Issue
Eur. Phys. J. Appl. Phys.
Volume 88, Number 3, December 2019
Advanced Electromagnetic Materials and Devices (META 2019)
Article Number 30901
Number of page(s) 10
Section Physics of Energy Transfer, Conversion and Storage
DOI https://doi.org/10.1051/epjap/2020190311
Published online 03 March 2020

© EDP Sciences, 2020

1 Introduction

Metamaterials (MTMs) are synthetic composites consisting of periodic or non-periodic subwavelength metal/dielectric particle arrays designed to achieve inherently inaccessible electromagnetic (EM) properties. Due to their strong ability to flexibly manipulate the propagation of electromagnetic waves, they have attracted considerable interest in both the scientific and engineering communities over the last 20 years.

Possible metamaterial characteristics have four quadrants in the permittivity (ε) − permeability (µ) domain. The first quadrant of the ε-µ diagram depicts right-handed materials (RHM) supporting the propagation of forwarding waves where ε > 0 and µ > 0 are most frequently discovered in nature. The magnetic field (H), the electric field (E), and the wave vector (K) form a right-handed system in this quadrant, as Maxwell's equations portray. The second quadrant depicts electrical plasmas that support waves of evanescence where ε < 0 and µ > 0 are present. The fourth quadrant also supports evanescent waves, but it represents magnetic plasmas in which ε > 0 and µ < 0. These are both single adverse (SNG) quadrants, meaning that negative is only one efficient parameter. Nevertheless, we concentrate on the third quadrant. The quadrant reflects the left-handed materials (LHM), where ε < 0 and µ < 0 over a defined frequency area simultaneously. In LHM, the electric field (E), the magnetic field (H), and the wave vector (k) form a left-handed scheme, which promotes backward wave propagation. The term backwards refers to the phase and group velocity of the opposite sign. The refractive index is negative. These types of materials, however, cannot be discovered in nature, which is why scientists artificially generate them.

The earliest metamaterials work dates back to Veselago's (1968) analysis of left-handed material (LHM) [1] in which the properties of a material with simultaneously negative permittivity and permeability are theoretically studied. Specific characteristics, such as negative refraction, backward propagation of waves, were also predicted. Nevertheless, no test can be performed to confirm the prediction due to the absence of natural LHMs. For almost 30 years, little attention was paid to the theory of LHMs until Pendry et al. realised negative permittivity using artificially formed metallic wires [2] in 1996. Using split-ring resonators (SRRs) [3], negative permeability was also reached later in 1999. Through integrating the two underlying artificial structures, Smith et al. simultaneously realised negative permittivity and permeability and for the first time in 2001 observed negative refraction in the experiment [4]. Much research has been done on LHMs since then [59], and since then, metamaterials are using for different types of applications successfully.

Metamaterials inspired antenna becomes more popular it helps to enhance the performance of the antenna by increasing the bandwidth, the gain or efficiency and also it helps to decrease the size of the antenna to make it more compact [10]. Designing microwave absorbers using metamaterial becomes more popular to increase the efficiency of solar cell and different applications [11]. Among various methods to decrease or control the value of specific absorption rate while using electromagnetic devices, metamaterials attract significant concentration [12]. To avoid possible interferences in a system, different passband filters have been designed utilising metamaterials at a specific frequency of operation [13]. As metamaterials can exhibit a better localisation and better fields enhancement, it can improve the sensor selectivity of detecting nonlinear substances, and it also enables detection of a small number of analytes [14]. For this, metamaterials have been using rapidly for different types of sensing applications [15]. A metamaterial is an implantable circuit element, thus now a day it has been using for different biomedical applications [16]. Furthermore, metamaterials have been using successfully to improve MRI imaging quality and also reduce the scan time [17]. Another advancement of metamaterial is that it is using for invisibility cloaking operation [18].

In 2011, Cheng et al. [19] developed a broadband planar SRR metamaterial resonating at 10.02 GHz where the metal-dielectric-metal structure exhibited strong electrical and magnetic responses simultaneously which leads to negative permittivity and negative permeability to make it negative index metamaterial (NIM). Cheng et al. [20] in 2012, demonstrated a 3D NIM composed by a 3D array of the cubic unit cell which has an operating region from 7.5 to 15 GHz. FR4 with dielectric constant (εr ) 4.9 was chosen as the dielectric cube. In 2014, Liu et al. [21] proposed multi-band terahertz metamaterial with combined ring pairs and cross pairs which work in terahertz frequency regime. The metamaterial resonates at 0.43 THz, 0.84 THz, and 1.32 THz, where the first two was for negative index bands and the last one was for positive index band. Later, Fang et al. [22] presented a numerical study on 3D broadband isotropic LHM. The 3D cube was composed by a dielectric cube where closed metallic rings were printed on all six sides of its. In comparison with 2D, the 3D structure had a broad passband from 7 to 13 GHz.

