High Gain Microstrip Antenna Design for BroadbandWireless Applications

Tayeb A. Denidni,1 Larbi Talbi2

1 Institut national de la recherche scientifique, Universite du Quebec, Place Bonaventure,800 De la Gauchetiere, Suite 6900, Montreal, Quebec, Canada H5A 1K62 Department of Computer Science and Engineering, Universite du Quebec-Hull, 101, rue St-Jean-Bosco, Case Postale 1250, Succursale B, Hull, Quebec, Canada J8X 3X7

Received 10 September 2002; accepted 18 June 2003

ABSTRACT: This article presents a new broadband microstrip antenna for personal com-munications systems (PCS) applications. Using multilayer substrate structure with aperture-coupled feed, a rectangular microstrip patch antenna operating at 1.9-GHz band is designedand experimentally validated. This antenna configuration uses a quarter-wave transformer toenhance the matching between the feed transmission line and the antenna patch. To demon-strate the design procedure, a first experimental broadband microstrip antenna prototype isdesigned and implemented. To analyse its performance, measurements are carried out andgood performances are achieved. However, this prototype has a low front-to-back ratio. Toovercome this drawback, an optimization process is proposed, and a second prototype isdesigned and successfully realized. To examine the effect of the optimization, experimentalinvestigations are carried out on the second prototype. Very good agreement is obtainedbetween numerical and measured results. Experimental results indicate that the proposedantenna 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. IntJ RF and Microwave CAE 13: 511–517, 2003.

Keywords: microstrip antenna; broadband antenna; radiation pattern

I. INTRODUCTION

For applications in wireless communications, such aspersonal communications systems (PCS), printed mi-crostrip antennas constitute a very attractive researchdomain. This importance is related to their advanta-geous characteristics, such as light weight, low pro-file, and ease of fabrication. In addition, they offerexcellent compatibility with monolithic microwaveintegrated circuits (MMIC). Particularly for futureapplications in mobile cellular networks, printed

patch antennas are well suited to assure good com-pactness and low cost in this area. In a single-elementform, they can be used to replace conventional anten-nas such us wire antennas. For applications in whichmore gain and space diversity are necessary, they canbe used as a radiating element in the development ofminiature antenna arrays. However, these antennasare characterized by the limitation of their bandwidth,which is of 1–2% [1]. This band is not sufficient forhigh-rate data transmission in wireless systems.

To resolve this problem, a rectangular patch an-tenna with large width has been proposed for broad-ening the bandwidth. However, this approach mayincrease surface-wave effects, which degrades perfor-mances significantly. To avoid this situation and im-prove the bandwidth, several methods based on

Correspondence to: T. Denidni; email: denidni@inrs-emt.uquebec.ca@

Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mmce.10109

© 2003 Wiley Periodicals, Inc.

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stacked patch [2] or parasitic-element configurations[3] have been proposed. Recently, Pozar [4] proposeda new feeding technique for microstrip antennas basedon the coupled-aperture concept in order to achieve alarge bandwidth. The technique consists of couplingenergy from the stripline through an aperture in theground plane. The radiating element is isolated fromthe feed network by the ground plane, which mini-mizes spurious radiation, and gives the designer thepossibility to select independent substrate materialsfor the feed and the patch. Furthermore, it offers agood compatibility with MMIC. For wideband wire-less applications, several related works based on thisidea have recently been proposed [10–12]. The re-ported bandwidths range from 10% to 20%. However,these configurations do not provide enough perfor-mance in terms of gain or front-to-back ratio. Thesetwo parameters are very important in wireless com-munications systems. In fact, the increasing of front-to-back ratio will reduce the backward radiation ofantenna that represents the radiation loss.

This article proposes a new approach where theabovementioned methods are extended in order toobtain an optimized antenna design that offers widebandwidth, high gain, and low back radiation. Ouraim, from these investigations, is to reach a compro-mise between the three important antenna criteria —bandwidth, gain, and front-to-back ratio — during theantenna design process.

Our approach uses a multilayer structure, an aper-ture-coupled feed, and a quarter-wave transformer inorder to design a novel antenna that offers a widebandwidth at a small size. During the design proce-dure, dielectric permittivity, substrate thickness, andthe size of the top patch were optimized. The quarter-wave transformer modifies an extra dimensional pa-rameter, which result in a very well-matched band-width microstrip patch antenna. Numerical results,taking into account the different physical parametersand operating conditions, are also studied and com-pared to validate the design.

