IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013 1169

Bandwidth Broadening of Dual-Slot Antenna UsingSubstrate Integrated Waveguide (SIW)

Moustapha Mbaye, Julien Hautcoeur, Larbi Talbi, Senior Member, IEEE, and Khelifa Hettak, Senior Member, IEEE

Abstract—This letter presents a dual-slot antenna etched on asubstrate integrated waveguide. This antenna is designed, fabri-cated, andmeasured to operate at X-band. It uses two unequal slotsin order to increase the bandwidth. The measured results show abandwidth of 8.5%, which is twice that of the standard dual-slotantennas. Attractive features including low cost, compact size, andplanar formmake this antenna easy to integrate within microwaveplanar circuits.

Index Terms—Antenna, bandwidth, dual slots, microwave, radiofrequency (RF), substrate integratedwaveguide (SIW), waveguide,wideband.

I. INTRODUCTION

S LOT antennas are one of the basic electromagnetic energyradiators. They have been widely used in radar and com-

munication systems. Recently, several novel techniques relyingon microwave integrated circuit fabrications, such as slot an-tennas and slot antenna arrays etched on substrate integratedwaveguide (SIW), have been proposed. The slots, etched in theground of the waveguide, cut and disturb the currents flowingbetween the waveguide walls to produce a resonance [1]. TheSIW technology, which is a compromise between both planarand waveguide technologies, features interesting characteristicsin terms of ease of integration to other circuit components, whileoffering components with a high quality factor [2].Previous studies have considered conventional dual-slot an-

tennas and a new-shaped slot integrated antenna [3]–[6]. Unfor-tunately these antennas have a narrow bandwidth close to 4%.Sellal et al. have proposed a round-ended banana-shaped slotantenna and a new V-shaped slot antenna with measured band-widths of 391 and 400 MHz at 10 GHz, respectively [3], [6].According to [5], a bandwidth of 360 MHz is achieved using adual-slot antenna.It is well known that the bandwidth of such arrays is deter-

mined by both the resonant properties of individual slots andthe bandwidth of the waveguide structure. Therefore, thesearrays suffer from severe bandwidth limitations, and a usablebandwidth of a few percent is usually the norm. Indeed, the

Manuscript received August 09, 2013; accepted August 30, 2013. Date ofpublication September 10, 2013; date of current version September 27, 2013.M. Mbaye, J. Hautcoeur, and L. Talbi are with the Department of Computer

Science and Engineering, University of Quebec in Outaouais (UQO), Gatineau,QC J8X 3X7, Canada (e-mail: mbam03@uqo.ca; julien.hautcoeur@uqo.ca;Larbi.talbi@uqo.ca).K. Hettak is with the Communication Research Center (CRC), Ottawa, ON

K2H 8S2, Canada (e-mail: Khelifa.hettak@crc.gc.ca).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LAWP.2013.2281295

Fig. 1. Layout of the dual-unequal-slot antenna design.

array suffers from a small frequency bandwidth because ofthe resonant nature of the slot and the interelement spacing inthe waveguide, which is defined in terms of the wavelength(e.g., for broadside radiation, with being the guidedwavelength). Techniques to improve the bandwidth of indi-vidual radiators include the use of wider slots and special slotgeometries, e.g., the so-called dumbbell slots [7]. Although thepractice of employing wideband radiators is widely used, noreference regarding the possible advantages of doing so is tobe found in the literature. The most effective way of improvingthe bandwidth of linear arrays is to use center-fed branches,which may be viewed as a form of subarraying. However, theimplementation of subarrays in planar slot arrays inevitablyresults in an increased complexity of the networks [8], [9].The common point of these works is that they have used lineararrays made of slots working at only one frequency. The keypoint of this work is to use two slots working at two distinctbut close resonant frequencies.In this letter, we present an SIW antenna using a dual unequal

slot in order to increase the bandwidth. The complexity of theantenna is not increased, and as a result, the bandwidth is in-creased up to 8.5%, which is twice in comparison to that of thestandard dual-slot antenna [3].

