215 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) ISSN (Print) 2313-4410, ISSN (Online) 2313-4402 © Global Society of Scientific Research and Researchers http://asrjetsjournal.org/ UWB Reflectarray Antenna for Chipless RFID Reader Gain Enhancement Amr H. Hussein a , Esraa H. Elgamal b* , Mahmoud A. Attia c , Fathi E. Abd El- Samie d a,b,c Electronics and Electrical Communications Engineering Dept., Faculty of Engineering, Tanta University, Tanta, Egypt d Electronics and Electrical Engineering Dept., Faculty of Engineering, Menoufia University, Menouf, Egypt a Email: amrvips@yahoo.com b Email: Esraa_Hassan254@yahoo.com c Email: fathi_sayed@yahoo.com Abstract The main limitation of chipless Radio Frequency Identification (RFID) systems is its short reading range which is generally less than as the amplitude of the back scattered tag signal is inversely proportional to the fourth root of the reading distance. In this paper, a design of reflectarray (RA) antenna consisting of unified unit cell. Five different unit cells structures centered at 6GHz for chipless RFID reader applications is introduced. The proposed RA has a narrow half power beam width (HPBW) and high gain which significantly enhance the reader sensitivity, maximize the reader reading range, reduce the multipath effects, and improve the tag localization. The proposed RA is realized on a rectangular single layer Rogers RT5880 lossy substrate of thickness and relative permittivity . radiating cells or elements with uniform element spacing are arranged on the rectangular substrate of dimensions ( ) and fed by a pyramidal horn antenna with gain of and HPBW equals 46.7°and 42.8° at E-plane and H-plane respectively. The simulation results showed that the proposed RA gives high gain up to which is greater than the feeder gain by and three times narrower HPBW of about .It operates over frequency range from to with fractional bandwidth (FBW) and has side lobe level, , which can't be achieved by the conventional antenna arrays. Keywords: Radio Frequency Identification(RFID); Reflectarray antenna(RA); Side lobe level (SLL). ------------------------------------------------------------------------ * Corresponding author. American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 216 1. Introduction A printed reflectarray antennas or flat reflectors can be considered as a new type of antennas. A reflectarray antenna is made up of an array of radiating elements that provides a focused pencil beam at a certain direction when illuminated by a feed. The horn antenna is the most commonly used feeder in reflectarray feeding due to its very low losses. Reflectarray antennas have the advantages of both phased antenna arrays and parabolic antennas. Compared to the phased array, it has the possibility of beam steering. On the other hand, it has a feeding mechanism as that of the parabolic antenna which decreases the design complexity and losses of phased array feeding network and avoids the manufacturing complexity of parabolic antennas [1].The main idea of reflectarray is generating a directive reflected beam from the reflectarray planar surface when it is illuminated in a certain direction by varying each element phase shift to collimate a pencil beam at the desired direction. That can be achieved by choosing a proper radiating element (unit cell) which satisfies two main conditions; first, it must span 360° at least to satisfy all required phase shifts. Second, the unit cell must have a linear phase curve with the cell phase controlled parameter, cell radius R, to reduce the phase errors which leads to high gain performance. The circular rings are better than the square ones as the square unit cell gives less phase response in terms of linearity [1,2]. While, the circular ring phase response is independent on the azimuth angle which results in less phase error and high efficiency. To get an overall collimated reflected beam from a reflectarray antenna at a certain direction with high gain and efficiency, the plane wave from a feeder when illuminates its planar surface elements, each element should reflect a beam with a certain phase shift depends on the element size and position on the array surface[2]. However, there is a main drawback with the RA antennas which is its narrow bandwidth behavior. Reflectarray bandwidth is limited mainly by two main factors. The first is the narrow band of the radiating elements and the second