Metamaterial Inspired Electrically Small Multiband Monopole Antenna Using Single DNG MTM and Ring Resonators


 This paper proposes a new metamaterial inspired electrically small multi-band monopole antenna. The proposed antenna is capable of operating at DCS 1800 in the lower band. At the same time, in the higher band, it covers two wireless local area network (WLAN) bands at 2.4 and 5.2 GHz. This paper describes the design and detailed analysis of an electrically small (ka = 0.64 < 1) antenna with a -10dB fractional bandwidth of 2.01%, 1.64% and 2.71% for triple-band operations with centre frequencies 1.80, 2.45 and 5.17 GHz. The compactness is achieved by the application of double negative metamaterial on a monopole antenna operating at 9 GHz. The proposed antenna has an overall compact electrical size 0.14 λ0 × 0.14λ0 × 0.01 λ0 at 1.8 GHz and physical dimensions 24 × 24 × 1.6 mm3 including the ground. In this proposed ESA a second DNG structure is also incorporated to enhance its gain. This enables reliable operations at DCS 1800 and WLAN frequencies 2.4 and 5.2 GHz.


INTRODUCTION
The requirement for greater compactness of antenna in the field of wireless communication systems is increasing every day. Electrically small antennas (ESAs) attract researchers because they fulfil this requirement to a great extent despite their limitations such as lack of wide band and high gain. The distinct advantages of ESAs include compactness of size, simplicity of structure etc. and hence they can be incorporated into small sized devices quite easily. However because of their small electrical size, they have disadvantages like high reactance and low radiation resistance.
ESAs are compact antennas having physical dimensions far below one wavelength at their lowest frequencies of operation. One of the widely used mathematical definitions developed for ESA is ka < 1 in which 'k' is the wave number (2π/λ) and 'a' is the spherical radius around the highest dimension of the unit cell. In ESAs decrease of the value of electrical size 'ka' closely correspond to the decrease of parameters like bandwidth, efficiency and gain.
Radiation pattern of most of the ESAs are omnidirectional, similar to that of dipole antennas. Different techniques such as employing a shorting post [1], microstrip fed slot techniques [2] and defected ground structure [3] have been proposed by researchers for developing ESAs for various applications. The performance of ESAs and larger antennas are more or less equal under matched conditions. However it is more difficult to achieve matching in ESAs when compared to larger antennas. In order to improve the impedance matching of ESAs two methods are commonly employed by researchers. In one method matching networks are used [4][5][6]. Here the entire system include the antenna and its matching network. Hence the entire system is cumbersome and deviates from the very concept of an ESA. A more preferable method of improving impedance matching is through structure modification of the antenna [7][8][9]. However there are certain difficulties in this method also. For example, even though modification of the structure of an ESA gives proper impedance matching with a 50 Ω system connected to it, its Quality factor (Q) becomes very high and goes beyond the Chu limit. The bandwidth and the Q factor of an antenna are inversely proportional and hence this method also imposes a limitation on the antenna's bandwidth [10]. Secondly, impedance matching networks have some inherent losses associated with them. These losses may dominate over the radiation resistance of an ESA degrading its efficiency below the desired level. Thirdly, the limit for gain bandwidth proposed by Fano-Bode comes in as a limitation when we use lossless techniques of passive matching networks [11][12][13]. The concept of MTM-inspired antennas was developed by Erentok and Ziolkowski [7].
MTM-inspired antenna is developed by placing an MTM unit-cell in the vicinity of the driven element, and utilise its features for enhancing the properties of the driven element. As a result the overall radiation efficiency and impedance matching of the antenna is improved without using additional matching networks [7][8][9]. The MTM unit-cell functions as the parasitic element in the near-field and becomes the dominant radiating component of the overall system at the desired resonant frequency [8].
MTM loading on monopole antennas function as additional capacitive and inductive elements in the near-field. However they reduce the bandwidth of the overall structure [14].
Electric-field-coupled and complementary electric-field-coupled resonators are parasitic structures used to load ESAs. But their bandwidth remain consistently below 2.55% [15]. ACSfed dual band flipped MIMO antenna integrated in an actual USB module, which achieves fractional bandwidths upto 22% has been proposed [16]. Active devices can also be employed to improve the bandwidth of ESAs [17]. Antenna bandwidth can also be increased by using modified electric coupled resonator with the disadvantage of large size and low gain [18]. Yet another ESA which operates at 2.4 GHz frequency is proposed [19]. It consists of a single parasitic MTM unit-cell attached to the ground plane, which reduces the size of the antenna considerably with good impedance matching. Moreover, in order to study the effect of NFRP element on structure of antenna, characteristic mode theory is also employed. Recently a very small antenna is designed using exclusively stamped metal for IoT/RFID applications with high radiation efficiency upto 80% and scalable to other operation bands 20].
In this paper, a compact triple-band MTM antenna design based on double negative metamaterial (DNG MTM) [20] and ring resonators is presented. Impedance matching and gain enhancement using alteration of ground plane and DNG loading are explored. An elaborate description and theoretical analysis of this proposed antenna are conducted and discussed in section 2, 3, 4 and 5 of this paper. In section 2 the evolution of the design of the antenna and geometrical parameters are described. Section 3 describes the extraction of DNG property in the MTM. Section 4 makes a parametric analysis of the various design parameters and identifies the optimum values for operating at the desired frequencies. In section 5 simulated and measured results are compared and analysed. Finally, in section 6 the advantages and novelty of the proposed antenna are described as the conclusion.

