High gain microstrip antenna design for broadband wireless applications

High Gain Microstrip Antenna Design for Broadband Wireless Applications Tayeb A. Denidni,1 Larbi Talbi2 1

Institut national de la recherche scientifique, Universite´ du Que´bec, Place Bonaventure, 800 De la Gauchetiere, Suite 6900, Montreal, Quebec, Canada H5A 1K6 2 Department of Computer Science and Engineering, Universite´ du Que´bec-Hull, 101, rue St-JeanBosco, Case Postale 1250, Succursale B, Hull, Que´bec, Canada J8X 3X7 Received 10 September 2002; accepted 18 June 2003

ABSTRACT: This article presents a new broadband microstrip antenna for personal communications systems (PCS) applications. Using multilayer substrate structure with aperturecoupled feed, a rectangular microstrip patch antenna operating at 1.9-GHz band is designed and experimentally validated. This antenna configuration uses a quarter-wave transformer to enhance the matching between the feed transmission line and the antenna patch. To demonstrate the design procedure, a first experimental broadband microstrip antenna prototype is designed and implemented. To analyse its performance, measurements are carried out and good performances are achieved. However, this prototype has a low front-to-back ratio. To overcome this drawback, an optimization process is proposed, and a second prototype is designed and successfully realized. To examine the effect of the optimization, experimental investigations are carried out on the second prototype. Very good agreement is obtained between numerical and measured results. Experimental results indicate that the proposed antenna achieves a bandwidth of 21%, a gain of 9.5 dB, and a front-to-back ratio of 20 dB, which are very sufficient for broadband wireless applications. © 2003 Wiley Periodicals, Inc. Int J RF and Microwave CAE 13: 511–517, 2003.

Keywords: microstrip antenna; broadband antenna; radiation pattern

I. INTRODUCTION For applications in wireless communications, such as personal communications systems (PCS), printed microstrip antennas constitute a very attractive research domain. This importance is related to their advantageous characteristics, such as light weight, low profile, and ease of fabrication. In addition, they offer excellent compatibility with monolithic microwave integrated circuits (MMIC). Particularly for future applications in mobile cellular networks, printed Correspondence to: T. Denidni; email: denidni@inrs-emt. uquebec.ca@ Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/mmce.10109

patch antennas are well suited to assure good compactness and low cost in this area. In a single-element form, they can be used to replace conventional antennas such us wire antennas. For applications in which more gain and space diversity are necessary, they can be used as a radiating element in the development of miniature antenna arrays. However, these antennas are characterized by the limitation of their bandwidth, which is of 1–2% [1]. This band is not sufficient for high-rate data transmission in wireless systems. To resolve this problem, a rectangular patch antenna with large width has been proposed for broadening the bandwidth. However, this approach may increase surface-wave effects, which degrades performances significantly. To avoid this situation and improve the bandwidth, several methods based on

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stacked patch [2] or parasitic-element configurations [3] have been proposed. Recently, Pozar [4] proposed a new feeding technique for microstrip antennas based on the coupled-aperture concept in order to achieve a large bandwidth. The technique consists of coupling energy from the stripline through an aperture in the ground plane. The radiating element is isolated from the feed network by the ground plane, which minimizes spurious radiation, and gives the designer the possibility to select independent substrate materials for the feed and the patch. Furthermore, it offers a good compatibility with MMIC. For wideband wireless applications, several related works based on this idea have recently been proposed [10 –12]. The reported bandwidths range from 10% to 20%. However, these configurations do not provide enough performance in terms of gain or front-to-back ratio. These two parameters are very important in wireless communications systems. In fact, the increasing of frontto-back ratio will reduce the backward radiation of antenna that represents the radiation loss. This article proposes a new approach where the abovementioned methods are extended in order to obtain an optimized antenna design that offers wide bandwidth, high gain, and low back radiation. Our aim, from these investigations, is to reach a compromise between the three important antenna criteria — bandwidth, gain, and front-to-back ratio — during the antenna design process. Our approach uses a multilayer structure, an aperture-coupled feed, and a quarter-wave transformer in order to design a novel antenna that offers a wide bandwidth at a small size. During the design procedure, dielectric permittivity, substrate thickness, and the size of the top patch were optimized. The quarterwave transformer modifies an extra dimensional parameter, which result in a very well-matched bandwidth microstrip patch antenna. Numerical results, taking into account the different physical parameters and operating conditions, are also studied and compared to validate the design. In this article, a broadband microstrip patch antenna for wireless systems is investigated. Section II presents the antenna design procedure to achieve a large bandwidth. To validate the design, numerical data for the return loss and the radiation pattern are presented. Experimental data of a first prototype are presented and analyzed. In section III, experimental results are discussed. In section IV, an optimization process to develop a second prototype to improve the front-to-back ratio of antenna pattern is described. Finally, the conclusion of this work is presented in section V.

