development of metamaterial inspired non-uniform circular

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Citation: Bhavani, K.D.; Madhav, B.T.P.; Das, S.; Hussain, N.; Ali, S.S.; Babu, K.V. Development of Metamaterial Inspired Non-Uniform Circular Array Superstate Antenna Using Characteristic Mode Analysis. Electronics 2022, 11, 2517. https:// doi.org/10.3390/electronics11162517 Academic Editor: Raed A. Abd-Alhameed Received: 1 July 2022 Accepted: 9 August 2022 Published: 11 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). electronics Article Development of Metamaterial Inspired Non-Uniform Circular Array Superstate Antenna Using Characteristic Mode Analysis Kothakonda Durga Bhavani 1 , Boddapati Taraka Phani Madhav 1 , Sudipta Das 2 , Niamat Hussain 3, * , Syed Samser Ali 4 and Kommanaboyina Vasu Babu 5 1 Antennas and Liquid Crystals Research Center, Department of ECE, Koneru Lakshmaiah Education Foundation, Vaddeswaram 522302, Andhra Pradesh, India 2 Department of Electronics & Communication Engineering, IMPS College of Engineering and Technology, Azimpur 732101, West Bengal, India 3 Department of Smart Device Engineering, Sejong University, Seoul 05006, Korea 4 Electronics and Communication Engineering Department, University Institute of Technology, Bardhaman 713104, West Bengal, India 5 Department of Electronics & Communication Engineering, Vasireddy Venkatadri Institute of Technology, Namburu 522508, Andhra Pradesh, India * Correspondence: [email protected] Abstract: In this work, using characteristic mode analysis, a multi-layered nonuniform metasurface structured antenna has been optimized. The driven patch of square structure and the parasitic patch elements of circular radiating cross-slotted meta-structure are used in the proposed model. The modal significance characteristic angles and surface currents are analyzed based on characteristic mode to optimize the nonuniform structures. The antenna is resonating between 5.5–6.1 GHz, covering WLAN applications with an average gain of 7.9 dBi and efficiency greater than 90%. Transient mode, terminal mode, and eigenmode-based analyses are performed on the proposed design, and comparative analysis has been presented in this work. The prototype model fabrication and real-time measurement analysis with simulation results matching are presented for application validation. Keywords: characteristic mode analysis (CMA); metasurface; eigenmode; WLAN 1. Introduction Nowadays, the demand for novel wideband antennas is attracting more and more attention from researchers towards wireless services such as WiMAX, Wi-Fi, and WLAN. The world has to put its efforts into the development of a wireless 5G system with the integration of advanced communication modules [1]. So, the researchers have to endorse a new design approach for the fifth-generation wireless systems, which must fulfill the needs such as higher data rates, reliability, and good connectivity [2,3]. On the other hand, the design challenges need to be addressed, and more focus should be on compatibility with the environment. The major challenge in the design of working communication systems will depend on the efficiency of the antenna system. The antenna needs to be reliable, and adoptable and should have high gain bandwidth with good impedance matching. The microstrip antenna technology is more suitable and preferable to cater to these needs with design simplicity and ease in impedance matching. Compared to other conventional antennas, the microstrip patch antennas are very popular due to their light weight, ease of fabrication, lower cost, and also provide wideband characteristics. One of the promising techniques to achieve the wideband characteristics is in the antenna structure loading a metasurface-layer, which leads to an increase in the bandwidth and also reduces the size of the antenna [4,5]. Normally the natural materials exhibit positive permittivity [r], permeability μ[r], and refractive index but metamaterials exhibit negative properties [6]. The metamaterial-based patch antenna for the improvement of Electronics 2022, 11, 2517. https://doi.org/10.3390/electronics11162517 https://www.mdpi.com/journal/electronics

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Citation: Bhavani, K.D.;

Madhav, B.T.P.; Das, S.; Hussain, N.;

Ali, S.S.; Babu, K.V. Development of

Metamaterial Inspired Non-Uniform

Circular Array Superstate Antenna

Using Characteristic Mode Analysis.

Electronics 2022, 11, 2517. https://

doi.org/10.3390/electronics11162517

Academic Editor: Raed A.

Abd-Alhameed

Received: 1 July 2022

Accepted: 9 August 2022

Published: 11 August 2022

Publisher’s Note: MDPI stays neutral

with regard to jurisdictional claims in

published maps and institutional affil-

iations.

Copyright: © 2022 by the authors.

Licensee MDPI, Basel, Switzerland.

This article is an open access article

distributed under the terms and

conditions of the Creative Commons

Attribution (CC BY) license (https://

creativecommons.org/licenses/by/

4.0/).

electronics

Article

Development of Metamaterial Inspired Non-Uniform CircularArray Superstate Antenna Using Characteristic Mode AnalysisKothakonda Durga Bhavani 1 , Boddapati Taraka Phani Madhav 1 , Sudipta Das 2 , Niamat Hussain 3,* ,Syed Samser Ali 4 and Kommanaboyina Vasu Babu 5

1 Antennas and Liquid Crystals Research Center, Department of ECE, Koneru Lakshmaiah EducationFoundation, Vaddeswaram 522302, Andhra Pradesh, India

2 Department of Electronics & Communication Engineering, IMPS College of Engineering and Technology,Azimpur 732101, West Bengal, India

3 Department of Smart Device Engineering, Sejong University, Seoul 05006, Korea4 Electronics and Communication Engineering Department, University Institute of Technology,

Bardhaman 713104, West Bengal, India5 Department of Electronics & Communication Engineering, Vasireddy Venkatadri Institute of Technology,

Namburu 522508, Andhra Pradesh, India* Correspondence: [email protected]

Abstract: In this work, using characteristic mode analysis, a multi-layered nonuniform metasurfacestructured antenna has been optimized. The driven patch of square structure and the parasitic patchelements of circular radiating cross-slotted meta-structure are used in the proposed model. The modalsignificance characteristic angles and surface currents are analyzed based on characteristic modeto optimize the nonuniform structures. The antenna is resonating between 5.5–6.1 GHz, coveringWLAN applications with an average gain of 7.9 dBi and efficiency greater than 90%. Transientmode, terminal mode, and eigenmode-based analyses are performed on the proposed design, andcomparative analysis has been presented in this work. The prototype model fabrication and real-timemeasurement analysis with simulation results matching are presented for application validation.

