- Email: [email protected]
Broadband µ-negative antenna using ELC unit cell
Journal Pre-proofs Regular paper Broadband µ-negative antenna using ELC unit cell R. Samson Daniel PII: DOI: Reference:
S1434-8411(19)33128-0 https://doi.org/10.1016/j.aeue.2020.153147 AEUE 153147
To appear in:
International Journal of Electronics and Communications
Received Date: Revised Date: Accepted Date:
20 December 2019 18 February 2020 26 February 2020
Please cite this article as: R. Samson Daniel, Broadband µ-negative antenna using ELC unit cell, International Journal of Electronics and Communications (2020), doi: https://doi.org/10.1016/j.aeue.2020.153147
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier GmbH.
Broadband µ-negative antenna using ELC unit cell R. Samson Daniel 1, 1,Assistant
Professor, Department of ECE, K. Ramakrishnan College of Engineering, Samayapuram, Tiruchirappalli, India
[email protected]
Abstract A broadband printed monopole antenna is designed by using ELC (Electric Inductive– Capacitive) based metamaterial element for WLAN applications. The proposed antenna comprises single ring hexagon loaded with ELC metamaterial for broadband radiation. ELC is designed with a square loop nested inside the ring hexagon structure which is coupled by the small metal stub. By introducing ELC, it significantly improves the impedance bandwidth of the antenna due to unique EM wave property. The band characteristics of the ELC and its metamaterial property (negative permeability) are explained through waveguide setup method. The designed antenna involves with dimensions 30 × 30 × 0.8mm3 and fabricated on a low-cost Flame Retardant-4 substrate having εr=4.4 and tan δ=0.02. It yields an impedance bandwidth of 2600 MHz (2.61 ― 5.21 GHz) with a dual resonance of 3.21 GHz and 4.89 GHz, which is usable for WiMAX, C-band and WLAN applications. The measured far-filed pattern exhibits a stable pattern. The proposed antenna has many benefits, such as comprising compact size, simple structure, lower S11 (dB) and capable of broadband characteristics.
Key words Broadband, ELC, Negative Permeability, WLAN.
*Corresponding Author Email Address: R. Samson Daniel: [email protected]
1. Introduction In recent scenario, broadband antennas are very essential to replace many antennas by a single antenna. The design of a miniaturized broadband antenna plays significant role in modern wireless devices due to small size and cost effective which covers all the specified frequencies [1]. Different design techniques have been observed in the design of broadband antenna such as symmetrical stubs [2], parasitic patch [3, 4], fractal structure [5] and bowtie dipole [6]. Metamaterial-inspired antennas have been emerging as an essential contributor in recent wireless communication. It yields multiband [7], broadband [8] and high gain [9] attributes along with miniaturization. These features can be revealed using unique electromagnetic wave properties of metamaterials such as ZOR (Zeroth Order Resonance), negative constitutive parameters (µ and ε ) and antiparallel phase and group velocity [10]. Metamaterials have been used in designing electromagnetic band-gap resonators [11], tilting antenna beam [12], reconfiguring antennas [13], artificial magnetic conductors, phase-correcting surfaces and beam steering surfaces [14]. ELC based metamaterial can be used to design phase shifters [15] and circular polarization [16]. ELC loading is used to create multiband resonance characteristics along with compactness due to quasi-static resonant behavior [17]. The structural orientation of the ELC governs the uniform electric field for eliminating cross polarization [18] and designing electrically small antennas [19]. Computing metamaterials are beneficial for providing a method to develop recent technologies such as chip-based analog computers and integrable computing elements [20]. Several electromagnetic technologies have been proposed for achieving optimum performance of the device properties. However, it suffers large dimensions, limited response control and narrow bandwidth. These limitations can be overcome by metasurface properties. Field and circuit theory [21] computes the equivalent circuit model elements such as capacitance and inductance
value for realizing the resonance behavior. Metamaterial element governs electric and magnetic currents to generate wideband characteristics [22]. In this paper, a miniaturized broadband antenna is developed and presented. In order to obtain broad bandwidth, an ELC metamaterial is introduced. The proposed antenna has an electrical size of 0.32 λ0×0.32 λ0×0.0085 λ0, where λ0 is the free space wavelength at f0=3.21 GHz. The designed antenna offers -10 dB impedance bandwidth from 2.61 GHz to 5.21 GHz with 81% of fractional bandwidth at f1=3.21 GHz and 53% fractional bandwidth at f2=4.89 GHz. The proposed technique is useful to recognize devices for sensing and telecommunication applications based on the band nature of the ELC and its negative permeability characteristics. A simple and good approach is utilized to achieve broad bandwidth within miniaturized antenna compared with previously listed antennas [2, 4, 6, 9, 16, 18, and 19].
2. Proposed antenna geometry and analysis The design starts with a closed ring resonator in a hexagonal fashion (A) as shown in Fig.1. To obtain broadband radiation from this closed ring resonator, the ELC is created as shown in the designed antenna (B) of Fig.1. ELC is feasible to produce magnetic resonance for governing broad bandwidth of 2600 MHz according to the ELC geometrical parameters.
Fig.1. Proposed antenna geometry.
