Multi-walled carbon nanotube-based RF antennas

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Nanotechnology 21 (2010) 045301 (10pp) doi:10.1088/0957-4484/21/4/045301

Multi-walled carbon nanotube-based RFantennasTaha A Elwi1, Hussain M Al-Rizzo1, Daniel G Rucker1,Enkeleda Dervishi2,3, Zhongrui Li2,3 and Alexandru S Biris2,3

1 Department of Systems Engineering, University of Arkansas at Little Rock,2801 South University Ave, Little Rock, AR 72204, USA2 Nanotechnology Center, University of Arkansas at Little Rock, Little Rock, AR 72204, USA3 Department of Applied Science, University of Arkansas at Little Rock,2801 South University Avenue, Little Rock, AR 72204, USA

E-mail: asbiris@ualr.edu and hmalrizzo@ualr.edu

Received 2 October 2009Published 10 December 2009Online at stacks.iop.org/Nano/21/045301

AbstractA novel application that utilizes conductive patches composed of purified multi-walled carbonnanotubes (MWCNTs) embedded in a sodium cholate composite thin film to create microstripantennas operating in the microwave frequency regime is proposed. The MWCNTs aresuspended in an adhesive solvent to form a conductive ink that is printed on flexible polymersubstrates. The DC conductivity of the printed patches was measured by the four probetechnique and the complex relative permittivity was measured by an Agilent E5071B probe.The commercial software package, CST Microwave Studio (MWS), was used to simulate theproposed antennas based on the measured constitutive parameters. An excellent agreement ofless than 0.2% difference in resonant frequency is shown. Simulated and measured results werealso compared against identical microstrip antennas that utilize copper conducting patches. Theproposed MWCNT-based antennas demonstrate a 5.6% to 2.2% increase in bandwidth, withrespect to their corresponding copper-based prototypes, without significant degradation in gainand/or far-field radiation patterns.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

The constitutive parameters of multi-walled carbon nanotubes(MWCNTs) in the microwave frequency range are notcurrently well understood [1]. Some studies have beenreported on measurements of the permittivity and conductivityof MWCNTs for frequencies ranging from 1 to 40 GHz [2, 3].The measured conductivities of different MWCNTs suspendedin different solvents and polymers were found to be in therange from 65 to 1 × 104 S m−1 [2–5]. On the other hand, themeasured values for the relative permittivity of different typesof MWCNTs vary from 9.3 up to 100 [6–8].

Several studies have recently theorized the possibility ofusing single MWCNTs as dipole antennas in the infrared andoptical regimes [1, 9], and as RF circuits, transmission lines,and single-electron microwave devices [10–14] based on dif-ferent numerical analyses and modeling approaches [15–19].To the best of the authors’ knowledge, antennas fabricated from

single and/or networks of MWCNTs that resonate in the mi-crowave frequency range have not yet been reported. The chal-lenge primarily exists because growing a single MWCNT, sev-eral centimeters long, with low resistance is still proving to bea challenge.

MWCNTs cannot easily be isolated from the bulk phase,and they are increasingly unstable at longer lengths. MWCNT-based antennas for low-power communication devices suchas wireless sensors operating in harsh chemical or gasenvironments are highly necessary where antennas made fromtraditional conductors may become corroded or oxidized inthese environments. CNT stability arises from the graphiticshells of carbon atoms that make up the CNTs’ strong andclosed structure in the shape of a seamless cylinder [9].CNT structures have demonstrated their ability to interactwith electromagnetic waves, which makes them attractive fornano-electronic applications [15]. CNT structures conductmicrowave energy through simultaneous motion of electrical

0957-4484/10/045301+10$30.00 © 2010 IOP Publishing Ltd Printed in the UK1

Nanotechnology 21 (2010) 045301 T A Elwi et al

Figure 1. (a) Low resolution TEM image of the MWCNTs obtained over the Fe–Co/CaCO3 catalyst, (b) high resolution TEM image of theMWCNTs, (c) the weight loss profile and the oxidation rate of the MWCNTs obtained from the Fe–Co/CaCO3 catalyst, (d) Raman scatteringspectra of the MWCNTs produced on the Fe–Co/CaCO3 catalyst with acetylene as the carbon source.

charges along their structure from atom to atom along theconjugate— π bonds [19]. This electron motion generatesmobile charges when exposed to an incident electromagneticwave [16].