Keywords: characteristic mode analysis (CMA); metasurface; eigenmode; WLAN

1. Introduction

Nowadays, the demand for novel wideband antennas is attracting more and moreattention from researchers towards wireless services such as WiMAX, Wi-Fi, and WLAN.The world has to put its efforts into the development of a wireless 5G system with theintegration of advanced communication modules [1]. So, the researchers have to endorse anew design approach for the fifth-generation wireless systems, which must fulfill the needssuch as higher data rates, reliability, and good connectivity [2,3]. On the other hand, thedesign challenges need to be addressed, and more focus should be on compatibility withthe environment. The major challenge in the design of working communication systemswill depend on the efficiency of the antenna system. The antenna needs to be reliable,and adoptable and should have high gain bandwidth with good impedance matching.The microstrip antenna technology is more suitable and preferable to cater to these needswith design simplicity and ease in impedance matching. Compared to other conventionalantennas, the microstrip patch antennas are very popular due to their light weight, ease offabrication, lower cost, and also provide wideband characteristics.

One of the promising techniques to achieve the wideband characteristics is in theantenna structure loading a metasurface-layer, which leads to an increase in the bandwidthand also reduces the size of the antenna [4,5]. Normally the natural materials exhibitpositive permittivity ε[r], permeability µ[r], and refractive index but metamaterials exhibitnegative properties [6]. The metamaterial-based patch antenna for the improvement of

Electronics 2022, 11, 2517. https://doi.org/10.3390/electronics11162517 https://www.mdpi.com/journal/electronics

Electronics 2022, 11, 2517 2 of 16

bandwidth has been studied by Wu [7]. The metasurface is considered a kind of two-dimensional metamaterial structure that contains a layer of electrically small scattersarranged in order by using certain rules [8]. Painam designed a circular microstrip patchantenna loaded with a metamaterial, and the size of the antenna is reduced to 74% comparedto other conventional microstrip antennas [9]. A non-uniform metasurface layer is formedby adjusting the cells of the antenna to improve the overall radiation performance [10–13].The unit cell size in a non-uniform metasurface layer is gradually increased from center tooutwards and achieved a wide bandwidth of 33.1% by Feng [14]. The wideband filtering ofthe antenna is achieved by adjusting the size of unit cells on two sides of the y-axis [15].In [16], the column unit cell size is adjusted on x-axis to achieve the wideband characteristicsof an antenna. Due to the lack of sufficient theoretical basics, the arrangement of unit cellsin a non-uniform metasurface layer to accomplish the wideband characteristics is a tediousand time-consuming task.

Considering the defects and difficulties in the above-mentioned design methods, thereis a need for more physical insights into the antenna design by engineers. The characteristicmode analysis (CMA) is the most popular and efficient method which is proposed byGarbacz [16] and modified by Harrington and Mautz [17–19] to optimize the antennaperformance characteristics. The characteristic mode analysis (CMA) furnishes an in-depthphysical insight into the antenna design aspect and its radiating properties. In recentyears, using characteristic mode analysis, different types of antennas are designed forvarious commercial and military communication applications, such as ultra-widebandantennas [20–22] and metasurface antennas [23–25]. In [26], a novel miniaturized meta-surface unit cell is proposed, and using this miniaturized unit cell, a 4 × 4 array antennastructure is composed. The design is analyzed using the theory of characteristic modesand realized circular polarization. In [27], 4 × 4 non-uniform array antenna is designedwith H-shape patch array elements. The current distributions of metasurface and radiatingproperties are analyzed and obtained wide impedance bandwidth of 38.8%. In [28], torealize the dual-band operation, a non-uniform 3 × 3 array antenna is designed usingsquare shape patch elements.

In this work, a new multi-layered design is proposed and also optimized for themetamaterial-inspired 3 × 3 non-uniform array antenna using characteristic mode analysis(CMA). The antenna modeling and the simulation performance characteristics are realizedon the commercial electromagnetic tool HFSS and presented in the subsequent sections.

The proposed design is analyzed in modal analysis, terminal, transient, eigenvalue,and characteristic mode analysis. All analysis method results for a particular design areplaced in this article. Characteristic mode analysis significance is clearly presented andshows how modes are resonating with respect to all parameters, such as modal significance,eigenvalue, and characteristic angle in the subsequent sections.

2. Antenna Design

The side and top views of the proposed metamaterial-inspired non-uniform arrayantenna design are presented in Figure 1. The designed antenna contains a non-uniformmetasurface, patch antenna on two substrates named sub1 and sub2. Both the substrates ofcircular shape are designed with the height of ‘h’ and radius of ‘R’. The substrate materialused in the design is PDMS material having a loss tangent of 0.013 and relative permittivityof 2.7 and the conducting material used in the antenna design is copper. The ground planealso has the same radius as ‘R’ and covers the sub2 bottom part.

