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Performance predictions of slotted graphene patch antenna for multi-band operation in terahertz regime
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Performance predictions of slotted graphene patch antenna for multi-band operation in terahertz regime
T
Shalini M*, Ganesh Madhan M Department of Electronics Engineering, MIT Campus, Anna University, Chennai, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Graphene patch Surface plasmon polaritons Multi-band antenna Defected ground structure
A compact multi-band graphene based patch antenna is proposed for THz applications. A single band antenna is initially designed without slot, where the lower resonant mode of the antenna operates only at single frequency of 1.9 THz with a bandwidth of 50 GHz. Dual-bands are realized by introducing slots in the graphene patch. The proposed structure makes the antenna simple in design and easy for fabrication by employing slots in the graphene patch. By adjusting the position of the slot in the patch, the antenna is made to radiate at dual frequencies of 1.96 THz and 4.83 THz with bandwidths of 80 GHz and 100 GHz respectively. FDTD based EM simulation predicts a return loss of -34 dB and-38 dB at 1.96 THz and 4.83 THz respectively and VSWR less than 1.5 at both frequency bands. Moreover, the antenna provides a significant gain of 4.75 dB and 4.3 dB over the operating bands and efficiency greater than 92 % is observed. Further, the antenna is made to operate at triple bands of 1.96 THz, 4.83 THz and 5.55 THz by introducing defects in the ground plane.
1. Introduction Terahertz spectra have received considerable attention due to increase in the demand for broadband wireless communication. Other applications include imaging, spectroscopy, bio-sensing, radar and security. In order to utilize the available spectrum efficiently, communication systems require antenna with compact size and multi-band characteristics. Dual-band antennas have the ability to provide strong and stable wireless connection especially in mobile applications. Two separate antennas are required, when two operating frequencies are far apart [1]. In such cases, a single dual-band antenna can be used to operate at two resonant frequencies, thereby reducing the number of antennas. Instead of using different antennas for each band, dual-frequency antenna can be employed to meet the requirements. This helps in reducing the space and weight while integrating with MMIC. Microstrip antennas are also widely desirable to have multiband characteristics. Several dual-band antennas have been proposed in RF/Microwave frequency range of the Electromagnetic spectrum using stacked patch [1], meta-materials [2], parasitic patch [3], slots in the ground plane [4], exciting various cavity modes on the asymmetric patch [5]. Dual-band antenna using stacked patch is implemented using several layers which leads to large antenna size. Parasitic patch dual-band antenna is implemented by forming four parasitic patches underneath a driven patch. However, this multi-stacked structure affects the coupling between the parasitic elements and decreases the efficiency of the antenna. Modal expansion technique is used to excite various cavity modes in order to achieve multiple bands. But, this technique is not desirable because the excited modes will have different polarization, different field and several feeding mechanisms are required to excite various modes. These RF/microwave antennas are radiated using copper conductor. However,
⁎
Corresponding author. E-mail address: [email protected] (S. M).
https://doi.org/10.1016/j.ijleo.2020.164223 Received 15 November 2019; Received in revised form 10 January 2020; Accepted 13 January 2020 0030-4026/ © 2020 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
S. M and G.M. M
terahertz antennas cannot be effectively implemented using copper, since it suffers from serious attenuation due to low conductivity and low mobility at terahertz frequencies [6]. Graphene is considered to be an efficient material for terahertz radiation, since it has high conductivity, high mobility and also supports surface plasmonic propagation. Graphene based dual-band antennas have been developed using several techniques. In [7], Dual band reconfigurable THz patch antenna is proposed with graphene stack based backing cavity. Beam steering is achieved by changing the applied voltage on the graphene stack. However, it has cavity in the dielectric substrate which will have its effect on the surface waves and return loss. Switches are incorporated in the reconfigurable antenna, thereby increasing the complexity of the antenna structure. Optically transparent dual band antenna using ITO (Indium Tin Oxide) has been developed for THz communication [8]. It is excited by aperture coupled feed. Use of transparent conducting material such as ITO will cause impedance mismatch and it is also expensive [9]. WCIP (Wave Concept Iterative Process) method is used to develop an antenna for dual-band THz operation in [10]. This method requires convergence study, which involves large number of iterations and increases the computational time. Graphene based antennas are developed for RFID applications [11]. However, it operates at a single resonant frequency. Dual band graphene antenna is also developed by varying the thickness of the substrate [12]. Meanwhile, surface wave losses increase thereby reducing radiation characteristics and bandwidth of the antenna, due to the excitation of higher order modes. Most of the antennas reported in the literature includes multiple-stacked patches, multilayer substrates, complicated matching network and single frequency antennas, which limits the application of antenna to multi-frequency operation. In this work, dual-band characteristics are