258 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009
A Broadband Dual-Mode Monopole Antenna UsingNRI-TL Metamaterial Loading
Marco A. Antoniades, Member, IEEE, and George V. Eleftheriades, Fellow, IEEE
Abstract—A printed monopole antenna is proposed which usesnegative-refractive-index transmission-line (NRI-TL) metamate-rial loading in order to achieve a broadband dual-mode operation.The metamaterial-loaded monopole supports a predominatelyeven-mode current at 5.5 GHz, which allows the antenna tobe modeled as a short folded monopole. Around 3.55 GHz, themetamaterial-loaded monopole acts as a balun for the groundplane currents, therefore rendering the entire top edge of theground plane as the main radiating element. This in turn radiatesa dipolar mode that is orthogonal to the folded-monopole mode at5.5 GHz. By virtue of the orthogonality between the two radiatingmodes, the metamaterial antenna exhibits a return-loss character-istic with a dual resonance, and therefore a very wide measuredimpedance bandwidth of 4.06 GHz. The total size of the antennais only 22 30 mm, and the measured efficiency is on the orderof 90% at both 3.55 and 5.5 GHz.
Index Terms—Folded monopole antenna, metamaterials, nega-tive refractive index.
I. INTRODUCTION
T HE recent inception of negative-refractive-index trans-mission-line (NRI-TL) metamaterials (MTM) [1], has
enabled the development of numerous new antenna designsthat take advantage of the unique characteristics that metama-terials have to offer. Notable among these are a compact andmulti-band patch antenna [2], a low-profile 0 ring antennawith vertical polarization [3], a meandered dipole with a po-larization that is orthogonal to its length [4], a dual-band ringantenna with a linear or circular polarization [5] and an infinitewavelength antenna with monopolar radiation patterns [6].
In this work, a printed monopole antenna that employsNRI-TL metamaterial loading is presented, that forms a naturalextension of the work carried out by the same authors in [3],[7], [8] and [9] for various electrically small MTM antennas.The different versions of the aforementioned MTM antennasconsisted of microstrip MTM unit cells arranged in a ringconfiguration over a truncated horizontal ground plane. Eachof the MTM unit cells was designed to incur a 0 phase shiftat the design frequency, thus ensuring that the currents on eachof the vertical vias within the unit cells were in phase. Thistechnique allows each of the MTM antennas to be modeled as
Manuscript received December 14, 2008; revised January 21, 2009. First pub-lished February 02, 2009; current version published May 06, 2009. This workwas supported by the Natural Sciences and Engineering Research Council ofCanada and Intel Corporation.
M. A. Antoniades and G. V. Eleftheriades are with the Edward S. Rogers Sr.Department of Electrical and Computer Engineering, University of Toronto,Toronto, ON M5S 3G4, Canada (e-mail: [email protected];[email protected]).
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.2009.2014402
a short multi-arm folded monopole that can be easily matchedto 50 without the use of an external matching network orbalun, and which radiates a vertical linear electric field [9].The antenna proposed herein and shown in Fig. 1 also employsMTM loading in order to implement the folded monopole tech-nique within the widely popular printed monopole structure. Asingle MTM unit cell is integrated directly onto the monopoleitself, which transforms the antenna into a folded monopolearound the frequency of 5.5 GHz, while at 3.55 GHz the MTMloading enables the ground plane of the antenna to radiate anorthogonal polarization. Thus, the MTM-loaded monopoleantenna achieves a very wideband dual-resonance responsein a compact and low-profile design that does not require theuse of any chip lumped-element components. Moreover, theantenna exhibits orthogonal polarizations in the 3.3–3.8 GHzWiMax and 5.15–5.85 GHz WiFi bands of interest, making itwell suited for use in low-cost multiple-input–multiple-output(MIMO) systems for wireless local area networks (WLAN).
II. ANTENNA DESIGN
The antennas considered in this work were designed on anFR4 substrate with parameters mm,and . As a reference comparison, an unloadedCPW-fed printed monopole antenna with a total length of
mm was first designed and simulated in Ansoft HFSS.This antenna had the same dimensions as the one shown in Fig. 1but without the MTM loading on the bottom side of the sub-strate. The simulated return-loss characteristics for the unloadedmonopole are shown in Fig. 2. It exhibits a matched resonancearound 6.3 GHz with a return-loss bandwidth below dB of2.36 GHz, from 5.31 to 7.67 GHz.
Since the performance of the unloaded monopole was abovethe range of interest for existing WLAN applications, a different
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ANTONIADES AND ELEFTHERIADES: BROADBAND DUAL-MODE MONOPOLE ANTENNA 259
Fig. 2. Simulated return-loss characteristics of the unloaded and MTM-loadedprinted monopole antennas from Ansoft HFSS.