The rectangular patch is printed on the sub 2 with a length of ‘Lp’ and width of ‘Wp’.To achieve 50 Ω impedance matching, a coaxial feeding technique is used, and a coaxialfeed probe is located on the y-axis at a distance of 4 mm from the origin. According to thetop view of antenna geometry shown, a non-uniform metasurface is designed with an arrayof 3 × 3 unit cells, and the space maintained between the unit cells on the metasurface is2 mm. Each unit cell covers an area of UL × UW and contains a circular radiating elementof radius ‘r’. To improve the coupling effect between the non-uniform metasurface layer

Electronics 2022, 11, 2517 3 of 16

and radiating antenna, the circular copper element is cut symmetrically into two parts by aslot width of ‘Ws’ and other cuts of having length ‘Lc’ and width of ‘Wc’. The angle ‘Ө’ isdefined as a rotation angle, which is the angle made between the x-axis and the center of theslot. The unit cells placed at the center and all four corners are having a counterclockwisedirection of rotation angle and the remaining unit cells are having the opposite direction ofthe rotation angle. The proposed antenna dimensions are given in Table 1.

Electronics 2022, 11, 2517 3 of 17

(a)

(b)

(c) (d)

(e) (f)

Figure 1. Non-uniform multilayered array antenna, (a) side view of the designed model. (b) Top view of simulated model. (c) Top view of the prototyped model. (d) Side view of the prototyped model. (e) Prototype-driven patch layer. (f) Antenna bottom view.

The rectangular patch is printed on the sub 2 with a length of ‘Lp’ and width of ‘Wp’. To achieve 50Ω impedance matching, a coaxial feeding technique is used, and a coaxial feed probe is located on the y-axis at a distance of 4mm from the origin. According to the top view of antenna geometry shown, a non-uniform metasurface is designed with an array of 3 × 3 unit cells, and the space maintained between the unit cells on the metasurface is 2 mm. Each unit cell covers an area of UL × UW and contains a circular radiating element

Figure 1. Non-uniform multilayered array antenna, (a) side view of the designed model. (b) Topview of simulated model. (c) Top view of the prototyped model. (d) Side view of the prototypedmodel. (e) Prototype-driven patch layer. (f) Antenna bottom view.

Electronics 2022, 11, 2517 4 of 16

Table 1. The non-uniform array antenna dimensions (unit: mm).

R h Lp Wp r Ws Lc Wc Ө Uw UL

27.5 2 13.27 13 6 2 2 2 40 14 14

Figure 2 represents modal, terminal, and transient analysis characteristics of thedesigned antenna array. It has been noticed that the transient mode analysis is contributinglesser bandwidth in comparison with the modal and terminal analysis and the impedancebandwidth of 52% is only attained for the transient analysis, whereas for other two, animpedance bandwidth of 105% has been obtained.

Electronics 2022, 11, 2517 4 of 17

of radius ‘r’. To improve the coupling effect between the non-uniform metasurface layer and radiating antenna, the circular copper element is cut symmetrically into two parts by a slot width of ‘Ws’ and other cuts of having length ‘Lc’ and width of ‘Wc’. The angle ‘Ө’ is defined as a rotation angle, which is the angle made between the x-axis and the center of the slot. The unit cells placed at the center and all four corners are having a counter-clockwise direction of rotation angle and the remaining unit cells are having the opposite direction of the rotation angle. The proposed antenna dimensions are given in Table 1.

Table 1. The non-uniform array antenna dimensions (unit: mm).

R h Lp Wp r Ws Lc Wc Ө Uw UL 27.5 2 13.27 13 6 2 2 2 400 14 14

Figure 2 represents modal, terminal, and transient analysis characteristics of the de-signed antenna array. It has been noticed that the transient mode analysis is contributing lesser bandwidth in comparison with the modal and terminal analysis and the impedance bandwidth of 52% is only attained for the transient analysis, whereas for other two, an impedance bandwidth of 105% has been obtained.

Figure 2. S11 response for modal, terminal, and transient analysis.

The eigenmode analysis with respect to five modes of Mode1, Mode2, Mode3, Mode4, and Mode5 has been presented in Figure 3. This is giving clear evidence regarding the resonant frequencies for each mode as per the operating frequency is concerned.

5.2 5.4 5.6 5.8 6.0 6.2-20

-18

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-14

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-10

-8

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-2

S11

(dB

)

5.5-6.1 GHz

5.6-5.9 GHz

Frequency (GHz)

Modal Analysis (simulated S11) Terminal Analysis (simulated S11) Transient Analysis (simulated S11) Measured S11 for Modal & Terminal Analysis

5.5-6.1 GHz

Figure 2. S11 response for modal, terminal, and transient analysis.

The eigenmode analysis with respect to five modes of Mode 1, Mode 2, Mode 3,Mode 4, and Mode 5 has been presented in Figure 3. This is giving clear evidence regardingthe resonant frequencies for each mode as per the operating frequency is concerned.

Electronics 2022, 11, 2517 5 of 17

Figure 3. Eigenmode analysis.

3. Unit Cell Design The proposed array antenna unit cell structure has been presented in Figure 4. The

unit cell with the port assignment and the cross-sectional view with respect to dimen-sional characteristics are presented in Figure 4a,b. Figures 5 and 6 represent the corre-sponding unit cell analysis parameters with respect to permittivity and permeability. The functional representation of these two parameters at the resonating band with negative values can be observed from the obtained results. The metamaterial behavior of the pro-posed design with negative characteristics gives strong motivation for the applicability of the given structure in the desired band.

(a) (b)

Figure 4. Unit cell (a) with port assignment (b) cross sectional view.

