Negative-Refractive-Index Transmission-Line

Metamaterial-Loaded Dipole Antennas

Marco A. Antoniades∗ George V. Eleftheriades†

Abstract — Several compact, fully-printed negative-refractive-index transmission-line (NRI-TL)metamaterial-loaded dipole antennas are pro-posed. The antennas achieve multi-band behaviourby loading a host dipole antenna with series capaci-tive gaps and shunt inductive strips. In an approachproposed by Schelkunoff, the two arms of thedipole antenna are modeled as the two conductorsof a biconical transmission line, and thus the richdispersion properties of the NRI-TL structure areexploited, while still creating an effective radiator.Both dual-band and tri-band versions of the anten-nas are shown, with individual bandwidths rangingfrom 40 MHz to 1.4 GHz. It is also demonstratedthat the antennas can achieve a size miniaturizationfactor of approximately two, and that they exhibituniform dipolar radiation patterns. Throughouttheir bands of operation the gain and radiationefficiency vary from approximately −3 dBi to2.3 dBi and 42% to 98%, respectively.

1 INTRODUCTION

The transmission-line (TL) approach to synthesiz-ing negative-refractive-index (NRI) metamaterials[1] has proven to be very useful for microwave ap-plications, and can be used to create structuresthat exhibit forward, backward as well as zero-phase propagation characteristics [2]. These versa-tile characteristics can be exploited to create multi-band antennas based on the NRI-TL metamaterialtopology. Of particular interest in this work aredifferentially-fed metamaterial-loaded dipole anten-nas, some examples of which can be found in [3–7].

In this paper, several compact, fully-printeddipole antennas are presented that are loaded ina NRI-TL metamaterial fashion to create two unit-cell NRI-TL metamaterial-loaded dipole antennas,as shown in Figure 1. The addition of the NRI-TL loading, and by modeling the two arms of thedipole antenna as the two conductors of a biconicaltransmission line, enables the creation of multipleoperating bands without increasing the footprint ofthe antenna. Furthermore, the antennas maintainhigh radiation efficiencies, have uniform linearly-polarized radiation patterns, are compact in size,

∗School of IT and Electrical Engineering, University ofQueensland, Brisbane, Queensland, 4072, Australia, e-mail:m.antoniades@uq.edu.au, tel.: +61-7-3365-4139.

†Dept. of Electrical and Computer Engineering, Univer-sity of Toronto, Toronto, Ontario, M5S 3G4, Canada, e-mail:gelefth@waves.utoronto.ca, tel.: +1-416-946- 3564.

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Figure 1: NRI-TL metamaterial-loaded dipole an-tenna.

have a low profile, and do not require any groundplane in order to operate.

2 ANTENNA EQUIVALENT CIRCUIT

The main objective of this work was to exploit theversatile phase-shifting properties of NRI-TL meta-material structures in order to create multi-banddifferential antennas with non-harmonically relatedoperating frequencies. To this end, the analysis ofthe propagation characteristics of NRI-TL meta-material media in [1, 2], outlines that they possessmultiple resonant frequencies, i.e. when the totalphase shift is equal to zero or integer multiples of𝜋, and that these frequencies are not harmonicallyrelated. One challenge in the realization of NRI-TL radiating structures, however, is the appropri-ate modeling of the right-handed transmission-linesections, such that these account for radiation whilestill providing the necessary phase shift. Herein,

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I(r)

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Figure 2: The evolution of Schelkunoff’s infinitebiconical antenna (a), into a cylindrical dipole an-tenna (b), and a planar dipole antenna (c).

an approach initially proposed by Schelkunoff wasused, where the two arms of a dipole antenna weremodeled as the two conductors of a biconical trans-mission line [8]. Subsequently, the dipole antennawas loaded in a NRI-TL metamaterial fashion us-ing series capacitive gaps and shunt inductive stripsin order to create the NRI-TL metamaterial-loadeddipole antenna shown in Figure 1.

For the purposes of this work, the biconical an-tenna of Figure 2 (a) was first approximated asthe cylindrical dipole antenna of Figure 2 (b), andthen as the planar dipole antenna of Figure 2 (c).This allowed the definition of an average charac-teristic impedance for the planar dipole antenna,given in [9]. Thus, referring to Figure 1, each pla-nar dipole section of the antenna was modeled asa transmission line with a characteristic impedance𝑍0, that was terminated in an appropriate terminalimpedance, 𝑍𝑡, which accounts for the finite lengthof the antenna and its cross-sectional shape [8, 9].Based on the geometry of Figure 1, a realistic equiv-alent circuit of the NRI-TL dipole antenna is shownin Figure 3.

