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1 This article was downloaded by: [Sejong University ] On: 02 January 2014, At: 17:53 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Journal of Electromagnetic Waves and Applications Publication details, including instructions for authors and subscription information: High-gain antenna using an intelligent artificial magnetic conductor ground plane I.Y. Park a & D. Kim a a Department of Electronic Engineering, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul , Korea Published online: 05 Aug To cite this article: I.Y. Park & D. Kim (2013) High-gain antenna using an intelligent artificial magnetic conductor ground plane, Journal of Electromagnetic Waves and Applications, 27:13, , DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views epressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, epenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is epressly forbidden. Terms & Conditions of access and use can be found at

2 Journal of Electromagnetic Waves and Applications, 2013 Vol. 27, No. 13, , High-gain antenna using an intelligent artificial magnetic conductor ground plane I.Y. Park and D. Kim Department of Electronic Engineering, Sejong University, 98 Gunja-dong, Gwangjin-gu, Seoul , Korea (Received 15 April 2013; accepted 18 June 2013) We propose a high-gain antenna using an intelligent ground plane composed of artificial magnetic conductors (AMCs). The main role of the ground plane is removing difference in phase delays caused by different path lengths from a signal-feeding dipole onto each AMC cell. Hence, the ground plane consists of AMC cells with different lengths corresponding to the phase delays to be compensated. As a consequence, we can obtain a planar wave front of a reflected wave, which is perpendicular to a boresight direction. By maimizing constructive interference between the reflected waves and direct waves from the dipole, we can significantly increase our antenna gain without deteriorating impedance matching bandwidth behavior. Eperimental data has shown good agreement with the prediction, which validates the accuracy and effectiveness of our proposal. 1. Introduction In general, the antenna plays a key role in wireless systems as an important front-end terminal device, which significantly affects overall performance of the systems. Among a variety of design parameters characterizing performance of antennas, antenna gain, impedance matching bandwidths, and radiation patterns are typical ones that are considered in almost all types of antennas.[1 4] In regards to the antenna gain, one of the conventional ways to increase the gain is using reflectors such as in parabola antennas or cassegrain antennas. With the help of reflectors, these types of antennas can provide very high-gain, but usually they occupy large space and cost a great deal.[3 5] Another well-known approach to acquire high-gain is an array technique, which places a number of antennas by an appropriate arrangement method. The array technique can offer very high-gain or desired radiation behavior according to well-organized amplitude and phase distribution in each antenna element. However, a large number of required antennas, complicated feed networks, and losses in feed signals are general weak points of the array technique.[6,7] To overcome mentioned drawbacks, recently some other types of approaches have been reported, which apply a phase-controllable ability of a superstrate or a substrate. First, regarding the superstrate, a partially reflective surface (PRS) is used in the superstrate, which forms a Fabry Perot cavity (FPC) together with a ground plane.[8,9] We can obtain very high-gain using the FPC approach. But, it is difficult to avoid an inherent narrow Corresponding author. dongkim@sejong.ac.kr 2013 Taylor & Francis

3 Journal of Electromagnetic Waves and Applications 1603 y Copper Dielectric( r1) a 12 w y1 n=1 n=2 a m2 a 22 a 12 a 12 a 22 a mn a 1n w 1 m=1 m=2 m=4 n=9 n=10 Unit set Figure 1. Geometry of the tapered AMC ground plane with w 1 = 360 mm, w y1 = 300 mm, and ɛ r1 = 3.5. bandwidth coming from the resonance property of the FPC. In order to more widen the impedance matching bandwidth, we have proposed a tapered superstrate and a reflection phase reversed superstrate.[10,11] Although, the proposed superstrates are helpful in epanding the bandwidth to a certain degree, but it is still not quite satisfactory. Net, as for the substrate, a tapered AMC ground plane has been proposed to widen a radiation bandwidth near an 8 GHz frequency band.[12] Even though the radiation bandwidth can be etended, but impedance matching bandwidth has not been considered. In [9], the tapered AMC is used to construct an FPC in cooperation with a PRS superstrate, which is able to enlarge an impedance bandwidth maintaining relatively high-gain. However, in [9,12], AMC cells are applied at high frequencies, which results in a relatively slow transition in the reflection phase of the AMC through a wide frequency range. This means, it is much easier to design the AMC ground than in a lower frequency region, to which our proposal applies. In addition, no rigorous analysis based on mathematical prediction is carried out in [9,12]. In this paper, we propose an intelligent ground plane consisting of different-sized AMC cells to largely increase antenna gain with no sacrifice of an impedance bandwidth. Target frequencies are in a 1.8 GHz band, which is challenging inasmuch as a reflection phase variation in the band is fairly sharp. Good agreement between the prediction and the eperiment proves that we can successfully increase antenna gain simply by compensating different phase delays generated from various propagation path lengths. All simulations are done with the CST Microwave studio.[13] 2. Design of an intelligent ground plane Overall structural composition of the proposed tapered AMC ground plane is depicted in Figure 1. The AMC ground plane consists of a total of 8 20 different-sized rectangular metallic patches that are etched on a 1.52 mm thick dielectric substrate with relative