The proposed antenna and its equivalent LC tank circuit model [7] are shown in Fig.2. The equivalent circuit is described for 50Ω microstrip transmission line, hexagonal closed ring resonator and ELC. Here Lf is the electrical length of a microstrip transmission line, Z0 is 50Ω characteristic impedance, Rf is loss resistance of feed line, lumped elements (R1, L1,C1) of the closed ring resonator and Cc is gap capacitance between hexagonal closed ring resonator and ELC. The loop inductance (L2) of ELC is influenced by conducting loops and capacitance (C2) is influenced by dielectric gap. Due to the SRR based ELC configuration, the inductance is coupled to the magnetic resonance and the capacitance is coupled to the electric resonance. Thus the proposed ELC is capable of attaining broadband radiation. The configuration of the designed antenna is depicted in Fig.3 and its geometrical values are given in Table.1.
Fig.2. Proposed antenna and its equivalent LC tank circuit model.
Fig.3.Designed antenna configuration. Table.1 Geometrical values of the proposed antenna
Parameters Ws Ls Wf Lf Lg S W1 L1 W2 L2 S1
Dimensions (mm) 30 30 1.6 7.61 5.25 14.6 14 10 4.5 3 1
Simulations are executed using Ansys HFSS V.14.0 EM software package. The simulated S11(dB) characteristics of the hexagonal closed ring resonator and proposed antenna are exposed in Fig.4. It is perceived that the hexagonal closed ring resonator produces a narrow resonance at 2.87 GHz with an impedance bandwidth of 1080 MHz (2.42 ― 3.5 GHz). This resonance frequency has been estimated using [17]
fr=
1.8412 × c 2πS εr
=
1.8412 × 3 × 108 2 × π × 14.6 × 10 ―3 × 4.4
= 2.87 GHz
(1)
Here εr is the dielectric constant of the substrate, c is the velocity of light and S represents the side length of the hexagonal closed ring. The proposed antenna covers an impedance bandwidth of 2600 MHz (2.61 ― 5.21 GHz) with a dual resonance of 3.21 GHz and 4.89 GHz. Thus the ELC based metamaterial can be increased an impedance bandwidth of 81% with respect to subwavelength magnetic resonance. After inserting ELC based metamaterial, the resonance frequency of the hexagonal closed ring resonator (at 2.87 GHz) is changed to an upper frequency of 3.21 GHz and also generates a new resonance at 4.89 GHz along with broadband radiation. Thus, the resonance frequency of
3.21 GHz is due to the conventional radiating element and 4.89 GHz is due to ELC. This ELC resonance frequency is computed by [8] fELC = 2π Where CELC =
C0 =
1
(2)
LELCCRELC
N―1 2
[2L-(2N-1) (W+S)]C0
K( 1 ― K2) ε0 K(k)
S
and k =
W+
LELC = 4µ0[L-(N-1) (S+W)] [ln (
2 S 2 0.98 ρ
) +1.84𝝆]
(N ― 1)(W + S)
𝝆 = 1 ― (N ― 1)(W + S) Here, ELC width (W), ELC length (L), number of radiating element (N), ELC dielectric gap (S) and K is the complete elliptic integral of the first kind identity K(𝑘). These empirical design equations are evaluated by a MATLAB code to compute the resonance frequency of ELC with respect to the inductance and capacitance values of ELC, which are given as, LELC = 1.9993×10 ―08 (Henry) CELC = 5.2833×10 ―14 (Farad) Hence, the ELC resonance frequency is 𝑓𝐸𝐿𝐶=
1 2𝜋 𝐿𝐸𝐿𝐶 𝐶𝐸𝐿𝐶
= 4.8994 GHz. From this lumped
circuit model, it is perceived that proposed ELC creates a resonance around 4.89 GHz. The evaluation of proposed antenna with existing designs shown in reference is represented in Table 2. From Table 2, it is examined that the proposed antenna contributes broad bandwidth with compact size compared with previously reported papers. This paper emphasizes LC tank circuit analysis and band characteristics along with µ-negative for generating broad bandwidth.
