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Optik 125 (2014) 5546–5549

Contents lists available at ScienceDirect

Optik

jo ur nal homepage: www.elsev ier .de / i j leo

raphene nanoribbon based terahertz antenna on polyimide substrate

. Ananda,∗, D. Sriram Kumara, Ren Jang Wub, Murthy Chavali c

Department of Electronics and Communication Engineering, National Institute of Technology, Tiruchirappalli 620015, IndiaDepartment of Applied Chemistry, Providence University, 200 Chungchi Road, Shalu, Taichung Hsien 433, Taiwan, ROCDivision of Chemistry, Department of Sciences & Humanities, Vignan University, Vadlamudi, Guntur 522213, India

r t i c l e i n f o

rticle history:eceived 5 October 2013ccepted 3 May 2014

a b s t r a c t

Graphene nano ribbon based terahertz patch antenna on polyimide substrate is designed and its radiationcharacteristics are investigated in the 725–775 GHz band. The terahertz communication system consists

eywords:raphene nanoribbonatch antennaerahertzraphene antenna

of higher data rate transmission, low transmit power with secured wireless communication. The proposedantenna consists of graphene nano ribbon as radiating patch and also the ground plane separated by a20 �m thin polyimide substrate. The antenna has achieved the broad impedance bandwidth (>5%) in theband of operation. The design has yielded a peak gain of 5.71dB at 750 GHz. The antenna is simulated byusing the finite element method based simulator Ansys - HFSS.

© 2014 Elsevier GmbH. All rights reserved.

. Introduction

Graphene is a promising material due to its excellent electricalonductivity, electromagnetism and electromechanical propertiesor next generation molecular electronics [1]. Graphene nanoibbon has potential applications of high frequency transistors2], modulators [3], wireless nano sensors [4], organic electron-cs [5] and devices operating in terahertz band [6]. Grapheneanoribbon-polymer composites based conformal antennas areeeded, because a metallic thin film is prone to fail due to microracks [7,8]. The next generation wireless communication systemequires ultra-broadband antenna with low transmitting power in

high mobility environment. For the terahertz frequency regime,he microstrip patch antenna is an essential device because ofts compatibility for miniaturization [9]. The microstrip patchntennas are broadly used in satellite and missile applications dueo its advantages of conformal to planar and nonplanar surfaces10]. The conformal patch antenna consists of parallel electriconductors, separated from the flexible dielectric material and cane fabricated using thin film deposition and nano lithography tech-

iques [11]. The flexible dielectric material is polyimide (εr = 3.5)ith tan ı = 0.008 is used here. The low dielectric permittivityaterial is needed for improving the radiation efficiency in the

∗ Corresponding author. Tel.: +91 9751 227 424.E-mail addresses: anand.s.krishna@gmail.com (S. Anand), srk@nitt.edu

D. Sriram Kumar), renjangwu@gmail.com (R.J. Wu), chavalim@gmail.comM. Chavali).

ttp://dx.doi.org/10.1016/j.ijleo.2014.06.085030-4026/© 2014 Elsevier GmbH. All rights reserved.

desired direction [12]. Previous researchers in this topic are unableto achieve high gain (≥2dB), broad impedance bandwidth (>5%)and better radiation efficiency [13].

The present work discusses the designing and radiation charac-teristics of a graphene nano ribbon (GNR) based microstrip patchantenna on the polyimide substrate in terahertz band. In Ref. [14],graphene based patch antenna has been designed and the surfaceconductivity of graphene sheet is analyzed. The organization of thepaper is as follows. Section 2 discusses the electrical conductivityof graphene sheet in the terahertz regime. Section 3 discusses theproposed GNR based microstrip patch antenna design. Section 4investigates the results of the impedance bandwidth and far-fieldradiation pattern of the proposed antenna. Conclusions are madein Section 5.

2. Graphene conductivity

The edge effects of the graphene would affect the electromag-netic radiation when the lateral dimensions of graphene sheet (GS)are less than 100 nm [15]. Calculation of the surface conductiv-ity of infinite GS by using the Kubo formalism is available in thepast literatures [2,14,16,17]. The frequency dependence of the con-ductivity of GS is presented in terms of intra band and inter bandcontributions [16].

The intraband conductivity,

�intra = e2gsgv�c

4��2KBT

((4 − i)

ω

)(1)

S. Anand et al. / Optik 125 (2014) 5546–5549 5547

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Table 1GNR based terahertz patch antenna design parameters.

Parameters Symbol Value

Operating frequencyPatch length andwidth

fL × W

(0.725–0.775) THz88.98 �m × 133.2 �m

Dielectric substratelength, width andthickness

Ls × WS × h 208.98 �m × 433.2 �m × 20 �m

Dielectric constant(polyimide)

εr 3.5

have an impedance mismatch under deformation, due to forma-tion of electrical resistance in the conducting patch [19]. But theGNR based flexible antenna has negligible resistance even underdeformation [20].

Fig. 1. Frequency dependence of conductivity of graphene sheet.

The interband conductivity,

inter = e2gsgv

4ω�

(4 − i

�

)(2)

here �c is the chemical potential, � is the reduced Planck’s con-tant, KB is the Boltzmann constant, T is the temperature, ω is therequency, gs and gv are spin and valley degeneracies of hexagonaltructured GS respectively. The total conductivity of GS is calcu-ated as, �total = �intra + �inter. Hence the total conductivity is giveny,

total = e2gsgv

2ωh

(�c(4 − i)�KBT

+ (4� − i))

(3)

he frequency dependence of real and imaginary parts of the intra-and, interband and total conductivity are shown in Fig. 1.