4 1604 I.Y. Park and D. Kim z z w 2 w 2 w 1 Dielectric( r2 ) l 1 h 1 w z l 2 Copper Copper Dielectric( r2) w z h 3 h 2 w 2 (a) h 4 (b) w 3 Figure 2. Geometry of the feeding antenna printed on a double-sided 1.52 mm thick substrate with w 2 = 69 mm, w z = mm, w 1 = 5 mm, w 2 = 6 mm, w 3 = 6 mm, l 1 = 25 mm, l 2 = 15 mm, h 1 = mm, h 2 = 16 mm, h 3 = 11 mm, h 4 = 10 mm, and ɛ r2 = 3.5: (a) the dipole antenna and (b) the hair-pin shaped feeder. permittivity, ɛ r = 3.5. The opposite side of the substrate is fully covered with copper so that we can use unique reflection properties of each AMC cell.[9,12] To generate symmetric radiation patterns and to collimate radiating beams toward a boresight direction (+z-direction), we build the AMC ground plane using mirrored replicas of a unit set, which is illustrated as a broken-lined bo enclosing a total of 4 10 patches. As a consequence, our ground plane is symmetric both in the - and y-directions. A dipole antenna installed on the center of the AMC ground plane is illustrated in Figure 2. An input signal is fed into the hair-pin-shaped feeder (see Figure 2(b)) directly from a 50 coaial cable. The T-shaped dipole structure (see Figure 2(a)) is printed on the opposite side of the feeder. The fed signal couples from the feeder onto the dipole, and the dipole radiates electromagnetic waves into the air. As is well-known, the height of any radiator from a ground plane largely affects antenna s performance properties such as gain, impedance matching bandwidth, etc. The height can be determined by h = c 2 f ( ) φr 2π + m, m = 0, 1, 2, 3... (1) where c is the speed of light, f is a frequency, and φ r is reflection phase on the center of the AMC ground plane. Substituting π and 0 into φ r and m gives us a well-known separation of λ/4 between a dipole and a PEC ground plane, which provides maimum gain. Based on the measured φ r value of 170,weseth to 45.2 mm. The operational principle eplaining how we can increase overall antenna gain with the help of the proposed AMC ground plane is illustrated in Figure 3. For convenience, we draw only a portion of the entire ground plane, which corresponds to the unit set enclosed by broken lines in Figure 1. In order to make reflected waves propagate only upwards (θ = 0 ), all the waves reflected by each AMC cell should have the same phase value at the moment of reflection, which constructs a wave front pointing toward the positive z-direction. For more detailed eplanation, now we follow wave travelling paths from the dipole via the AMC cells into the air in consecutive order. As shown in Figure 3, all paths from the dipole to the AMC cells have different lengths, which produce different phase delays. For the path from the dipole to the center of the (m, n) unit cell, the phase delay is epressed by

5 Journal of Electromagnetic Waves and Applications 1605 z Dipole antenna h 1 y l 11 l 3n l 41 a 11 a 21 a 31 a 41 a 1n a 2n a 3n a 4n Figure 3. Principle of antenna gain enhancement using the tapered AMC ground plane. y m= n= Center of (3, 2) AMC unit cell Ground plane Figure 4. Distribution of E-field phase on the entire AMC ground plane. φ ideal mn = β l mn + φ 0 = 2π f c l mn + φ 0 (2) where β is a phase constant, φ 0 is an initial phase offset of a transmitted wave from the dipole antenna, and l mn is a distance from the dipole to the center of the (m, n) unit cell. Now, if we can compensate for each phase delay of φmn ideal using reflection phase values of each AMC cell, then we can make all reflected waves departing from each cell have eactly the same phase delay. Consequently, we can successfully collimate a reflected wave toward the positive z-direction, which is obviously helpful to considerably increase antenna gain. For this purpose, each AMC cell should have a different length in the -direction so that it provides the required reflection phase according to its length.[10] Based on the reference distance l 11, we can calculate the required reflection phase value of each AMC cell, which is necessary to set all phase values of reflected waves to be identical. The desirable AMC phase can be determined by