Table 2. Evaluation of proposed antenna with existing designs. Reference
Patch Detail
[2]
Circular patch with symmetrical stubs Grid-slotted radiation patch Bowtie dipole with parasitic cross-slots Rectangular patch and CSRR Fractal Koch Slot Antenna with ELC ELC metamaterial resonator
[4]
[6]
[9]
[16]
[18]
Substrate Dimensions L×W (mm2) 30 × 30
Impedance Bandwidth (MHz)
Equivalent Metamaterial Circuit Analysis property verification
2100 MHz (1.9 ― 4 GHz)
Not Analyzed
Not Verified
78 × 92
1000 MHz (3.31 ― 4.31 GHz)
Not Analyzed
Not Verified
55 × 55
4070 MHz (2.39 ― 6.46 GHz)
Not Analyzed
Not Verified
40 × 46
100 MHz (2.4 ― 2.5 GHz)
Not Analyzed
Not Verified
40 × 40
Bandwidth Percentage: 1.9%, 14% and 5% 1240 MHz (3.28 ― 4.52 GHz) and 670 MHz (5.20 ― 5.87 GHz) 60 MHz (2.49 ― 2.55 GHz), 680 MHz (3.0 ― 3.68 GHz), and 1010 MHz (5.03 ― 6.04 GHz). 2600 MHz (2.61 ― 5.21 GHz)
Not Analyzed
Not Verified
Not Analyzed
Not Verified
Not Analyzed
Not Verified
Analyzed
Verified
21 × 30
[19]
Monopole radiator with ELC
35 × 35
Proposed antenna
ELC is nested inside the ring hexagon structure
30 × 30
Fig.4. Simulated S11(dB) characteristics of the hexagonal closed ring resonator and proposed antenna. 3. Operating Mechanism The hexagonal closed ring resonator supports the fundamental radiating mode TM10 at 2.87 GHz, which is estimated using cavity model by [23] 1
𝑓𝑟(𝑚𝑛)= 2𝜋
𝜇𝜀
𝑚𝜋 2 𝐿
𝑛𝜋 2 𝑊
( ) ( ) +
Here 𝜀=𝜀0 𝜀𝑒𝑓𝑓 𝜇 = 4π×10 ―7 h/m 𝜀0= 8.8419×10 ―12 F/m m=0, 1, 2….; n=0, 1, 2……. L=W=Side length of the hexagonal closed ring resonator
(3)
Where 𝜀𝑒𝑓𝑓 is effective dielectric constant and L=W=14.6 mm. Real and imaginary parts of the proposed antenna input impedance are illustrated in Fig. 5. It has two resonant modes TM10 (at 3.21 GHz) and TM12 (at 4.89 GHz) in the operating band. Due to ELC loading, the fundamental radiating mode TM10 (at 2.87 GHz) gets affected and produces the modified modes TM10 and TM12 towards the upper frequencies at 3.21 GHz and 4.89 GHz, respectively. These modified modes are responsible for generating dual resonance characteristics in the wider impedance bandwidth.
Fig. 5. Input resistance and reactance of the proposed antenna.
4. Parameter extraction of ELC To validate the results, the ELC band characteristics are discussed in this section. The effective medium theory is employed to examine the S-parameters for evaluating, reflection
(S11) coefficient and transmission (S21) coefficient of ELC [24]. The reflection coefficient induces pass band behavior with respect to band pass filter for producing a broadband radiation
and transmission coefficient induces stop band behavior with respect to band stop filter. Fig.6 (a) represents the pass band (S11) and stop band (S21) behavior of ELC structure. It is inferred that antenna with ELC has stop band at 2.4 GHz and wider pass band characteristics. This pass band accounts for broadband radiation in the S11(dB) characteristics. An effective permeability (µ) of the ELC is illustrated
in
(b),
the
where
Fig.6
permeability (µ) becomes negative at 2.4 GHz due to S21(dB) characteristics of the ELC structure.
(a)
(b) Fig.6.(a) S-parameters (S11 and S21) of ELC, (b) Permeability of ELC. Fig.7 describes the reflection coefficient (S11) of the hexagonal closed ring resonator with ELC and without ELC. Here, metamaterial unit cell ELC is kept within the waveguide of proper axis, which is enclosed by the Perfect Magnetic Conductor (PMC) and Perfect Electric Conductor
(PEC)
boundary
conditions.
ELC produces
a magnetic
response by the
incident
wave from one
port and S-
parameter (S11)
is revealed
from
port of the
another
waveguide. It is observed that antenna without ELC has no wider pass band, but antenna with ELC has wider pass band characteristics. This pass band is responsible for generating a broadband radiation in the S11(dB) characteristics of the proposed antenna.
Fig.7.S11(dB) characteristics of hexagonal closed ring resonator with ELC and without ELC.
5. Parametric Study
The geometrical values of the antenna are optimized by examining S11(dB) characteristics of the microstrip feed width(Wf), length of the ground(Lg) and dielectric gap(S1). Simulated S11(dB) characteristics for various feed widths are shown in Fig.8. The feed width is varied from 0.8 mm to 1.6 mm with step size of 0.2 mm. As the feed width (Wf) increases, S11 (dB) values are improved. However, the optimum feed width Wf=1.6 mm is assigned for
50Ω
characteristic impedance to obtain better impedance matching. The length of the ground plane performs significant factor on antenna impedance and radiation properties. The ground plane length (Lg) is changed from 7.25 mm to 4.25 mm, and the corresponding S11(dB) plot is depicted in Fig.9. It is inferred that by decreasing the length of the ground plane, the bandwidth of the antenna is enhanced. Hence optimum length Lg= 5.25 mm is allocated for the proposed antenna.
Fig. 8. S11
(dB) of antenna for various feed width.
Fig.9. S11(dB) of antenna for different ground plane length.
Fig.10. S11(dB) of antenna for different dielectric gap.