3 NRI-TL DIPOLE ANTENNA #1

The first NRI-TL metamaterial-loaded dipole an-tenna investigated had a total length of 50 mm andwas designed on an FR4 substrate with parameters𝜖𝑟 = 4.34 and tan 𝛿 = 0.016. Referring to Fig-ure 1, the dimensions of the antenna (in mm) were:𝐿1 = 28, 𝐿2 = 50, 𝑊 = 10, 𝑊𝑓 = 0.79, 𝑔 = 0.1,𝐿𝑠1 = 28, 𝑊𝑠1 = 0.1, 𝐿𝑠2 = 50, 𝑊𝑠2 = 0.1, 𝑠 = 2.9,ℎ𝑠𝑢𝑏 = 0.79, via � = 0.1. For this antenna, the

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Figure 3: Realistic equivalent circuit of the NRI-TLdipole antenna of Figure 1.

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Figure 4: ADS simulated ∣𝑆11∣ responses of theequivalent circuit of NRI-TL dipole antenna #1 fordifferent values of the inductive strip length 𝐿𝑠2.

effect of changing the length of the shunt inductivestrip 𝐿𝑠2 was investigated, which determines thevalue of the loading inductance 𝐿02.Figure 4 shows the ∣𝑆11∣ responses obtained from

the Agilent-ADS circuit simulator for the equiv-alent circuit of the dipole antenna of Figure 3for different values of the inductive strip length𝐿𝑠2. The following circuit parameters extractedfrom Figure 1 were used: 𝐶01 = 𝐶02 = 0.2 pF,𝐿𝑠1 = 28 mm, 𝑊𝑠1 = 0.1 mm, 𝐿𝑠2 = 35→ 50 mm,𝑊𝑠2 = 0.1 mm, 𝑍01 = 312.6 Ω, 𝑍02 = 355.2 Ω,𝑙1 = 13.6 mm, 𝑙2 = 10.9 mm, and 𝛽𝑙1 = 49∘ and𝛽𝑙2 = 39.2∘ at 3 GHz. It can be observed that allthe antenna responses exhibit multi-band charac-teristics, with three distinct resonances in the fre-quency range shown. Figure 5 shows the full-wavesimulated ∣𝑆11∣ responses obtained from Ansoft-HFSS for the NRI-TL dipole antenna of Figure 1 forthe same values of the bottom inductive strip 𝐿𝑠2.It can be observed that the general performance ofthe antenna equivalent circuit matches well withthe performance obtained from the full-wave re-sults, albeit the location of the high-frequency res-onance is more sensitive to the variation of 𝐿𝑠2 inthe full-wave results.In order to provide a practical feed for the dipole

antennas, a broadband microstrip to broadside-

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Figure 5: HFSS simulated ∣𝑆11∣ responses of NRI-TL dipole antenna #1 for different values of thebottom inductive strip length, 𝐿𝑠2.

coupled co-planar strip transition was added to theNRI-TL dipole antenna, as shown in Figure 6 (a).Figure 6 (b) shows a 3-D radiation pattern at2.86 GHz for the antenna with 𝐿𝑠2 = 50 mm, whereit can be observed that the antenna has a dipo-lar radiation pattern with a vertical linear electricfield polarization. This is representative of the pat-terns throughout the operating bands of the an-tenna. In Figure 6 (c) a tri-band variation is shownwith −10 dB bandwidths of 49 MHz, 494 MHz and208 MHz, while in Figure 6 (d) a dual-band vari-ation of the antenna is shown, with bandwidths of39 MHz and 810 MHz. For the latter antenna with𝐿𝑠2 = 50 mm, the three resonant frequencies are1.11 GHz, 2.86 GHz and 3.34 GHz, and at each ofthese frequencies the corresponding gain and effi-ciency are −3.06 dBi (48%), 2.14 dBi (94%) and1.35 dBi (75%).

4 NRI-TL DIPOLE ANTENNA #2

The second NRI-TL metamaterial-loaded dipoleantenna investigated had a total length of 30 mmand was also designed on an FR4 substrate. Thedimensions of the antenna (in mm) were: 𝐿1 = 17,𝐿2 = 30, 𝑊 = 6, 𝑊𝑓 = 0.79, 𝑔 = 0.1, 𝐿𝑠1 = 17,𝑊𝑠1 = 0.1, 𝐿𝑠2 = 30,𝑊𝑠2 = 0.1, 𝑠 = 1, ℎ𝑠𝑢𝑏 = 0.79,via � = 0.1. For this antenna, the effect of changingthe relative sizes of the two unit cells was investi-gated by changing the length of the first unit cell𝐿1 while keeping the total length of the antenna at𝐿2 = 30 mm. This had the effect of changing thelengths of both of the transmission line sections 𝑙1and 𝑙2, as well as the length of the bottom inductivestrip 𝐿𝑠1, which determines the value of the loadinginductance 𝐿01.Figure 7 shows the HFSS simulated ∣𝑆11∣ re-

sponses for the NRI-TL dipole antenna of Figure 1for different values of the length 𝐿1. It can be ob-