In this article, we present a novel miniaturised EF-structured metamaterial loaded with split ring resonator (SRR) along with the addition of the simulated results with experimental results operating at microwave frequency band. The self and mutual coupling effects of the resonator strips leads to multi-resonant phenomena of the developed metamaterial. The EF structure metamaterial also exhibited double-negative characteristics. Therefore, the developed metamaterial can be used for modern portable electronic communications as well as with satellite communications.

2 Design procedure of metamaterial unit cell and it's equivalent circuit

The main approach of this study is to develop a miniaturised metamaterial for advanced practical applications of the microwave frequency range. Finite Integration Techniques based software has been utilised for the design layout and elaborate electromagnetic analysis. Metamaterials are the only materials that affect electromagnetic waves which interact with its structural arrangement or features. After a series of development, miniaturisation has been achieved by increasing the inductive loading, i.e. increasing the metal strips to develop EF-shaped resonator, which is opposite to each other. After that, SRR is added at the outer side of the resonators to bring the capacitance effect. The main reason for incorporating SRRs with EF-structure is for a better electric and magnetic response. Because SRRs possess strong magnetic response which leads to negative permeability and our developed EF-structure possess strong electrical response which leads to negative permittivity. The evolution of the geometry of EF-structured unit is shown in Figure 1.

All the resonators are made by copper having a thickness of 0.035 mm and etched on the upper side of Flame Retardent-4 (FR4) dielectric substrate material. FR4 is a low-cost dielectric material which loss tangent, dielectric constant, and thickness of the substrate is 0.008, 4.2, and 1.6 mm, respectively. SRR behaves like LC resonator as the splits represent the capacitance and the metal strip represents the inductance as well as the double inverse E-shaped metal strip. Splits gap (g) and the metal resonator width (W1) are set to 0.2 mm and 0.45 mm, respectively, after parametric studies of these two parameters. The other parameters are listed in Table 1. The resonance frequency is set by both the inductance and the capacitance of the structure by interacting with each other. For the validation of the designed structure, an array of the developed unit cell has been fabricated because metamaterial works as an assembly of multiple individual elements. An array of 20 × 20 unit cells has been fabricated. The size of the metamaterial slab is 220.20 × 220.20 mm2. Figure 2a–c presents the geometrical parameters and fabricated experimental prototype of the unit cell along with scale measurement and the array metamaterial prototype, respectively.

As capacitive and inductive element develops the metamaterial structure, thus it can be represented by the electrical inductive-capacitive circuit. The splits represent the capacitance, and the metal strips represent the inductance.

Equation for capacitance formed among the splits or gaps can be given by quasi-static theory as [23]:(1)

Thus, the equation for total capacitance (CT ) is:(2)and, the equation for total inductance (LT ) is:(3)where, ε 0 = free-space permittivity = 8.854 × 10−12 F/m, μ 0 = free-space permeability = 4π × 10−7 H/m.

The splits or gaps of the structure behaves like capacitive element which denotes as C1, C2, C3, C4, C5, and C6. The inductance elements were metal strips denotes as L1, L2, L3, L4, L5, L6, L7, and L8. The equivalent electrical circuit is drawn in Figure 3.

thumbnail Fig. 1

Evolution of the design of the developed metamaterial structure.

Table 1

Parameters of the EF-structured unit cell metamaterial.

thumbnail Fig. 2

(a) Geometrical configurations of the EF-structure resonator, (b) fabricated unit cell, and (c) fabricated 20 × 20 array of metamaterial with unit cell enlarged view.

thumbnail Fig. 3

An equivalent capacitive-inductive circuit for the EF-shaped structure.

3 Method and technique

In this article, all the simulations and analysis, including SAR analysis, has been performed using the CST Microwave Studio simulation package. Figure 4 exhibits the simulation set up to find out the S-parameters of the metamaterial. The resonator has been tested on the electromagnetic field to verify its working range of frequency. The two wave-guide ports have been put at the positive and negative end, on the z-axis of the resonator, and the electromagnetic wave w spread between the ports. The electrical field has been given towards the x-axis, and the magnetic field has been given towards the y-axis.