is the differential spatial phase delay resulting from the different paths from the feeder to each point on the wave front of the radiated beam[3]. In this paper, a single layer substrate based reflectarray antenna (RA) design with high gain, high efficiency, and ultra-wide bandwidth is proposed for reading range extension of chipless RFID readers. To ensure the phase linearity characteristic of the array, five different circular unit cell ( )designs that satisfy all the frequency, polarization, bandwidth, gain, and efficiency requirements are introduced. The proposed RAs are composed of a unified cell from each of the five different circular unit cells arranged on a rectangular plane. Each RA is build up with , ( ) , elements or with dimensions ( ) centered at .The RAs aperture profiles are limited to( ) . The RAs are fed with a center feeding rectangular horn antenna with focal length 2. Chipless RFID System Limitations Chipless RFID system is the further cheaper solution which emerges from the difficulties of achieving low cost chipped RFID system. It is expected to replace the bar code technology at 2020 [4].That results in the need for new efficient RFID readers with high gain, high sensitivity, and large reading range. A Chipless tag can't generate a signal without the reader sending an interrogation signal to the tag itself. Therefore, the reader acts as a Master and the tag as a Slave[5].The main drawback of Chipless RFID system is its limited reading range where the back scattered tag signal is subjected to the fourth power reduction in magnitude with the reading distance as in Eq.(1). Besides, they also suffer from the reader low sensitivity, multipath effects, and low American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 217 efficient tag localization [6]. So, Chipless RFID reader must generate a high gain direct beam over a wide band of frequencies to accommodate multiple bits, reduce the effects of multipath propagation, and enhance the reader sensitivity which in turn leads to increasing the reader reading range which can't be achieved using the conventional antenna arrays [4]. = √ (1) where is the RFID reading range, is the reader transmitting antenna gain, is the reader receiving antenna gain, λ is the wavelength, σ is the tag radar cross section (RCS), is the transmitted power, and is the reader sensitivity. 3. Proposed Unit Cells Structures The basic building element of a reflectarray antenna is the reflecting .All must give high reflection coefficients equal or close to at the center frequency . It must span 360° at least to satisfy all the required phase shifts and must have a linear phase curve with the cell radius to reduce the phase errors which leads to high gain and wide bandwidth. According to Chipless RFID system applications operating at a center frequency , a five circular are proposed and designed with ground plane of thickness and fixed substrate dimensions of( ).The Rogers RT5880(lossy) substrate of thickness and relative permittivity is utilized to reduce the material loss as shown in figure 1(a) shows the general side view of the UC layers. While the other sub-figures show the front views of the proposed circular ring based unit cells, .The unit cell dimensions are listed in Table 1. With resonator boundary conditions, the cells reflection loss curves against the frequency are plotted as shown in figure 2. All the unit cells give high reflection co-efficient near to over the entire operating frequency range from to . At the center frequency , the calculated reflection coefficients are very close to as summarized in Table 2. Also, applying resonator boundary conditions introduced in[1], the cells reflection phase curves against the cell phase control parameter are plotted as shown in figure 3. Figure 3 (a) shows that gives the most linear phase curve but with phase span less than almost equals . In this case, the can't be separately used in full RA design although its linearity. Figure 3(b) and figure 3(c) show that both and give acceptable linear phase curves with phase span more than one cycle which equals and respectively. So, they can be used individually in full RA design. Figure 3(d) and figure 3(e) show that both and give more than two cycles span with different linear parts of phase curves which makes them efficient choices for reflectarray design. A comparison between the phase curves of the proposed five UCs at the center