IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010 379

A Simple Approach for Reducing Mutual Couplingin Two Closely Spaced Metamaterial-Inspired

Monopole AntennasJiang Zhu, Student Member, IEEE, and George V. Eleftheriades, Fellow, IEEE

Abstract—A simple approach for reducing the mutual cou-pling in two closely spaced electrically small antennas (ESAs)is proposed based on the general idea of field cancellation. Theantenna array consists of two metamaterial-inspired small printedmonopole antennas. The mutual coupling is reduced by self-can-cellation of the induced common ground and near-field currents,without introducing additional structures. A fabricated proto-type of a two-element array with an edge-to-edge separation of� �� � (or center-to-center separation of � �� �) yields a

measured mutual coupling of �� better than �� dB acrossthe impedance band of �� �� dB, which is significantlybetter than the � dB level of the original array.

Index Terms—Antenna arrays, electrically small antennas(ESAs), metamaterials, mutual coupling, printed monopole.

I. INTRODUCTION

T HE development of next-generation wireless communi-cation systems requires broadband and multiband devices

for faster data transfers. Meanwhile, there is a trend toward theminiaturization of handheld devices. These conflicting require-ments must be met using low-cost solutions that simultaneouslymaintain a high radiation efficiency. Transmission-line metama-terials (TL-MTM) provide a conceptual route for implementingsmall resonant antennas. Typically, TL-MTM antennas sufferfrom narrow bandwidths. Recently, [1] addressed the bandwidthproblem by proposing a two-arm TL-MTM antenna resonatingat closely spaced frequencies. Furthermore, a compact tribandmonopole antenna with single-cell metamaterial loading wasdemonstrated in [2] and [3] for WiFi and WiMAX applications,whereas a dual-band metamaterial antenna was proposed in [4]for WiFi applications.

Another TL-MTM type of a dual-band electrically smallantenna (ESA), fabricated on an FR4 board with thicknessof 1.6 mm, has been recently reported in [5]. This antennais based on the planar coplanar waveguide (CPW) monopoletopology, but with a single-cell metamaterial loading, as shownin Fig. 1(a). The antenna comprises a two-arm fork-like

Manuscript received January 26, 2010; revised March 09, 2010; acceptedApril 12, 2010. Date of publication April 22, 2010; date of current version May10, 2010. This work was supported in part by an IEEE Antenna and PropagationSociety Graduate Fellowship.

The authors are with the Edward S. Rogers, Sr. Department of Electrical andComputer Engineering, University of Toronto, Toronto, ON M5S 3G4, Canada(e-mail: jiangzhu@waves.utoronto.ca; gelefth@waves.utoronto.ca).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LAWP.2010.2048691

Fig. 1. A dual-band MTM-inspired small antenna. (a) Geometry (theground plane is rescaled). All dimensions are in millimeters. (b) Simulated�-parameters.

monopole with a thin-strip inductor loaded on top of themonopole and an interdigital capacitor loaded on the right-sidearm. This type of loading creates a second resonance coveringthe lower WiFi band of 2.40–2.48 GHz, in addition to themonopole resonance over the 5.15–5.80 GHz upper WiFi band.At the lower WiFi band, the antenna no longer acts as a regularmonopole along the vertical direction, but rather as a slotalong the horizontal direction. The proposed loading forces thecurrent to wrap around the slot perimeter and induce an E-fielddistribution along the horizontal direction within the slot, bothcontributing to the slot-mode radiation. The monopole elementhas dimensions of 8.5 5.7 mm (orat 2.45 GHz). A dual-band performance can be clearly seenfrom the HFSS simulation shown in Fig. 1(b). The simulated

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380 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010

radiation efficiencies are above 60% and 90% at the lower andhigher WiFi bands, respectively. This design is single-layeredand via-free and can therefore be easily fabricated at a low cost.

Multiple-input–multiple-output (MIMO) communicationsystems require good isolation between antenna elements. Itis challenging to design a closely spaced antenna system withgood isolation [6]. Recently, in [7], a suspended neutralizationstrip that is physically connected to the antenna elements wasinserted for improving isolation, and in [8], a coupling elementlocated between adjacent antennas was introduced to create anadditional coupling path. The idea behind these two methodsis to use additional conducting strips to cancel out the existingmutual coupling.

In this letter, we describe a simple technique to reduce themutual coupling between two closely spaced antennas havingthe previously described topology shown in Fig. 1. This is im-portant for MIMO applications in handheld units. Some prelim-inary simulation results have been reported in [9].

II. PROPOSED MUTUAL COUPLING REDUCTION TECHNIQUE

A simple approach is proposed to enhance the isolation fora closely spaced antenna array that consists of two antennas ofthe kind shown in Fig. 1(a). This approach exploits the generalidea of field cancellation, but without the aid of any additionalconducting strips, which in turn simplifies the design and fabri-cation process.

A. Two Major Sources Contributing to Mutual Coupling

Before introducing the isolation-enhanced design, it is worth-while to examine the contribution of mutual coupling from dif-ferent sources. As shown in Fig. 2, the two antennas of Fig. 1are placed symmetrically along the central axis of the array andseparated by an edge-to-edge distance of 4 mm or at2.45 GHz and a center-to-center distance of 9.7 mm orat the same frequency. The current distribution at the lower WiFiband (2.40–2.48 GHz) is sketched using HFSS and is shown inFig. 2(a). The induced currents at Antenna B, when Antenna A isfed with a sinusoidal current, are mainly due to: 1) the near-fieldcoupling from Antenna A; and 2) the common-ground currentshared with Antenna A. Since the antennas are very close toeach other, these induced currents due to mechanisms 1) and2) on Antenna B are approximately in phase. Moreover, as canbe seen from Fig. 2(b), first, and are identical dueto symmetry and second, the antenna array exhibits a simu-lated mutual coupling of worse than dB across the

dB impedance band from 2.45 to 2.54 GHz, withthe peak mutual coupling of dB occurring at the resonanceof ( GHz).