achieved in the graphene antenna by etching a rectangular slot in the radiating patch element. Embedding slot in the radiator is one of the most robust and effective method of exciting various modes in the antenna to produce multi-band operation. This technique also improves the impedance characteristics of the antenna. Defected ground structure is used to obtain multi-band notch characteristics in microwave regime [13]. Further, the antenna is made to operate at triple bands of 1.96 THz, 4.83 THz and 5.55 THz by introducing defects in the ground plane. Therefore, considering the demand for simple and an effective multi-band antenna in terahertz region, graphene based antenna is proposed which consists of a square shaped slot on the radiator to achieve dual band operation and defected ground structure is used to obtain triple bands. Moreover, miniaturization is also be obtained by etching slots in the antenna and it can be economical. Compact design and multi-band operation are required for future wireless applications [14]. This work presents a novel miniaturized slotted tri-band antenna for Terahertz applications. High data rate can be achieved using terahertz band and implemented using graphene based antennas, whose chemical potential can be adjusted to achieve desirable tuning characteristics [15]. Multiband antennas can be used to improve the bandwidth, capacity and data rate of terahertz communication system. Slots are used in the antenna design for bandwidth enhancement, miniaturization and to produce multi-frequency operation with desirable tuning characteristics. Slot antenna is preferred because of its simple structure [16]. Square-shaped slot is used to have low profile. Terahertz spectrum are widely used in imaging, spectroscopy and bio-medical applications due to its unique characteristics. Terahertz wave has low photon energy, non-ionizing and it doesn't cause any harmful effects to human body. Terahertz spectroscopy can be used in pharmaceutical applications such as chemical analysis of tablets to detect the defects present in the tablet coating, industrial applications such as product inspection for identification of faults and medical applications such as cancer detection [17]. A number of single band antennas are reported in terahertz frequency range. However, designing a multi-band antenna in terahertz frequency range is a challenging task. Metamaterials, Photonic band gap structure and Split Ring Resonators are used to design multi-band antennas [18]. These structures involve tedious procedures, which makes the antenna design more complicated. Hence, to overcome these limitations, a simple structure is proposed using graphene material for multi-band operation in terahertz regime. The paper is organized as follows. Evolution of dual band Graphene antenna and the effect of slot on antenna characteristics are illustrated in Section III. The impact of defects in the ground plane is discussed in Section IV. 2. Evolution of dual-band graphene antenna 2.1. Single band antenna The basic antenna is designed using conventional microstrip patch radiator without embedding slot in it, for single resonant frequency of 1.9 THz. The substrate used is Silicon-di-oxide (Silica) with dielectric constant of 3.9 and a thickness of 30 μm. An inset feed microstrip line is chosen to excite the antenna for impedance matching. Graphene is used to model the patch radiator for efficient transmission of terahertz waves. The conductivity of mono-layer graphene should be determined to know about the propagation of surface plasmons, which generates terahertz waves. Kubo's formula gives the approximation for calculating Drude-like intraband contribution of conductivity, as it dominates below 5 THz [20]. It is given by,
σintraband (S ) = j
2kT0 eq2 ℏ(j г−1ℏ + ℏω)
μ ⎞ 2 lncosh ⎛ ⎝ 2kT0 ⎠ ⎜
⎟
(1)
J/K is the Boltzmann constant, eq is the electron charge, ℏ is the reduced Plank’s constant, μ is the chemical where k = 1.38 x potential, T0 is the temperature and г is the relaxation time.
10−23
Г=
μn eq. vF2
(2)
where, vF is the Fermi velocity whose magnitude is 106m / s and n = 10 4cm2V −1s−1 is the electron mobility [20]. The volume conductivity of graphene is obtained by dividing the surface conductivity by the thickness of graphene (0.34 nm) 2
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
S. M and G.M. M
Fig. 1. Return loss characteristics of the antenna without slot.
[20].
σintraband (V) =
σintraband (S) thickness of patch
(3)
Standard design procedure is followed to calculate the length (pL) and width (p W ) of patch antenna [12].
c
pL =
2f 0
pW =
εe =
c 2f 0
p ⎡ (ε e + 0.3) hw + 0.264 − 2 ⎢0.412 t Wp ⎢ εe (ε e − 0.258) h + 0.8 ⎣
(
(
) ⎤⎥
) ⎥⎦
(4)
εr + 1 2
(5)
⎛ (εr + 1) (ε − 1) 1 + r ⎜ 2 2 ⎜ 1 + 12 pt W ⎝
⎞ ⎟ ⎟ ⎠
(6)
where, pL is the length of the patch and p W is the width of the patch, t is the thickness of the substrate and εe denotes the effective dielectric constant. The geometry and return loss of the graphene antenna without slot is shown in Fig. 1. The resonance occurs at only a single frequency of 1.9 THz and the maximum return loss is about 14 dB. The bandwidth is found to be 50 GHz. 2.2. Dual-band operation The single band antenna is modified by embedding slot in the radiating patch for dual-band operation. The dimension of slot is chosen as one-fourth of guided wavelength, λ g . It is given by,
Fig. 2. Configuration of the proposed dual-band antenna. 3
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
S. M and G.M. M
Table 1 Dimensions of the proposed antenna. Parameter
Dimensions (μm)
Length of the patch (pl) Width of the patch (pw) Length of the substrate (LS) Width of the substrate(WS) Height of the substrate (t) Width of inset-feed gap (a) Length of inset-feed gap (b)
20 25 50 50 30 1 5
Fig. 3. Effect of slot on return loss characteristics.