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

5.0

5.2

5.4

5.6

5.8

6.0

6.2

Freq

uenc

y (G

Hz)

Eigen Value

Mode 1 Mode 2 Mode 3 Mode 4 Mode 5

Figure 3. Eigenmode analysis.

3. Unit Cell Design

The proposed array antenna unit cell structure has been presented in Figure 4. Theunit cell with the port assignment and the cross-sectional view with respect to dimensionalcharacteristics are presented in Figure 4a,b. Figures 5 and 6 represent the correspondingunit cell analysis parameters with respect to permittivity and permeability. The functional

Electronics 2022, 11, 2517 5 of 16

representation of these two parameters at the resonating band with negative values can beobserved from the obtained results. The metamaterial behavior of the proposed design withnegative characteristics gives strong motivation for the applicability of the given structurein the desired band.

Electronics 2022, 11, 2517 5 of 17

Figure 3. Eigenmode analysis.

3. Unit Cell Design The proposed array antenna unit cell structure has been presented in Figure 4. The

unit cell with the port assignment and the cross-sectional view with respect to dimen-sional characteristics are presented in Figure 4a,b. Figures 5 and 6 represent the corre-sponding unit cell analysis parameters with respect to permittivity and permeability. The functional representation of these two parameters at the resonating band with negative values can be observed from the obtained results. The metamaterial behavior of the pro-posed design with negative characteristics gives strong motivation for the applicability of the given structure in the desired band.

(a) (b)

Figure 4. Unit cell (a) with port assignment (b) cross sectional view.

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

5.0

5.2

5.4

5.6

5.8

6.0

6.2

Freq

uenc

y (G

Hz)

Eigen Value

Mode 1 Mode 2 Mode 3 Mode 4 Mode 5

Figure 4. Unit cell (a) with port assignment (b) cross sectional view.

Electronics 2022, 11, 2517 6 of 17

Figure 5. Frequency versus permittivity plot.

Figure 6. Frequency versus permeability plot.

4. Characteristic Mode Analysis The analysis of characteristic mode became a popular method to design the mi-

crostrip patch antenna, which provides more physical insight into antenna resonance and radiation analysis. The characteristic modes are current modes that are associated with eigenvalues that can be calculated numerically for arbitrarily shaped perfect electric con-ducting bodies (PEC). The theory of characteristic modes is dependent only on the shape and size of a conducting object and is independent of any kind of excitation to achieve good performance of an antenna design. The CMA provides an inevitable approach for an antenna design, which is more efficient than cut-and-try methods or time-consuming optimizations [29]. The design of the antenna can be performed in two steps using char-acteristics. In Step 1, the size and shape of conducting elements are optimized. In Step 2, based on the current distributions provided by CMA, a suitable feeding configuration is chosen to excite desired characteristic modes. If the size of the element is changed, then the resonant frequency and radiating properties also will change. The derivations of

4.0 4.5 5.0 5.5-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Imag

ε[r]

Frequency (GHz)

Real

5.0 5.5 6.0 6.5 7.0-10

-8

-6

-4

-2

0

2

4

6

Imagμ[r]

Frequency (GHz)

Real

Figure 5. Frequency versus permittivity plot.

Electronics 2022, 11, 2517 6 of 17

Figure 5. Frequency versus permittivity plot.

Figure 6. Frequency versus permeability plot.

4. Characteristic Mode Analysis The analysis of characteristic mode became a popular method to design the mi-

crostrip patch antenna, which provides more physical insight into antenna resonance and radiation analysis. The characteristic modes are current modes that are associated with eigenvalues that can be calculated numerically for arbitrarily shaped perfect electric con-ducting bodies (PEC). The theory of characteristic modes is dependent only on the shape and size of a conducting object and is independent of any kind of excitation to achieve good performance of an antenna design. The CMA provides an inevitable approach for an antenna design, which is more efficient than cut-and-try methods or time-consuming optimizations [29]. The design of the antenna can be performed in two steps using char-acteristics. In Step 1, the size and shape of conducting elements are optimized. In Step 2, based on the current distributions provided by CMA, a suitable feeding configuration is chosen to excite desired characteristic modes. If the size of the element is changed, then the resonant frequency and radiating properties also will change. The derivations of

4.0 4.5 5.0 5.5-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Imag

ε[r]

Frequency (GHz)

Real

5.0 5.5 6.0 6.5 7.0-10

-8

-6

-4

-2

0

2

4

6

Imagμ[r]

Frequency (GHz)

Real

Figure 6. Frequency versus permeability plot.

Electronics 2022, 11, 2517 6 of 16

4. Characteristic Mode Analysis

The analysis of characteristic mode became a popular method to design the microstrippatch antenna, which provides more physical insight into antenna resonance and radiationanalysis. The characteristic modes are current modes that are associated with eigenvaluesthat can be calculated numerically for arbitrarily shaped perfect electric conducting bodies(PEC). The theory of characteristic modes is dependent only on the shape and size of aconducting object and is independent of any kind of excitation to achieve good performanceof an antenna design. The CMA provides an inevitable approach for an antenna design,which is more efficient than cut-and-try methods or time-consuming optimizations [29].The design of the antenna can be performed in two steps using characteristics. In Step 1,the size and shape of conducting elements are optimized. In Step 2, based on the currentdistributions provided by CMA, a suitable feeding configuration is chosen to excite desiredcharacteristic modes. If the size of the element is changed, then the resonant frequency andradiating properties also will change. The derivations of characteristic modes and theirvarious applications in antenna design are presented in [30]. The characteristic currents arederived by using eigenvalue equation,

X[→

J n

]= λnR

[→J n

](1)

where λn represents the eigenvalues,→J n are nothing but the eigen currents or eigen

functions, n is the mode order and R and X are the real and imaginary parts of impedancematrix is [31],

Z = R + JX (2)

The eigenvalue is one of the utmost essential parameters because its magnitudeprovides valuable information about the resonant frequency and radiation informationof the characteristic mode [32,33]. Consider a mode is resonating, the eigenvalue (λn)associated with a mode is zero, i.e., |λn = 0|, it is stated that the mode radiates moreefficiently when the magnitude of the eigenvalue is smaller.

when λn = 0, the mode is externally resonant.

