8/12/2019 05067699

1/5

Overview of Resonant Metamaterial Antennas

C. Caloz and A. Rennings#

Poly-Grames Research Center, Ecole Polytechnique de Montr eal, Montr eal, H3T 1J4, Quebec, Canada#General and Theoretical Electrical Engineering (ATE), University of Duisburg-Essen, 47048 Duisburg, Germany

Abstract Composite right/left-handed (CRLH) resonant an-tennas (RAs) have been less studied than their leaky-waveantennas (LWAs) counterpart. However, they exhibit equallyinteresting features, which are in fact complementary to thoseof LWAs. This paper presents an overview of recent works onCRLH RAs, in comparison with CRLH LWAs.

I. INTRODUCTION

Metamaterials represent a new paradigm in electromagnetic

science and technology [1], [2]. They have already leadto many unprecedented microwave applications, which may

be classified in three categories: guided-wave, radiated-wave

and refracted-wave applications [1]. Radiated-wave applica-

tions cover several types of novel antennas and reflectors,

which may be 1D or 2D, passive or active, and static or

dynamically tuned [3]. Metamaterial antennas in 1D and 2D

configurations may be of two types. The first type consists of

conventional radiators (e.g. patches or dipoles) placed above

artificial dielectrics [4], where the main benefits of the artifi-

cial dielectric metamaterials are compactness [5] and slightly

enhanced bandwidth in electromagnetically small antennas

[6]. The second type consists in artificial transmission line

(TL) metamaterial structures, which exhibit more diverse andoriginal properties, such as for instance equivalent negative

index of refraction or infinite1 wavelength propagation [1].

In this paper, we consider only the latter type. These TL

metamaterials generally exhibit a composite right/left-handed

(CRLH) dispersive response [1], or related responses when

the line integrates additional lumped elements in its unit cell

(higher order TL) [7]. Although most CRLH antennas reported

to date have been leaky-wave antennas (LWAs), much work

has been recently done on CRLH resonant antennas (RAs) [3],

since the introduction of the CRLH resonator in 2003 [8]. This

paper presents an overview on CRLH RAs, including a brief

theoretical recall, a comparison with LWAs, a description of

the different properties and types of RAs, and an enumeration

of their potential applications.

1Rigorously, the electromagnetic field along the structure is given by asuperposition of Bloch-Floquet space harmonics. However, in the metama-terial regime (p g), the contribution of the |m| > 0 space harmonicsis negligible, and only the fundamental space harmonic plays a significantrole: m() =0() + 2m/p 0(). The CRLH infinite wavelengthrigorously refers to the fundamental space harmonic (m= 0) of the structure.This is true at the macroscopic (metamaterial) level, while very small phasevariations naturally exist across the unit cell at the microscopic level.

I I . THEORETICAL R ECALL

A CRLH TL metamaterial is an artificial TL structure

composed of the periodic repetition of a CRLH unit cell, as

shown in Fig. 1. Such a TL exhibits in the lossless case the

dispersion/attenuation relation [1]

cos[(j)p] = 1 + ZY2

= 1 (2 2se)(2 2sh)

222R,

(1)

where se = 1/LRCL, sh = 1/

LLCR, and R =

1/LRCR. Under the so-called balanced condition (se =

sh 0), the dispersion relation () can be explicitlyderived from inversion of the general relation (1) as

() =

20+

2Rsin

2

p

2

+ Rsin

p

2

. (2)

This dispersion curve is plotted in Fig. 2. In the metamaterial

frequency range (p g), the artificial CRLH structurebehaves as a uniform TL [() /R L/, whereL= 1/

LLCL], and may therefore be transformed, like any

TL, into a TL resonator by using discontinuous (short/open)

terminations. A difference with a uniform TL however is

that, due to the CRLH pass-band characteristic, only 2N

1

resonances, whereNis the number of unit cells, can fit in thetransmission band. These resonances naturally correspond to