Fig. 4. a) E-plane and H-plane characteristics at 1.96 THz. b) E-plane and H-plane characteristics at 4.83 THz.
λg =
c fc
(7)
εe
where, fc is the cut-off frequency and εe is effective dielectric constant. The configuration of the proposed antenna with square shaped slot in the patch is shown in Fig. 2. The position of slot is determined such that it matches the feed impedance. The propagation of SPP (Surface plasmon polaritons) waves in the metal4
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
S. M and G.M. M
Fig. 5. a) 3D Radiation pattern of dual-band antenna at 1.96 THz. b) Gain of the proposed dual-band antenna.
dielectric interface makes the antenna to resonate at first frequency; the slot in the antenna introduces the second resonant frequency. This makes the antenna to radiate at two resonant frequencies (1.96 THz and 4.83 THz respectively). The dimensions of the antenna are shown in Table 1. As described, additional resonance is created by introducing slot, thereby the antenna radiates at second frequency (4.83 THz). Embedding slot in the radiator varies the electrical length of patch, thereby producing additional resonance at 4.83 THz. When the slot length (s) is varied, it is found to have minimal effect on first resonance frequency and the second resonant frequency is found to 5
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
S. M and G.M. M
Table 2 Comparison of dual-band antenna characteristics with similar design. Techniques
Operating frequencies(THz)
Return loss(dB)
Gain(dBi)
Radiation efficiency(%)
Graphene based stacked cavity [7]
4.46 7 0.75 1.1 2.5 5.061 2.47 3.35 2.17 2.59 1.96 4.83
−42 −30 −22 −28 −26.57 −28.52 −17 −22 −28 −35 −34 −38
Not mentioned
Not mentioned
7.33 10.3 Not mentioned
88.4 91.2 Not mentioned
2.7 6.03 4.01 5.03 4.75 4.3
87.3 53.47 64.12 60.8 93.7 95.3
ITO based dual band antenna [9] Wave concept iterative process [10] Varying substrate height for dual-band operation [12] Dual-band reconfigurable graphene based patch antenna [19] Proposed work- Slot induced dual-band antenna
Fig. 6. Return loss for various h.
Table 3 Modified dimensions of triple band antenna. Parameter
Dimensions (μm)
Reduced ground length(h) slot in ground(g)
10 5
shift towards right. The resonant frequencies of the antenna also decrease with increase in length of loaded square slot. The effect of slot length is shown in Fig. 3. The optimum dimension of slot length is found to be 6 μm. The Voltage Standing Wave Ratio (VSWR) for the proposed dual band antenna using graphene radiator are 1.12 at 1.96 THz and 1.16 at 4.83 THz. E-plane and H-plane characteristics of the antenna are shown in Fig. 4(a) and (b). The main-lobe magnitude is around 4.7 dB and the angular width is 90° for 1.96 THz. The main lobe magnitude is around 4 dB and angular width is 121° for 4.83 THz. 3D radiation pattern is shown in Fig. 5(a). The proposed antenna achieves peak gain of 4.75 dB and 4.3 dB at 1.96 THz and 4.83 THz respectively, as shown in Fig. 5(b). The path of the surface current flows around the slot in the patch and the length of current path are increased. The proposed dual-band antenna has been compared with the existing literature in Table 2. The antenna is implemented with simple technique compared to the existing methods as shown in the table. Further, the proposed antenna achieves higher efficiency with better return loss compared to other techniques. 3. Design of triple band antenna using defected ground structure The performance of the dual band graphene antenna is investigated by introducing defects in the ground plane. When the length 6
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
S. M and G.M. M
Fig. 7. Return loss curve and configuration (back view of antenna) of triple band operation.