Similarly, Fig. 10 represents the reflection coefficient of proposed antenna with dielectric gap(S1). It indicates that, as the dielectric gap increases from 0.25 mm to 1 mm, the bandwidth is also expanded gradually. However, the maximum possible value of S1=1 mm is selected to resonate from 2.61 GHz to 5.21 GHz. 6. Results and discussion The snapshot of the fabricated antenna is depicted in Fig.11. The S11(dB) of the antenna is measured using Anritsu Vector Network Analyzer MS46122B. Fig.12 describes the simulated and measured S11(dB) characteristics of the proposed antenna. The evaluation of wider band characteristics is exhibited in Table.3 numerically, both data equivalent to each other. Fig.13 illustrates the far-field radiation pattern in the elevation plane and azimuthal plane over the impedance bandwidth. The behavior in the YZ plane (E-Plane) is a dipole pattern and XZ plane (H-plane) is an omnidirectional pattern. Thus the required characteristics are observed in E-plane due to an electric field in Y-direction and H-plane due to a magnetic field in Z-direction. Both Eplane (ϕ = 90°) and H-plane (ϕ = 0°) exhibits maximum radiation in desired directions.
The gain of the antenna has been measured using gain transfer method [8]. The proposed antenna gain plot is shown in Fig.14. The measured peak gains of 3.44 dBi and 3.61 dBi are realized at 3.12 GHz and 4.64 GHz respectively. This antenna design, the ELC is used as the main radiating element, which enables miniaturization. When the antenna is miniaturized, its radiation characteristics are affected, leading to a reduction in the gain.
The proposed antenna has focused on the bandwidth improvement of hexagonal patch antenna only. The proposed technique can also be useful for other radiating structures. In future, the proposed antenna can also be carried out with CSRR based metamaterial structures.
Fig.11.Snapshot of the fabricated antenna (a) Top view (b) Bottom view.
Fig. 12. S11(dB) characteristics of the proposed antenna. Table 3 Comparison of performance of the antenna Proposed antenna
Operating Frequency (GHz)
S11(dB)
Impedance bandwidth (MHz)
3.2
-36
2600 MHz (2.61 ― 5.21 GHz)
4.9
-46
3.12
-47
4.64
-47
Simulated
Measured
2400 MHz (2.63 ― 5.03 GHz)
Fig. 13.Far-field radiation patterns (a) 3.12 GHz (b) 4.94 GHz.
Fig. 14 Gain plot of the proposed antenna.
7. Conclusion An electrically small broad bandwidth antenna based on ELC metamaterial is designed using cavity model analysis and effective medium theory. It reveals that ELC metamaterial is substantial in addition to the hexagonal closed ring resonator, to produce a broad bandwidth without increasing the size of the substrate. Impedance bandwidth is increased from 38% to 81%. The prototype antenna has an electrical size of 0.32 λ0×0.32 λ0×0.0085 λ0.The prototype antenna has been fabricated and its characteristics are measured. The experimental data covers the 3 GHz WiMAX, 4 GHz C-band and 5 GHz WLAN applications.
References [1] B. Murugeshwari R. Samson Daniel S. Raghavan. A compact dual band antenna based on metamaterial‑inspired split ring structure andhexagonal complementary split‑ring resonator for ISM/WiMAX/WLAN applications. Applied Physics A: Materials Science and Processing2019; 125:628. [2] Immaculate S, Singaravalu Raghavan. Split ring loaded broadband monopole with SAR reduction. Microw Opt Technol Lett 2015; 58(1):158–62. [3] PratapN.Shinde, Jayashree P. Shinde. Design of compact pentagonal slot antenna with bandwidth enhancement for multiband wireless applications. AEU Int J Electron C 2015; 69:1489–94. [4] BoCheng, ZhengweiDu, Daiwei Huang. A broadband low-profile multimode microstrip antenna. IEEE Antennas Wirel Propag Lett 2019; 18:1332–36. [5] A. Gorai, R. Ghatak. Utilization of Shorted Fractal Resonator topology for high isolation and ELC resonator for band suppression in compact MIMO UWB Antenna. AEU Int J Electron C 2019; 113:152978. [6] Guirong Feng, Lei Chen, Xinwei Wang, Xingsi Xue, Xiaowei Shi. Broadband Circularly Polarized Crossed Bowtie Dipole Antenna Loaded with Parasitic Elements. IEEE Antennas Wirel Propag Lett 2018; 17:114–17. [7] Samson Daniel R, Pandeeswari R, Raghavan S. A compact metamaterial loaded monopole antenna with offset-fed microstrip line for wireless applications. AEU Int J Electron C 2018; 83:88–94.
[8] Samson Daniel R, Pandeeswari R, Raghavan S. Offset-fed Complementary Split Ring Resonators loaded monopole antenna for multiband operations. AEU Int J Electron C 2017; 78:72–8. [9] Pandeeswari R, Raghavan S. Microstrip antenna with complementary split ring resonator loaded ground plane for gain enhancement. Microwave Opt Technol Lett 2015; 57(2):292– 96. [10] Christophe Caloz, Tatsuo Itoh. Electromagnetic metamaterials: Transmission line theory and microwave applications. New York: John Wiley & Sons, Inc.; 2006. [11] Ali Lalbakhsh, Muhammad U.Afzal, Karu P.Esselle, Stephanie L.Smith, and Basit A. Zeb. Single-dielectric Wideband Partially Reflecting Surface with Variable Reflection Components for Realization of a Compact High-gain Resonant Cavity Antenna. IEEE Transactions on Antennas and Propagation 2019; 67(3): 1916–21. [12] Muhammad U. Afzal, Karu P.Esselle, Ali Lalbakhsh. A Metasurface to Focus Antenna Beam at Offset Angle. URSI Atlantic Radio Science Meeting (AT-RASC) 2018; DOI: 10.23919/URSI-AT-RASC.2018.8471483. [13] Priyanka Das, Kaushik Mandal, Ali Lalbakhsh. Single-layer polarization-insensitive frequency selective surface for beam reconfigurability of monopole antennas. Journal of Electromagnetic Waves and Applications 2020; 34(1): 1–18. [14] Muhammad U.Afzal, Ali Lalbakhsh, Karu P.Esselle. Electromagnetic-wave beam-scanning antenna using near-field rotatable graded-dielectric plates. Journal of Applied Physics 2018; 124(23): 234901-11.