TL lengths = ng/2, and therefore to propagation constantsn = n/ = n/(Np), with the CRLH particularitythat n can be both positive (RH range) and negative (LHrange), and even zero (transition frequency)2. The Bloch

(periodic structure) impedance is given in general by ZB =Z/Y

1 + ZY/4 (see immittances in Fig. 1), and reduces

in the balanced case to [1], [3]

ZB() = ZL

1 +1

4

R L

2+

, (3)

where ZL =LL/CL, and where = (R,G) is a dissipa-

tive term, which is much smaller than the reactive terms. In the

metamaterial range, this impedance may be assimilated to a

TL characteristic impedance, which is frequency-independent

(ZB Zc= ZL= ZR, where ZR=LR/CR).

2 These resonances, labelled n here, are really the resonances of the overallmetamaterial structure, of length . They should not be confused with theeigenfrequencies of the periodic structure, labelledm in the previous footnote,which are out of the metamaterial frequency range < BZE =/p shownin Fig. 2.

615

8/12/2019 05067699

2/5

radiation

1 2 N

ppp

Zin

Zc, Zt Zt

Fig. 1. CRLH TL resonant antenna (RA), formed by repeating N times aunit cell of size p, where the terminations are either short-ended (Zt 0)or open-ended (Zt ), including unit cell equivalent circuit and typicalinterdigital/stub implementation.

()

0p

+

p+

+2

+3

2

3

0

+1

+2

+3

12

3

LH (highpass) GAP

RH (lowpass) GAP

BRILLOUIN ZONE

cL

cR

Fig. 2. Resonances of the (balanced) CRLH TL resonator shown in Fig. 1.The length and the period p of the resonator are related by = N p, whichresults in Brillouin zone edges ofBZE = /p = N/.

In summary, a CRLH resonator supports 2N 1 resonancefrequencies,N1in the LH band, N1in the RH band, and1at the transition frequency 0 (or seor sh, depending on theterminations, if the CRLH structure is unbalanced, se=sh).These frequencies are obtained by sampling Eq. (1) at the pointsn= n/Np (n= 1, 2, . . .2N 1)

n=

20+

2Rsin

2n

2

+ Rsin

n2

. (4)

The Bloch impedance ZB given by Eq. (3) is the input

impedance when the structure is infinitely periodic or termi-nated by a resistor of value ZB. Since the terminations Zt ofthe CRLH resonator (Fig. 1) are short or open circuits, the

impedance at the input of the structure is naturally different

from ZB. However, as pointed out in the previous paragraph,ZB constitutes the equivalent TL characteristic impedance Zcin the metamaterial regime (p/g 0), and is thereforeessential for coupling energy into (i.e. matching) the resonant

structure. The resistive elements R and G in Fig. 1 represent

in general the radiation resistance in the antenna, in addition

to the conductor and dielectric losses, respectively.

III. COMPARISON WITHLWAS

CRLH LWAs have been extensively studied and have led

to many applications. Among the most recent applications,

one may cite an analog real-time spectrum analyzer [9] and a

super-compact and low-loss direction of arrival (DOA) system

[10].CRLH RAs have some similarities with CRLH LWAs. Both

share the same guided wavelength g = 2/ and dispersionrelation (), which is discrete for the RA (dots in Fig. 2)and continuous for the LWA (curve in Fig. 2). A more subtle

similarity is that when, at a fixed frequency (typically 0), itssize is progressively increased beyond several wavelengths, a

CRLH RA behaves more and more like a LWA, because the

power injected into the antenna has completely radiated out

before it may see the discontinuity at the termination [11].