Current technologies allow the growth of a singleMWCNT with a maximum length in the range of a fewmicrometers [20]. To design an antenna in the microwavefrequency range requires a conductor length of the orderof a few centimeters, which makes it very difficult tofabricate an effective microwave antenna from a singleMWCNT. However, this paper demonstrates that by using inkconstructed from MWCNTs, it is possible to build antennasbearing the desirable properties of MWCNTs without facingthe instabilities of single MWCNTs. There are severalpotential applications for this MWCNT-based ink such asinterconnecting microelectronic chips, RFIDs, transmissionlines, and patch antennas. The antennas proposed in this paperresonate at the desirable microwave frequencies, are easy todesign, fabricate, and analyze using the traditional Maxwell’sequations, and have the potential for new applications inwireless communications and sensor networks.

In this work, the concept of manufacturing antennasfrom MWCNTs is demonstrated for six microstrip antennaprototypes formed from plural networks of MWCNTsdeposited on three different substrates. The manufacturingprocess was conducted under atmospheric conditions. Five keyissues are addressed in this paper: finding the best formulation

of the MWCNT ink and method for fabrication, investigatingthe reliability of classical electromagnetic theory to explain thebehavior of the MWCNT antennas, testing the performanceof the fabricated antennas, comparing the performance of thefabricated MWCNT antennas to the same geometries withcopper as a conductive material, and finally comparing thesimulated results against measured values.

2. Production and characterization of the MWCNTink

The MWCNTs used in this study were produced on a Fe–Co/CaCO3 catalyst with a Fe:Co:CaCO3 weight ratio of2.5:2.5:95 using acetylene as the carbon source at 720 ◦C. Theyield was found to be around 80%. The low resolution andhigh resolution TEM images of the MWCNTs are shown infigures 1(a) and (b), respectively. Thermogravimetric analysis(TGA) was performed to characterize the purity of the purifiedMWCNTs in an airflow rate of 150 ml min−1. The firstderivative of the TGA curve determines the decompositiontemperature of the sample. Figure 1(c) shows the weightloss profile of the purified nanotubes, which were heated from25 to 850 ◦C at a rate of 5 ◦C min−1. The normalized TGAcurve and its first derivative indicate a significant mass drop ataround 551 ◦C, which corresponds to the weight loss due to thecombustion of the MWCNTs.

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Nanotechnology 21 (2010) 045301 T A Elwi et al

Figure 2. (a) Different printed geometries with MWCNT ink on a filter paper; (b) printed antenna patches on a solid substrate (RO3210);(c) different printed geometries with MWCNT ink on a transparency sheet; (d) printed dipole antenna on a flexible transparent substrate.

The quantitative analysis revealed that after the single-step purification in HCl, the purity of the MWCNT productwas higher than 98%. The Raman scattering spectrum of theMWCNTs grown on Fe–Co/CaCO3 is shown in figure 1(d).The characteristic bands for the MWCNTs are the D band, Gband and the 2D band. The D band is present between 1305and 1330 cm−1 and is related to the presence of defects andimpurities in the MWCNT. The G band, present between 1500and 1605 cm−1, is also known as the tangential band and arisesfrom the E2g mode of the graphite plane. The G band positionis relatively constant for MWCNT material excited at differentenergies [21–23].

The last important mode observed in the Raman spectrumof MWCNTs is the 2D band, or the second-order harmonicof the D band, which is often present between 2450 and2650 cm−1. The 2D band is also highly dispersive andassociated with the degree of MWCNT crystallinity. Therelative intensities between the G and the D band (IG/ID), andbetween the 2D and G band (I2D/IG), are found to be 0.81and 0.63, respectively. These values indicate an inter-planardistance of 0.342 nm between the graphite layers, as shownin [23].