[15] A. Hamidia, M. Amin Honarvar, Bal S. Virdee. Multi-way and poly-phase wideband differential phase shifter based on metamaterial technology. AEU Int J Electron C 2019; 110:152872. [16] HosseinRajabloo, VahidAmiriKooshki, HomayoonOraizi. Compact microstrip fractal Koch slot antenna with ELC coupling load for triple band application. AEU Int J Electron C 2016; 73:144–49. [17] R. Samson Daniel, R. Pandeeswari, S. Raghavan. Dual-band monopole antenna loaded with ELC metamaterial resonator for WiMAX and WLAN applications. Applied Physics A: Materials Science and Processing 2018;124:570. [18] R. Boopathi Rani, S.K. Pandey. ELC metamaterial based CPW-fed printed dual-band antenna. Microwave Opt Technol Lett 2017;59: 304–07. [19] Ke Li, Cheng Zhu, Long Li, Yuan-Ming Cai, Chang-Hong Liang. Design of electrically small metamaterial antenna with ELC and EBG loading. IEEE Antennas Wirel Propag Lett 2013; 12: 678–81. [20] Nasim Mohammadi Estakhri, Brian Edwards, Nader Engheta. Inverse-designed metastructures that solve equations. Science 2019; 363(6433): 1333–38. [21] Luigi La Spada. Metasurfaces for Advanced Sensing and Diagnostics. Sensors 2019; 19 (2), 355: 1–15. [22] Luigi La Spada1, Lucio Vegni. Metamaterial-based wideband electromagnetic wave absorber. Optics Express 2016; 24 (6): 5763–72. [23] A. Ghosal, S. Kumar Das, A. Das. Multifrequency Rectangular Microstrip Antenna with array of L-slots. AEU Int J Electron C 2019; 111:152890.
[24] Samson Daniel R, Pandeeswari R, Raghavan S. A miniaturized printed monopole antenna loaded with hexagonal complementary split ring resonators for multiband operations. Int J RF Microw Comput Aided Eng. 2018; e21401.
S1434-8411(19)33128-0 https://doi.org/10.1016/j.aeue.2020.153147 AEUE 153147
To appear in:
International Journal of Electronics and Communications
Received Date: Revised Date: Accepted Date:
20 December 2019 18 February 2020 26 February 2020
Please cite this article as: R. Samson Daniel, Broadband µ-negative antenna using ELC unit cell, International Journal of Electronics and Communications (2020), doi: https://doi.org/10.1016/j.aeue.2020.153147
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier GmbH.
Broadband µ-negative antenna using ELC unit cell R. Samson Daniel 1, 1,Assistant
Professor, Department of ECE, K. Ramakrishnan College of Engineering, Samayapuram, Tiruchirappalli, India
[email protected]
Abstract A broadband printed monopole antenna is designed by using ELC (Electric Inductive– Capacitive) based metamaterial element for WLAN applications. The proposed antenna comprises single ring hexagon loaded with ELC metamaterial for broadband radiation. ELC is designed with a square loop nested inside the ring hexagon structure which is coupled by the small metal stub. By introducing ELC, it significantly improves the impedance bandwidth of the antenna due to unique EM wave property. The band characteristics of the ELC and its metamaterial property (negative permeability) are explained through waveguide setup method. The designed antenna involves with dimensions 30 × 30 × 0.8mm3 and fabricated on a low-cost Flame Retardant-4 substrate having εr=4.4 and tan δ=0.02. It yields an impedance bandwidth of 2600 MHz (2.61 ― 5.21 GHz) with a dual resonance of 3.21 GHz and 4.89 GHz, which is usable for WiMAX, C-band and WLAN applications. The measured far-filed pattern exhibits a stable pattern. The proposed antenna has many benefits, such as comprising compact size, simple structure, lower S11 (dB) and capable of broadband characteristics.
Key words Broadband, ELC, Negative Permeability, WLAN.
