4 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 5, 2006

A Compact Dual-Band Cavity-Backed Slot AntennaYijun Liu, Zhongxiang Shen, Senior Member, IEEE, and Choi Look Law, Senior Member, IEEE

Abstract—A compact design for dual- and wide-band cavity-backed slot antenna is presented in this letter. The slot antenna isfed by two microstrip lines on the top of the substrate for 900- and1800-MHz bands, respectively. With the use of a U-type slot anda ridged slot backed by one rectangular cavity, a compact designis realized for a dual- and wide-band slot antenna. A prototypeis fabricated and tested in an anechoic chamber. The measured10-dB return loss bandwidth is 28% for 900-MHz band and 31%for 1800-MHz band. The measured gain is 5.0 dBi at 1000 MHz,5.5 dBi at 1800 MHz.

Index Terms—Cavity-backed slot antenna, dual-band, mi-crostrip-fed, wide-band.

I. INTRODUCTION

I N many applications such as mobile or satellite communi-cations, large bandwidth for two or more frequency bands,

compact size and high gain are required for antenna elementwhen used in array configurations. For such applications,several designs have been reported for dual- and broad-bandoperations [1]–[3]. For the reported design in [1], a dual-patchantenna excited by two L-probes shows a 10-dB return lossbandwidth of 20.8% and 17.9% at 900- and 1800-MHz bands,respectively. However, this antenna is composed of two non-planar patches, which is large in size and not suitable forintegration with other circuits. In [2], the bandwidth of an aper-ture-stacked patch antenna reaches 124 MHz (14.4%) at lowerband (860 MHz) and 663 MHz (33.8%) at the upper-band(1960 MHz). The front-to-back ratio is inherently low forthis design because of its feeding structure. Furthermore, theantenna uses a structure of multilayered patches, which isstructurally complicated. A dual-band dual-polarized antennareported in [3] covers triple bands for mobile phone basestations. This antenna consists of six slots: four slots for thelower band, and two crossed slots for the upper band. Theantenna structure is also complicated and not suitable for arrayapplications. Furthermore, the lower band is not completelycovered for the design in [3].

In this letter, a new compact design is presented for dual-band (900/1800 MHz) operation using the cavity-backed slotantenna. The proposed antenna has two radiating slots printedon the same substrate and backed by one rectangular cavity.In order to realize a compact size, a U-type slot is used for900-MHz band and a ridged slot is employed for the 1800-MHzband. The two slots are placed near to each other for a com-pact size and are backed by a metallic cavity to obtain unidirec-tional radiation [4]. Two microstrip feed lines are used to excite

Manuscript received July 28, 2005; revised October 13, 2005.The authors are with the School of Electrical and Electronic Engineering,

Nanyang Technological University, Singapore 639798, Singapore (e-mail:ezxshen@ntu.edu.sg).

Digital Object Identifier 10.1109/LAWP.2005.863611

Fig. 1. Geometry of a dual-band microstrip-fed cavity-backed slot antenna(L = 160 mm, W = 120 mm, H = 35 mm, h = 1:575 mm, Ls =

144mm, Ws = 60mm, Lp = 22mm, Wp = 10mm, W = 4:6mm,S = 10mm, S = 10mm,Ls = 78mm,Ws = 20mm,Lp = 5mm,Wp = 6 mm).

the two slots and they are etched on the upper side of the sub-strate to realize the inverted configuration so that a higher gainand greater design flexibility can be achieved [5], [6]. Based onthis design, a prototype is fabricated and tested. The measured10-dB return loss bandwidths are 28% and 31% for 900- and1800-MHz bands, respectively. Measured radiation patterns arealso given and they show that this slot antenna element is verysuitable for dual-band array configurations. Measured gains are5 and 5.5 dBi at 1000 and 1800 MHz, respectively.

II. ANTENNA CONFIGURATION

Fig. 1 shows the geometry of the proposed compact design ofa dual- and wide-band slot antenna backed by a metallic cavity.The U-type slot is employed to reduce the overall length of theslot resonating at 900 MHz. A smaller slot loaded with ridgesis used to realize the resonance at 1800 MHz. The ridges areintroduced at the center of the slot to reduce the slot length sothat a certain clearance can be maintained between the two slots

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LIU et al.: DUAL-BAND CAVITY-BACKED SLOT ANTENNA 5

Fig. 2. Simulated and measured return loss results of the dual-band cavity-backed slot antenna at the two feeding ports.

for improving the isolation performance between the bands. Thetwo slots are etched on the bottom layer of a Taconic substrate[5] and are backed by one rectangular cavity to suppress thebackward radiation. Two open-ended microstrip feed lines areetched on the upper layer of the substrate to realize the invertedconfiguration for higher gain and greater adjustment flexibility[5], [6].