The imaginary parts of both intraband and total conductivity arexactly overlapping. Interband conductivity is very less comparedo intraband conductivity. At higher frequencies the intrabandonductivity is dominating the interband component in the totalonductivity. In this region, it is therefore approximated that theonductivity of GS depends only on the intraband conductivity,hich is an assumption [14].

. Graphene patch antenna

The GNR based microstrip patch antenna is designed and hasimensions in the order of few micrometres. The basic structuref the microstrip patch antenna consists of a conducting patch, around plane separated by a thin dielectric substrate. The squarend rectangular shapes of radiating patch are more preferred

ecause of easy analysis and fabrication. As indicated in Fig. 2, GNRs radiating patch (patch thickness (t) is very much less than the0) and also ground plane. The radiating patch and ground planere separated by a 20 �m thin polyimide substrate. The thickness

Fig. 2. GNR loaded microstrip patch antenna.

Microstrip line length L1 × L2 43.02 �m × 16.98 �mMicrostrip line width W1 × W2 20 �m × 4 �m

of the dielectric substrate (h) should be less than the free spacewavelength (0.003 �0 ≤ h ≤ 0.05 �0) [18].

The fringing fields are the main sources of electromagnetic radi-ation. The excited electric fields at the edges of the patch undergofringing. The substrate having smaller values of dielectric permit-tivity is preferred, because it enhances the fringing fields. Themicrostrip line feed technique has been used for impedance transi-tion between rectangular patch and coaxial line. The antenna is fedby a 50 � microstrip transmission line. The width of the microstripline is optimized to match the antenna impedance with negligiblemismatch loss. The proposed antenna structure is designed to res-onate at 0.75 THz. The dimensions of the GNR based patch antennais listed in Table 1.

4. Results and discussion

The THz antenna is required for enabling the ultra-broadbandand secured data transfer in the future wireless communicationsystems. The proposed antenna is designed and its radiation char-acteristics are investigated in the band of 0.725–0.775 THz, usingthe Ansys - HFSS simulation tool.

The return loss (S11) of the GNR based patch antenna is shown inFig. 3. The antenna has achieved a -10dB impedance bandwidth of6.67% in the band of operation. The conventional flexible antennas

Fig. 3. Return loss (S11).

Table 2Radiation performances of the GNR based patch antenna.

Frequency (GHz) 725 740 750 770

Gain (dB) 4.93 5.05 5.09 5.07Directivity (dB) 5.54 5.67 5.71 5.70Radiation efficiency (%) 86.85 86.69 86.58 86.43Front to back lobe ratio 79.80 74.23 70.27 62.10

5548 S. Anand et al. / Optik 125 (2014) 5546–5549

Table 3Radiation characteristics of the GNR based patch antenna in comparison with that of the existing antennas performances.

Parameters Graphene based antennas Conventional antennas

Proposed antenna [21] [22] [23] [24]

Resonant frequency, fr (GHz) 750 1000 800 1425 2150Gain (dB) 5.09 −1 – 1.65 4.97Directivity (dB) 5.71 – 0 – –Radiation efficiency (%) 86.58 – 20 43 79.6Front to back lobe ratio 70.27 – – – –

taaa

aF

Fp

Fig. 4. Radiation efficiency (%).

Fig. 4 has demonstrated the antenna radiation efficiency in rela-ion to the frequency. The peak radiation efficiency of the proposedntenna is 86.85% at 725 GHz. It is also shown that the proposedntenna has better radiation efficiency than the conventional patch

ntennas [13].

The E and H planes far-field radiation patterns of the proposedntenna at 725 GHz, 740 GHz, 750 GHz and 770 GHz are shown inig. 5(a)–(d). The E-plane radiation plot shows that the antenna has

ig. 5. E-plane (solid line) and H-plane (dash line) far field radiation patterns of theroposed antenna at (a) 725 GHz, (b) 740 GHz, (c) 750 GHz and (d) 770 GHz.

Fig. 6. Antenna gain (dB).

minimum back lobe radiation in the desired band. So better frontto back lobe ratio is achieved, and discussed in Table 2.

It is observed from Fig. 6, GNR based patch antenna has yieldeda peak gain of 5.71dB at 750 GHz. This is a very encouraging resultas compared to conventional patch antenna gain (≤2dB) [13]. Theradiation characteristics of the proposed antenna are comparedwith that of the previously reported graphene based antennasand conventional antennas operating in terahertz regime, issummarized in Table 3. The proposed GNR based patch antennais not optically transparent. In future, graphene based opticallytransparent antenna should be tried for GHz and THz applications.

5. Conclusion

The radiation characteristics of graphene nanoribbonbased microstrip patch antenna have been investigated inthe 725–775 GHz band. Graphene nanoribbon based terahertzpatch antenna is simulated by using the finite element methodbased simulator Ansys - HFSS. The antenna achieved the -10dBimpedance bandwidth of 6.67% in the band of operation. Theproposed antenna is shown to produce the peak gain of 5.71dB at750 GHz and the 5.54dB is the minimum gain observed at 725 GHz.The antenna has minimum back lobe in the desired band, hencebetter front to back lobe ratio. The radiation characteristics of theproposed antenna is listed in Table 2, and it is compared with thatof the previously reported graphene based antennas and also withconventional patch antennas in Table 3.

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