Fig. 8. VSWR curve of triple band antenna. Table 4 Parameters of triple band antenna. Triple band frequencies (THz)
Return loss(dB)
VSWR
Gain(dBi)
Radiation efficiency (%)
1.9551 4.83 5.4474
−26.329 −24.849 −19.619
1.1207 1.1412 1.2326
4.79 5.05 5.53
93.6 95.9 98.08
of the ground plane is reduced by a value of h, only minimal effect is observed on first resonant frequency and the second resonant frequency gets shifted towards right as shown in Fig. 6. Furthermore, as the ground plane length is reduced to 40 μm, a square shaped slot is inserted in the center of the ground plane for triple band operation. This slot disturbs the current distribution in the ground plane by offering some capacitance and inductance, which produces additional resonance at 5.5 THz. The value of inductance (L) and capacitance (C) depends on resonant frequency (f0) and cut-off frequency (fc). The design equations are given by,
L=
C=
1 4 π 2f02 C
(8)
fc 2Z0(f 2 − f 2 ) 0
(9)
c
7
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
S. M and G.M. M
Hence, the antenna is modified to operate at 1.96 THz, 4.83 THz and 5.5 THz leading to triple band operation. The dimensions of the proposed triple band antenna are shown in Table 3. The configuration of the proposed tri-band antenna (back view) and its S11 results are shown in Fig. 7 and its corresponding VSWR are shown in the Fig. 8. Table 4 gives the information on various parameters of triple band antenna. The antenna has a peak gain of 4.79dBi, 5.05dBi and 5.53 dBi at 1.9551 THz, 4.83 THz and 5.4474 THz respectively. It also achieves radiation efficiency greater than 92 % at all three frequencies. Gaurav et al. [21] has reported a triband bow-tie terahertz antenna operating at 7.1 THz, 11.1 THz and 13.1 THz. This antenna uses very thin substrate, which is not suitable for terahertz communication, as it will break down when it is excited by plane wave. The efficiency obtained is only 75 % which is less than the proposed tri-band antenna (> 92 %). 4. Conclusion A single element microstrip antenna using graphene is proposed for multi-frequency operation at Terahertz band. This is achieved by embedding slot in the radiating patch to excite additional resonant modes in the antenna. The proposed antenna resonates at dual frequencies of 1.96 THz and 4.83 THz respectively. Return loss better than -30 dB and VSWR less than 1.5 are achieved at both the frequency bands. The antenna achieves significant gain of 4.75 dB and 4.3 dB at 1.96 THz and 4.83 THz respectively. Further, the antenna is made to operate at triple bands of 1.96 THz, 4.83 THz and 5.5 THz, by embedding defects in the ground plane and efficiency greater than 92 % is obtained. Thus, a single element antenna is designed with compact size, which offers multiband performance at terahertz frequency range. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References [1] S. Maci, G.B. Gentili, Dual-frequency patch antennas, IEEE Antennas Propag. Mag. 39 (6) (1997) 13–20. [2] Braden P. Smyth, Stuart Barth, Ashwin K. Iyer, Dual-band microstrip patch antenna using integrated uniplanar metamaterial-based EBGs, IEEE Trans. Antennas Propag. 64 (12) (2016) 5046–5053. [3] S.A. Long, M. Walton, A dual-frequency stacked circular-disc antenna, IEEE Trans. Antennas Propag. 27 (2) (1979) 270–273. [4] Younkyu Chung, Seong-Sik Jeon, Dal Ahn, Jae-Ick Choi, Tatsuo Itoh, High isolation dual-polarized patch antenna using integrated defected ground structure, Ieee Microw. Wirel. Compon. Lett. 14 (1) (2004) 4–6. [5] Frederic Croq, David M. Pozar, Multifrequency operation of microstrip antennas using aperture-coupled parallel resonators, IEEE Trans. Antennas Propag. 40 (11) (1992) 1367–1374. [6] I. Llatser, C. Kremers, A. Cabellos-Aparicio, J.M. Jornet, E. Alarcón, D.N. Chigrin, Graphene-based nano-patch antenna for terahertz radiation, Photon. Nanostruct-Fundam. Appl. 10 (4) (2012) 353–358. [7] Yanfei Dong, Peiguo Liu, Dingwang Yu, Gaosheng Li, Feng Tao, Dual-band reconfigurable terahertz patch antenna with graphene-stack-based backing cavity, IEEE Antennas Wirel. Propag. Lett. 15 (2016) 1541–1544. [8] Tale Saeidi, Idris Ismail, Adam R.H. Alhawari, Aduwati Sali, Alyani Ismail, High gain dual-band couple feed transparent THz antenna for satellite communications, 2016 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE), IEEE, 2016, pp. 11–15. [9] Anand Sreekantan Thampy, Mayur Sudesh Darak, Sriram Kumar Dhamodharan, Analysis of graphene based optically transparent patch antenna for terahertz communications, Physica E Low. Syst. Nanostruct. 66 (2015) 67–73. [10] Aymen Hlali, Zied Houaneb, Hassen Zairi, Dual-band reconfigurable graphene-based patch antenna in terahertz band: design, analysis and modeling using wcip method, Prog. Electromagn. Res. 87 (2018) 213–226. [11] Mitra Akbari, M. Waqas, A. Khan, Masoumeh Hasani, Toni Björninen, Lauri Sydänheimo, Leena Ukkonen, Fabrication and characterization of graphene antenna for low-cost and environmentally friendly RFID tags, IEEE Antennas Wirel. Propag. Lett. 15 (2015) 1569–1572. [12] Jemima Nissiyah George, M. Ganesh Madhan, Analysis of single band and dual band graphene based patch antenna for terahertz region, Physica E Low. Syst. Nanostruct. 