Figure 1. Geometry of the proposed antenna.

II. ANTENNA DESIGN The main objective is to design a broadband microstrip patch antenna for a wireless system, particularly in the PCS band (1.85–1.99 GHz). The geometric configuration of the proposed antenna is shown in Figure 1. In this design, two dielectric substrate layers and one foam layer are stacked together. On the first layer, the rectangular patch is etched. Utilization of the foam layer provides the possibility to realize an antenna with a tick substrate and a very low dielectric constant. But it is impossible to etch the patch directly on the foam because of its porosity. For this reason, a thin dielectric substrate is employed to support the patch antenna. The third layer supports the microstrip feed line on one side, and the ground plane with a coupling aperture on the other. The antenna input impedance is matched to 50⍀ using a microstrip quarterwave matching that was printed on the same substrate as the feed line. To understand the choice of the parameter selection, leading to the optimal bandwidth in the design process, some elements can be mentioned. According to [5, 6], the dielectric constant of the foam affects the bandwidth and radiation efficiency of the antenna. Lower permittivity gives wider impedance bandwidth and minimizes surface-wave excitation. Foam thickness influences the bandwidth and coupling level. Thicker foam results in a wider bandwidth, but less coupling for a given aperture size. The microstrip patch length determines the resonant frequency while its width affects the antennas’ resistance at resonance frequency. For instance, a wider patch gives a lower resistance. The feed-substrate dielectric constant should be selected for good microstrip circuit qualities. Thinner microstrip feed substrates result in less spurious radiation from feed lines, but a higher loss. The length of the coupling slot primarily determines the coupling level, as well as the back radiation level. The slot should therefore be made no larger than it is

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Figure 2. Top view of the antenna.

Figure 4. Photograph of the first antenna prototype.

Figure 3. Return loss vs. frequency.

required for impedance matching. The width of the slot also affects the coupling level, but to a much less degree. The width of the feed line controls the impedance of the feed line and affects the coupling to the slot. The tuning stub is used to tune the excess reactance of the antenna, and its length is slightly less then a quarter wavelength. Shortening the stub will move the impedance locus in the capacitive direction on the Smith chart.

Since there are important interactions between these different parameters involved in the design process, the Ensembe software package [7] is used here as a CAD tool to determine the layout of the proposed antenna. This software is based on the full-wave method to solve a mixed-potential integral equation, which takes into account the effects of discontinuities, surfaces waves, and spurious radiations. Especially for antenna design, this package seems to be an excellent tool to calculate and optimize the patch dimensions. Using this approach, a first antenna is designed and its geometric configuration is shown in Figure 1. The first layer is an RT/Duroı¨d 5880 substrate with a dielectric constant ␧r ⫽ 2.2- and 0.787-mm thickness. It supports the microstrip feed line and the quarterwavelength transformer on one side, and the ground plane with coupling aperture on the other. This layer is followed by a hard foam of 12.7-mm thickness with a low dielectric constant ␧r ⫽ 1.07. On the top of the

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Figure 5. Measured and simulated reflection coefficient.

foam, a thin layer, RT/Duroı¨d substrate with dielectric constant ␧r ⫽ 2.2-and 0.127-mm thickness is used to support the etched microstrip antenna. The dimensions of the microstrip feed line, the coupling aperture, and the patch are shown in Figure 2 (in mm dimensions). To determine the performances of the proposed design, simulations were carried out. Figure 3 shows the input-return loss. From this curve it can be noted that this antenna has a bandwidth of about 22%, which is enough to cover the PCS band. To validate the design and the simulation results, experimental measurement will be presented in the next section.

III. EXPERIMENTAL RESULTS According to the geometry configuration and the specifications given in the previous section, a first antenna prototype has been fabricated and tested. Figure 4 shows the photograph of the first antenna prototype. To evaluate its experimental performances, measurements were carried out using HP8719 network analyzer. The measured and simulated input reflection coefficient is shown in Figure 5. The comparison between measured data and simulated ones indicate a good agreement. Referring to the curves shown in Figure 5, this first prototype has a bandwidth of 300 MHz, over which the VSWR is less than 2:1, which represents 22% of the center frequency (1.9 GHz). This frequency band is very sufficient to cover the PCS band. In addition, to analyse the radiating proprieties of this prototypes, the E-plane and H-plane radiation patterns were measured at 1.90 GHz. Figure 6 shows the measured E-plane and H-plane patterns. As shown, this prototype has a HPBW of 61° in the

Figure 6. Measured far-field radiation patterns of the second antenna prototype at center frequency f0 ⫽ 1.9 GHz: (a) E-plane; (b) H-plane. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

E-plane and 73° in the H-plane. From the radiation pattern, this antenna has a 9.4-dB gain. However, this prototype has an important back lobe, with a poor front-to-back ratio of 10 dB. This back radiation level is an undesired characteristic in PCS applications for two reasons: first, a part of the electromagnetic energy is radiated in a nondesired direction, which represents some radiation-power loss. Second, if the antenna


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Figure 7. Photograph of the second antenna prototype.

