown in Figures 14 and 15. The 3 dB beamwidth of the H plane for State 1 was 46°, and for State 2 the beamwidth was 48°. For the E plane, when the upper switches of phase shifter were open, the half-power beamwidth was 69.3° (State 2), and otherwise the beamwidth was 73.6° (State 1). In accordance with the simulation results, a directivity of 12.1 dB and a 20 dB difference of the co- and cross-polar levels at boresight was obtained at 867 MHz, from the measurements. The measured results showed that antenna could be steered ±40°.
"toh Antenna btt<2
E plane 00
,-..It Aattft... Sfatt2
0 0 0
A 2 x 2 UHF RFID antenna-array system, which consisted of a phase shifter, power dividers, and microstrip-patch antenna element, operating at a frequency of 867 MHz, was designed, imple-
4. Experimental Results A test bench for assessing the performance of a phased-array antenna in an actual RFID system, to confirm the radiation-pattern measurements completed in the compact range, was established. For testing purposes, an Alien bistatic ALR-8800 model reader and passive UHF ALN-9554 tags were used . The array antenna was employed in the receiver port and a standard patch was used for transmission. For different positions of the receiver antennas, and for the standard patch antenna and the antenna array in two different states, the measurement results affirmed the extended coverage and gain of the antenna array as compared to the coverage and gain of the patch antenna (Figure 16). Due to the limited area for measurements, the transmitted power level decreased by 6 dB from the maximum power level obtainable (2 W), in order to minimize the coverage.
0 '-0--0 a 0 0 0 0 0 <&>
Figure 16. Readable location information of UHF passive tags. mented, and measured. The main beam of the antenna array could be switched between two directions designed to be 80° apart. The measured input impedance was well matched with two different beam-pointing directions with return losses of 30 dB. The radiation pattern of the antenna was measured and plotted in a compact range. To corroborate the results, the performance of the antenna array was tested in an actual UHF RFID system.
6. Acknowledgment 100 This work was supported by the Turkish Scientific and Technology Research Institution TUBITAK. Grant 104E123. The authors also wish to acknowledge Dr. Nazli Candan and Dr. Bahattin Turetken for their help in antenna-gain measurements, performed in the TUBITAK. UEKAE Labs.
Figure 15. The measured co- and cross-polarizations (E-plane) of the phased-array antenna.
1. K. Finkenzeller, RFID Handbook, 2nd Edition, New York, John Wiley & Sons, 2003.
IEEE Antennas and Propagation Magazine, Vol. 51, No.1, February 2009
2. G. De Vita and G. Iannaccone, "Design Criteria for the RF Section of UHF and Microwave Passive RFID Transponders," IEEE Transactions on Microwave Theory and Techniques, 53, 9, September 2005, pp. 2978-2990.
Introducing the Authors
3. Jong-Wook Lee, Hongil Kwon, and Bomson Lee, "Design Consideration of UHF RFID Tag for Increased Reading Range," IEEE MTT-S International Symposium Digest, June 2006, pp. 1588-1591. 4. Joshua D. Griffin and Gregory D. Durgin, "Gains for RF Tags Using Multiple Antennas," IEEE Transactions on Antennas and Propagation, AP-56, 2, February 2008, pp. 563-570. 5. Leena Ukkonen, Lauri Sydanheimo, and Markku Kivikoski, "Read Range Performance Comparison of Compact Reader Antennas for a Handheld UHF RFID Reader," IEEE International Conference on RFID 2007, March 26-28, 2007, pp. 63-70. 6. Constantine Balanis, Antenna Theory, New York, John Wiley & Sons, Inc. 7. E. Levine, G. Malamud, S. Shtrikman, and D. Treves, "A Study of Microstrip Array Antennas with the Feed Network," IEEE Transactions on Antennas and Propagation, AP-37, 4, April 1989, pp. 426-434. 8. P. S. Hall and C. M. Hall, "Co-Planar Corporate Feed Effects in Microstrip Patch Array Design," Inst. Elect. Eng. Proc., 135H, June 1988, pp. 180-186. 9. P. Daniel, G. Dubost, C. Tenet, J. Citeme, and M. Drissi, "Research on Planar Antennas and Arrays: 'Structure Rayonnantes'," IEEE Antennas and Propagation Magazine, Vol. 35, 1, February 1993, pp. 14-38. 10. D. Y. Kim, B. J. Jang, H. G. Yoon, J. S. Park, and 1. G. Yook, "Effects of Reader Interference on the RPID Interrogation Range," Proceedings of the 37th European Microwave Conference (EuMC'07), Munich, October 2007, pp. 728-731. 11. RFID system: http://www.alientechnology.com.
IEEE Antennas and Propagation MagaZine, Vol. 51, No.1, February 2009
Mehmet Abbak received his BSc degree from the Telecommunications Engineering Department of Sabanci University in 2006. He is currently continuing his MSc degree at the Electronics Engineering Program of Faculty of Engineering and Natural Sciences of Sabanci University, Istanbul, Turkey. His research interests are printed microstrip antennas and arrays, and radiofrequency circuit design. Ibrahim Tekin received his BS and MS degrees from Electrical and Electronics Engineering Department of Middle East Technical University (METU) in 1990 and 1992, respectively. From 1993 to 1997, he was with the Electrical Engineering Department of the Ohio State University (OSU), where he received his PhD degree in 1997. During 1990-1993, he was a research assistant at METU, and from 1993 to 1997 he worked as a Graduate Research Associate at the ElectroScience Laboratory, OSU. From 1997 to 2000, he worked as a researcher in the Wireless Technology Lab of Bell Laboratories, Lucent Technologies. He is now with the telecommunications program at SabancI University, Istanbul. His research interests are RF and microwave design, UWB antennas and circuits, and numerical methods in electromagnetics. He is a member of the IEEE Antennas and Propagation and Communications Societies. C1!)