However, when the electrical size of the CRLH structure

does not exceed a few wavelengths, the RA exhibits a behavior

which is very different and complementary to that of the

LWA. Due to the termination discontinuities (Fig. 1), the RA

supports a standing wave as opposed to a travelling wave,

and therefore radiates exclusively at discrete narrow resonance

frequencies (dots in Fig. 2), which do not need to be in the

fast-wave region of the dispersion diagram. Due to resonance,

the RA, as a single radiating element, cannot perform beam

scanning (i.e. (), characteristic of the LWA), and supportsonly broadside radiation. However, when only broadside is

targeted, the RA suffers of less beam squinting (/), andis therefore preferable in precision applications, such as point-

to-point communication and synthetic aperture radar imaging.

Finally, an RA is always preferable to a LWA for electrically

small antennas. In such short antennas, an LWA dissipatesmuch power in the load, which ruins the radiation efficiency;

in contrast, an RA may be optimized to preserve optimal

efficiency-bandwidth performances, while preserving its size-

frequency independence, within Chu-Harrington physical lim-

itations [12].

IV. PROPERTIES ANDT YPES OFR AS

A. Implementations

As all CRLH metamaterial structures, CRLH RAs may

be implemented in different technologies [planar hybrid or

MMIC, LTCC, hollow waveguide], in different waveguide or

TL configurations [microstrip, coplanar waveguide (CPW),coplanar strip-line (CPS), coaxial, waveguide, etc.], and using

different LC elements [printed or chip form; interdigital (ID)

or metal-insulator-metal (MIM) capacitors; straight, spiral,

meander inductors]. Some typical implementations are shown

in Fig. 3. The properties of the resulting antennas including

polarization, radiation patterns, efficiency, directivity depend

not only on the selection of the CRLH resonant mode, but

also on the choice of the technology, configuration, and LC

elements.

616

8/12/2019 05067699

3/5

CPW ID CPW MIM Meander LCoaxial

microstrip microstrip MIM

CPS MIMR-WG

MMIC

Fig. 3. Typical 1D CRLH TL implementations.

B. Half-Wavelength Resonances

A CRLH resonator (Fig. 1) may be excited in any of its

2N 1 resonancesn, given by Eq. (4) and shown in Fig. 2.Compared to the|n| > 1 resonances, the n =1, (i.e. half-wavelength) resonances are particularly useful because they

provide broadside radiation and exhibit high efficiency. Fig. 4

presents an interdigital half-wavelength open-ended CRLH RAwith longitudinal polarization. The current distribution on this

antenna is similar to that on a half-wavelength patch antenna,

but the physical size of the antenna is essentially related to the

LC loads of the structure and not on the operation frequency,

which provides new degrees of flexibilities (see next sections).

The prototype shown is excited by a coaxial probe connected

on the ground plane and shifted from the center of the structure

to exhibit50 matching. The antenna may be operated eitherat the n =1 resonance (f1) or at the n = +1 resonance(f+1), the latter providing naturally higher directivity fromits larger effective aperture. Due to their relatively similar

g/2 field distributions, the f1 and f+1 resonances may be

simultaneously matched, leading to dual-band operation. Dueto the open-ended configuration, only the fsh (n = 0) shunt(and not the series) mode is excited between thef1 andf+1resonances. However, in the present prototype, this mode was

not optimized in terms of matching and does not radiate, due

to the mutually cancelling current contributions in the anti-

parallel stubs, designed to minimize cross-polarization.

C. Zeroth Order Resonance

The zeroth order resonance, which corresponds to the

mode(s)n= 0, is a quite unique property of CRLH structures.In contrast to CRLH LWAs which require fulfilment of the

balance condition, se = sh

0, to radiate at broadside,

CRLH RAs operate in the standing wave regime and worktherefore independently on whether the CRLH structure is

balanced or not. If the structure is unbalanced, the two distinct

zeroth order frequencies, se and sh, are possible, but onlyone of them will be excited depending on the terminations.