In this article, a broadband microstrip patch an-tenna for wireless systems is investigated. Section IIpresents the antenna design procedure to achieve alarge bandwidth. To validate the design, numericaldata for the return loss and the radiation pattern arepresented. Experimental data of a first prototype arepresented and analyzed. In section III, experimentalresults are discussed. In section IV, an optimizationprocess to develop a second prototype to improve thefront-to-back ratio of antenna pattern is described.Finally, the conclusion of this work is presented insection V.

II. ANTENNA DESIGN

The main objective is to design a broadband micro-strip patch antenna for a wireless system, particularlyin the PCS band (1.85–1.99 GHz). The geometricconfiguration of the proposed antenna is shown inFigure 1. In this design, two dielectric substrate layersand one foam layer are stacked together. On the firstlayer, the rectangular patch is etched. Utilization ofthe foam layer provides the possibility to realize anantenna with a tick substrate and a very low dielectricconstant. But it is impossible to etch the patch directlyon the foam because of its porosity. For this reason, athin dielectric substrate is employed to support thepatch antenna. The third layer supports the microstripfeed line on one side, and the ground plane with acoupling aperture on the other. The antenna inputimpedance is matched to 50� using a microstripquarterwave matching that was printed on the samesubstrate as the feed line.

To understand the choice of the parameter selec-tion, leading to the optimal bandwidth in the designprocess, some elements can be mentioned. Accordingto [5, 6], the dielectric constant of the foam affects thebandwidth and radiation efficiency of the antenna.Lower permittivity gives wider impedance bandwidthand minimizes surface-wave excitation. Foam thick-ness influences the bandwidth and coupling level.Thicker foam results in a wider bandwidth, but lesscoupling for a given aperture size. The microstrippatch length determines the resonant frequency whileits width affects the antennas’ resistance at resonancefrequency. For instance, a wider patch gives a lowerresistance. The feed-substrate dielectric constantshould be selected for good microstrip circuit quali-ties. Thinner microstrip feed substrates result in lessspurious radiation from feed lines, but a higher loss.The length of the coupling slot primarily determinesthe coupling level, as well as the back radiation level.The slot should therefore be made no larger than it is

Figure 1. Geometry of the proposed antenna.

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required for impedance matching. The width of theslot also affects the coupling level, but to a much lessdegree. The width of the feed line controls the imped-ance of the feed line and affects the coupling to theslot. The tuning stub is used to tune the excess reac-tance of the antenna, and its length is slightly less thena quarter wavelength. Shortening the stub will movethe impedance locus in the capacitive direction on theSmith chart.

Since there are important interactions betweenthese different parameters involved in the design pro-cess, the Ensembe software package [7] is used hereas a CAD tool to determine the layout of the proposedantenna. This software is based on the full-wavemethod to solve a mixed-potential integral equation,which takes into account the effects of discontinuities,surfaces waves, and spurious radiations. Especiallyfor antenna design, this package seems to be an ex-cellent tool to calculate and optimize the patch dimen-sions.

Using this approach, a first antenna is designed andits geometric configuration is shown in Figure 1. Thefirst layer is an RT/Duroıd 5880 substrate with adielectric constant �r � 2.2- and 0.787-mm thickness.It supports the microstrip feed line and the quarter-wavelength transformer on one side, and the groundplane with coupling aperture on the other. This layeris followed by a hard foam of 12.7-mm thickness witha low dielectric constant �r � 1.07. On the top of the

Figure 2. Top view of the antenna.

Figure 3. Return loss vs. frequency.

Figure 4. Photograph of the first antenna prototype.

High Gain Microstrip Antenna Design 513

foam, a thin layer, RT/Duroıd substrate with dielectricconstant �r � 2.2-and 0.127-mm thickness is used tosupport the etched microstrip antenna. The dimen-sions of the microstrip feed line, the coupling aper-ture, and the patch are shown in Figure 2 (in mmdimensions).