94 (2017) 126–131. [13] A. Kamalaveni, M. Ganesh Madhan, Halve dumbbell shaped DGS tapered ring antenna for dual-band notch characteristics, Electromagnetics 38 (3) (2018) 189–199. [14] Y.C. Lee, J.S. Sun, Compact printed slot antennas for wireless dual and multi-band operations, Prog. Electromagn. Res. B Pier B 88 (2008) 289–305. [15] Meisam Esfandiyari, Saughar Jarchi, Mohsen Ghaffari-Miab, Channel capacity enhancement by adjustable graphene-based MIMO antenna in THz band, Opt. Quantum Electron. 51 (5) (2019) 137. [16] Satish Kumar Sharma, Lotfollah Shafai, N. Jacob, Investigation of wide-band microstrip slot antenna, IEEE Trans. Antennas Propag. 52 (3) (2004) 865–872. [17] Ashish Y. Pawar, Deepak D. Sonawane, Kiran B. Erande, Deelip V. Derle, Terahertz technology and its applications, Drug Invent. Today 5 (2) (2013) 157–163. [18] Ritesh Kumar Kushwaha, P. Karuppanan, Yogesh Srivastava, Proximity feed multiband patch antenna array with SRR and PBG for THz applications, Optik 175 (2018) 78–86. [19] Sirisha Mrunalini, Arun Manoharan, Dual-band re-configurable graphene-based patch antenna in terahertz band for wireless network-on-chip applications, Iet Microw. Antennas Propag. 11 (14) (2017) 2104–2108. [20] M. Shalini, M. Ganesh Madhan, Design and analysis of a dual-polarized graphene based microstrip patch antenna for terahertz applications, Optik 194 (2019) 163050. [21] Gaurav Bansal, Amanpreet Singh, Rajni Bala, A triband slotted bow-tie wideband THz antenna design using graphene for wireless applications, Optik 185 (2019) 1163–1171.
8
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Performance predictions of slotted graphene patch antenna for multi-band operation in terahertz regime
T
Shalini M*, Ganesh Madhan M Department of Electronics Engineering, MIT Campus, Anna University, Chennai, India
A R T IC LE I N F O
ABS TRA CT
Keywords: Graphene patch Surface plasmon polaritons Multi-band antenna Defected ground structure
A compact multi-band graphene based patch antenna is proposed for THz applications. A single band antenna is initially designed without slot, where the lower resonant mode of the antenna operates only at single frequency of 1.9 THz with a bandwidth of 50 GHz. Dual-bands are realized by introducing slots in the graphene patch. The proposed structure makes the antenna simple in design and easy for fabrication by employing slots in the graphene patch. By adjusting the position of the slot in the patch, the antenna is made to radiate at dual frequencies of 1.96 THz and 4.83 THz with bandwidths of 80 GHz and 100 GHz respectively. FDTD based EM simulation predicts a return loss of -34 dB and-38 dB at 1.96 THz and 4.83 THz respectively and VSWR less than 1.5 at both frequency bands. Moreover, the antenna provides a significant gain of 4.75 dB and 4.3 dB over the operating bands and efficiency greater than 92 % is observed. Further, the antenna is made to operate at triple bands of 1.96 THz, 4.83 THz and 5.55 THz by introducing defects in the ground plane.
1. Introduction Terahertz spectra have received considerable attention due to increase in the demand for broadband wireless communication. Other applications include imaging, spectroscopy, bio-sensing, radar and security. In order to utilize the available spectrum efficiently, communication systems require antenna with compact size and multi-band characteristics. Dual-band antennas have the ability to provide strong and stable wireless connection especially in mobile applications. Two separate antennas are required, when two operating frequencies are far apart [1]. In such cases, a single dual-band antenna can be used to operate at two resonant frequencies, thereby reducing the number of antennas. Instead of using different antennas for each band, dual-frequency antenna can be employed to meet the requirements. This helps in reducing the space and weight while integrating with MMIC. Microstrip antennas are also widely desirable to have multiband characteristics. Several dual-band antennas have been proposed in RF/Microwave frequency range of the Electromagnetic spectrum using stacked patch [1], meta-materials [2], parasitic patch [3], slots in the ground plane [4], exciting various cavity modes on the asymmetric patch [5]. Dual-band antenna using stacked patch is implemented using several layers which leads to large antenna size. Parasitic patch dual-band antenna is implemented by forming four parasitic patches underneath a driven patch. However, this multi-stacked structure affects the coupling between the parasitic elements and decreases the efficiency of the antenna. Modal expansion technique is used to excite various cavity modes in order to achieve multiple bands. But, this technique is not desirable because the excited modes will have different polarization, different field and several feeding mechanisms are required to excite various modes. These RF/microwave antennas are radiated using copper conductor. However,
⁎
Corresponding author. E-mail address: [email protected] (S. M).