frequency is summarized in figure 4. The slow phase variations provides immunity against manufacturing tolerance and truncation errors [7]. None of the unit cells has a perfect linear phase characteristic. But, each has limited linear regions over the radius span ⁄ where the substrate dimensions are ⁄ ⁄ . With the aid of the second derivative phase curves shown in figure 5, the most linear regions of each curve can be identified by its closeness to zero. From this point of view, a RA can be designed using different UCs utilizing their linear regions to achieve the desired phase shifts that maximize the array gain. American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 218 (b) (c) (d) (e) (f) Figure 1: Unit cell structure (a) cell side view,(b) front view,(c) front view,(d) front view, (e) front view, and (f) front view. Table 1: Unit cells parameters Parameter g h Dimensions in 0.8158 0.952 1.0186 1.57 0.07 (a) American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 219 Figure 2: Reflection loss versus frequency for the five unit cells. Table 2: Reflection loss for the five unit cells at the center frequency . Reflection Loss in at Unit Cells 0.053 0.00071 0.00058 0.001 0.00664 (a) (b) American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 220 (c) (d) (e) Figure 3: Phase characteristic curves for each unit cell against its radius at 6GHz (a) phase curve, (b) phase curve, (c) phase curve, (d) phase curve,and (e) phase curve. Figure 4: A comparison between the phase curves against the unit cells Radius of the proposed five UCs at the center frequency . American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 221 4. Reflectarray Antenna Design There are seven steps for an efficient reflectarray (RA) antenna design which can be summarized as follows:  Design a UC that meets all the aforementioned requirements at the desired center frequency.  Estimate the UC phase curve against the cell phase controlling radius parameter (R).  Find the desired phase shift from each array UC according to Eq.(2) introduced in [8]. ( ) ( ( ) (2) (a) (b) (c) (d) Figure 5: Feeder horn antenna (a) E-field radiation pattern at the center frequency ,(b) H-field radiation pattern at the center frequency , (c) Feeding horn antenna dimensions, and (d) The feeding horn antenna 3-D radiation pattern at the center frequency . where ( )is the required phase shift from the ( ) unit cell. is the free space wave number and is the free space wavelength which depends on the resonance frequency . is the distance American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 222 between the face center of the feed horn antenna and the center of ( ) cell.( ) are the ( ) cell coordinates. and are the Elevation and Azimuth angles of the incident plane wave.  Determine each cell radius (R) according to the required phase shift from its reflection phase curve.  Repeat 4 & 5 steps for each element on the array.  Build the array of cells with different radii that satisfy the required phase shifts according to their positions on the array surface according to Eq. (2). All the designed RAs are fed with a pyramidal horn antenna. The feeder horn antenna has and HPBW equals 46.7°and 42.8° at the E-plane and H-plane, respectively as shown in figure 5(a) and figure 5(b). The horn dimensions are ( ) using the standard ATH1G18A waveguide ( ) for high gain horn antenna as shown in figure 5(c). Figure 5(d) shows the horn antenna 3-D radiation pattern at the center frequency . 5. Reflectarray Antenna Specifications (a) (b) (C) Figure 6: (a) based RA structure , (b) radiation pattern of the RA at , and (c) S- parameter ( ) over the entire bandwidth from to . American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 223 The unified reflectarray antenna (URA) is a RA that is based on a single UC to achieve all the required phase shifts. is excluded as it doesn't span 360° and it can't give all the required phase shifts to build the RA. Four URAs are designed based on the four unit cells , , , and where their structures, radiation patterns, and scattering parameters are shown in figure 6, figure 7, figure 8, and figure 9, respectively. The radiation patterns parameters of the RAs at the center frequency are summarized in Table 3 . (a) (b) (c) Figure 7: (a) based RA structure , (b) radiation pattern of the RA at , and (c) S- parameter ( ) over the entire bandwidth from to . American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 