*Corresponding Author Email Address: R. Samson Daniel: [email protected]
1. Introduction In recent scenario, broadband antennas are very essential to replace many antennas by a single antenna. The design of a miniaturized broadband antenna plays significant role in modern wireless devices due to small size and cost effective which covers all the specified frequencies [1]. Different design techniques have been observed in the design of broadband antenna such as symmetrical stubs [2], parasitic patch [3, 4], fractal structure [5] and bowtie dipole [6]. Metamaterial-inspired antennas have been emerging as an essential contributor in recent wireless communication. It yields multiband [7], broadband [8] and high gain [9] attributes along with miniaturization. These features can be revealed using unique electromagnetic wave properties of metamaterials such as ZOR (Zeroth Order Resonance), negative constitutive parameters (µ and ε ) and antiparallel phase and group velocity [10]. Metamaterials have been used in designing electromagnetic band-gap resonators [11], tilting antenna beam [12], reconfiguring antennas [13], artificial magnetic conductors, phase-correcting surfaces and beam steering surfaces [14]. ELC based metamaterial can be used to design phase shifters [15] and circular polarization [16]. ELC loading is used to create multiband resonance characteristics along with compactness due to quasi-static resonant behavior [17]. The structural orientation of the ELC governs the uniform electric field for eliminating cross polarization [18] and designing electrically small antennas [19]. Computing metamaterials are beneficial for providing a method to develop recent technologies such as chip-based analog computers and integrable computing elements [20]. Several electromagnetic technologies have been proposed for achieving optimum performance of the device properties. However, it suffers large dimensions, limited response control and narrow bandwidth. These limitations can be overcome by metasurface properties. Field and circuit theory [21] computes the equivalent circuit model elements such as capacitance and inductance
value for realizing the resonance behavior. Metamaterial element governs electric and magnetic currents to generate wideband characteristics [22]. In this paper, a miniaturized broadband antenna is developed and presented. In order to obtain broad bandwidth, an ELC metamaterial is introduced. The proposed antenna has an electrical size of 0.32 λ0×0.32 λ0×0.0085 λ0, where λ0 is the free space wavelength at f0=3.21 GHz. The designed antenna offers -10 dB impedance bandwidth from 2.61 GHz to 5.21 GHz with 81% of fractional bandwidth at f1=3.21 GHz and 53% fractional bandwidth at f2=4.89 GHz. The proposed technique is useful to recognize devices for sensing and telecommunication applications based on the band nature of the ELC and its negative permeability characteristics. A simple and good approach is utilized to achieve broad bandwidth within miniaturized antenna compared with previously listed antennas [2, 4, 6, 9, 16, 18, and 19].
2. Proposed antenna geometry and analysis The design starts with a closed ring resonator in a hexagonal fashion (A) as shown in Fig.1. To obtain broadband radiation from this closed ring resonator, the ELC is created as shown in the designed antenna (B) of Fig.1. ELC is feasible to produce magnetic resonance for governing broad bandwidth of 2600 MHz according to the ELC geometrical parameters.
Fig.1. Proposed antenna geometry.
The proposed antenna and its equivalent LC tank circuit model [7] are shown in Fig.2. The equivalent circuit is described for 50Ω microstrip transmission line, hexagonal closed ring resonator and ELC. Here Lf is the electrical length of a microstrip transmission line, Z0 is 50Ω characteristic impedance, Rf is loss resistance of feed line, lumped elements (R1, L1,C1) of the closed ring resonator and Cc is gap capacitance between hexagonal closed ring resonator and ELC. The loop inductance (L2) of ELC is influenced by conducting loops and capacitance (C2) is influenced by dielectric gap. Due to the SRR based ELC configuration, the inductance is coupled to the magnetic resonance and the capacitance is coupled to the electric resonance. Thus the proposed ELC is capable of attaining broadband radiation. The configuration of the designed antenna is depicted in Fig.3 and its geometrical values are given in Table.1.
Fig.2. Proposed antenna and its equivalent LC tank circuit model.
Fig.3.Designed antenna configuration. Table.1 Geometrical values of the proposed antenna
Parameters Ws Ls Wf Lf Lg S W1 L1 W2 L2 S1
Dimensions (mm) 30 30 1.6 7.61 5.25 14.6 14 10 4.5 3 1
Simulations are executed using Ansys HFSS V.14.0 EM software package. The simulated S11(dB) characteristics of the hexagonal closed ring resonator and proposed antenna are exposed in Fig.4. It is perceived that the hexagonal closed ring resonator produces a narrow resonance at 2.87 GHz with an impedance bandwidth of 1080 MHz (2.42 ― 3.5 GHz). This resonance frequency has been estimated using [17]
fr=
1.8412 × c 2πS εr
=
1.8412 × 3 × 108 2 × π × 14.6 × 10 ―3 × 4.4
= 2.87 GHz
(1)
Here εr is the dielectric constant of the substrate, c is the velocity of light and S represents the side length of the hexagonal closed ring. The proposed antenna covers an impedance bandwidth of 2600 MHz (2.61 ― 5.21 GHz) with a dual resonance of 3.21 GHz and 4.89 GHz. Thus the ELC based metamaterial can be increased an impedance bandwidth of 81% with respect to subwavelength magnetic resonance. After inserting ELC based metamaterial, the resonance frequency of the hexagonal closed ring resonator (at 2.87 GHz) is changed to an upper frequency of 3.21 GHz and also generates a new resonance at 4.89 GHz along with broadband radiation. Thus, the resonance frequency of
3.21 GHz is due to the conventional radiating element and 4.89 GHz is due to ELC. This ELC resonance frequency is computed by [8] fELC = 2π Where CELC =
C0 =
1
(2)
LELCCRELC
N―1 2
[2L-(2N-1) (W+S)]C0
K( 1 ― K2) ε0 K(k)
S
and k =
W+
LELC = 4µ0[L-(N-1) (S+W)] [ln (
2 S 2 0.98 ρ
) +1.84𝝆]
(N ― 1)(W + S)
𝝆 = 1 ― (N ― 1)(W + S) Here, ELC width (W), ELC length (L), number of radiating element (N), ELC dielectric gap (S) and K is the complete elliptic integral of the first kind identity K(𝑘). These empirical design equations are evaluated by a MATLAB code to compute the resonance frequency of ELC with respect to the inductance and capacitance values of ELC, which are given as, LELC = 1.9993×10 ―08 (Henry) CELC = 5.2833×10 ―14 (Farad) Hence, the ELC resonance frequency is 𝑓𝐸𝐿𝐶=
1 2𝜋 𝐿𝐸𝐿𝐶 𝐶𝐸𝐿𝐶
= 4.8994 GHz. From this lumped
circuit model, it is perceived that proposed ELC creates a resonance around 4.89 GHz. The evaluation of proposed antenna with existing designs shown in reference is represented in Table 2. From Table 2, it is examined that the proposed antenna contributes broad bandwidth with compact size compared with previously reported papers. This paper emphasizes LC tank circuit analysis and band characteristics along with µ-negative for generating broad bandwidth.