https://doi.org/10.1016/j.ijleo.2020.164223 Received 15 November 2019; Received in revised form 10 January 2020; Accepted 13 January 2020 0030-4026/ © 2020 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
S. M and G.M. M
terahertz antennas cannot be effectively implemented using copper, since it suffers from serious attenuation due to low conductivity and low mobility at terahertz frequencies [6]. Graphene is considered to be an efficient material for terahertz radiation, since it has high conductivity, high mobility and also supports surface plasmonic propagation. Graphene based dual-band antennas have been developed using several techniques. In [7], Dual band reconfigurable THz patch antenna is proposed with graphene stack based backing cavity. Beam steering is achieved by changing the applied voltage on the graphene stack. However, it has cavity in the dielectric substrate which will have its effect on the surface waves and return loss. Switches are incorporated in the reconfigurable antenna, thereby increasing the complexity of the antenna structure. Optically transparent dual band antenna using ITO (Indium Tin Oxide) has been developed for THz communication [8]. It is excited by aperture coupled feed. Use of transparent conducting material such as ITO will cause impedance mismatch and it is also expensive [9]. WCIP (Wave Concept Iterative Process) method is used to develop an antenna for dual-band THz operation in [10]. This method requires convergence study, which involves large number of iterations and increases the computational time. Graphene based antennas are developed for RFID applications [11]. However, it operates at a single resonant frequency. Dual band graphene antenna is also developed by varying the thickness of the substrate [12]. Meanwhile, surface wave losses increase thereby reducing radiation characteristics and bandwidth of the antenna, due to the excitation of higher order modes. Most of the antennas reported in the literature includes multiple-stacked patches, multilayer substrates, complicated matching network and single frequency antennas, which limits the application of antenna to multi-frequency operation. In this work, dual-band characteristics are achieved in the graphene antenna by etching a rectangular slot in the radiating patch element. Embedding slot in the radiator is one of the most robust and effective method of exciting various modes in the antenna to produce multi-band operation. This technique also improves the impedance characteristics of the antenna. Defected ground structure is used to obtain multi-band notch characteristics in microwave regime [13]. Further, the antenna is made to operate at triple bands of 1.96 THz, 4.83 THz and 5.55 THz by introducing defects in the ground plane. Therefore, considering the demand for simple and an effective multi-band antenna in terahertz region, graphene based antenna is proposed which consists of a square shaped slot on the radiator to achieve dual band operation and defected ground structure is used to obtain triple bands. Moreover, miniaturization is also be obtained by etching slots in the antenna and it can be economical. Compact design and multi-band operation are required for future wireless applications [14]. This work presents a novel miniaturized slotted tri-band antenna for Terahertz applications. High data rate can be achieved using terahertz band and implemented using graphene based antennas, whose chemical potential can be adjusted to achieve desirable tuning characteristics [15]. Multiband antennas can be used to improve the bandwidth, capacity and data rate of terahertz communication system. Slots are used in the antenna design for bandwidth enhancement, miniaturization and to produce multi-frequency operation with desirable tuning characteristics. Slot antenna is preferred because of its simple structure [16]. Square-shaped slot is used to have low profile. Terahertz spectrum are widely used in imaging, spectroscopy and bio-medical applications due to its unique characteristics. Terahertz wave has low photon energy, non-ionizing and it doesn't cause any harmful effects to human body. Terahertz spectroscopy can be used in pharmaceutical applications such as chemical analysis of tablets to detect the defects present in the tablet coating, industrial applications such as product inspection for identification of faults and medical applications such as cancer detection [17]. A number of single band antennas are reported in terahertz frequency range. However, designing a multi-band antenna in terahertz frequency range is a challenging task. Metamaterials, Photonic band gap structure and Split Ring Resonators are used to design multi-band antennas [18]. These structures involve tedious procedures, which makes the antenna design more complicated. Hence, to overcome these limitations, a simple structure is proposed using graphene material for multi-band operation in terahertz regime. The paper is organized as follows. Evolution of dual band Graphene antenna and the effect of slot on antenna characteristics are illustrated in Section III. The impact of defects in the ground plane is discussed in Section IV. 2. Evolution of dual-band graphene antenna 2.1. Single band antenna The basic antenna is designed using conventional microstrip patch radiator without embedding slot in it, for single resonant frequency of 1.9 THz. The substrate used is Silicon-di-oxide (Silica) with dielectric constant of 3.9 and a thickness of 30 μm. An inset feed microstrip line is chosen to excite the antenna for impedance matching. Graphene is used to model the patch radiator for efficient transmission of terahertz waves. The conductivity of mono-layer graphene should be determined to know about the propagation of surface plasmons, which generates terahertz waves. Kubo's formula gives the approximation for calculating Drude-like intraband contribution of conductivity, as it dominates below 5 THz [20]. It is given by,
σintraband (S ) = j
2kT0 eq2 ℏ(j г−1ℏ + ℏω)
μ ⎞ 2 lncosh ⎛ ⎝ 2kT0 ⎠ ⎜
⎟
(1)
J/K is the Boltzmann constant, eq is the electron charge, ℏ is the reduced Plank’s constant, μ is the chemical where k = 1.38 x potential, T0 is the temperature and г is the relaxation time.