224 (a) (b) (c) Figure 8: (a) based RA structure and (b) radiation pattern of the RA at , and (c) S- parameter ( ) over the entire bandwidth from to . (a) (b) (c) Figure 9: (a) based RA structure and (b) radiation pattern of the RA at , and (c) S- parameter ( ) over the entire bandwidth from to . American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 225 Table 3: Comparison between the four unified UC based RAs at the center frequency . RA RA Antenna Parameters Gain ( ) HPBW SLL ( ) based RA 16.5 13.6° -5.7 based RA 16.3 14.9° -7.4 based RA 16.3 10.4° -5 based RA 16.8 10° -4.5 It is clear that the unified RAs have higher gains around for based RA with acceptable SLL equals . While the lower SLL of the unified RAs is with high gain which reaches for based RA. That is because the unified UC based RA has to use the non linear portions of the utilized UC phase curve to meet the required phase shifts. The far field gain against frequency for all proposed RAs are shown in figure 10. ensures that all the RAs give the maximum gain at the designed frequency. Figure 10: Far field gain versus frequency for all proposed RAs. 6. Reflectarray Performance There are three factors should be taken into consideration for RA efficient design: 6.1. Feeder Blockage Can be described as the shadowing with the feed in the path of rays arriving at or departing from the aperture of the antenna or interfering with the radiation. To avoid feeder blockage based RA offset feed, offset beam, and center feed are designed and compared as shown in Table 4 . American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 226 Table 4: Comparison between offset beam, center beam center feed, and offset feed based RAs. Table 4 shows that the based RA center feeding center beam gives the lowest SLL which is -5.7 . Where the offset beam based RA SLL equals -3.5 and the offset feed based RA gives -2.4 SLL. And, on the other hand the based RA gives also the highest gain up to 16.5 . 6.2. Element Spacing Efficient element spacing must be used to enhance gain, BW and minimize the SLL according to Eq.(3). ≤ (3) where, is the element spacing, λ is the wavelength, and θ is the incident angle from the feeder. Table 5 shows that ⁄ is the sufficient element spacing that gives a high gain and minimum SLL [7]. Three different element spacing based RAs are designed and compared as shown in Table 5. The first RA is designed with = 0.3λ, the second is designed with =0.5λ, and the third RA is designed with = λ. Table 5: Comparison between the three based RAs with different element spacing 0.3λ, 0.5λ, and λ. From Table , it is clear that the RA with = 0.5λ gives the best SLL and the highest gain which is 5 greater than that of the based RA with = λ and 1 greater than that of the . 6.3. Ratio The feeder position and orientation are analytically calculated to produce a taper of the RA panel [3], maximizing the focal length leads to maximize the array BW and minimizes the SLL with trading of spill over and tapper efficiency. Where F is the focal length (the distance between the center of the feeder to the center of RA Antenna Parameters ARRAYS SLL( ) HPBW Gain( ) -3.5 13° 16.5 Offset beam RA with -5.7 13.6° 16.5 Center feed center beam RA with -2.4 14.9° 15.5 Offset feed RA with RA Antenna Parameters Array SLL HPBW Gain -3.6 13.9° 15.5 RA with = 0.3λ -5.7 13.6° 16.5 RA with = 0.5λ -0.4 9.4° 11.6 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 227 the array surface) and D is the RA surface diameter[2]. based RA is designed at , , and . Table 6 shows that gives the highest performance in gain and with at least 1 increase in both gain and SLL compared to the other ratios. Table 7: Comparison of , , and based RA. 8. Comparison With Related Work Related work in RA antenna design (NSL: Number of substrate Layers, FBW: Fractional Bandwidth). Table 8 Ref Related RA Work Results Type of element NSL Element size(λ) Freq.