The use of MWCNT ink to fabricate antennas couldrepresent a new way to develop multifunctional antennas giventhe good electrical conductivity of the carbon-nanostructuredmaterials. MWCNTs consist of seamlessly rolled-up graphenesheets of carbon with π -conjugative and highly hydrophobicsidewalls that can interact with, for example, surfactants andsome kinds of aromatic compounds through hydrophobic orπ–π electronic interaction(s) [13]. The MWCNTs can bemade into uniform solutions after proper surface treatment.The purified MWCNT was first dispersed in sodium cholate(NaCh) aqueous solution (MWCNT: sodium cholate) 1:1 wt,5 mg l−1 [24]. The solution was deposited by ink jet deliveryover the flexible polymer substrates, which allowed for a fastevaporation of the solvent.

3. Antenna manufacturing and geometry

3.1. MWCNT ink printing

There are several methods to make thin films of MWCNTnetworks, such as filtration, spin coating, drying from solvent,and Langmuir–Blodgett deposition [25]. However, it is verydifficult to achieve the desirable antenna dimensions andshapes with high precision using these traditional methods. Ithas been recently reported that MWCNTs were successfullyprinted on flexible polymer substrates using an ink jet printingmethod [23]. The ink jet process is a popular industrialprinting method due to the fast pattern generation, non-contactinjection, solvent providing effects, high repeatability, quality,and scalability that are beneficial to large patterns [24]. Inaddition, ink jet printing of MWCNTs controls the patternthickness, geometrical dimensions and homogeneity of theprinted pattern. The ink jet printing process adopted inthe research reported in this paper demonstrated an excellentability to print MWCNT films of controllable geometries andsizes on different substrates, as shown in figure 2.

An ink jet printing process based on a microspray nozzlecontrolled by piezoelectric actuation, common in commercialink jet printers, was used to deposit the MWCNT ink onto aflexible polymer substrate. The MWCNTs have been added toa sodium cholate solution to form a mixture with a MWCNTconcentration of approximately 5 mg l−1. A homogeneous inkwas obtained by using a sonicator for two hours to separate anyMWCNT bundles. At this stage, the ink cartridge is filled withthe homogeneous ink to be printed on flexible substrates. Toprepare the polymer flexible substrates for printing, an initialtreatment by oxygen plasma for 2 min was applied to inducea high degree of hydrophilicity, thus increasing the adhesionquality of the MWCNT ink [22]. The plasma treatment processis performed inside a cylindrical reactor pipe that is connectedto a rotary pump in series with a RF power source. The reactor

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Figure 3. Antenna geometry used for the simulations conductedusing CST Microwave Studio; (a) top view; and (b) side view.

environment was adjusted to a base pressure of 0.002 Torrbefore introducing oxygen inside the reactor. The plasmatreatment was carried out at 0.2 kW for 2 min under an oxygenpressure of 0.4 Torr.

3.2. Antenna geometry

Six microstrip antennas were constructed from squareMWCNT patches. Two patch sizes were considered withW × L of 10 × 10 mm2 and 20 × 20 mm2 with the samethickness of 0.5 mm. The dimensions of the substrate andthe ground plane are 30 × 30 mm2. Figure 3 depicts theantenna geometry along with the inset feeding structure, whilethe detailed parameters are listed in table 1.

These antennas were formed on three different substrates.The first one is Rogers RO3210 with a relative permittivity of10.2, the second is Copper Clad CCFR4 Circuit Board 1/32with a relative permittivity of 5.2, and the third is RogersTMM4 with a relative permittivity of 4.5. The MWCNT ink

Table 1. Geometrical dimensions for several antennas.

10 × 10 (mm2) 20 × 20 (mm2)Dimension(mm) RO3210 CCFR4 TMM4 RO3210 CCFR4 TMM4

H 1.5 1 0.5 1.5 1 0.5Xp 3 3.5 4 8 8.5 9Yp 3 3 3 6 6 6Wp 1.43 1.2 1 1.43 1.2 1

was deposited on these substrates horizontally. The MWCNTpatch was fed through a 50 � copper microstrip line andconnected to the patch by a silver adhesive which dries atroom temperature. A SMA connector is soldered to the coppermicrostrip line. The MWCNT-based antennas and copper-based antennas that were manufactured for this study aredisplayed in figures 4(a) and (b).