• λn > 0, the mode is inductive, which means the energy is stored in a magnetic field.• λn < 0, the mode is capacitive, which means the energy is stored in an electric field.

The second important parameter in characteristic mode analysis is the characteristicangle, it is an angle that represents the phase lag between the electric field and surfacecurrent on a conductor object [34,35]. Mathematically, it is represented as,

αn = 180− tan−1 λn (3)

• When αn = 180 deg, the mode is externally resonant.• When 90 deg < αn < 180 deg, the mode is inductive.• When 180 deg < αn < 180 deg, the mode is capacitive.

Modal significance is the parameter used in CMA to find the resonant frequency andradiating bands of a specified mode. It is represented as,

MS =1

|1 + Jλn|(4)

In characteristic mode analysis a mode can be considered resonant when λn = 0,MS = 1 and αn = 180.

The frequency vs. characteristic angle plots are presented in Figure 7. The rotationangle of the circular slots is varied from 0 deg to 90 deg and −20 deg to −60 deg andanalyzed the change in characteristic angle and presented the resonant frequency for theparticular mode in Table 2. In Mode 1 for a rotation angle of 0 deg, the resonant frequencyis varied between 3.7 to 5.6 GHz. In Mode 2, the resonant frequency varied between 4.4 to

Electronics 2022, 11, 2517 7 of 16

6.1 GHz and at 90 deg and −60 deg, there is no resonance from the antenna. In Mode 3 foronly a rotation angle of 20 deg, the antenna resonates at 3.8 GHz and for other angles, thereis no resonance from the antenna.

Electronics 2022, 11, 2517 8 of 17

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Figure 7. Characteristic angle plot of proposed antenna (a) Ө = 0° (b) Ө = 20° (c) Ө= 40° (d) Ө = 60° (e) Ө = 90° (f) Ө = −20° (g) Ө = −40° (h) Ө = −60°.

4.0 4.5 5.0 5.5 6.0 6.5 7.0100

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80100120140160180200220240260280

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Figure 7. Characteristic angle plot of proposed antenna (a) Ө = 0 (b) Ө = 20 (c) Ө= 40 (d) Ө = 60

(e) Ө = 90 (f) Ө = −20 (g) Ө = −40 (h) Ө = −60.

Electronics 2022, 11, 2517 8 of 16

Table 2. Mode resonating frequencies with respect to characteristic angle.

Angle Ө = 0 Ө = 20 Ө = 40 Ө = 60 Ө = 90 Ө = −20 Ө = −40 Ө = −60

Mode 1 5.6 3.7 5.0 5.0 5.49 3.7 4.7 5.09Mode 2 5.7 5.4 5.5 6.1 - 4.48 5.8 -Mode 3 - 5.6 - - - - - -

The model significance analysis for the proposed antenna structure with reference tofrequency of operation is indicated in Figure 8. Mode 1, Mode 2 and Mode 3 are analyzedwith resonant frequency and presented the same in this section.

The change in the rotation angle of the slots in the radiating element varied from 0degrees to 90 degrees and from −20 degrees to −60 degrees. For 20 degrees there are threeresonant frequencies for three modes at 3.7, 5.4, and 3.8 GHz, respectively.

For 90 degrees and −60 degrees, there is only a single resonant frequency at 5.4 and5 GHz, respectively. For 0, 20, 40, 60, −20, and −40 degrees, both Mode 1 and Mode 2provide different resonant frequencies of operation from Table 3.

The eigenvalue analysis with respect to resonant frequency is presented in Figure 9for different angles of rotation. Table 4. Except for 90 deg and −60 deg, single resonantmode and for 20 deg triple resonant modes are observed. A similar kind of observation isattained for eigenvalues from Table 4 and modal significance analysis from Table 3.

Electronics 2022, 11, 2517 9 of 17

Table 2. Mode resonating frequencies with respect to characteristic angle.

Angle Ө = 0° Ө = 20° Ө = 40° Ө = 60° Ө = 90° Ө = −20° Ө = −40° Ө = −60° Mode 1 5.6 3.7 5.0 5.0 5.49 3.7 4.7 5.09 Mode 2 5.7 5.4 5.5 6.1 - 4.48 5.8 - Mode 3 - 5.6 - - - - - -

The model significance analysis for the proposed antenna structure with reference to frequency of operation is indicated in Figure 8. Mode 1, Mode 2 and Mode 3 are analyzed with resonant frequency and presented the same in this section.

(a) (b)

(c) (d)

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Frequency (Ghz)

Mode 1 Mode 2 Mode 3

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.00.0

0.2

0.4

0.6

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1.0M

odal

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nific

ance

Frequency (GHz)

Mode 1 Mode 2 Mode 3

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.00.0

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Frequency (GHz)

Mode 1 Mode 2 Mode 3

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.00.0

0.2

0.4

0.6

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Mode 1 Mode 2 Mode 3

Figure 8. Cont.