B. Mutual Coupling Reduction Exploiting Field Cancellation

The idea of reducing the mutual coupling between the twoantennas shown in Fig. 2(a) is to make the contributions tomutual coupling from the above-mentioned induced currentscancel each other. This can be achieved by simply flipping An-tenna B along the central axis of its CPW feedline as shownin Fig. 3(a). When this is done, the induced ground currentsretain their original phase, but the induced near-field currents

Fig. 2. A closely spaced antenna array composed of two small antennas placedsymmetrically along the central axis of the array. (a) Current distribution atlower WiFi band. (b) Simulated �-parameters.

reverse their phase. This is due to the asymmetric loading ca-pacitor. At the lower WiFi band, the loading capacitor results incircular currents along the loop that is composed of the two-armfork-like monopole and the loading inductor above it [5]. Sincethe currents on the CPW feedline flow into/out of the arm withthe least impedance (that is, the arm that does not contain theinterdigital capacitor), the induced near-field currents and theinduced common-ground currents are contradirectional on thecentral conductor of the CPW feedline. This leads to a very lowmutual coupling, as can be verified from the simulation shownin Fig. 3(b) ( dB at 2.46 GHz, the resonant fre-quency of ). It should be noted that this method is not ap-plicable for the isolation enhancement at the higher WiFi band(5.15–5.80 GHz). At that band, the loading capacitor becomeselectrically shorted, and the inductor becomes an open, leavingthe in-phase currents, instead of the circular currents, flowingon the two arms.

C. Final Design

It should also be noticed from Fig. 3(b) that the resonant fre-quency of Antenna B is shifted by 70 MHz toward the lowerfrequency from the resonant frequency of Antenna A. This isbecause the antenna array is now asymmetric along its central

ZHU AND ELEFTHERIADES: REDUCING MUTUAL COUPLING IN TWO CLOSELY SPACED METAMATERIAL-INSPIRED MONOPOLE ANTENNAS 381

Fig. 3. A closely-spaced antenna array composed of two small antennas.Antenna B is flipped along the central axis of its CPW feedline as comparedto Fig. 2(a). (a) Current distribution at lower WiFi band. (b) Simulated�-parameters.

axis, and the performance of each small antenna is sensitive tothe corresponding size of its left and right CPW ground plane.The resonance of Antenna B can be fine-tuned using the offsetlength as shown in Fig. 4(a), until its resonance is alignedwith that of Antenna A when mm. In Fig. 4(b), thesimulated mutual coupling is better than dB across theband of dB from 2.40 to 2.50 GHz. The mutualcoupling at the resonance of is dB, which is signifi-cantly better than the corresponding dB level of the orig-inal array shown in Fig. 2. The HFSS simulated current distri-bution is shown in Fig. 5, where contradirectional currents onthe central conductor of the CPW feedline of Antenna B can beclearly seen.

III. EXPERIMENTAL RESULTS

The design of a closely spaced antenna array composed oftwo small antennas with the proposed isolation enhancementscheme shown in Fig. 4(a) was fabricated and tested. The fab-ricated prototype is shown in Fig. 6(a), and the measured -pa-rameters are shown in Fig. 6(b). The two-element antenna arrayexhibits a measured impedance bandwidth of 110 MHz from2.47 to 2.58 GHz for dB, and a bandwidth of

Fig. 4. The final design of a closely spaced antenna array composed of twosmall antennas with isolation enhancements. (a) Geometry. (b) Simulated�-parameters.

Fig. 5. The current distribution of the final design of a closely spaced antennaarray composed of two small antennas with isolation enhancements at 2.46 GHz.

70 MHz from 2.47 to 2.54 GHz for dB. The res-onant frequencies of and are shifted by about 70 MHztoward the higher frequency, and there is a 35-MHz offset be-tween the resonances of and . These discrepancies be-tween the simulated and the measured results are likely due toa variation of the dielectric constant of the FR4 substrate andalso due to a slightly nonuniform milling of the interdigital ca-pacitors and strip inductors on the antennas. The measured mu-tual coupling of is better than dB across the band of

382 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010

Fig. 6. The final design of a closely spaced antenna array composed of twosmall antennas with isolation enhancements. (a) The fabricated prototype.(b) The measured �-parameters.

dB with a mutual coupling of dB at the res-onant frequency of ( GHz). This is in line with theobservation from the simulated results shown in Figs. 3(b) and4(b).

IV. CONCLUSION

A simple technique has been proposed to reduce the mu-tual coupling between two MTM-inspired electrically smallmonopole antennas that are closely spaced. This techniquerelies on the self-cancellation of the induced common-groundand near-field currents and does not require any additionalstructures, which in turn simplifies the design process andreduces the fabrication cost. The theoretical performance is ver-ified by full-wave simulations and experimental data. We haveshown that for two small antennas sizedand spaced edge-to-edge or center-to-center ,the peak mutual coupling (within the dB band)is reduced from dB of the original array to dBin HFSS simulation and dB in measurement for themutual-coupling reduced array. These attributes make the pro-posed antenna well suited for emerging multiantenna wirelessapplications.

REFERENCES

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[3] J. Zhu, M. A. Antoniades, and G. V. Eleftheriades, “A compact tri-bandmonopole antenna with single-cell metamaterial loading,” IEEE Trans.Antennas Propag., vol. 58, no. 4, pp. 1031–1038, Apr. 2010.

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