4. Results and discussions

4.1. Constitutive parameters of the MWCNT ink

Difficulty arises in fully controlling the electrical propertiesof the MWCNT ink given the variations in concentration,chirality, dimension, and length amongst distributions ofnanotubes. Generally, MWCNTs possess metallic properties,which highly impact their interaction with electromagneticfields. A reason for picking multi-wall nanotubes for thisstudy is because they have better electrical conductivitycompared with the semiconductor-type single wall nanotubes.Several earlier studies reported a strong interaction betweenRF radiation and CNTs of various configurations [15, 19].These results were attributed to the good electrical conductivityof the nanotubes and their unique electronic structure. Aspecial attention should be given to the crystallinity of thenanotubes, since the missing carbon atoms from the graphiticstructure of the nanotubes would negatively impact theircorresponding electrical properties. In order for the proposedantennas to have a set of replicable properties it is essentialfor the thickness of the nanotube films that are composing

Figure 4. The fabricated square antenna patches; (a) 10 × 10 mm2 (top) and 20 × 20 mm2 (bottom) from MWCNT film right to left: RO3210,CCFR4, and TMM4; (b) 10 × 10 mm2 (top) and 20 × 20 mm2 (bottom) from copper right to left: RO3210, CCFR4, and TMM4.

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Nanotechnology 21 (2010) 045301 T A Elwi et al

Figure 5. Set up for the DC conductivity measurement.

the antennas to be uniform throughout the entire surface ofthe devices. Moreover, the use of solvents that enhance theelectrical conductivity of the nanotubes would be desirablesince such properties along with the thickness of the filmswould influence the behavior of the corresponding antennas.The real advantage of using carbon nanotube-based filmsto form RF antennas is that they can be formed in anyshape, form, thickness and particularly deposited on flexiblesubstrates, which makes them extremely advantageous forspecial communication applications.

In order to design an antenna and simulate itsperformance, the electrical properties of the antenna materialmust be fully understood and characterized. The four-probe resistivity measurement technique is a widely acceptedprocedure for the measurement of the electrical resistivity ofthin sheet samples [26]. A current source provides a constantlyincreasing current I , while I is passing though the two outerprobes; the resulting voltage drop V across the two innerprobes is measured by a voltmeter, as seen in figure 5.

The measurement process has been conducted on ninesamples of the MWCNT ink. The width of each sample wasfixed at 1 cm. Three heights were considered for each sample:100, 150, and 200 nm. For each sample, we have consideredthree lengths: 1, 5, and 10 cm. The measured DC conductivityfor the MWCNT ink was found to fluctuate around an averagevalue of (2.2 ± 0.5) × 104 S m−1. The small fluctuations arosedue to the difficulty in obtaining two identical homogeneoussurfaces with the same MWCNT density and the same cross-sectional area.

Generally, the relative permittivity is a complex numberwith the real and imaginary parts depending upon frequency asin [27]

εr( f ) = ε′r( f ) − jε′′

r ( f ) (1)

where εr( f ) is the complex relative permittivity, ε′r( f ) is the

real part of the relative permittivity, and ε′′r ( f ) is the dielectric

loss factor. The loss tangent, tan δ, is another derived parameter

Figure 6. Measured dielectric properties of the MWCNT; (a) tan δ;(b) ε′

r and ε′′r .

used in the simulations of the antennas and it is defined by thefollowing equation [27]:

tan δ = ε′′r

ε′r

. (2)