In this study, the Nicolson-Ross-Weir [NRW] method has been utilised throughout the work to extract extraordinary characteristics. In this method, permittivity (ε) and permeability (µ) are obtained using simulated or measured S-parameters which is more convenient than any other techniques. The simplified equations of NRW method [24] are as follows:(4) (5) (6) (7) (8)

These equations [48] has been adopted to determine the metamaterial retrieval parameters, i.e. effective permittivity, permeability, and refractive index.

An Agilent N5227 vector network analyser (VNA) addition with an Agilent N4694–60001 calibrator has been utilised to find out the S-parameters of the metamaterial. A set of waveguides has been used for measurement purpose. These waveguides were 510WCAS, 340WCAS, 187WCAS, 137WCAS, 75WCAS for the frequency ranges 1.45–2.20 GHz, 2.20–3.30 GHz, 3.95–5.85 GHz, 5.85–8.20 GHz, 10–15 GHz, respectively. The fabricated EF-structure metamaterial has been set in between the two waveguides for measurement, as presented in Figure 5.

thumbnail Fig. 4

Simulation setup for the metamaterial.

thumbnail Fig. 5

Experimental measurement of S-parameters by waveguides.

4 Results and discussions

The reflection characteristics (S 11) and transmission characteristics (S 21) of the metamaterial unit cell has been analysed by applying the finite integration technique (FIT) of the simulation software. A series of different arrangements and simulations have been executed for the development of the unit cell. For the sake of clarity, development procedures and other results are not included here.

The simulated reflection and transmission characteristics of the unit cell metamaterial are shown in Figure 6a and b, respectively. Besides, the multi-resonance performance is also obtained despite achieving miniaturisation of the unit cell. This is caused because of the mutual coupling of metallic strips. As a metamaterial, five resonance frequencies were observed from the transmission coefficient at 1.82 GHz, 2.22 GHz, 4.82 GHz, 5.89 GHz and 10.54 GHz with the transmission drop of −24.66 dB, −14.61 dB, −23.64 dB, −21.97 dB, −15.97 dB, respectively.

As Figure 7 represents the measured results, the resonance frequency obtained at 1.80 GHz, 2.20 GHz, 4.78 GHz, 5.87 GHz, 10.52 GHz, respectively. There are minor deviations seen if we compare the simulation results to the measured results. However, these variations might be due to the calibration error, a mutual resonance between the transmitting and receiving ends of the waveguide, minor fabrication errors etc. The resonance frequency also depends on the permittivity of the dielectric substrate. Conjunction with dielectric constant, capacitance values are changed and also responsible for shifting of the resonance frequency. The breaks in Figure 7 are used to clarify that different waveguides have been used for the specific frequency range.

The effective medium parameters such as the permittivity (ε), permeability (µ), and refractive index (η) of the engineered structure are computed from the scattering parameters (S 11 and S 21) using the NRW method already discussed in Section 3. The extracted real and imaginary parts of the effective the permittivity (ε), permeability (µ), and refractive index (η), were shown in Figure 8a–c, respectively. From Figure 8a and b, it can be seen that the metamaterial shows negative permittivity from 1.63 to 2.01 GHz, 3.27 to 4.99 GHz, 5.31 to 5.94 GHz, 6.58 to 8.19, negative permeability from 5.97 to 10.27 GHz. It also shows negative refractive index characteristics from 3.6 to 4.6 GHz, 5.48 to 8.12 GHz, 8.39 to 8.79 GHz, as presented in Figure 8c.

To explore the electromagnetic characteristics of the EF-structure metamaterial, the surface current, electrical field, and magnetic field distributions at the five resonance frequencies are shown in Figure 9a–c, sequentially to exhibit the physical properties of achieving the electrical and magnetic resonance from the developed metamaterial. The mutual coupling effects among the electrical fields and the gaps which behave as capacitive element were responsible for the electric resonances. On the other hand, the mutual coupling effects among the metal strips, which acts as an inductive element and the magnetic fields are responsible for the magnetic resonances. These electric and magnetic resonances originate when the electromagnetic wave propagation applied and passes through the metamaterial structure. From Figure 9a, it can be seen that the current circulation at the lower resonance was very high contrasted with the mid and upper frequencies. Also, a clockwise and anti-clockwise current flow in the metallic structure of spiral strips, preventing the excitation of magnetic resonance. Therefore, an electric resonance behaviour is achieved for both lower and middle resonance frequencies. From Figure 9a, for higher resonance frequency, a relatively high dispersive field noticed in contrast with lower resonance. Furthermore, the weakest surface current circulation is observed at upper frequency. Because of higher dispersive field distributions together with weak surface current, the developed metamaterial creates magnetic resonance at the upper resonance frequency.