The Taconic substrate is of thickness andits dielectric constant is 2.49. In order to obtain a compactsize and an easy assembly, the substrate is placed imme-diately above the metallic cavity (with an internal size of160 mm 120 mm 35 mm) with the slot side facing down to-ward the cavity. The feed lines of the same width(for 50 ohms characteristic impedance) are extended by 56 mmand 21 mm from the slots’ longer edge for 900- and 1800-MHzbands, respectively. The offsets are 17.4 mm and 7.4 mm fromthe slots’ shorter edge for 900- and 1800-MHz bands, respec-tively. For this slot antenna element, two SMA connectors aresoldered to the two microstrip feed lines for the measurementpurpose. When the slot antenna is used in an array configura-tion, a feeding network can be easily designed on the upperlayer of the substrate.

III. MEASURED RESULTS

The dual-band cavity-backed slot antenna is initially simu-lated using Ansoft’s Ensemble. The slot lengths as well as thedimensions of their bended sections and loaded ridges are ad-justed for obtaining the center frequencies of dual-band opera-tion. The slot widths and separation between them are varied forachieving the broad-band performance with a satisfactory isola-tion. The cavity length and width should also be tuned for good

Fig. 3. Simulated and measured isolation between two feeding ports of thedual-band cavity-backed slot antenna.

Fig. 4. Measured radiation patterns of the dual-band cavity-backed slotantenna when excited at the two feeding ports for two different frequencybands.

return loss and isolation results. After the simulation, a proto-type is then fabricated and tested.

Fig. 2 presents the measured and simulated return loss resultsof the fabricated dual-band cavity-backed slot antenna at twofeeding ports. It is seen that the slot antenna can operate overtwo frequency bands: 840–1100 MHz and 1630–2200 MHz.This corresponds to the 10-dB return loss bandwidth of 28%at the center frequency 920 MHz and 31% at the center fre-quency 1795 MHz, respectively. Fig. 3 shows the measured andsimulated isolation between the two feeding ports. The isola-tion degrades slightly at the 1800-MHz band, compared to the900-MHz band. This may be attributed to the coupling betweenthe fundamental resonant mode of the small slot and the secondresonant mode of the large slot. The isolation is at1100 MHz and at 1700 MHz. From Figs. 2 and 3, we

6 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 5, 2006

can see that the agreement between simulated results and mea-sured ones is very good.

Fig. 4 provides the co-polarized and cross-polarized radia-tion patterns measured in an anechoic chamber at 1000 and1800 MHz for both - and - planes. It can be seen that thecross polarization in the boresight is below for both -and -plane patterns at 1000 MHz and below for both

- and -plane patterns at 1800 MHz. The degradation of thecross-polarization level at 1800 MHz is due to the ridges intro-duced at the center of the smaller slot and the coupling betweenthe edge of the smaller slot and the bended section of the largerslot. The measured gains are 5.0 dB at 1000 MHz and 5.5 dB at1800 MHz, respectively.

IV. CONCLUSION

A compact dual- and wide-band cavity-backed slot antennahas been presented in this letter. A U-type slot has been used forthe 900-MHz band and a ridged slot has been employed for the1800-MHz band. These two slots are then fed by two microstriplines printed on the upper layer of the substrate for higher gainand greater design flexibility. The designed antenna has been

measured and it shows that the 10-dB return loss bandwidth is28% for 900 -MHz band and 31% for 1800-MHz band, respec-tively. The measured gains are 5.0 dB at 1000 MHz and 5.5 dBat 1800 MHz, respectively. Because of its relatively high gainand good radiation patterns at both frequency bands, this de-scribed slot antenna should be very useful for dual-band arrayapplications.

REFERENCES

[1] K. M. Luk, C. H. Lai, and K. F. Lee, “Wideband L probe feed patchantenna with dual band operation for GSM/PCS base stations,” Electron.Lett., vol. 35, no. 14, pp. 1123–1124, 1999.

[2] K. Oh, B. Kim, and J. Choi, “Design of dual and wideband aperturestacked patch antenna with double-sided notches,” Electron. Lett., vol.40, no. 11, pp. 643–645, 2004.

[3] V. Deillon, J.-F. Zurcher, and A. K. Skrivervik, “A compact dual-banddual-polarized antenna element for GSM/DCS/UMTS base stations,”Microw. Opt. Technol. Lett., vol. 40, no. 1, pp. 29–33, 2004.

[4] J. Hirokawa, H. Arai, and N. Goto, “Cavity-backed wide slot antenna,”Inst. Elect. Eng. Proc., pt. H, vol. 136, no. 1, pp. 29–33, Feb. 1989.

[5] Q. Li and Z. Shen, “Inverted microstrip-fed cavity-backed slot an-tennas,” IEEE Antennas Wireless Propag. Lett., vol. 1, pp. 98–101,2002.

[6] , “An inverted microstrip-fed cavity-backed slot antenna for cir-cular polarization,” IEEE Antennas Wireless Propag. Lett., vol. 1, pp.190–192, 2002.