Fig. 5 presents a zeroth order (n = 0, g = 2/ =)series-mode (fse) CRLH RA with longitudinal polarization. Inorder to operate in the zeroth order series mode, the antenna is

shorted at its end by via holes. This antenna is excited at one

of its end through a microstrip section transforming the 50

0

2.25 2.5 2.75 3 3.25 3.530

25

20

15

10

5

Measurement

Simulation

xx

x

y

y

zfeed

frequency (GHz)

S11

(dB)

f1

f1

f+1

f+1

(f0)

Fig. 4. Half-wavelength (n= 1) open-ended CRLH RA with longitudinal

polarization (electrical field along the x-direction).

of the source to a very low impedance value ( 3 ) at the feedpoint of the antenna. The spectrum of the structure includes all

the resonances shown in Fig. 2, except fshwhich requires openterminations. Replacing the short ends in the zeroth order RA

by open ends resonates the structure in its shunt mode (fsh).Fig. 6 shows such an antenna. In the shunt mode, the energy

resonates in the stubs and therefore a different configuration

than that of Fig. 4, with single stubs, is required for radiation.

The antenna subsequently exhibits transverse polarization.

D. Comparison of Half-Wavelength and Zeroth Order Reso-

nances

The CRLH half-wavelength RAs (Sec. IV-B) and zeroth

order RAs (Sec. IV-C) have distinct features. In fact, as

previously anticipated [1], it turns out that the zeroth order

RA exhibits a higher radiation efficiency, due to the higher

uniformity of the fields along the structure [13]. This fact is

verified in Fig. 7, where an efficiency comparable to that of a

patch antenna is achieved, however with the clear advantage

that the physical size of the antenna can be tuned at a given

frequency (see Secs. IV-F and IV-G), with subsequent control

on directivity. A particularly interesting property of the zeroth

order RA is that the length of the antenna (or its number

of cells N) may be increased for higher directivity withoutany change in frequency, due to the uniformity of the zeroth

order field. In contrast, the half-wavelength antenna requires

small tunings (smaller and smaller as increases due to theincreasingly packed resonances within the Brillouin zone)

[14], as may be understood from Fig. 2.

E. Multi-band Operation

The inherent dispersion of CRLH structures allows multi-

band operation. A standard CRLH TL is inherently dual-band,

617

8/12/2019 05067699

4/5

2 2.2 2.4 2.6 2.8 340

35

30

25

20

15

10

5

0

Measurement

Simulation

frequency (GHz)

S11

(dB)

(f1) (f+1)fse

excitationshort terminations using via holes

Fig. 5. Zeroth order (n= 0) series-mode (fse, short-ended) CRLH RA withlongitudinal polarization.

due to its four fundamental parameters (LR, CR, LL, CL)compared to just two parameters (LR, CR) in a conventionaltransmission line [1]. In fact, dual-band operation was im-

plicitly demonstrated in Sec. IV-B. Using higher-order of

structuring, i.e. a unit cell incorporating more LC elements,

CRLH structures may also accommodate tri-band or quad-band regimes, both in components and antennas, being un-

derstood that higher order structures are more challenging to

design due to the larger number of parasitics to control and

higher constraints [7]. However, a standard CRLH structure

may operate as a tri-band antenna when the balance condition

is relaxed, which is possible in RAs (but not in LWAs, where

balanced resonances are conditional to broadside radiation).

Such an antenna, in metal-insulator-metal (MIM) technology,

was presented in [15].

open ends excitation

Fig. 6. Zeroth order (n= 0) shunt-mode (fsh, open-ended) CRLH RA withtransverse polarization.

Fig. 7. Comparative performances of typical interdigital half-wavelength(n= 1) and zeroth order (n= 0) RAs, also compared to a patch antenna onthe same substrate.