Figure 9b which represents the electric field distribution, it can be seen that the field is highly concentrated towards the gap along the direction of the incident electrical field when compared to higher resonance in the lower resonance. The capacitive coupling of the unit cell is increased because of the large number of concentrated charges. As a result, comparing lower resonance to the middle resonance, a strong electrical resonance property was observed for the lower resonance. On the other hand, because of the dispersive field distributions, a weak electrical resonance property was seen at mid-resonances.

Figure 9c depicted the magnetic field distributions at the resonance frequencies. It can be seen that the magnetic fields are perpendicular with the electric fields, which are equivalent to a magnetic moment of the dipole. The magnetic moment of the dipole produced by the electrical field was responsible for generating the artificial magnetism, which resulted in an effective negative permeability.

The SRR loaded EF-structure metamaterial is composed of a metallic layer over a dielectric substrate, where both of the elements are nonmagnetic. The only cause of magnetic resonance is, therefore, the moving currents powered by the capacitance between the SRR splits or gaps between the metal strip. Usually, negative permeability results from a strong resonance response to an external magnetic field, whereas negative permittivity can be obtained through both a plasmonic and external electric field response. Both the electrical field and the magnetic field interacted in the opposite direction as electromagnetic waves travelled through the structure, consistent with the distribution of surface current.

A parametric study is presented with numerical simulations to determine the effect of changing metallic strip width (w), split gap (g) and different dielectric substrate on the transmission characteristics of the EF-structure metamaterial. Figure 10a–c represents the transmission response of the developed metamaterial structure by varying w, g, and different dielectric substrates, respectively. From Figure 10a, it can be noticed that increasing the width of the metallic resonator strip results in self-inductance enhancement of the EF-structure resonator, which tends to shift the operational frequency to lower frequency, however on this case, the size of the unit cell increases with the increase of the width. Decreasing the width results in higher resonance frequency by lowering the self-inductance.

Furthermore, it can be noticed from Figure 10b that the operational frequency is almost stable for the split gap 0.1, 0.2, 0.3 and 0.5, which confirms the conformity of the design. Slight resonance shifting due to increasing the difference happens because of the gap capacitance decreases. Thus, the precise values of w and g were fixed at 0.2 and 0.4 mm, respectively, to confirm the miniaturised size of the metamaterial structure and also to confirm the stability of the structure that can be applied different specified applications.

Figure 10c shows the transmission results of the developed metamaterial structure on different dielectric substrates. The dielectric substrate material is an essential element for designing the metamaterial. The dielectric constant of a substance depends on the material's internal structure. Increasing the dielectric constant of the material decreased the conductivity of the material. However, the metamaterial structure shows stable results for FR4 dielectric substrate, where is there is noise, fluctuations for some other conventional dielectric substrate, like Rogers RT5880, Rogers RO4003C and Taconic RF-10 etc.

thumbnail Fig. 6

Simulated characteristics in dB versus frequency (a) reflection coefficient (S11), and (b) transmission coefficient (S21).

thumbnail Fig. 7

Measured transmission curve (S21) of the unit cell from 1 to 11 GHz.

thumbnail Fig. 8

Extracted real and imaginary values (a) permittivity (ε), (b) permeability (µ), and (c) refractive index (η) of the engineered structure.

thumbnail Fig. 9

Instantaneous distribution of (a) surface current, (b) electric field and (c) magnetic field at 1.82 GHz, 2.22 GHz, 4.82 GHz, 5.89 GHz and 10.54 GHz.

thumbnail Fig. 10

Transmission (S 21) response for the developed metamaterial at different values of (a) metallic strip width (w), (b) split gap (g), and (c) different dielectric substrates.

5 Specific absorption rate (SAR) analysis

As the metamaterial is designed for communication application as DCS, 5G application, thus we have investigated the performance of MTM using it with the head phantom for the measurement of SAR reduction. A hand and a mobile phone with a PIFA antenna were also attached with the hand phantom. The stimulated power output from the mobile was considered as 0.5 W for measuring the SAR values. We have done SAR evaluations at GSM 900 MHz and GSM 1800 MHz because these GSM bands are cellular frequencies designed by the International Telecommunication Union (ITU) for the operation of mobile phones all over the world.

For the measurement of SAR at the head phantom, the unit cell of the metamaterial has been placed in the circuit board of the mobile phone model. SAR value is evaluated for GSM 900 Band and GSM 1800 Band, and it is measured while the mobile phone is in check position. Significant reduction of SAR is achieved, which is shown in Table 2.