Table 2. Evaluation of proposed antenna with existing designs. Reference
Patch Detail
[2]
Circular patch with symmetrical stubs Grid-slotted radiation patch Bowtie dipole with parasitic cross-slots Rectangular patch and CSRR Fractal Koch Slot Antenna with ELC ELC metamaterial resonator
[4]
[6]
[9]
[16]
[18]
Substrate Dimensions L×W (mm2) 30 × 30
Impedance Bandwidth (MHz)
Equivalent Metamaterial Circuit Analysis property verification
2100 MHz (1.9 ― 4 GHz)
Not Analyzed
Not Verified
78 × 92
1000 MHz (3.31 ― 4.31 GHz)
Not Analyzed
Not Verified
55 × 55
4070 MHz (2.39 ― 6.46 GHz)
Not Analyzed
Not Verified
40 × 46
100 MHz (2.4 ― 2.5 GHz)
Not Analyzed
Not Verified
40 × 40
Bandwidth Percentage: 1.9%, 14% and 5% 1240 MHz (3.28 ― 4.52 GHz) and 670 MHz (5.20 ― 5.87 GHz) 60 MHz (2.49 ― 2.55 GHz), 680 MHz (3.0 ― 3.68 GHz), and 1010 MHz (5.03 ― 6.04 GHz). 2600 MHz (2.61 ― 5.21 GHz)
Not Analyzed
Not Verified
Not Analyzed
Not Verified
Not Analyzed
Not Verified
Analyzed
Verified
21 × 30
[19]
Monopole radiator with ELC
35 × 35
Proposed antenna
ELC is nested inside the ring hexagon structure
30 × 30
Fig.4. Simulated S11(dB) characteristics of the hexagonal closed ring resonator and proposed antenna. 3. Operating Mechanism The hexagonal closed ring resonator supports the fundamental radiating mode TM10 at 2.87 GHz, which is estimated using cavity model by [23] 1
𝑓𝑟(𝑚𝑛)= 2𝜋
𝜇𝜀
𝑚𝜋 2 𝐿
𝑛𝜋 2 𝑊
( ) ( ) +
Here 𝜀=𝜀0 𝜀𝑒𝑓𝑓 𝜇 = 4π×10 ―7 h/m 𝜀0= 8.8419×10 ―12 F/m m=0, 1, 2….; n=0, 1, 2……. L=W=Side length of the hexagonal closed ring resonator
(3)
Where 𝜀𝑒𝑓𝑓 is effective dielectric constant and L=W=14.6 mm. Real and imaginary parts of the proposed antenna input impedance are illustrated in Fig. 5. It has two resonant modes TM10 (at 3.21 GHz) and TM12 (at 4.89 GHz) in the operating band. Due to ELC loading, the fundamental radiating mode TM10 (at 2.87 GHz) gets affected and produces the modified modes TM10 and TM12 towards the upper frequencies at 3.21 GHz and 4.89 GHz, respectively. These modified modes are responsible for generating dual resonance characteristics in the wider impedance bandwidth.
Fig. 5. Input resistance and reactance of the proposed antenna.
4. Parameter extraction of ELC To validate the results, the ELC band characteristics are discussed in this section. The effective medium theory is employed to examine the S-parameters for evaluating, reflection
(S11) coefficient and transmission (S21) coefficient of ELC [24]. The reflection coefficient induces pass band behavior with respect to band pass filter for producing a broadband radiation
and transmission coefficient induces stop band behavior with respect to band stop filter. Fig.6 (a) represents the pass band (S11) and stop band (S21) behavior of ELC structure. It is inferred that antenna with ELC has stop band at 2.4 GHz and wider pass band characteristics. This pass band accounts for broadband radiation in the S11(dB) characteristics. An effective permeability (µ) of the ELC is illustrated
in
(b),
the
where
Fig.6
permeability (µ) becomes negative at 2.4 GHz due to S21(dB) characteristics of the ELC structure.