ANTENNA GEOMETRY AND DESIGN
The triple-band MTM antenna structure proposed here is simulated and fabricated using low-cost FR-4 substrate with relative permittivity (εr) 4.4, loss tangent (tan δ) 0.02 and substrate height (h) 1.6 mm. The overall dimensions of the antenna are 24 × 24 × 1.6 mm 3 . The design of the proposed antenna is evolved through different stages in order to operate at all the desired frequency bands 1.8/2.4/5.2 GHz by the optimization of various parameters. Figure 1 depicts the design stages.
The proposed design is evolved from a monopole operating at 9 GHz with truncated ground (step 1 of Figure 1). In the next stage, this monopole is converted into metamaterial inspired monopole yielding two operating bands at 7 GHz and 9 GHz (step 2 of Figure 1)by incorporating a MTM structure exhibiting DNG property in C-band [21].
The MTM inspired monopole structure of step 2 is incorporated with two closed ring resonators with inner radius R2 and R3 respectively (Table 1) The configuration of the designed antenna is shown in Figure 3. Figures 3(a) and 3(b) show front view and back view of the final optimised structure of ESA operating at multifrequency bands. The final optimized parameters of the proposed antenna are given in Table.1.

EXTRACTION OF THE DOUBLE NEGATIVE PROPERTY IN MTM
The negative permittivity and negative permeability are retrieved from the S-parameters based on [21]. To extract the S11 and S21 characteristics of MTM, a two-port waveguide configuration as shown in Figure 4 can be used. For this boundary conditions are to be applied in such a way that the walls of the waveguide use a pair each, of both perfect magnetic conductor (PMC) and perfect electric conductor (PEC) along the planes x-y and y-z respectively. MTM structure is modelled as PEC on a dielectric slab located at the centre of the waveguide.
Two waveguide ports are placed parallel to x-z planes. Then the input wave is launched from port1 and S11 and S21 are determined. Using these values, permittivity, permeability, refractive index and impedance can be retrieved by S-parameter retrieval method using the following equations. These equations are implemented in MATLAB™. (2) Where 'k0 'is the wavenumber in free space and 'm' represents branch index of refractive index which is an integer. Relative permittivity ɛ and relative permeability µ can be calculated using the relations (4)

Effect of the Gap Length (G1)
Gap length (G1) is one of the significant parameters because it sustains the DNG property of the metamaterial structure used in the proposed ESA. In figure 6(a) the impact of gap length on return loss (S11) characteristics of the designed ESA is plotted. It can be found that the desired resonances remain unchanged as the G1 between the stubs of DNG is changed from 10 mm to 0.2 mm. Yet, the optimum gap of G1 = 0.2 mm is chosen because all the required frequency bands resonate with good matching and maximum gain at this gap length.