Fig. 3. NRI-TL metamaterial � unit cell.
approach using metamaterial loading was pursued in order tomeet the WLAN specifications. Capitalizing on the reducedsize that a MTM folded monopole has to offer [9], the printedmonopole was loaded on the bottom side of the substrate ina left-handed fashion as shown in Fig. 1. This type of loadingwas inspired by the NRI-TL metamaterial unit cell shown inFig. 3, and enables the MTM-loaded monopole to maintain itssmall form-factor while decreasing its operating frequency andextending its bandwidth. Other monopole loading techniqueshave also been reported in the literature, for example in [10] aprinted monopole was loaded with an H-shaped conductor tocreate a band notch in the response of a UWB antenna.
In the MTM-loaded monopole of Fig. 1 the loading was car-ried out in an asymmetric fashion, where the series capacitance
was formed between the monopole on the top of the substrateand the rectangular patch on the bottom of the substrate, theshunt inductance was formed at the base of the monopole,while the shunt inductance was formed by the thin inductivestrip and via that join the bottom rectangular patch to the CPWground plane. The length of the monopole itself and the bottompatch and thin inductive strip form the TL sections in Fig. 3,with approximate parameters of andat 5.5 GHz. The capacitance was adjusted by changing thelength of the bottom patch and the inductance was ad-justed by changing the width of the thin inductive strip inorder to obtain in-phase currents along the top monopole sectionand along the thin bottom strip at 5.5 GHz, thus forming a com-pact folded monopole. The final dimensions of the optimizedMTM monopole antenna are outlined in Fig. 1. Using these di-mensions, the extracted values for the loading elements were inthe range of 0.2 nH for , 3 nH for , and 0.5 pF for ,which when inserted into [9, eq. (1)] result in a phase shift closeto 0 .
Fig. 4. Surface current distributions on the conductors of the MTM-loadedmonopole antenna. (a) 5.5 GHz. (b) 3.55 GHz.
The simulated return-loss characteristics for the unloaded andthe MTM-loaded monopole antennas are both shown in Fig. 2.It can be observed that while the unloaded monopole exhibits asingle resonance around 6.3 GHz, the MTM-loaded monopoleexhibits a broadband dual-resonance, comprising the desiredresonance around 5.5 GHz together with an additional reso-nance around 3.55 GHz. The MTM antenna has a simulatedreturn-loss bandwidth below dB of 3.84 GHz, from 3.15to 6.99 GHz. Thus, it can be observed that the addition of theMTM loading to the short monopole not only reduces the oper-ating frequency from 6.3 to 5.5 GHz, but also introduces anotherresonance around 3.55 GHz, which is located at the center of the3.3–3.8 GHz WiMax band. The resonance around 3.55 GHz canbe tuned by changing the width of the ground plane , whilethe length of the ground plane can be used to change theinput impedance level.
A. Principle of Operation
The dual-mode operation of the antenna can be explained byconsidering the current distribution on the MTM-loaded antennaat each of the resonant frequencies, as shown in Fig. 4. Thesefigures were sketched from the surface current distributions ob-served in Ansoft HFSS. At 5.5 GHz, the MTM loading wasadjusted such that the current along the monopole and alongthe bottom thin inductive strip were in phase. Thus, at this fre-quency the MTM loading was used to create a two-arm foldedmonopole, similar to the four-arm folded monopole of [9]. Aswas outlined in [9], by adjusting the value of the loading in-ductance it is possible to effectively eliminate the odd-modecurrent on the monopole, enabling the even-mode current alongthe -direction to radiate, as shown in Fig. 4(a). Additionally, at5.5 GHz, the currents along the top edges of the two groundplanes are out of phase and the balanced CPW mode is pre-served, therefore these currents do not contribute to any radi-ation.
At 3.55 GHz, the antenna no longer acts as a folded monopolealong the -axis, but rather as a dipole oriented along the -axis.This is a result of the in-phase currents along the top edges ofboth the ground plane sections as shown in Fig. 4(b), whichrender the ground plane as the main radiating element at this fre-quency. This current distribution can be explained with the aidof the circuit diagram of Fig. 3. At low frequencies the host TLsections can be considered negligible. Assuming that the feedis placed at the base of the shunt inductor , the entire cir-cuit is simply transformed into a series resonator formed be-tween the loading capacitance and the loading inductance
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260 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009
Fig. 5. Measured and simulated return-loss characteristics of the unloadedmonopole antenna, shown in the inset photograph.
. Around 3.55 GHz the series resonator, which rep-resents the MTM-loaded monopole, forms a short circuit andtherefore acts as a balun for the currents on the ground plane.The phase of the current flowing on the right ground plane istherefore reversed, and is aligned with the current on the leftground plane. This in turn enables the entire top edge of theground plane to radiate in a dipolar fashion. As a consequence,the MTM-loaded monopole cannot be integrated onto a largerground plane and must be used as a stand-alone unit in order topreserve the radiating mode at 3.55 GHz. As will be shown inthe next section, the dual-radiating nature of the antenna at 3.55and 5.5 GHz will also be verified by the orthogonal polariza-tions radiated at each of the aforementioned frequencies.