We have measured ε′r( f ), ε′′

r ( f ), and tan δ using anAgilent E5071B, 300 kHz–8.5 GHz ENA series NetworkAnalyzer, SN05499505 coaxial cable, and an Agilent 85070Bdielectric probe kit. The regular calibration procedure isperformed under open circuit and short circuit with distilledwater at a temperature of 25 ◦C. The dielectric probe software85070 E2.00 was used to calculate the dielectric constant andthe dielectric loss factor in the frequency range of the 85070E2network analyzer. Figure 6 shows ε′

r and ε′′r of the MWCNT as

a function of frequency in the range from 300 kHz to 8.5 GHz.It should be noted that the measured ε′

r and ε′′r are rather high

as compared to copper metallic films.It is well known that ε′

r of traditional conductors in theRF frequency range is very close to unity [27]. Two separatemechanisms may explain the measured relative permittivity ofthe MWCNT layer. One is the macroscopic structure of thepolymer used to facilitate adhesion to the substrate. Another

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Nanotechnology 21 (2010) 045301 T A Elwi et al

Figure 7. Simulated antenna characteristics versus conductivity, (a) S11 of the 10 × 10 mm2 patch on the TMM4 substrate, (b) S11 of the20 × 20 mm2 patch on the RO3210 substrate, (c) gain of the 10 × 10 mm2 patch on the TMM4 substrate, (d) gain of the 20 × 20 mm2 patchon the RO3210 substrate.

explanation is based on the fact that the phase velocity inside asingle MWCNT is about 0.01 of the speed of light in vacuum.These two aspects are expected to have a considerable effecton ε′

r. In this case, the electrons may take several pathswithin the MWCNT without exhibiting diffusive or bondingbehavior. Electron hopping between these paths is possible,which reduces the speed of the wave propagation inside theMWCNT, making ε′

r different from unity.

4.2. Numerical simulations

Numerical simulations based on CST MWS [28] wereperformed to design and characterize the performance ofthe antenna geometries shown in figure 4, in terms of thereturn loss (S11), resonant frequency, bandwidth, gain, and

far-field radiation patterns. The simulations are based ontwo steps: firstly, a parametric study was applied to monitorthe performance of the antenna while changing the electricalconductivity of the patch; secondly, the antenna performancewas simulated using our measurements of the electricalproperties of the MWCNT ink.

4.2.1. Parametric study. A parametric study was conductedby changing the electrical conductivity of the patch whilemonitoring S11 and bore-sight gain on two patch antennas withsizes 10 × 10 mm2 on a TMM4 substrate and 20 × 20 mm2 ona RO3210 substrate. The electrical conductivity was increasedfrom 22 S m−1 up to 22×1010 S m−1 while maintaining ε′

r = 5.The simulated S11 and gain are displayed in figures 7(a), (b)

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Nanotechnology 21 (2010) 045301 T A Elwi et al

and (c), (d), respectively. The S11 curves do not show arecognizable resonance mode when the electrical conductivitychanges from 22 S m−1 up to 220 S m−1. However, infigures 7(a) and (b), the S11 curves display approximatelythe same resonant frequency, but with varying magnitudesof S11 when the conductivity changes from 22 × 102 up to22 × 106 S m−1. Increasing the conductivity over 22 ×106 S m−1 does not show any noticeable difference in theresonant frequency of the dominant mode. The simulated gainexceeds 0 dB for conductivities above 7.5 × 103 S m−1 and5 × 104 S m−1 for the 10 × 10 mm2 and 20 × 20 mm2 patches,respectively. The simulated gain versus conductivity shown infigure 7(c) has a greater rate of change than the case presentedin figure 7(d) because the antenna patch presented in figure 7(c)is smaller than the one presented in figure 7(d) with respect tothe same ground plane.

4.2.2. Simulation of the MWCNT-based antennas usingmeasured constitutive parameters of the MWCNT ink Themeasured DC conductivity (σ = 2.2 × 104 S m−1) andpermittivity (ε′

r = 5) of the MWCNT ink were used to evaluatethe gain, S11, and resonant frequency of the antennas shownin figure 3. S11 and the resonant frequency are presented infigure 8. The antennas bore-sight gain values are listed intable 2 as well as the magnitude of S11, the resonant frequency,and the bandwidth.

4.3. Comparison of the simulated performance of theMWCNT-based antennas versus copper antennas

The proposed antenna designs were simulated using copperfor the conducting patch with an electrical conductivity of

Figure 8. Simulated S11 of the MWCNT-based antennas,(a) 10 × 10 mm2, and (b) 20 × 20 mm2.