Electronics 2022, 11, 2517 9 of 16Electronics 2022, 11, 2517 10 of 17

(e) (f)

(g) (h)

Figure 8. Modal significance plot of proposed antenna (a) Ө = 0° (b) Ө = 20° (c) Ө = 40° (d) Ө = 60° (e) Ө = 90° (f) Ө = −20° (g) Ө = −40° (h) Ө = −60°.

The change in the rotation angle of the slots in the radiating element varied from 0 degrees to 90 degrees and from −20 degrees to −60 degrees. For 20 degrees there are three resonant frequencies for three modes at 3.7, 5.4, and 3.8 GHz, respectively.

For 90 degrees and −60 degrees, there is only a single resonant frequency at 5.4 and 5 GHz, respectively. For 0, 20, 40, 60, −20, and −40 degrees, both Mode1 and Mode2 provide different resonant frequencies of operation from Table 3.

Table 3. Mode resonating frequencies with respect to modal significance.

Modal Sig-nificance (MS = 1)

Ө = 0° Ө = 20° Ө = 40° Ө = 60° Ө = 90° Ө = −20° Ө = −40° Ө = −60°

Mode 1 5.6 3.7 5.0 5.0 5.49 3.7 4.7 5.09 Mode 2 5.7 5.4 5.5 6.1 - 4.48 5.8 - Mode 3 - 5.6 - - - - - -

The eigenvalue analysis with respect to resonant frequency is presented in Figure 9 for different angles of rotation. Table 4. Except for 90 deg and −60 deg, single resonant mode and for 20 deg triple resonant modes are observed. A similar kind of observation is attained for eigenvalues from Table 4 and modal significance analysis from Table 3.

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

0.0

0.2

0.4

0.6

0.8

1.0

Mod

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Frequency (GHz)

Mode 1 Mode 2 Mode 3

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.00.0

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0.4

0.6

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Frequency (GHz)

Mode 1 Mode 2 Mode 3

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.00.0

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Frequency (GHz)

Mode 1 Mode 2 Mode 3

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.00.0

0.2

0.4

0.6

0.8

1.0

Mod

al S

igni

fican

ce

Frequency (GHz)

Mode 1 Mode 2 Mode 3

Figure 8. Modal significance plot of proposed antenna (a) Ө = 0 (b) Ө = 20 (c) Ө = 40 (d) Ө = 60

(e) Ө = 90 (f) Ө = −20 (g) Ө = −40 (h) Ө = −60.

Table 3. Mode resonating frequencies with respect to modal significance.

Modal Significance (MS = 1) Ө = 0 Ө = 20 Ө = 40 Ө = 60 Ө = 90 Ө = −20 Ө = −40 Ө = −60

Mode 1 5.6 3.7 5.0 5.0 5.49 3.7 4.7 5.09Mode 2 5.7 5.4 5.5 6.1 - 4.48 5.8 -Mode 3 - 5.6 - - - - - -

The surface current distribution analysis of the designed antenna for different rotationangles is presented in Figure 10. The circular orientation of current elements’ directionin all the cases gives evidence of polarization diversity, and in each unit cell, the strengthof the current is consistent. The direction of current in Mode 1 is along the y-axis and inMode 2 is along the x-axis.

The three-dimensional and 2D gain plots of the current antenna are presented inFigure 11. The gain plots at different resonant frequencies of various modes are analyzedand presented. At 5.4 GHz of Mode 2 (Ө = 20), a maximum gain of 7.5 dB is attained.At 5.5 GHz of Mode 2 (Ө = 40), a maximum gain of 7.7 dB is attained. At 5.6 GHz ofMode 1 (Ө = 0), a maximum gain of 7.8 dB is attained. At 5.7 GHz of Mode 2 (Ө = 0), amaximum gain of 7.9 dB is attained. At 5.8 GHz of Mode 2 (Ө = −40), a maximum gain of8.1 dB is attained. At 6.1 GHz of Mode 2 (Ө = 60), a maximum gain of 8.2 dB is attained.

Electronics 2022, 11, 2517 10 of 16Electronics 2022, 11, 2517 11 of 17

(a) (b)

(c) (d)

(e) (f)

(g) (h)

Figure 9. Frequency vs. eigenvalue plot of proposed antenna (a) Ө = 0° (b) Ө = 20° (c) Ө = 40° (d) Ө = 60° (e) Ө = 90° (f) Ө = −20° (g) Ө = −40° (h) Ө = −60°.

4.0 4.5 5.0 5.5 6.0 6.5 7.0-10

-8

-6

-4

-2

0

2

Eige

n V

alue

Frequency (GHz)

Mode 1 Mode 2 Mode 3

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0-60

-50

-40

-30

-20

-10

0

10

Eige

n V

alue

Frequency (GHz)

Mode 1 Mode 2 Mode 3

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

-10

0

10

20

30

40

Eige

n V

alue

Frequency (GHz)

Mode 1 Mode 2 Mode 3

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

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0

10

20

30

40

50

Eige

n V

alue

Frequency (GHz)

Mode 1 Mode 2 Mode 3

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0-10

-5

0

5

10

15

20

25

30

35

Eige

n V

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Frequency (GHz)

Mode 1 Mode 2 Mode 3

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0

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20

30

40

50

Eige

n V

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Mode 1 Mode 2 Mode 3

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20

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40

50

Eige

n V

alue

Frequency (GHz)

Mode 1 Mode 2 Mode 3

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0-10

0

10

20

30

40

50

Eige

n V

alue

Frequency (GHz)

Mode 1 Mode 2 Mode 3

Figure 9. Frequency vs. eigenvalue plot of proposed antenna (a) Ө = 0 (b) Ө = 20 (c) Ө = 40

(d) Ө = 60 (e) Ө = 90 (f) Ө = −20 (g) Ө = −40 (h) Ө = −60.