10−23
Г=
μn eq. vF2
(2)
where, vF is the Fermi velocity whose magnitude is 106m / s and n = 10 4cm2V −1s−1 is the electron mobility [20]. The volume conductivity of graphene is obtained by dividing the surface conductivity by the thickness of graphene (0.34 nm) 2
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
S. M and G.M. M
Fig. 1. Return loss characteristics of the antenna without slot.
[20].
σintraband (V) =
σintraband (S) thickness of patch
(3)
Standard design procedure is followed to calculate the length (pL) and width (p W ) of patch antenna [12].
c
pL =
2f 0
pW =
εe =
c 2f 0
p ⎡ (ε e + 0.3) hw + 0.264 − 2 ⎢0.412 t Wp ⎢ εe (ε e − 0.258) h + 0.8 ⎣
(
(
) ⎤⎥
) ⎥⎦
(4)
εr + 1 2
(5)
⎛ (εr + 1) (ε − 1) 1 + r ⎜ 2 2 ⎜ 1 + 12 pt W ⎝
⎞ ⎟ ⎟ ⎠
(6)
where, pL is the length of the patch and p W is the width of the patch, t is the thickness of the substrate and εe denotes the effective dielectric constant. The geometry and return loss of the graphene antenna without slot is shown in Fig. 1. The resonance occurs at only a single frequency of 1.9 THz and the maximum return loss is about 14 dB. The bandwidth is found to be 50 GHz. 2.2. Dual-band operation The single band antenna is modified by embedding slot in the radiating patch for dual-band operation. The dimension of slot is chosen as one-fourth of guided wavelength, λ g . It is given by,
Fig. 2. Configuration of the proposed dual-band antenna. 3
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
S. M and G.M. M
Table 1 Dimensions of the proposed antenna. Parameter
Dimensions (μm)
Length of the patch (pl) Width of the patch (pw) Length of the substrate (LS) Width of the substrate(WS) Height of the substrate (t) Width of inset-feed gap (a) Length of inset-feed gap (b)
20 25 50 50 30 1 5
Fig. 3. Effect of slot on return loss characteristics.
Fig. 4. a) E-plane and H-plane characteristics at 1.96 THz. b) E-plane and H-plane characteristics at 4.83 THz.
λg =
c fc
(7)
εe
where, fc is the cut-off frequency and εe is effective dielectric constant. The configuration of the proposed antenna with square shaped slot in the patch is shown in Fig. 2. The position of slot is determined such that it matches the feed impedance. The propagation of SPP (Surface plasmon polaritons) waves in the metal4
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
S. M and G.M. M
Fig. 5. a) 3D Radiation pattern of dual-band antenna at 1.96 THz. b) Gain of the proposed dual-band antenna.
dielectric interface makes the antenna to resonate at first frequency; the slot in the antenna introduces the second resonant frequency. This makes the antenna to radiate at two resonant frequencies (1.96 THz and 4.83 THz respectively). The dimensions of the antenna are shown in Table 1. As described, additional resonance is created by introducing slot, thereby the antenna radiates at second frequency (4.83 THz). Embedding slot in the radiator varies the electrical length of patch, thereby producing additional resonance at 4.83 THz. When the slot length (s) is varied, it is found to have minimal effect on first resonance frequency and the second resonant frequency is found to 5
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S. M and G.M. M
Table 2 Comparison of dual-band antenna characteristics with similar design. Techniques
Operating frequencies(THz)
Return loss(dB)
Gain(dBi)
Radiation efficiency(%)
Graphene based stacked cavity [7]
4.46 7 0.75 1.1 2.5 5.061 2.47 3.35 2.17 2.59 1.96 4.83
−42 −30 −22 −28 −26.57 −28.52 −17 −22 −28 −35 −34 −38
Not mentioned
Not mentioned
7.33 10.3 Not mentioned
88.4 91.2 Not mentioned
2.7 6.03 4.01 5.03 4.75 4.3
87.3 53.47 64.12 60.8 93.7 95.3
ITO based dual band antenna [9] Wave concept iterative process [10] Varying substrate height for dual-band operation [12] Dual-band reconfigurable graphene based patch antenna [19] Proposed work- Slot induced dual-band antenna
Fig. 6. Return loss for various h.