5.8 × 107 S m−1 and ε′r = 1. Figure 9 compares the simulated

results of S11 versus frequency for both the MWCNT andcopper antennas. The differences in S11, resonant frequency,bandwidth, and gain for the MWCNT versus copper antennaswere found to be consistent. The antenna gains of both cases

Figure 9. Simulated S11 of the MWCNT versus copper antennas; (a) 10 × 10 mm2 with a RO3210 substrate; (b) 10 × 10 mm2 with a CCFR4substrate; (c) 10 × 10 mm2 with a TMM4 substrate; (d) 20 × 20 mm2 with a RO3210 substrate; (e) 20 × 20 mm2 with a CCFR4 substrate;(f) 20 × 20 mm2 with a TMM4 substrate.

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Nanotechnology 21 (2010) 045301 T A Elwi et al

Figure 10. Measured S11 of the MWCNT and copper antennas; (a) 10 × 10 mm2 with a TMM4 substrate; (b) 10 × 10 mm2 with a CCFR4substrate; (c) 10 × 10 mm2 with a RO3210 substrate; (d) 20 × 20 mm2 with a TMM4 substrate; (e) 20 × 20 mm2 with a CCFR4 substrate;(f) 20 × 20 mm2 with a RO3210 substrate.

Table 2. Simulated antenna characteristics based on the measuredMWCNT paint constitutive parameters.

10 × 10 (mm2)Gain(dBi)

S11

(dB)

Resonantfrequency(GHz)

Bandwidth(%)

TMM4 5.0 −24.4 6.73 2.23CCFR4 3.2 −27.3 6.30 2.22RO3210 0.2 −15.3 4.50 2.81

20 × 20 (mm2)Gain(dBi)

S11

(dB)

Resonantfrequency(GHz)

Bandwidth(%)

TMM4 3.54 −17.3 3.40 3.53CCFR4 1.46 −28.5 3.20 4.38RO3210 −9.6 −19.5 2.32 5.60

are listed in table 3. This comparison shows the significance ofthe material properties on the antenna performance in terms ofresonant frequency, bandwidth, and gain.

The low conductivity of the MWCNT patches reducedthe antenna gain in comparison to the copper patch, as shownin cases 3 and 6. However, some of the MWCNT antennaspossess high gain even with the relatively lower conductivitypatches, due to the large ground plane with respect to thepatch at the first resonant frequency. This leads to a highergain, as shown in cases 1 and 4. The reduced electricalconductivity of the MWCNT patches, however, leads to anincreased bandwidth for all cases considered.

4.4. Experimental measurements

The S11 and S12/S12 were measured using an Agilent E5071BENA series Network Analyzer. We characterized S11 of the

antennas fabricated from the MWCNT ink and copper on thethree substrates according to the geometry and dimensionsgiven in section 2. The same measurements are repeated forthe antennas made from copper. The S11 was measured forthe twelve antenna prototypes (MWCNT and copper) shown infigures 4(a) and (b). For gain measurements, we selected twoantennas corresponding to those with the highest and lowestgain provided by the simulations as depicted in table 4. Themeasured values of S11 for MWCNT and copper antennas aredisplayed in figures 10(a)–(f).

For gain measurements, the Friis transmission equation (3)is considered in the far-field region [29]:

G = −1.629 + 10 log(R f ) + 0.5(S12) (3)

where G is the measured gain in dBi, S12 or S21 is the measuredtransmission parameter in dB, R is the separation betweenantennas in cm and f is the resonant frequency of each antennain GHz. In the above equation, the measured S12 at thereceiving antenna and S21 at the transmitting antenna shouldbe equal. G is the gain for both the transmitting and receivingantennas: they should be identical. They should also havethe same resonant frequency and S11. For R, it should bein the far field of each antenna. The far-field distance in ourmeasurements is found by using the following equation [30].