II. DESIGN PROCESS

Details of the dual-unequal-slot antenna design are shown inFig. 1. It was fabricated on a Rogers RT/Duroid 5880 with per-mittivity and thickness of 0.508 mm. It is made of anSIWwaveguide operating at X-band. Themetallic side walls aremade of an array of metallized vias whose diameter equals1 mm and pitch equals 2 mm. These values are calculatedusing the design rules given in [2]. To operate at X-band, the

1536-1225 © 2013 IEEE

1170 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013

Fig. 2. Equivalent circuit model of the dual unequal slots in the SIW.

width of the SIW equals 12.1 mm. In the proposed an-tenna, two rectangular slots are etched in the top ground planeof the SIW. The lengths and of the slots are chosen tobe different in order to generate two distinct but close resonantfrequencies: GHz and GHz. This resulted toincrease the bandwidth of the antenna compared to a standarddual-slot antenna at X-band. The half-wavelength slots lengthsare calculated using respectively [10]

and (1)

These are of mm and mm. The widthsof the slots are identical and equal to 0.7 mm. The SIW is ter-minated by a short circuit. It is well known that a resonanceat a selected frequency is obtained when the center of eachslot is placed at an electrical distance offrom the end of the SIW waveguide [11]. This leads to

mm and mm, respec-tively. The matching of the antenna is controlled by the offset

mm from the axis of the SIW and the center of eachslot. Indeed, the effective shunt conductance of the SIWis [10]

(2)

with

where and are the normalized conductance of each slot.The equivalent circuit model of the two unequal slots in theSIW is shown in Fig. 2. Each slot is modeled by a shunt con-ductance and a susceptance. As at each resonant frequency, therespective susceptance equals zero, to achieve a good matching,and the sum of the normalized shunt conductance of the twoslots was set to unity at 10.5 GHz, which is the middle of thebandwidth [10]

The SIW is fed by a coaxial probe through a microstrip-to-SIWtransition. To limit its effect on the radiation pattern of the an-tenna, the microstrip-to-SIW transition is located on the oppo-site side of the dual unequal slots, as shown in Fig. 3.

Fig. 3. Photograph of the prototype dual-unequal-slot antenna design: (a) topview; (b) bottom view.

Fig. 4. Simulated and measured reflection coefficients of the dual-unequal-slotantenna.

III. EXPERIMENTAL RESULTS AND DISCUSSION

The dual-unequal-slot antenna was designed and optimizedusing the electromagnetic simulator HFSS at 10 GHz. Thedesigned antenna was built using a standard printed circuitboard (PCB) process and measured with an MS4647A Anritsuvector analyzer network (VNA). Fig. 4 shows the simulatedand measured reflection coefficients , which exhibit thesame behavior. The measured reflection coefficient is under10 dB from 10.10 to 11.0 GHz, while the simulated reflection

coefficient is under 10 dB from 10.21 to 11.0 GHz. Thesevalues correspond to a measured bandwidth of 8.5% and a sim-ulated bandwidth of 7.5%. The variation between the simulatedand measured reflection coefficients is due to the presence ofthe connector, which was not included in the simulation, and tothe fabrication and measurement errors.The E-plane and H-plane radiation patterns are shown in

Fig. 5. The dual-unequal-slot antenna exhibits an experimentalmaximum gain of 8.14 dBi, very close to the theoreticalgain, which equals 8.57 dBi. According to [9], this measuredgain value leads to an efficiency of approximately 75%,which corresponds to that of a standard dual-slot antenna.A bandwidth comparison has been conducted between thedual-unequal-slot antenna and those of the literature, i.e., the

MBAYE et al.: BANDWIDTH BROADENING OF DUAL-SLOT ANTENNA USING SIW 1171

Fig. 5. Radiation patterns of the dual-unequal-slot antenna. (a) E-plane.(b) H-plane.