6 1606 I.Y. Park and D. Kim y a mn p y b Metallic strip dipole r1 p Figure 5. Geometry of the unit cell constructing the AMC ground plane with p p y = 15 mm, and b = 12 mm. = 45 mm, φamc ideal = φideal 11 φmn ideal = 2π λ (l mn l 11 ). (3) In Equations (2) and (3), we use an ideal propagation constant in the air. However, in practice, each phase delay is slightly different from the ideal one in a near-field environment. Thus, to get more accurate phase, we measure each phase value at the center of each AMC unit cell using a simulation. By applying this measurement (φmn meas ), we correct the required reflection phase of the AMC unit cells like the following: φamc meas = φmeas 11 φmn meas. (4) The Equation (4) tells us that the required reflection phase of a farther AMC cell should be greater than that of a nearer cell. This criterion produces unusual tapered shapes in AMC cells as shown in Figure 1. Measured phase values on the entire AMC ground plane are given in Figure 4. The thick border line means the outermost boundary of the AMC ground plane. And, all points of intersection of the broken lines are the center points of each AMC cell. For eample, the small circle located on the first quadrant of the figure implies the center of the (3, 2) AMC cell. Instead of directly using the ideal values of Equation (2), we pick up the measured phase values at these intersection points to calculate the required reflection phase of the AMC cells, which are described in Equation (4). At right below the dipole antenna, the phase value is about 145. As we can epect, the farther cells result in greater phase delays. The geometry of the rectangular AMC unit cell is depicted in Figure 5. All cells have a fied width b = 12 mm and periodicity of p = 45 mm and p y = 15 mm in the - and y-directions, respectively. By changing the length a mn, we can obtain required reflection phase values. The reflection phase responses for some different cells are shown in Figure 6. The zero reflection phase means that the AMC unit cell operates as a perfect magnetic conductor. As a frequency increase, the reflection phase sharply decreases from a certain positive value to a negative one passing through the zero phase. And, the shorter cell produces the larger reflection phase at the same frequency point. We use this unique property of AMC cells to compensate different phase delays from the dipole to each AMC cell, which constructs distinctive arrangements of cells on the ground plane as shown in Figure 1.[9]

7 Journal of Electromagnetic Waves and Applications 1607 Figure 6. Reflection phase values of AMC unit cells with some different lengths a mn. Figure 7. Photograph of the proposed high-gain antenna installed on the intelligent AMC ground plane. Table 1. The lengths a mn (mm) of the AMC unit cells. n = m = Fabrication and eperimental verification A photograph of the fabricated antenna structure is given in Figure 7. The lengths a mn of the entire AMC unit cells calculated by Equations (2) (4) are listed in Table 1. We compare the measured input reflection coefficient (S11) with the predicted one, which is shown in Figure 8. For a more realistic comparison, we also show the input reflection coefficient of a dipole antenna installed above a normal ground plane, which is drawn by blue broken lines. In the prediction data, we can find that the proposed AMC ground plane does not much affect impedance matching behavior, which implies the tapered ground is of practical use. The predicted 10 db bandwidths are both slightly greater than 200 MHz, which corresponds to a fractional bandwidth (FBW) of about 11%. The measured 10 db

8 1608 I.Y. Park and D. Kim Figure 8. Comparison of the input reflection coefficients with the AMC ground and the PEC ground. Table 2. Comparison of the 10 db bandwidths of the input reflection coefficient of an AMC ground and a PEC ground plane. Items PEC ground AMC ground Simulation Eperiment Impedance BW (MHz) (FBW) 220 (11.5 %) 210 (11.0 %) 210 (10.8 %) (freq. range (GHz)) ( ) ( ) ( ) Maimum 6.6 dbi dbi dbi realized gain (freq.) (1.86 GHz) (1.83 GHz) (1.88 GHz) bandwidth is about 210 MHz, which corresponds to about 10.8% FBW. All detailed values are summarized in Table 2. Although overall frequency response slightly shifts towards high frequencies, we can see that the measurement agrees well with the prediction. Realized gain properties are also compared in Figure 9. Here again, for a more convenient comparison, we present gain of the same dipole antenna placed above a normal PEC ground plane (see the blue broken line in Figure 9). Comparison of the two predicted gain behaviors reveals that our approach to enhance gain is very effective over a relatively wide frequency region. Specifically, considering that the highest realized gain of the normal dipole antenna above the PEC ground plane is 6.6 dbi, it is very important to note that our antenna gain is higher than 6.6 dbi over a wide frequency band ranging from GHz to 2.1 GHz. Even though we design AMC cells at a single target operation frequency of 1.84 GHz, we can obtain relatively high-gain over 300 MHz. This result is important in practical use of the proposed antenna. Though a little frequency shift is also found in measurement, overall gain property agrees well with the prediction. The measured maimum gain is dbi at 1.88 GHz, which is fairly close to the target frequency of 1.84 GHz. In Figure 9, we can find one peculiar characteristic in the gain. The measured and predicted gains are all greater than that with a normal ground plane. However, around a 1.75 GHz region, the two gains are lower than the blue broken lines. This can be eplained by the following: near 1.75 GHz, a contrast in reflection phase values of AMC cells is maimized. In other words, the difference in reflection phase values between the longer and the shorter cells comes close to 180, which produces strong destructive interference among reflected waves. The difference is in its maimum around 1.75 GHz, which yields