To determine the performances of the proposeddesign, simulations were carried out. Figure 3 showsthe input-return loss. From this curve it can be notedthat this antenna has a bandwidth of about 22%,which is enough to cover the PCS band. To validatethe design and the simulation results, experimentalmeasurement will be presented in the next section.

III. EXPERIMENTAL RESULTS

According to the geometry configuration and thespecifications given in the previous section, a firstantenna prototype has been fabricated and tested. Fig-ure 4 shows the photograph of the first antenna pro-totype. To evaluate its experimental performances,measurements were carried out using HP8719 net-work analyzer. The measured and simulated inputreflection coefficient is shown in Figure 5. The com-parison between measured data and simulated onesindicate a good agreement. Referring to the curvesshown in Figure 5, this first prototype has a bandwidthof 300 MHz, over which the VSWR is less than 2:1,which represents 22% of the center frequency (1.9GHz). This frequency band is very sufficient to coverthe PCS band. In addition, to analyse the radiatingproprieties of this prototypes, the E-plane and H-planeradiation patterns were measured at 1.90 GHz. Figure6 shows the measured E-plane and H-plane patterns.As shown, this prototype has a HPBW of 61° in the

E-plane and 73° in the H-plane. From the radiationpattern, this antenna has a 9.4-dB gain. However, thisprototype has an important back lobe, with a poorfront-to-back ratio of 10 dB. This back radiation levelis an undesired characteristic in PCS applications fortwo reasons: first, a part of the electromagnetic energyis radiated in a nondesired direction, which representssome radiation-power loss. Second, if the antenna

Figure 5. Measured and simulated reflection coefficient.

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

514 Denidni and Talbi

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

Figure 7. Photograph of the second antenna prototype.

Figure 8. Measured and simulated reflection coefficient ofsecond the antenna prototype.

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

High Gain Microstrip Antenna Design 515

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

IV. OPTIMIZATION

Although the characteristics of the antenna presentedin the prior section are not too bad, it is possible toimprove some of the antenna’s weak points such asthe front-to-back ratio. A technique used to minimizethe back lobe consists of adding a second back plane,distanced by a foam layer (12.7 mm) from the previ-ous configuration. Using the Ensemble software as adesign tool, the antennas’ dimensions were again ad-justed in order to keep the center frequency and band-width at the desired values. First, the length of thepatch radiator was reduced approximately by 0.5 mmto correct the center frequency. Second, the length ofthe tuning stub was adjusted by 0.2 mm to tune theinput antenna impedance. Therefore, a second micro-strip patch antenna was designed, fabricated, andtested. Figure 7 shows a photograph of the secondantenna prototype. Similarly to the first case, mea-surements were also carried out. Figure 8 presents themeasured and computed input reflection coefficient.From these curves it can be seen that a bandwidth of21% has been achieved. The comparison between theexperimental and the computed data indicates quitegood agreement. In addition, for antenna radiationcharacterization, the E-plane and H-plane patterns ofthe second antenna are plotted, and Figure 9 showsthe measured E-plane and H-plane patterns, respec-tively. Referring to these antenna patterns, the newantenna has a 9.5-dB gain, and HPBW of 59° in theE-plane, and 74° in the H-plane, respectively. Accord-ing to the antenna patterns shown in Figure 9, thecomparison between the front-lobe level and the back-lobe level gives a ratio of 20 dB, which represents asignificant improvement to the reduction of back ra-diation. From these results, it can be concluded theoptimized antenna prototype has achieved a band-

width of 21%, a gain of 9.5 dB, and a front-to-backratio level of 20 dB, which are very sufficient for PCSsystems and other wireless applications. In addition, ifmore bandwidth is needed for special applicationssuch as ultra-wideband systems, another patch layercan be added to the proposed structure (second reso-nator). In this case, however, more dielectric layersand resonators will lead to a complex configurationdesign.

Table I compares the performances of the proposedantenna and those of the similar antenna reported in[10–12]. The latter antennas were selected becausethey were designed and fabricated within the sameband as that of the proposed antenna. This tablesummarizes the quantitative performances compari-son between the four configurations in terms of band-width, gain, and front-to-back ratio. Antenna 1 [10]and Antenna 2 [11] exhibit a lower bandwidth thanthat of Antenna 3 [12], and to achieve those band-widths, the Antenna 1 and Antenna 2 configurationshave used two patches. However, additional patchlayers will make the antenna design and fabricationmore complex and expensive. From Table I it can beseen that the proposed antenna offers a relatively lessbandwidth than that of Antenna 3, but provides animportant gain and a high front-to-back ratio. Withthis approach, the design was improved and optimizedin terms of gain, bandwidth, and front-to-back ratio.