TABLE ICOMPARISON OF DUAL-UNEQUAL, RECTANGULAR, V-SHAPED, ANDSQUARE-ENDED BANANA-SHAPED SLOT ANTENNA BANDWIDTHS

FOR dB

rectangular dual-slot [3]–[5], the V-shaped slot, and the ba-nana-shaped slot antennas [6]. This is summarized in Table I.The dual-unequal-slot antenna exhibits a bandwidth of 8.5%at X-band, which is twice as large as the other slot antennaspreviously reported. Consequently, the use of unequal slots indual-slot SIW antennas design enhances the bandwidth withoutimpacting the radiation efficiency, the size, and the cost.

IV. CONCLUSION

In this letter, a dual-unequal-slot antenna, suitable for high-density microwave integrated circuit applications, has been de-signed and fabricated using a standard PCB process. A goodagreement has been noticed between the experimental and sim-ulated results, and a bandwidth of 8.5% was measured. Thisletter demonstrates that using dual unequal slots may increasethe bandwidth of the antenna up to 100% compared of that of astandard dual-slot antenna. This low-cost, compact-size antennaandwider bandwidth are attractive candidates for SIW antennas.

ACKNOWLEDGMENT

The authors would like to thank the technicians of CREER,Montreal, QC, Canada, for their assistance and support in thefabrication and measurement processes. They would also liketo thank Rogers Corporation for supplying sample materials.

REFERENCES[1] W. H. Watson, “Resonant slots,” Proc. Inst. Elect., Pt. III-A, vol. 93,

pp. 747–777, 1946.[2] M. Mbaye, L. Talbi, K. Hettak, and A. Kabiri, “Design of 15 dB direc-

tional coupler using substrate integrated waveguide technology,” Mi-crow. Opt. Technol. Lett., vol. 54, pp. 970–9733, 2012.

[3] L. Talbi, K. Sellal, J. Lebel, T. A. Denidni, and T. A. , “Study ofa round-ended banana-shaped slot integrated antenna at X-band,” inProc. IEEE AP/S Symp., San Diego, CA, USA, Jul. 2008, pp. 1–4.

[4] D. Stephen, P. R. Young, and I. D. Robertson, “W-band substrate in-tegrated waveguide slot antenna,” Electron. Lett., vol. 41, no. 4, pp.165–167, Feb. 2005.

[5] A. J. Farrall and P. R. Young, “Integrated waveguide slot antennas,”Electron. Lett., vol. 40, pp. 974–975, May 2004.

[6] K. Sellal and L. Talbi, “Experimental study of new-shaped slot inte-grated antenna at X-band,” inProc. IEEEAPSURSI, Jun. 2009, pp. 1–4.

[7] P. K. Park and I. P. Yu, “Characterization of dumbbell slots in rect-angular waveguide by method of moments,” in Proc. Antennas Appl.Symp., Sep. 1984, pp. 1–6.

[8] P. N. Richardson and H. Y. Lee, “Design and analysis of slotted wave-guide arrays,” Microw. J., vol. 31, pp. 109–125, 1984.

[9] M. Zhang, J. Hirokawa, and M. Ando, “An E-band partially corporatefeed uniform slot array with laminated quasi double-layer waveguideand virtual PMC terminations,” IEEE Trans. Antennas Propag., vol.59, no. 5, pp. 1521–1527, May 2011.

[10] C. A. Balanis, Analysis and Design, 2nd ed. New York, NY, USA:Wiley, 1997.

[11] J. Hautcoeur, E. M. Cruz, J. Bartholomew, J. Sarrazin, Y. Mahé, andS. Toutain, “Low-cost printed antenna array built with hybrid feed forurban microwave links,”Microw., Antennas Propag., vol. 4, no. 9, pp.1320–1326, 2010.