9 Journal of Electromagnetic Waves and Applications 1609 Figure 9. Comparison of the realized gain with the AMC ground and the PEC ground. (a) Figure 10. Radiation patterns (realized gain) at 1.83 GHz (simulation) and 1.88 GHz (eperiment) : (a) E-plane (φ = 0 ) and (b) H-plane (φ = 90 ). unusual gain sink as can be seen from Figure 9. This unusual frequency band with the high contrast among the reflection phases of AMC cells eactly accords with the rising point of the input reflection coefficient shown in Figure 8. For the same reason, we can also eplain why the gain of the proposed antenna becomes similar to that with a normal ground plain below 1.7 GHz. The radiation patterns in an E-plane and an H-plane at maimum gain frequencies (1.83 GHz for the simulation and 1.88 GHz for the eperiment) are compared in Figure 10. The measured half power beam widths in the E- and H-planes are both 40.Wealso summarize overall performance of our antenna in Table 2. All eperimental data show relatively good agreement with the predicted ones, which prove validity and accuracy of our proposal. (b) 4. Conclusions We have proposed a high-gain dipole antenna installed above a tapered AMC ground plane. We can prove that the antenna gain can be largely enhanced by only using the proposed AMC ground plane without any superstrate that often brings a major obstacle to enlarge

10 1610 I.Y. Park and D. Kim an impedance bandwidth. For rigorous analysis, we consider each wave propagation path, and install AMC cells with different lengths, which compensate various phase delays to a fied value. Hence, we can make the all phase values of waves reflected by AMC cells to be the same. Consequently, we can obtain a planar wave front directing a boresight direction, which leads considerable enhancement of antenna gain with no sacrifice of the impedance bandwidth. It is important that our approach does not aggravate the impedance matching bandwidth. Therefore, the proposed technique is quite valuable in practical use. References [1] Balanis C. Antenna theory: a review. Proc. IEEE. 1992;80:7 23. [2] Godara L. Applications of antenna arrays to mobile communications, Part I: performance improvement, feasibility, and system considerations. Proc. IEEE. 1997;85: [3] Balanis C. Modern antenna handbook. New York: Wiley; [4] Stutzman W, Thiele G. Antenna theory and design. New York: Wiley; [5] Theunissen W, Yoon H-T, Burnside W, Washington G. Reconfigurable contour beam-reflector antenna synthesis using a mechanical finite-element description of the adjustable surface. IEEE Trans. Antennas Propag. 2002;49: [6] Maillou R. Antenna array architecture. Proc. IEEE. 1992;80: [7] Ge Z-C, Zhang W-X, Liu Z-G, Gu Y-Y. Broadband and high-gain printed antennas constructed from Fabry Perot resonator structure using EBG or FSS cover. Microwave Opt. Tech. Lett. 2006;48: [8] Feresidis A, Vardaoglou J. High gain planar antenna using optimized partially reflective surfaces. IEE Microwave Antennas Propag. 2001;148: [9] Kim D, Yeo J. Novel design of a high-gain and wideband Fabry Perot cavity antenna using a tapered AMC substrate. Int. J. Infrared Milli. Waves. 2009;30: [10] Kim D, Ju J-H, Choi J-I. A broadband Fabry Perot cavity antenna designed using an improved resonance prediction method. Microwave Opt. Tech. Lett. 2011;53: [11] Kim D, Ju J-H, Choi J-I. A mobile communication base station antenna using a genetic algorithm based Fabry Perot resonance optimization. IEEE Trans. Antennas Propag. 2012;60: [12] Kim D, Yeo J. Design of a wideband artificial magnetic conductor (AMC) ground plane for low-profile antennas. J. Electromagn. Waves Appl. 2008;22: [13] CST Microwave Studio: Workflow & Solver Overview, CST Studio Suite 2012, CST-GmbH, 2012.