Table 3 Modified dimensions of triple band antenna. Parameter
Dimensions (μm)
Reduced ground length(h) slot in ground(g)
10 5
shift towards right. The resonant frequencies of the antenna also decrease with increase in length of loaded square slot. The effect of slot length is shown in Fig. 3. The optimum dimension of slot length is found to be 6 μm. The Voltage Standing Wave Ratio (VSWR) for the proposed dual band antenna using graphene radiator are 1.12 at 1.96 THz and 1.16 at 4.83 THz. E-plane and H-plane characteristics of the antenna are shown in Fig. 4(a) and (b). The main-lobe magnitude is around 4.7 dB and the angular width is 90° for 1.96 THz. The main lobe magnitude is around 4 dB and angular width is 121° for 4.83 THz. 3D radiation pattern is shown in Fig. 5(a). The proposed antenna achieves peak gain of 4.75 dB and 4.3 dB at 1.96 THz and 4.83 THz respectively, as shown in Fig. 5(b). The path of the surface current flows around the slot in the patch and the length of current path are increased. The proposed dual-band antenna has been compared with the existing literature in Table 2. The antenna is implemented with simple technique compared to the existing methods as shown in the table. Further, the proposed antenna achieves higher efficiency with better return loss compared to other techniques. 3. Design of triple band antenna using defected ground structure The performance of the dual band graphene antenna is investigated by introducing defects in the ground plane. When the length 6
Optik - International Journal for Light and Electron Optics 204 (2020) 164223
S. M and G.M. M
Fig. 7. Return loss curve and configuration (back view of antenna) of triple band operation.
Fig. 8. VSWR curve of triple band antenna. Table 4 Parameters of triple band antenna. Triple band frequencies (THz)
Return loss(dB)
VSWR
Gain(dBi)
Radiation efficiency (%)
1.9551 4.83 5.4474
−26.329 −24.849 −19.619
1.1207 1.1412 1.2326
4.79 5.05 5.53
93.6 95.9 98.08
of the ground plane is reduced by a value of h, only minimal effect is observed on first resonant frequency and the second resonant frequency gets shifted towards right as shown in Fig. 6. Furthermore, as the ground plane length is reduced to 40 μm, a square shaped slot is inserted in the center of the ground plane for triple band operation. This slot disturbs the current distribution in the ground plane by offering some capacitance and inductance, which produces additional resonance at 5.5 THz. The value of inductance (L) and capacitance (C) depends on resonant frequency (f0) and cut-off frequency (fc). The design equations are given by,
L=
C=
1 4 π 2f02 C
(8)
fc 2Z0(f 2 − f 2 ) 0
(9)
c
7
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S. M and G.M. M
Hence, the antenna is modified to operate at 1.96 THz, 4.83 THz and 5.5 THz leading to triple band operation. The dimensions of the proposed triple band antenna are shown in Table 3. The configuration of the proposed tri-band antenna (back view) and its S11 results are shown in Fig. 7 and its corresponding VSWR are shown in the Fig. 8. Table 4 gives the information on various parameters of triple band antenna. The antenna has a peak gain of 4.79dBi, 5.05dBi and 5.53 dBi at 1.9551 THz, 4.83 THz and 5.4474 THz respectively. It also achieves radiation efficiency greater than 92 % at all three frequencies. Gaurav et al. [21] has reported a triband bow-tie terahertz antenna operating at 7.1 THz, 11.1 THz and 13.1 THz. This antenna uses very thin substrate, which is not suitable for terahertz communication, as it will break down when it is excited by plane wave. The efficiency obtained is only 75 % which is less than the proposed tri-band antenna (> 92 %). 4. Conclusion A single element microstrip antenna using graphene is proposed for multi-frequency operation at Terahertz band. This is achieved by embedding slot in the radiating patch to excite additional resonant modes in the antenna. The proposed antenna resonates at dual frequencies of 1.96 THz and 4.83 THz respectively. Return loss better than -30 dB and VSWR less than 1.5 are achieved at both the frequency bands. The antenna achieves significant gain of 4.75 dB and 4.3 dB at 1.96 THz and 4.83 THz respectively. Further, the antenna is made to operate at triple bands of 1.96 THz, 4.83 THz and 5.5 THz, by embedding defects in the ground plane and efficiency greater than 92 % is obtained. Thus, a single element antenna is designed with compact size, which offers multiband performance at terahertz frequency range. 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