(a)
(b) Fig.6.(a) S-parameters (S11 and S21) of ELC, (b) Permeability of ELC. Fig.7 describes the reflection coefficient (S11) of the hexagonal closed ring resonator with ELC and without ELC. Here, metamaterial unit cell ELC is kept within the waveguide of proper axis, which is enclosed by the Perfect Magnetic Conductor (PMC) and Perfect Electric Conductor
(PEC)
boundary
conditions.
ELC produces
a magnetic
response by the
incident
wave from one
port and S-
parameter (S11)
is revealed
from
port of the
another
waveguide. It is observed that antenna without ELC has no wider pass band, but antenna with ELC has wider pass band characteristics. This pass band is responsible for generating a broadband radiation in the S11(dB) characteristics of the proposed antenna.
Fig.7.S11(dB) characteristics of hexagonal closed ring resonator with ELC and without ELC.
5. Parametric Study
The geometrical values of the antenna are optimized by examining S11(dB) characteristics of the microstrip feed width(Wf), length of the ground(Lg) and dielectric gap(S1). Simulated S11(dB) characteristics for various feed widths are shown in Fig.8. The feed width is varied from 0.8 mm to 1.6 mm with step size of 0.2 mm. As the feed width (Wf) increases, S11 (dB) values are improved. However, the optimum feed width Wf=1.6 mm is assigned for
50Ω
characteristic impedance to obtain better impedance matching. The length of the ground plane performs significant factor on antenna impedance and radiation properties. The ground plane length (Lg) is changed from 7.25 mm to 4.25 mm, and the corresponding S11(dB) plot is depicted in Fig.9. It is inferred that by decreasing the length of the ground plane, the bandwidth of the antenna is enhanced. Hence optimum length Lg= 5.25 mm is allocated for the proposed antenna.
Fig. 8. S11
(dB) of antenna for various feed width.
Fig.9. S11(dB) of antenna for different ground plane length.
Fig.10. S11(dB) of antenna for different dielectric gap.
Similarly, Fig. 10 represents the reflection coefficient of proposed antenna with dielectric gap(S1). It indicates that, as the dielectric gap increases from 0.25 mm to 1 mm, the bandwidth is also expanded gradually. However, the maximum possible value of S1=1 mm is selected to resonate from 2.61 GHz to 5.21 GHz. 6. Results and discussion The snapshot of the fabricated antenna is depicted in Fig.11. The S11(dB) of the antenna is measured using Anritsu Vector Network Analyzer MS46122B. Fig.12 describes the simulated and measured S11(dB) characteristics of the proposed antenna. The evaluation of wider band characteristics is exhibited in Table.3 numerically, both data equivalent to each other. Fig.13 illustrates the far-field radiation pattern in the elevation plane and azimuthal plane over the impedance bandwidth. The behavior in the YZ plane (E-Plane) is a dipole pattern and XZ plane (H-plane) is an omnidirectional pattern. Thus the required characteristics are observed in E-plane due to an electric field in Y-direction and H-plane due to a magnetic field in Z-direction. Both Eplane (ϕ = 90°) and H-plane (ϕ = 0°) exhibits maximum radiation in desired directions.
The gain of the antenna has been measured using gain transfer method [8]. The proposed antenna gain plot is shown in Fig.14. The measured peak gains of 3.44 dBi and 3.61 dBi are realized at 3.12 GHz and 4.64 GHz respectively. This antenna design, the ELC is used as the main radiating element, which enables miniaturization. When the antenna is miniaturized, its radiation characteristics are affected, leading to a reduction in the gain.
The proposed antenna has focused on the bandwidth improvement of hexagonal patch antenna only. The proposed technique can also be useful for other radiating structures. In future, the proposed antenna can also be carried out with CSRR based metamaterial structures.
Fig.11.Snapshot of the fabricated antenna (a) Top view (b) Bottom view.
Fig. 12. S11(dB) characteristics of the proposed antenna. Table 3 Comparison of performance of the antenna Proposed antenna
Operating Frequency (GHz)
S11(dB)
Impedance bandwidth (MHz)
3.2
-36
2600 MHz (2.61 ― 5.21 GHz)
4.9
-46
3.12
-47
4.64
-47
Simulated
Measured
2400 MHz (2.63 ― 5.03 GHz)
Fig. 13.Far-field radiation patterns (a) 3.12 GHz (b) 4.94 GHz.
Fig. 14 Gain plot of the proposed antenna.
7. Conclusion An electrically small broad bandwidth antenna based on ELC metamaterial is designed using cavity model analysis and effective medium theory. It reveals that ELC metamaterial is substantial in addition to the hexagonal closed ring resonator, to produce a broad bandwidth without increasing the size of the substrate. Impedance bandwidth is increased from 38% to 81%. The prototype antenna has an electrical size of 0.32 λ0×0.32 λ0×0.0085 λ0.The prototype antenna has been fabricated and its characteristics are measured. The experimental data covers the 3 GHz WiMAX, 4 GHz C-band and 5 GHz WLAN applications.
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