Electronics 2022, 11, 2517 11 of 16

Table 4. Mode resonating frequencies with respect to eigenvalue.

Eigenvalue(λn=0) Ө = 0 Ө = 20 Ө = 40 Ө = 60 Ө = 90 Ө = −20 Ө = −40 Ө = −60

Mode 1 5.6 3.7 5.0 5.0 5.49 3.7 4.7 5.09Mode 2 5.7 5.4 5.5 6.1 - 4.48 5.8 -Mode 3 - 5.6 - - - - - -

Electronics 2022, 11, 2517 12 of 17

Table 4. Mode resonating frequencies with respect to eigenvalue.

Eigenvalue (𝝀𝒏 = 0) Ө = 0° Ө = 20° Ө = 40° Ө = 60° Ө = 90° Ө =− 20° Ө =− 40° Ө =− 60°

Mode 1 5.6 3.7 5.0 5.0 5.49 3.7 4.7 5.09 Mode 2 5.7 5.4 5.5 6.1 - 4.48 5.8 - Mode 3 - 5.6 - - - - - -

The surface current distribution analysis of the designed antenna for different rota-tion angles is presented in Figure 10. The circular orientation of current elements’ direc-tion in all the cases gives evidence of polarization diversity, and in each unit cell, the strength of the current is consistent. The direction of current in Mode 1 is along the y-axis and in Mode 2 is along the x-axis.

(a) (b)

(c) (d)

(e) (f)

Electronics 2022, 11, 2517 13 of 17

(g) (h)

Figure 10. Surface current distributions of proposed antenna (a) Ө = 0° (b) Ө = 20° (c) Ө = 40° (d) Ө = 60° (e) Ө = −90° (f) Ө = −20° (g) Ө = −40° (h) Ө = −60°.

The three-dimensional and 2D gain plots of the current antenna are presented in Fig-ure 11. The gain plots at different resonant frequencies of various modes are analyzed and presented. At 5.4 GHz of Mode 2(Ө = 20°), a maximum gain of 7.5 dB is attained. At 5.5 GHz of Mode 2(Ө = 40°), a maximum gain of 7.7 dB is attained. At 5.6 GHz of Mode 1(Ө = 0°), a maximum gain of 7.8 dB is attained. At 5.7 GHz of Mode 2(Ө = 0°), a maximum gain of 7.9 dB is attained. At 5.8 GHz of Mode 2(Ө = −40°), a maximum gain of 8.1 dB is attained. At 6.1 GHz of Mode 2(Ө = 60°), a maximum gain of 8.2 dB is attained.

(a)

(b)

Figure 10. Surface current distributions of proposed antenna (a) Ө = 0 (b) Ө = 20 (c) Ө = 40

(d) Ө = 60 (e) Ө = −90 (f) Ө = −20 (g) Ө = −40 (h) Ө = −60.

Electronics 2022, 11, 2517 12 of 16

Electronics 2022, 11, 2517 13 of 17

(g) (h)

Figure 10. Surface current distributions of proposed antenna (a) Ө = 0° (b) Ө = 20° (c) Ө = 40° (d) Ө = 60° (e) Ө = −90° (f) Ө = −20° (g) Ө = −40° (h) Ө = −60°.

The three-dimensional and 2D gain plots of the current antenna are presented in Fig-ure 11. The gain plots at different resonant frequencies of various modes are analyzed and presented. At 5.4 GHz of Mode 2(Ө = 20°), a maximum gain of 7.5 dB is attained. At 5.5 GHz of Mode 2(Ө = 40°), a maximum gain of 7.7 dB is attained. At 5.6 GHz of Mode 1(Ө = 0°), a maximum gain of 7.8 dB is attained. At 5.7 GHz of Mode 2(Ө = 0°), a maximum gain of 7.9 dB is attained. At 5.8 GHz of Mode 2(Ө = −40°), a maximum gain of 8.1 dB is attained. At 6.1 GHz of Mode 2(Ө = 60°), a maximum gain of 8.2 dB is attained.

(a)

(b)

Electronics 2022, 11, 2517 14 of 17

(c)

(d)

(e)

(f)

Figure 11. Three-dimensional and measured vs. simulated two-dimensional radiation patterns (a) 5.4 GHz (b) 5.5 GHz (c) 5.6 GHz (d) 5.7 GHz € 5.8 GHz (f) 6.1 GHz.

Figure 11. Cont.

Electronics 2022, 11, 2517 13 of 16

Electronics 2022, 11, 2517 14 of 17

(c)

(d)

(e)

(f)

Figure 11. Three-dimensional and measured vs. simulated two-dimensional radiation patterns (a) 5.4 GHz (b) 5.5 GHz (c) 5.6 GHz (d) 5.7 GHz € 5.8 GHz (f) 6.1 GHz.

Figure 11. Three-dimensional and measured vs. simulated two-dimensional radiation patterns(a) 5.4 GHz (b) 5.5 GHz (c) 5.6 GHz (d) 5.7 GHz (e) 5.8 GHz (f) 6.1 GHz.

Figure 12 providing the simulated and measured reflection coefficient of the antennawith perfect matching between them. The gain and efficiency plot in Figure 13 providesaverage gain of 7.9 dBi and an efficiency of more than 90%.

Figure 14 provides the measurement setup of the proposed antenna. The comparativeanalytical study of the proposed model with the literature is tabulated in Table 5. The gainand efficiency are showing better performance characteristics when compared with existingantenna models.