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Figure 6: (a) NRI-TL dipole antenna #1, fed usinga broadband transition. (b) 3-D radiation patternat 2.86 GHz with 𝐿𝑠2 = 50 mm. (c) Simulated∣𝑆11∣ response with 𝐿𝑠2 = 40 mm, compared to theresponse of a reference unloaded dipole antenna.(d) Simulated ∣𝑆11∣ response with 𝐿𝑠2 = 50 mm.

served that the this antenna also exhibits multi-band characteristics, however in this case both thehigh-frequency resonances shift as 𝐿1 increases andthe relative sizes of the two unit cells change.The NRI-TL dipole antenna with the broadband

transition is shown in Figure 8 (a). Figure 8 (b)shows a 3-D radiation pattern at 4.78 GHz for theantenna with 𝐿1 = 17 mm, where it can be ob-served that this antenna also has a dipolar radi-ation pattern, which is representative of the pat-terns throughout the operating bands. In Fig-ure 8 (c) a tri-band variation is shown with −10 dBbandwidths of 56 MHz, 880 MHz and 307 MHz,

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L1= 10mm

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Figure 7: HFSS simulated ∣𝑆11∣ responses of NRI-TL dipole antenna #2 for different values of thefirst unit cell size 𝐿1.

while in Figure 8 (d) a dual-band variation of theantenna is shown, with −10 dB bandwidths of49 MHz and 1380 MHz. For the latter antennawith 𝐿1 = 17 mm, the three resonant frequenciesare 1.86 GHz, 4.78 GHz and 5.60 GHz, and at eachof these frequencies the corresponding gain and ef-ficiency are −2.72 dBi (42%), 2.33 dBi (98%) and1.57 dBi (68%).

References

[1] G.V. Eleftheriades, A.K. Iyer, and P.C. Kremer,“Planar negative refractive index media us-ing periodically L-C loaded transmission lines,”IEEE Trans. Microwave Theory Tech., vol. 50,no. 12, pp. 2702–2712, Dec. 2002.

[2] M.A. Antoniades and G.V. Eleftheriades,“Compact linear lead/lag metamaterial phaseshifters for broadband applications,” IEEE An-tennas Wireless Propagat. Lett., vol. 2, no. 1,pp. 103–106, 2003.

[3] R. Ziolkowski and A. Erentok, “Metamaterial-based efficient electrically small antennas,”IEEE Trans. Antennas Propagat., vol. 54, no. 7,pp. 2113–2130, July 2006.

[4] F.J. Herraiz-Martinez et al., “Dual-frequencyprinted dipole loaded with metamaterial parti-cles,” in Proc. IEEE Int. Symp. Antennas andPropag., San Diego, CA, July 2008, pp. 1–4.

[5] H. Iizuka and P.S. Hall, “Left-handed dipoleantennas and their implementations,” IEEETrans. Antennas Propagat., vol. 55, no. 55, pp.1246–1253, May 2007.

[6] M.A. Antoniades and G.V. Eleftheriades,“A NRI-TL metamaterial-loaded bow-tie an-tenna,” in Proc. Fifth European Conf. Antennasand Propag., Rome, Italy, Apr. 2011, pp. 1–4.

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Figure 8: (a) NRI-TL dipole antenna #2, fed usinga broadband transition. (b) 3-D radiation patternat 4.78 GHz with 𝐿1 = 17 mm. (c) Simulated ∣𝑆11∣response with 𝐿1 = 20 mm, compared to the re-sponse of a reference unloaded dipole antenna. (d)Simulated ∣𝑆11∣ response with 𝐿1 = 17 mm.

[7] M.A. Antoniades and G.V. Eleftheriades, “Amulti-band NRI-TL metamaterial-loaded bow-tie antenna,” in Proc. IEEE AP-S Int. Symp.Antennas and Propag., Spokane, WA, July2011, pp. 1–4.

[8] S.A. Schelkunoff, Electromagnetic Waves. NewYork: Van Nostrand, 1943, ch. 11.

[9] M.A. Antoniades and G.V. Eleftheriades,“Multi-band compact printed dipole antennasusing NRI-TL metamaterial loading,” IEEETrans. Antennas Propagat., accepted for pub-lication, 22 Apr. 2012.

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