Figure 9. Measured far-field radiation patterns of the second antenna prototype at center frequency f0 ⫽ 1.9 GHz: (a) E-plane; (b) H-plane. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley. com.]

Figure 8. Measured and simulated reflection coefficient of second the antenna prototype.

with an important back lobe is used at the handset, there is an electromagnetic energy exposure risk for mobile phone users. This represents a very important parameter in antenna design if the specific absorption rate (SAR) is considered [8, 9], which is defined as the rate of energy absorption by body tissues close to antenna. To overcome this phenomenon, the back lobe of this antenna must be reduced and optimized.

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TABLE I.

Comparison Between the Proposed Antenna and Similar Antennas Reported By [10 –12]

Antenna geometry Number of patch used in antenna Operating frequency (GHz) Gain (dBi) Bandwidth Front-to-back ratio (dB)

Proposed Antenna

Antenna 1 [10]

Antenna 2 [11]

Antenna 3 [12]

Rectangular

Rectangular

Rectangular

1 1.9 9.5 21% 20

2 2.4 9.32 14.4% 18

2 1.901 4.2 20.4% NA

Rectangular with E-shaped slot 1 1.9 6.7 30.3% 12

In this perspective, an optimization process is proposed in the next section to increase the front-to-back lobe ratio and ensure a low-SAR antenna.

IV. OPTIMIZATION Although the characteristics of the antenna presented in the prior section are not too bad, it is possible to improve some of the antenna’s weak points such as the front-to-back ratio. A technique used to minimize the back lobe consists of adding a second back plane, distanced by a foam layer (12.7 mm) from the previous configuration. Using the Ensemble software as a design tool, the antennas’ dimensions were again adjusted in order to keep the center frequency and bandwidth at the desired values. First, the length of the patch radiator was reduced approximately by 0.5 mm to correct the center frequency. Second, the length of the tuning stub was adjusted by 0.2 mm to tune the input antenna impedance. Therefore, a second microstrip patch antenna was designed, fabricated, and tested. Figure 7 shows a photograph of the second antenna prototype. Similarly to the first case, measurements were also carried out. Figure 8 presents the measured and computed input reflection coefficient. From these curves it can be seen that a bandwidth of 21% has been achieved. The comparison between the experimental and the computed data indicates quite good agreement. In addition, for antenna radiation characterization, the E-plane and H-plane patterns of the second antenna are plotted, and Figure 9 shows the measured E-plane and H-plane patterns, respectively. Referring to these antenna patterns, the new antenna has a 9.5-dB gain, and HPBW of 59° in the E-plane, and 74° in the H-plane, respectively. According to the antenna patterns shown in Figure 9, the comparison between the front-lobe level and the backlobe level gives a ratio of 20 dB, which represents a significant improvement to the reduction of back radiation. From these results, it can be concluded the optimized antenna prototype has achieved a band-

width of 21%, a gain of 9.5 dB, and a front-to-back ratio level of 20 dB, which are very sufficient for PCS systems and other wireless applications. In addition, if more bandwidth is needed for special applications such as ultra-wideband systems, another patch layer can be added to the proposed structure (second resonator). In this case, however, more dielectric layers and resonators will lead to a complex configuration design. Table I compares the performances of the proposed antenna and those of the similar antenna reported in [10 –12]. The latter antennas were selected because they were designed and fabricated within the same band as that of the proposed antenna. This table summarizes the quantitative performances comparison between the four configurations in terms of bandwidth, gain, and front-to-back ratio. Antenna 1 [10] and Antenna 2 [11] exhibit a lower bandwidth than that of Antenna 3 [12], and to achieve those bandwidths, the Antenna 1 and Antenna 2 configurations have used two patches. However, additional patch layers will make the antenna design and fabrication more complex and expensive. From Table I it can be seen that the proposed antenna offers a relatively less bandwidth than that of Antenna 3, but provides an important gain and a high front-to-back ratio. With this approach, the design was improved and optimized in terms of gain, bandwidth, and front-to-back ratio.