V. CONCLUSION

In this article, a broadband microstrip patch antennahas been designed, fabricated, and tested at the PCSband. Design and implementation considerations weregiven for two microstrip antenna prototypes. The firstgives a good bandwidth of 21% at a center frequencyof 1.9 GHz, but a low front-to-back ratio of 10 dB. Toreduce the radiation back lobe of the antenna pattern,a second optimized antenna prototype, with a 9.5-dBgain, 21% of bandwidth, and a 20-dB front-to-backratio, which gives a low-SAR antenna, was proposed

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

ProposedAntenna Antenna 1 [10] Antenna 2 [11] Antenna 3 [12]

Antenna geometry Rectangular Rectangular Rectangular Rectangular withE-shaped slot

Number of patch used in antenna 1 2 2 1Operating frequency (GHz) 1.9 2.4 1.901 1.9Gain (dBi) 9.5 9.32 4.2 6.7Bandwidth 21% 14.4% 20.4% 30.3%Front-to-back ratio (dB) 20 18 NA 12

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and realized. Measurements of the return loss and theradiation pattern were presented and discussed. Thecomparison between experimental and numerical re-sults has shown good agreement.

REFERENCES

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USAF Symp Antennas, 1953.2. H.K. Smith and P.E. Mayes, Stacking resonators to

increase the bandwidth of low-profile antennas, IEEETrans Antennas Propagat 35 (1987), 1473–1476.

3. C. Wood, Improved bandwidth of microstrip antennausing parasitic elements, IEE Proc Microwaves Opticsand Acoustics 127 (1980), 231–234.

4. D.M. Pozar, A microstrip antenna aperture coupled to amicrostrip line, Electron Lett 21, (1985), 49–50.

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concept for high-performance broadband planar an-tenna, Electron Lett EL-24 (1988), 1433–1435.

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BIOGRAPHIES

Tayeb A. Denidni received a B. Sc. degreein electronic engineering from the Universityof Setif, Algeria, in 1986, and M.Sc. andPh.D. degrees in electrical engineering fromLaval University, Quebec, Canada, in 1990and 1994, respectively. From 1994 to 1996,he was an Assistant Professor with the En-gineering Department of the Universite duQuebec in Rimouski (UQAR), Quebec, Can-

ada. From 1996 to 2000, he was also an Associate Professor atUQAR, where he founded the Communications Research Labora-tory. Since August 2000, he has been with the Personal Commu-nications Staff, Institut National de la recherche scientifique(INRS), Universite du Quebec in Montreal, Canada. His currentresearch interests are adaptive antennas array, phased array, mi-crostrip antenna, microwave and RF design for wireless applica-tions, and development for communications systems. He is a Mem-ber of the Order of Engineers of the Province of Quebec, Memberof URSI (Commission C), and Member of IEEE.

Larbi Talbi received a Diplme d’Ingnieurd’Etat from the National Institute of Elec-tronics (INE), Setif, Algeria, in 1986, andM.Sc. and Ph.D. degrees from Laval Univer-sity, Quebec, P.Q., Canada, in 1989 and1994, respectively, both in electrical engi-neering. He completed a post-doctoral fel-lowship at INRS-Telecommunications,within the Personal Communications Sys-

tems Group, P.Q., Canada. From 1995 to 1998, he was an AssistantProfessor in the Electronics Engineering Department, Riyadh Col-lege of Technology, Saudi Arabia. From 1998 to 1999 he was anInvited Professor with the Electrical Engineering Department, La-val University, Canada. Since 1999, he has been a professor at theUniversite du Quebec, Hull, QC, Canada. His research interestsinclude the numerical techniques applied to electromagnetics, UHF,and millimeter indoor-radio-propagation channel characterizationand measurement, design of microwave integrated circuits for wire-less communication systems, and radar cross sections.

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