Effect of DNG Radius (R1)
The effect of DNG radius (R1) on return loss is plotted in Figure 6(b). We can observe that the first and third resonances are unaffected as the DNG radius is altered. On the other hand, increase or decrease in the DNG radius has considerable impact on the second resonant frequency, which decreases as R1 increases.

Figures 7(a) and (b)
show the effect of R2 and R3 on resonance frequencies. R2 significantly affects the second resonance whereas R3 has its greatest effect on the first resonance.
Alterations in both R2 and R3 cause minor shift in the third resonance also.

RESULTS AND DISCUSSIONS
As per the values proposed in Table 1, a prototype has been fabricated and tested. The photographs of the fabricated antenna and the experimental setup for its measurement are shown in Figure 8. The simulations are done using Ansoft HFSS and measurements are made using Vector Network Analyzer. The simulated and measured S11, plotted in Figure 9, are in good agreement. The -10 dB impedance bandwidths at three operating bands 1.8 GHz, 2.4 GHz and 5.2 GHz are 2.01%, 1.64% and 2.71% respectively, which cover the bands of DCS and some bands of WLAN simultaneously. To understand the influence of DNG structure on the monopole antenna, simulated surface current distribution at 1.8 GHz, 2.4 GHz, and 5.2 GHz frequencies are illustrated in Figure   10. The bands centered at 5.17 and 2.45 GHz are caused by the DNG structure, as maximum surface current density is found in the DNG at these frequencies. The first band at 1.8 GHz is caused by the presence of outer ring resonator, which contains maximum surface current density at this frequency. It also shows the coupling effect between ground plane and feed as well as coupling between DNG structure and second ring resonator. Coupling of the surface currents at the ground plane and feed also provide proper matching at first two bands. The bandwidth of the proposed ESA is delimited by the theoretical limit for ESAs proposed by Chu [22]. This theoretical limit of quality factor ( ) for ESAs, is expressed by Chu as where 'k' stands for the free space wave number, 'a' stands for the spherical radius around the highest dimension of the radiating part of the antenna.
The maximum bandwidth can also be expressed using Chu limit as Where FBWmax represents the maximum fractional bandwidth [23]. For the proposed ESA, k = 37.67 rad/m at the first resonant frequency 1.8 GHz, a = 16.97 mm and therefore ka = 0.64 < 1. Hence, by definition, the proposed antenna is electrically small. On applying (6) and (7), the FBWmax that can be achieved at VSWR = 2, is 13.15% and the bandwidth of 2.01% obtained is well below the maximum limit of bandwidth.
The 2D radiation patterns at 1.  It should be borne in mind that all the radiating energy of an ESA is found in its reactive nearfield, which seriously delimits its radiation efficiency. The reactive field can be converted into a propagating far-field by perturbing the near-field using periodic inclusions. Ziolkowski proposed a double negative metamaterial sphere for covering an ESA in order enhance its radiation efficiency [24]. The proposed antenna shows stable S11 characteristics. However, the antenna shows peak gains of -7.25 dBi at 1.8 GHz, -1.8 dBi at 2.4 GHz and 0.12 dBi at 5.2 GHz, respectively. This low gain can be improved using an additional DNG structure without altering return loss characteristics. Figure 12 shows the comparative evaluation of return loss characteristics of the proposed antenna and its modified design. The inductance of the 15 additionally used DNG MTM can be used for matching ESA's high capacitive reactance. The modified structure is shown in Figure 13. Gain comparison of basic ESA with modified structure is shown in Figure 14. Peak gains obtained for the modified structure are -5.6 dBi at 1.8 GHz, 2.9 dBi at 2.4 GHz and 1.7 dBi at 5.2 GHz, respectively. Also the realisable gain of an ESA, is delimited by "Harrington bound" [25] which can be expressed using Equation (8) = ( ) 2 + 2 The maximum feasible gain according to this equation is 1.68 dBi. The observed gain of the modified antenna design at 1.8 GHz frequency is only -5.6 dBi which is well within the Harrington bound. Therefore the proposed ESA is found to be applicable in DCS 1800 and WLAN bands at 2.4 and 5.2 GHz.

Funding
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Availability of data and materials
All data generated or analysed in this study are included in this published article.

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