V. CONCLUSION In this article, a broadband microstrip patch antenna has been designed, fabricated, and tested at the PCS band. Design and implementation considerations were given for two microstrip antenna prototypes. The first gives a good bandwidth of 21% at a center frequency of 1.9 GHz, but a low front-to-back ratio of 10 dB. To reduce the radiation back lobe of the antenna pattern, a second optimized antenna prototype, with a 9.5-dB gain, 21% of bandwidth, and a 20-dB front-to-back ratio, which gives a low-SAR antenna, was proposed


and realized. Measurements of the return loss and the radiation pattern were presented and discussed. The comparison between experimental and numerical results has shown good agreement.

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REFERENCES

9.

1. G.A. Deschamps, Microstrip microwave antennas, 3rd USAF Symp Antennas, 1953. 2. H.K. Smith and P.E. Mayes, Stacking resonators to increase the bandwidth of low-profile antennas, IEEE Trans Antennas Propagat 35 (1987), 1473–1476. 3. C. Wood, Improved bandwidth of microstrip antenna using parasitic elements, IEE Proc Microwaves Optics and Acoustics 127 (1980), 231–234. 4. D.M. Pozar, A microstrip antenna aperture coupled to a microstrip line, Electron Lett 21, (1985), 49 –50. 5. R Gard, P. Bhartia, I. Bahl, and A. Ittipiboon, Microstrip antenna design handbook, Artech House, Norwood, MA, 2000. 6. J.F. Zu¨ rcher and F.E. Gardiol, The SSFIP: A global

8.

10.

11.

12.

517

concept for high-performance broadband planar antenna, Electron Lett EL-24 (1988), 1433–1435. ENSEMBLE 7.0, User’s Guide, Ansoft Corporation, Pittsburgh, PA. Q. Balzano, O. Garay, and T.J. Manning, Electromagnetic energy exposure of simulated users of portable cellular phones, IEEE Trans Veh Technol 44 (1995), 390 – 403. M. Okoniewski and M.A. Stukly, A study of the Handset antenna and human body interaction, IEEE Trans Microwave Theory Tech 44 (1996), 1855– 1864. L.K. Chung and A.S. Mohan, Gain and bandwidth enhancement of a 2.4 GHz singly-fed cross-aperture coupled patch antenna, in IEEE AP-S Int Symp Dig 1 (2002), 410 – 413. Y.J. Wang, C.K. Lee, and W.J. Koh, Design of small and broadband internal antennas for IMT-mobile handsets, IEEE Trans Microwave Theory Tech 49 (2001), 1398 –1403. F. Yang, X. Zhang, X. Ye, and Y. Rahmat-Samii, Wide-band E-shaped patch antennas for wireless communications, IEEE Trans Antennas Propagat 49 (2001), 1094 –1100.

BIOGRAPHIES Tayeb A. Denidni received a B. Sc. degree in electronic engineering from the University of Setif, Algeria, in 1986, and M.Sc. and Ph.D. degrees in electrical engineering from Laval University, Quebec, Canada, in 1990 and 1994, respectively. From 1994 to 1996, he was an Assistant Professor with the Engineering Department of the Universite´ du Que´ bec in Rimouski (UQAR), Que´ bec, Canada. From 1996 to 2000, he was also an Associate Professor at UQAR, where he founded the Communications Research Laboratory. Since August 2000, he has been with the Personal Communications Staff, Institut National de la recherche´ scientifique (INRS), Universite´ du Que´ bec in Montreal, Canada. His current research interests are adaptive antennas array, phased array, microstrip antenna, microwave and RF design for wireless applications, and development for communications systems. He is a Member of the Order of Engineers of the Province of Que´ bec, Member of URSI (Commission C), and Member of IEEE.

Larbi Talbi received a Diplme d’Ingnieur d’Etat from the National Institute of Electronics (INE), Setif, Algeria, in 1986, and M.Sc. and Ph.D. degrees from Laval University, Quebec, P.Q., Canada, in 1989 and 1994, respectively, both in electrical engineering. He completed a post-doctoral fellowship at INRS-Telecommunications, within the Personal Communications Systems Group, P.Q., Canada. From 1995 to 1998, he was an Assistant Professor in the Electronics Engineering Department, Riyadh College of Technology, Saudi Arabia. From 1998 to 1999 he was an Invited Professor with the Electrical Engineering Department, Laval University, Canada. Since 1999, he has been a professor at the Universite´ du Quebec, Hull, QC, Canada. His research interests include the numerical techniques applied to electromagnetics, UHF, and millimeter indoor-radio-propagation channel characterization and measurement, design of microwave integrated circuits for wireless communication systems, and radar cross sections.