F. Super-High Directivity

Due to their frequency-independent physical size, CRLH

RAs, as pointed out in Sec. IV-D may have a physical

size much larger than the free space wavelength and may

subsequently exhibit much higher directivity compared to a

convention resonant element. In fact, a CRLH RA, like a

CRLH LWA, when electrically large, behaves like a conven-

tional array antenna in terms of directivity, with the distinct

advantage that it does not require a typical complex corporate

feeding network. The result of Fig. 7 do not show this property,

since it uses CRLH antennas of the same size as the patch

antenna. However, directivites increased by 5 dB may beachieved.

G. Electromagnetically Small Antennas

Electromagnetically small antennas have been the holy grail

for many academic and industrial antenna developers. How-

ever, the laws of physics cannot be broken: electromagnetically

small antennas always exhibit small radiation efficiency and

narrow bandwidth [12]. Metamaterials do not change this fact.

However, due to their frequency-independent physical size and

unique uniform profile, CRLH zeroth order resonant antennas

may provide modest benefits compared to conventional solu-

tions in terms of radiation efficiency. A sure fact is that RAs

are systematically superior to LWAs for small antennas, since

small LWAs waste a lot of power in the matched load at its

end, unless a special power recycling mechanism is found to

avoid this problem.

H. Electric and Magnetic Loop Monopoles

CRLH loops provide particularly interesting monopole an-

tennas, both of electric and magnetic nature [16]. The CRLH

magnetic and electric monopoles are based on the series and

618

8/12/2019 05067699

5/5

shunt CRLH zeroth order resonances, respectively, as shown

in Fig. 8. In contrast to the non-looped structures described in

the previous sections, the CRLH loop structures are infinitely

periodic and do not have any termination. What determines

the resonances that are excited (se or sh) is then theexcitations, as illustrated in Fig. 8, where two different exci-

tation slots excite separately the series and shunt resonances.

Consequently, the series and shunt may exists simultaneously

in a given design, yielding a dual-monopole antenna, and even

at the same frequency when the CRLH structure is balanced.

Moreover, when the series/shunt frequencies are tuned so as

to radiate quadrature fields, the dual-monopole antenna may

provide circular polarization in the far-field.

excitation slot

excitation slot

Fig. 8. Magnetic and electric monopole RAs using the series and shunt CRLHzeroth order resonances, respectively. Principle and current distributions.

V. SPECIFICA PPLICATIONS

CRLH RAs may find several applications, complementary

to those of CRLH LWAs. CRLH LWAs are particularly ground

breaking for scanning applications, where beam steering is

achieved without requiring a lossy and cumbersome corporate

feeding network with expensive phase shifters. Thanks to their

multi-band capability with minimal beam-squint, they may

be applied to several multi-band wireless systems, such asWLANs and WPANs. Thanks to their inherent high directivity,

they may benefit to the spectral efficiency of point-to-point

communication systems, as for instance WiMAX (internet or

voice over IP), at a low cost. Thanks to their frequency-

independent size, they may be designed electromagnetically

large for dramatically enhanced directivity at millimeter-wave

frequencies, or electromagnetically small for modestly en-

hanced radiation efficiency at radio frequencies. Finally, thanks

to their unique zeroth order uniform field property, CRLH RAs

may be used in a diversity of exotic applications, such as for

instance MRI [17].

VI . CONCLUSIONS

An overview on CRLH RAs has been presented. CRLH

RAs have been less reported than their CRLH LWAs coun-

terpart, but are equally interesting and exhibit complementary

properties. Different types of CRLH RAs (half-wavelength,

zeroth-order, multi-band, highly directive, electromagnetically

small, and electric/magnetic loop monopole RAs) have been

presented, and specific applications have been suggested.

REFERENCES

[1] C. Caloz and T. Itoh,Electromagnetic Metamaterials, Transmission LineTheory and Microwave Applications. Wiley and IEEE Press, 2005.

[2] N. Engheta and R. W. Ziolkowski (eds.), Electromagnetic Metamaterials:Physics and Engineering Explorations. Wiley and IEEE Press, 2006.