Table 2 shows the summaries SAR values including without metamaterial attachment and with metamaterial attachment.

The SAR reduction was accomplished by attaching a 3×4 metamaterial unit cell because of the size limitation of mobile phones.

Table 3 represents a comparison of the developed metamaterial with the existing metamaterials designed by the different researchers throughout the world.

Mallik et al. [21] presented two-rectangular U-shaped metamaterial that showed negative permeability and negative permittivity simultaneously. He found resonance frequency at 6.02 GHz and only applicable for C-band. Then, Islam et al. [22] developed a modified H-shaped metamaterial with a split gap which showed resonance at 2.74 GHz, 7.12 GHz, 10.853 GHz and 14.337 GHz that is applicable for multi-band applications. However, his metamaterial size was 30 × 30 mm2 that is not suitable modern small portable electronic communication. Hossain et al. [23] presented a new double negative metamaterial with different arrays working in S and C band whose size is 12 × 12 mm2. Hasan et al. [24] developed a 10 × 10 mm2 S-shaped metamaterial which works in X-band only. Hossain et al. [25] developed a double C-shaped metamaterial with different array configurations which showed a resonance at 3.36 GHz, 8.574 GHz, and 11.57 GHz.

Table 2

SAR values without and with metamaterial.

Table 3

Comparison between developed metamaterial and existing metamaterials.

6 Conclusion

In this article, a novel SRR loaded EF-shaped metamaterial is presented, which is applicable in multi-band frequency applications. Despite the size miniaturisation, it has five resonance frequencies, which are so vital for modern electronic communications. The developed metamaterial has a highly effective medium ratio of about 14.82. The metamaterial also exhibits double-negative properties along with negative refractive index over multiple frequency ranges. The surface current distribution and electrical field distribution also discussed the effectiveness behaviour of the metamaterial. As the metamaterial was for electronic communications, thus the specific absorption rate also calculated. With metamaterial, for 900 MHz, SAR reduction found about 43%, and for 1800 MHz, it is about 44%. Overall, a novel unit cell metamaterial has been presented where the compactness of the size of the metamaterial unit cell and its performances makes it compatible to use for the corresponding mentioned applications.

Author contribution statement

Ahmed Mahfuz Tamim made substantial contributions to conception, design analysis, and writing. Mohammad Rashed Iqbal Faruque participated for critical revision of the article. Mohammad Tariqul Islam and Sabirin Abdullah provided necessary instructions for experimental purposes.

Acknowledgments

This work was supported by Fundamental Research Grant Scheme (FRGS), MOE, Malaysia, Code: FRGS/1/2018/TK04/UKM/02/13.

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Cite this article as: Ahmed Mahfuz Tamim, Mohammad Rashed Iqbal Faruque, Mohammad Tariqul Islam, Sabirin Abdullah, Split ring resonator loaded EF-structured left-handed metamaterial for modern electronic communications, Eur. Phys. J. Appl. Phys. 88, 30901 (2019)

All Tables

Table 1

Parameters of the EF-structured unit cell metamaterial.

Table 2

SAR values without and with metamaterial.

Table 3

Comparison between developed metamaterial and existing metamaterials.

All Figures

thumbnail Fig. 1

Evolution of the design of the developed metamaterial structure.

In the text
thumbnail Fig. 2

(a) Geometrical configurations of the EF-structure resonator, (b) fabricated unit cell, and (c) fabricated 20 × 20 array of metamaterial with unit cell enlarged view.

In the text
thumbnail Fig. 3

An equivalent capacitive-inductive circuit for the EF-shaped structure.

In the text
thumbnail Fig. 4

Simulation setup for the metamaterial.

In the text
thumbnail Fig. 5

Experimental measurement of S-parameters by waveguides.

In the text
thumbnail Fig. 6

Simulated characteristics in dB versus frequency (a) reflection coefficient (S11), and (b) transmission coefficient (S21).

In the text
thumbnail Fig. 7

Measured transmission curve (S21) of the unit cell from 1 to 11 GHz.

In the text
thumbnail Fig. 8

Extracted real and imaginary values (a) permittivity (ε), (b) permeability (µ), and (c) refractive index (η) of the engineered structure.

In the text
thumbnail Fig. 9

Instantaneous distribution of (a) surface current, (b) electric field and (c) magnetic field at 1.82 GHz, 2.22 GHz, 4.82 GHz, 5.89 GHz and 10.54 GHz.

In the text
thumbnail Fig. 10

Transmission (S 21) response for the developed metamaterial at different values of (a) metallic strip width (w), (b) split gap (g), and (c) different dielectric substrates.

In the text

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