(GHz) FBW [5] Double circular rings 2 0.5 5 37% [7] Stacked patches 2 0.56 12 16.7%(1.5dB drop) [8] Stacked patches 3 0.56 12 10% (0.5dB drop) [9] Patch Loaded With Slot 1 0.7 12.5 4% [10] Coupled Structures 1 0.55 35 4.8% (3dB drop) [11] 3 Parallel Dipoles 1 0.5 300 13% [12] Double Cross Rings 1 0.44 22 10% (1dB drop) [13] split-slotted-dipole 2 0.45 5.4 21% This Work Unified unit cell RA 1 0.5 6 43% 9. Conclusion In this paper, a RA antenna is introduced for Chipless RFID readers. A high gain pencil beam proposed RA antenna is used to increase the reading range and improve the tag detection. Some considerations are exploited to enhance the operating , minimize losses and simplify the design. The simulation results shows a of the UWB RA antenna equals 43% satisfies Chipless RFID reader applications. The radiated beam is 14.9° three time narrower the feeder beam and covers all the feeder , 16.3 gain and -7.4 RA Antenna Parameters Array SLL HPBW Gain -4 19.1° 15.2 RA with F/D=0.8 -5.7 13.6° 16.5 RA with F/D=1 -3.7 13.1° 14 RA with F/D=1.5 American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 228 References [1] J. Huang, J. A. Encinar, Reflectarray Antennas, A John Wiley & Sons, Inc., Publication, 2008. [2] M. Zebrowski, “Illumination and Spillover Efficiency Calculations for Rectangular Reflectarray Antennas,” High Frequency Design, pp. 28-38, December, 2012. [3] J. Shaker, M.R. Chaharmir, and J. Ethier, Reflectarray Antennas: Analysis, Design, Fabrication, and Measurement:, Antennas and Propagation. Artech House, 2013. [4] M. Khaliel, A. Fawky, M. El-Hadidy, T. Kaiser, “ UWB Reflectarray Antenna for Chipless RFID Applications,” 31st National Radio Science Conference of Egypt, pp. 17-20, April, 2014. [5] S. Preradovic and N.C. Karmakar, Multiresonator-Based Chipless RFID: 9 Barcode of the Future, DOI 10.1007/978-1-4614-2095-8_2, © Springer Science Business Media, LLC 2012. [6] Y. F. Weng, S. W. Cheung, T. I. Yuk and L. Liu, “Design of Chipless UWB RFID System Using A CPW Multi-Resonator,” IEEE Antennas and Propagation Magazine, Vol. 55, No. 1, Feb. 2013, pp. 13–31. [7] M. Khaliel, A. Fawky, M. El-Hadidy, and T. Kaiser, “Long Reading Range Chipless RFID System Based on Reflectarray Antennas in,”11th European Conference on Antennas and Propagation (EUCAP), 2017. [8] M. Niroo-Jazi, M. R. Chaharmir, J. Shaker, and A. R. Sebak,” Reflectarray antennas using single layer polarization independent multi-resonant unit cells,” IEEE Antennas and Propagation Society International Symposium (APSURSI), 2014. [9] J. A. Encinar, "Design of Two-Layer Printed Reflectarrays using Patches of Variable Size." IEEE Trans. Antennas Propag., Vol. 49, No. 10, pp. 1403-1410, Oct. 2001. [10] J. A. Encinar and J. A. Zornoza. "Broadband Design of Three-Layer Printed Reflectarrays." IEEE Trans. Antennas Propag., Vol. 51, No. 7, pp. 1662-1664, July 2003. [11] D. Cadoret, A. Laisne, R. Gillard, L. Le Coq, H. Legay, “Design and measurement of new reflectarray antenna using microstrip patches loaded with slot”, Electronics Letters, Vol. 41, No. 11, pp. 623–624, 2005. Forum for Electromagnetic Research Methods and Application Technologies (FERMAT) [12] S. Dieter, C. Fischer, W. Menzel, “Single–layer unit cells with optimized phase angle behaviour”, in Proc. 3rd European Conference on Antennas and Propag., EuCAP 2009, pp. 1149–1153, Mar., 2009. [13] F. Rossi, J. A. Encinar, A. Freni, “Design of a reflectarray antenna at 300 GHz using parallel dipoles of variable size printed on a quartz wafer”, in Proc. 5th ESA Workshop on Millimeter Wave Technology and American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) (2019) Volume 59, No 1, pp 215-229 229 Applications & 31st ESA Antenna Workshop, at ESA/ESTEC, The Netherlands, May 2009. [14] M. R. Chaharmir, J. Shaker, M. Cuhaci and A. Ittipiboon., "Broadband Reflectarray Antenna with Double Cross Loops." Electronics Letters Vol. 42, No. 2, pp. 65-66, 2006. [15] H. Khaled, K. Maher, E. Mohamed, and K. Thomas, " Design an Adaptive Electronically Beamsteering Reflectarray Antenna for RFID Systems" Duisburg-Essen University, Institute of Digital Signal Processing, Bismarckstrasse Sl, 47057 Duisburg Germany, 978-1-4799-7815-1/15/$31.00©20151EEE. [16] R. Elsharkawy, A. R. Sebak, M. Hindy, O. M. Haraz, A. Saleeb, and E. M. El-Rabaie, (2015a). Polarization insensitive Ka-band reflectarray antenna. In The proceedings of the AP-S, pp. 2483–2484, 2015. [17] R. Elsharkawy, A. R. Sebak, M. Hindy, O. M. Haraz, A. Saleeb, and E. M. El-Rabaie, (2015b). Single layer polarization independent reflectarray antenna for future 5-G cellular applications. In Proceedings of International Conference on Information and Communication Technology Research, pp. 9–12, 2015. [18] R. Elsharkawy, A. R. Sebak, M. Hindy, A. Saleeb, and E. M. El-Rabaie, (2017). A Reflectarray with octagonal unit cells for 5-G applications. Wireless Personal Communication, 97(2), pp. 2999–3016, 2017.