III. SIMULATION AND EXPERIMENTAL RESULTS
Both the unloaded and the MTM-loaded monopole antennaswere fabricated and tested, and their photographs and return-loss characteristics are shown in Figs. 5 and 6. In both casesthe simulated and measured responses match quite well, withthe unloaded monopole exhibiting a measured dB band-width of 2.48 GHz, from 5.35 to 7.83 GHz, while the MTM-loaded monopole exhibits an increased measured bandwidth of4.06 GHz, from 3.14 to 7.20 GHz.
The measured and simulated radiation patterns for theMTM-loaded monopole antenna for the three principal planesat 5.5 GHz are shown in Fig. 7. At 5.5 GHz, the antenna exhibitsradiation patterns with a horizontal -directed linear electricfield polarization, consistent with -directed currents alongthe monopole and the bottom thin inductive strip, as shown inFig. 4(a). Thus, the radiation patterns verify that at 5.5 GHzthe metamaterial loading of the monopole antenna enables it tooperate as a short folded monopole.
The radiation patterns in the - and the -planes, whichcorrespond to the two E-planes of the folded monopole, re-veal that the patterns are not completely symmetric. This is anexpected result, since the ground plane acts to partially blockradiation in the direction. As such, the observed gain ofthe antenna is higher in the half-space region, i.e., in thebottom-half of Fig. 7(a) and in the left-half of Fig. 7(b). Inthe -plane, which corresponds to the H-plane of the foldedmonopole, the radiation pattern is as expected omnidirectional.The simulated efficiency from HFSS at 5.5 GHz was 91.0%,
Fig. 6. Measured and simulated return-loss characteristics of the MTM-loadedmonopole antenna, shown in the inset photographs.
compared to measured efficiencies of 89.2% using the gain com-parison method and 92.6% using the Wheeler cap method [11],verifying that they are in good agreement.
Fig. 8 shows the measured and simulated radiation patternsfor the MTM-loaded monopole for the three principal planesat 3.55 GHz. Interestingly, at this frequency the antenna ex-hibits radiation patterns with a horizontal -directed linear elec-tric field polarization, consistent with a -directed current alongthe ground plane of the structure, as shown in Fig. 2. This indi-cates that around 3.55 GHz, the ground plane acts as the mainradiating element for the antenna, providing an orthogonal po-larization to the one observed at 5.5 GHz.
The radiation patterns in the - and the -planes, whichcorrespond to the two E-planes of the ground-plane radiatingmode at 3.55 GHz, indicate that the structure radiates in adipolar fashion at this frequency. In fact, at 3.55 GHz the widthof the ground plane, or equivalently the length of the radiatingedge, is approximately equal to . Since it is the groundplane itself that is radiating, there is no radiation blockageobserved as in the case of the E-plane patterns at 5.5 GHz.There is, however, a partial filling of the null around inthe -plane, that can be attributed to constructive interferencefrom the two -directed currents on the monopole and thebottom thin inductive strip. This additional current also mani-fests itself in the cross-polarization data of the -plane in the
direction. In the -plane, which corresponds to theH-plane of the radiating ground plane, the radiation pattern is asexpected omnidirectional. The simulated efficiency from HFSSat 3.55 GHz was 88.7% compared to measured efficiencies of87.7% using the gain comparison method and 91.6% using theWheeler cap method, verifying that at this frequency they arealso in good agreement.
As a final note, it should be mentioned that throughout eachof the 3.3–3.8 GHz WiMax and 5.15–5.85 GHz WiFi bands theradiation patterns exhibit similar characteristics to the ones pre-sented in Figs. 8 and 7, respectively.
IV. CONCLUSION
A compact and broadband antenna has been presented,which employs metamaterial loading on a conventional printedmonopole design in order to create a dual-mode antenna. Itwas demonstrated that the addition of the metamaterial loading
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ANTONIADES AND ELEFTHERIADES: BROADBAND DUAL-MODE MONOPOLE ANTENNA 261
Fig. 7. Measured and simulated radiation patterns for the MTM-loaded monopole antenna at 5.5 GHz. Solid blue line: measured copolarization, dashed blackline: simulated copolarization, solid red line: measured cross-polarization, and dash-dot black line: simulated cross-polarization.
Fig. 8. Measured and simulated radiation patterns for the MTM-loaded monopole antenna at 3.55 GHz. Solid blue line: measured copolarization, dashed blackline: simulated copolarization, solid red line: measured cross-polarization, and dash-dot black line: simulated cross-polarization.
allows the antenna to be modeled as a short folded monopoleat 5.5 GHz, while at 3.55 GHz the loading enables the entiretop edge of the ground plane to radiate. Thus, the metama-terial-loaded antenna achieves orthogonal pattern diversityin both the 3.3–3.8 GHz WiMax and 5.15–5.85 GHz WiFibands. Additionally, the antenna exhibits a measured dBreturn-loss bandwidth of 4.06 GHz, while maintaining a veryhigh efficiency in the order of 90% in both the bands of interest.It is therefore well suited for MIMO diversity systems foremerging wireless LAN applications.
ACKNOWLEDGMENT
The authors would like thank D. Choudhury from Intel Cor-poration for many useful discussions.
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