Electronics 2022, 11, 2517 15 of 17

Figure 12 providing the simulated and measured reflection coefficient of the antenna with perfect matching between them. The gain and efficiency plot in Figure 13 provides average gain of 7.9 dBi and an efficiency of more than 90%.

Figure 12. The measured and simulated reflection coefficient of the antenna.

Figure 13. The measured and simulated gain and efficiency of the antenna.

Figure 14 provides the measurement setup of the proposed antenna. The compara-tive analytical study of the proposed model with the literature is tabulated in Table 5. The gain and efficiency are showing better performance characteristics when compared with existing antenna models.

Figure 14. The measurement setup.

5.2 5.4 5.6 5.8 6.0 6.2-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

S 11(d

B)

Frequency (GHz)

Simulated Measured

5.5 5.6 5.7 5.8 5.9 6.0 6.17.47.57.67.77.87.98.08.18.28.38.48.5

Simulated Gain Measured Gain Simulated Efficiency Measured Efficiency

Frequency (GHz)

Gai

n (d

B)

90.0

90.1

90.2

90.3

90.4

90.5

90.6

90.7

90.8

90.9

91.0

Effic

ienc

y (%

)

Figure 12. The measured and simulated reflection coefficient of the antenna.

Electronics 2022, 11, 2517 14 of 16

Electronics 2022, 11, 2517 15 of 17

Figure 12 providing the simulated and measured reflection coefficient of the antenna with perfect matching between them. The gain and efficiency plot in Figure 13 provides average gain of 7.9 dBi and an efficiency of more than 90%.

Figure 12. The measured and simulated reflection coefficient of the antenna.

Figure 13. The measured and simulated gain and efficiency of the antenna.

Figure 14 provides the measurement setup of the proposed antenna. The compara-tive analytical study of the proposed model with the literature is tabulated in Table 5. The gain and efficiency are showing better performance characteristics when compared with existing antenna models.

Figure 14. The measurement setup.

5.2 5.4 5.6 5.8 6.0 6.2-20

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-8

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S 11(d

B)

Frequency (GHz)

Simulated Measured

5.5 5.6 5.7 5.8 5.9 6.0 6.17.47.57.67.77.87.98.08.18.28.38.48.5

Simulated Gain Measured Gain Simulated Efficiency Measured Efficiency

Frequency (GHz)

Gai

n (d

B)

90.0

90.1

90.2

90.3

90.4

90.5

90.6

90.7

90.8

90.9

91.0

Effic

ienc

y (%

)

Figure 13. The measured and simulated gain and efficiency of the antenna.

Electronics 2022, 11, 2517 15 of 17

Figure 12 providing the simulated and measured reflection coefficient of the antenna with perfect matching between them. The gain and efficiency plot in Figure 13 provides average gain of 7.9 dBi and an efficiency of more than 90%.

Figure 12. The measured and simulated reflection coefficient of the antenna.

Figure 13. The measured and simulated gain and efficiency of the antenna.

Figure 14 provides the measurement setup of the proposed antenna. The compara-tive analytical study of the proposed model with the literature is tabulated in Table 5. The gain and efficiency are showing better performance characteristics when compared with existing antenna models.

Figure 14. The measurement setup.

5.2 5.4 5.6 5.8 6.0 6.2-20

-18

-16

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-12

-10

-8

-6

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S 11(d

B)

Frequency (GHz)

Simulated Measured

5.5 5.6 5.7 5.8 5.9 6.0 6.17.47.57.67.77.87.98.08.18.28.38.48.5

Simulated Gain Measured Gain Simulated Efficiency Measured Efficiency

Frequency (GHz)G

ain

(dB)

90.0

90.1

90.2

90.3

90.4

90.5

90.6

90.7

90.8

90.9

91.0

Effic

ienc

y (%

)

Figure 14. The measurement setup.

Table 5. Comparison of proposed and other antenna designs (NR: not reported, DB: dual band).

Reference Dimensions(mm ×mm)

Bandwidth(GHz)

AverageGain (dBi)

AverageEfficiency

[5] 132 × 132 5.71–5.88 13.7 NR

[15] 78 × 78 4.20–5.59 8.2 95%

[32] 80 × 60 3.27–4.66 7.7 NR

[33] 60 × 60 4.9–5.1 11.6 NR

[34] 27.5 × 27.5 × π 5.07–5.94 7.63 >80%

[35] 20 × 20 9.798–10.20214.09–15.91

8.249.65

82%87%

Proposed 27.52 × π 5.5–6.1 7.9 >90%

5. Conclusions

A non-uniform slotted array with a multilayer structured antenna is designed in thiswork for WLAN communication applications. Characteristic mode analysis has been exam-ined to optimize the antenna performance characteristics as per the application specification.The slotted structure metamaterial characteristics are analyzed for the designed model withrespect to negative permittivity and negative permeability at the targeted operating band.Characteristic angle, modal significance parameters, and eigenvalues are analyzed andpresented with respect to the resonant frequency in the current work. An average gain of7.9 dBi, impedance bandwidth of 105%, and efficiency of more than 90% is attained withgood matching between simulation and measurement results.

Electronics 2022, 11, 2517 15 of 16

Author Contributions: Conceptualization: B.T.P.M. and S.D.; methodology: S.S.A.; software: K.V.B.;validation: K.D.B. and B.T.P.M.; writing—original draft preparation: K.D.B.; writing—review andediting: S.D., N.H. and S.S.A.; project administration: N.H.; All authors have read and agreed to thepublished version of the manuscript.

Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: The data presented in this research are available on request from thecorresponding author.

Acknowledgments: Thanks to DST FIST- SR/FST/ET-II/2019/450 for providing research facilities.

Conflicts of Interest: The authors declare no conflict of interest.

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