R � 2a2/λ (4)

where d is the largest linear dimension of the antenna, and λ

is the wavelength at the fundamental resonant frequency. Wehave measured S12 at four different distances in the far-fieldregion for both the MWCNT and copper antennas. The antennagain measurements versus frequency of the highest and lowest

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Nanotechnology 21 (2010) 045301 T A Elwi et al

Figure 11. Measured and simulated gain versus frequency of the MWCNT-based antennas versus copper antennas; (a) 20 × 20 mm2 with aRO3210 substrate and MWCNT-based patch; (b) 10 × 10 mm2 with a TMM4 substrate and MWCNT-based patch; (c) 20 × 20 mm2 with aRO3210 substrate and copper patch; (d) 10 × 10 mm2 with a TMM4 substrate and copper patch.

Table 3. Simulated gain of the MWCNT versus copper antennas.

Case 10 × 10 (mm2) MWCNT antenna gain (dBi) Bandwidth (%) Copper antenna gain (dBi) Bandwidth (%)

1 TMM4 5.0 2.23 6.78 1.172 CCFR4 3.2 2.22 5.67 1.153 RO3210 0.2 2.81 3.87 1.52

Case 20 × 20 (mm2) MWCNT antenna gain (dBi) Bandwidth (%) Copper antenna gain (dBi) Bandwidth (%)

4 TMM4 3.54 3.53 5.35 1.935 CCFR4 1.46 4.38 4.65 2.156 RO3210 −9.6 5.60 2.93 3.01

gain of the proposed antennas for both MWCNT and copperare presented in figure 11.

Table 4 shows the measured bore-sight gain of the selectedantennas of the 10 × 10 mm2 and 20 × 20 mm2 patches basedon TMM4 and RO3210 substrates of both MWCNT and copperpatches at the corresponding fundamental resonant frequency.An excellent agreement has been achieved in terms of S11

between the simulated results presented in figure 9 and themeasured results in figure 10. The measured bore-sight gaindisplays a good agreement against the simulated values, asshown in figure 11. The simulated radiation patterns are shownin figure 12.

5. Conclusions

We have demonstrated the feasibility of fabricating microstripantennas based on MWCNT by printing a conductive MWCNTink. The measured DC conductivity of the MWCNT ink is2.2 × 104 S m−1. The measured complex relative permittivityranged from 6.5–j0.9 to 4.5–j1.1 over the frequency rangefrom 300 kHz to 8.5 GHz. The MWCNT patches havebeen deposited on the substrates using an ink jet printingprocess. The proposed antennas have been simulated basedon the measured constitutive parameters. The simulatedantenna models are validated by the excellent agreementagainst measured results. The proposed MWCNT antennas

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Nanotechnology 21 (2010) 045301 T A Elwi et al

Figure 12. Simulated radiation patterns of the MWCNT-based antennas versus copper antennas at ϕ = 90◦ and θ = (0◦–360◦);(a) 10 × 10 mm2 with a TMM4 substrate; (b) 20 × 20 mm2 with a RO3210 substrate.

Table 4. Antenna gain measurements of the MWCNT-basedantennas versus copper antennas.

10 × 10 (mm2)MWCNT antennagain (dBi)

Copper antennagain (dBi)

TMM4 4.45 6.65

20 × 20 (mm2)MWCNT antennagain (dBi)

Copper antennagain (dBi)

RO3210 −8.3 2.86

demonstrated a remarkable enhancement in the bandwidthranging from 2.2% to 5.6% at the first mode, as comparedto 1.1% to 3.0% for their identical copper-based counterparts.This is approximately 45% more bandwidth than the antennaswith copper patches at their corresponding fundamentalresonant mode. The MWCNT antennas possess a lowergain compared to their copper counterparts due to thelower conductivity of the patch; however, the shape ofthe far-field radiation patterns is unchanged. The ease offabrication, enhanced bandwidth, and moderate gain of theproposed MWCNT-based antennas demonstrate their viabilityfor short and medium range wireless communication andsensor networks in environments which are not suitable forconventional conductors.

Acknowledgment

The authors are grateful for the financial support receivedfrom the Arkansas Science and Technology Authority, (ASTA)Grant No. 08-CAT-03.

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