[3] C. Caloz, T. Itoh, and A. Rennings, CRLH traveling-wave and resonantmetamaterial antennas, Antennas Propagat. Magazine, vol. 50, no. 5,pp. 2539, Oct. 2008.

[4] R. E. Collin, Field Theory of Guided Waves, second Ed., Wiley-Interscience, 1991, chap. 12.

[5] M. Coulombe, H. V. Nguyen, and C. Caloz, Substrate integrated artificial

dielectric (SIAD) structure for miniaturized microstrip circuits, Antennasand Wireless Propagat. Lett., vol. 6, no. 6, pp. 575579, 2007.

[6] P. M. T. Ikonen, K. N. Rozanov, A. V. Osipov, P. Alitalo, andS. A. Tretyakov, Magnetodielectric substrates in antenna miniaturization:Potential and limitations, Trans. Antennas and Propagat., vol. 54, no. 6,pp. 33913396, Nov. 2006.

[7] A. Rennings, S. Otto, J. Mosig, C. Caloz, and I. Wolff, Extendedcomposite right/left-handed (E-CRLH) metamaterial and its applicationas a quadband quarter wavelength transmission line, in Proc. Asia PacificMicrowave Conf. (APMC), Yokohama, Japan, Dec. 2006.

[8] A. Sanada, C. Caloz, and T. Itoh, A novel zeroth order resonancein composite right/left-handed transmission line resonators, in Proc.Asia Pacific Microwave Conf. (APMC), Seoul, Korea, pp. 15881592,Nov. 2003.

[9] S. Gupta and C. Caloz, Leaky-wave based spectrum analyzer with unre-stricted time-frequency resolution, in Proc. IEEE MTT-S Int. MicrowaveSymp. Dig., Atlanta, GA, pp. 807810, June 2008.

[10] A. Abielmona, H. V. Nguyen, S. Farzaneh, and C. Caloz, Super-compact and low-loss DOA system based on a CRLH leaky-wave antennausing beam-space MUSIC algorithm, IEEE Trans. Antennas Propagat.,submitted.

[11] T. Liebig, A Rennings, S. Otto, C. Caloz, and D. Erni, Comparisonbetween CRLH zeroth-order antenna (ZORA) and series-fed patch array,in Proc. European Conference on Antennas and Propagat. (EuCAP),Berlin, Germany, March. 2008 (this conference).

[12] R. C. Hansen, Electrically small, superdirective, and superconductingantennas. Wiley, 2006.

[13] A. Rennings, T. Liebig, S. Otto, C. Caloz, and I. Wolff, Highly directiveresonator antennas based on composite right/left-handed (CRLH) trans-mission lines, in Proc. 2nd International ITG Conference on Antennas(INICA), Munich, Germany, March 2007, pp. 190194.

[14] A. Rennings, S. Otto, C. Caloz, and P. Waldow,, Enlarged half-wavelength resonator antenna with enhanced gain, in Proc. EEE AP-SInternational Symposium USNC/URSI National Radio Science Meeting,

Washington, USA, June 2005.[15] A. Rennings, T. Liebig, S. Abielmona, C. Caloz, and P. Waldow,,

Tri-band and dual-polarized antenna based on (unbalanced) CRLHtransmission line, in Proc. 37th European Microwave Conf. (EuMC),Munich, Germany, Oct. 2007, 720-723.

[16] S. Otto, A. Rennings, C. Caloz, and P. Waldow, Dual-mode zeroth orderring resonator with tuning capability and selective mode excitation, inProc. 35th European Microwave Conf. (EuMC), Paris, France, Oct. 2005,149152.

[17] A. Rennings, J. Mosig, A. Bahr, C. Caloz, M. E. Ladd, and D. Erni,A CRLH metamaterial based RF coil element for magnetic resonanceimaging at 7 tesla, in Proc. European Conference on Antennas andPropagat. (EuCAP), Berlin, Germany, March. 2008 (this conference).

619