A Microstrip Antenna Using I-Shaped Metamaterial Superstrate with Enhanced Gain for Multiband Wireless Systems

This paper presents the design of a rectangular microstrip patch antenna (MPA) using the I-shaped metamaterial (MTM) superstrate. A seven × seven array of the I-shaped MTM unit cell is used as the superstrate to enhance the antenna performance. The antenna is fed by a microstrip feeding technique and a 50 Ω coaxial connector. An in-phase electric field area is created on the top layer of the superstrate to improve the performance of the antenna. The proposed I-shaped MTM-based rectangular MPA produces three operating frequencies at 6.18 GHz, 9.65 GHz, and 11.45 GHz. The gain values of the proposed antenna at 6.18 GHz, 9.65 GHz and 11.45 GHz are 4.19 dBi, 2.4 dBi, and 5.68 dBi, respectively. The obtained bandwidth at frequencies 6.18 GHz, 9.65 GHz and 11.45 GHz are 240 MHz (3.88%), 850 MHz (8.8%), and 1010 MHz (8.82%), respectively. The design and simulation of the antenna are done using the Computer Simulation Technology (CST) studio suite and MATLAB. The proposed I-shaped MTM-based rectangular MPA is fabricated on a low-cost FR-4 substrate and measured using the Agilent 8719ET network analyzer. The proposed antenna has an overall dimension of 70 × 70 × 1.6 mm3. A significant improvement in the gain of the antenna up to 74.28% is achieved. The obtained results confirm that the proposed multiband antenna has a high gain, and enhancement in bandwidth and radiation efficiency. These properties make the proposed antenna suitable for the multiband wireless communications systems such as Wi-Fi devices, radar systems, short- and long-range tracking systems, etc.


Introduction
Microstrip patch antennas (MPAs) have been widely used in various wireless and mobile communication systems since 2002 after the Federal Communication Commission (FCC) opened its use to civilians [1]. An antenna is widely acknowledged to be a critical component in communication systems. To keep up with the development trend, a high-performance ultra-wideband antenna with a compact structure must be designed. In comparison to other conventional antennas, due to its high efficiency, wide bandwidth, miniaturized size, low spectral density, affordability, and flexibility, the ultra-wideband antenna has received much attention recently. Apart from the numerous benefits of MPAs, there are a few drawbacks, including low gain, low efficiency, low power handling, narrow impedance, and excessive radiation from feeds [2][3][4][5]. Numerous researchers have conducted extensive research on metamaterial structures to mitigate these constraints by enhancing the MPA performance in terms of bandwidth, gain, directivity, and efficiency.
MTMs have been discovered to be excellent candidates for improving antenna characteristics. MTMs are artificially designed structures with negative permeability and permittivity properties at specific resonant frequencies [6,7]. Tunable MTMs can be designed using liquid crystals. The tunability in liquid crystals-based metamaterials can be achieved employing various strategies such as external voltage, power tunability of liquid four split gaps and the inner ring having a single cut. The antenna gain and directivity were increased from 4.04 dBi to 5.3 dBi and 5.8 dBi to 6.7 dBi, respectively. A quintuple circular SRR was designed to enhance the performance of a rectangular MPA by G.Al. Duhni [31]. The SRR-loaded MPA produced five resonance frequencies at 8.16 GHz,9.08 GHz, 10.06 GHz, 10.73 GHz, and 11.34 GHz.
This research work builds on our initial design in [30] and incorporates a significantly improved design. Considering the reduction in the operating frequency band reported in [16], low gain in some antennas such as in [21,23,28], and low gain enhancement reported in [22,30], this work aims to provide a robust approach to tackle these limitations. The uniqueness of this work lies in the design of the proposed I-shaped MTM array superstrate utilized by the proposed antenna for multi-band applications. The I-shaped MTM array is designed using the I-shaped MTM cell [32]. In this work, the design, simulation, fabrication, and measurement of the rectangular MPA using an I-shaped MTM array superstrate are presented. The proposed model of the I-shaped MTM array superstrate is integrated with the rectangular MPA, and this shows a significant improvement in terms of gain, bandwidth, and efficiency. A maximum gain enhancement of 74.28% is achieved. The rest of the paper is organized as follows: the design and geometry of the rectangular MPA, I-shaped MTM array and the rectangular MSA with the I-shaped superstrate are presented in Section 2. Section 3 presents the results and discussion. The conclusion of the research work is presented in Section 4.

Design and Geometry of the I-Shaped MTM based MPA
This section describes the design process for the proposed I-shaped MTM-based MPA. The I-shaped MTM structure is analyzed and used to improve the performance of the designed I-shaped MTM-based MPA.

Geometry of the I-Shaped MTM Superstrate
The geometry and fabricated structure of the 7 × 7 I-shaped MTM array is illustrated in Figure 1. The MTM array is printed on the FR-4 substrate with a dielectric constant of 4.3 and a loss tangent (tanδ) of 0.025. The thickness of the substrate (sh) and the thickness of the annealed copper (hc) used for the split ring resonator are 1.6 mm and 0.035 mm, respectively. The primary function of splits in the ring resonators is to ensure that the inductance and capacitance interact with one another to determine the operating frequency. The unit cell's total optimum size is 10 × 10 × 1.6 mm 3 (0.2λ 0 × 0.2λ 0 × 0.03λ 0 ) [32]. The dimensional parameters of the 7 × 7 I-shaped MTM array are presented in Table 1.  The effective parameters of the MTM can be determined by carefully placing the structure between two ports of the waveguides, with an electromagnetic wave with magnetic and electric fields along the y-and x-axes, respectively. This implies that the first port transmits the reflecting signal, while the second port serves as the receiving end. The effective medium ratio is proportional to the unit cell dimension, and the wavelength must be less than the working wavelength. The reflection and transmission coefficient can be given as [33,34]: where S11, S21 and Z represent the reflection coefficient, transmission coefficient and impedance, respectively. By using Equations (2) and (3), the intermediate parameters V 1 and V 2 can be defined as [33,34]: The Nicolson-Ross-Weir (NRW) method is adopted to determine the electric permittivity (ε r ) and the magnetic permeability (µ r ) of the MTM as given below [33,34]: where f and S h denote the operating frequency and the height of the substrate, respectively. The scattering parameters of the MTM unit cell and array can be analyzed by using the above equations. The S-parameters and the effective parameters of the proposed 7 × 7 I-shaped MTM array superstrate are shown in Figure 2. The simulated S-parameters for the 7 × 7 I-shaped MTM array are presented in Figure 2a. where 11, 21 and represent the reflection coefficient, transmission coefficient and impedance, respectively. By using Equations (2) and (3), the intermediate parameters and can be defined as [33,34]: The Nicolson-Ross-Weir (NRW) method is adopted to determine the electric permittivity ( ) and the magnetic permeability ( ) of the MTM as given below [33,34]: where and denote the operating frequency and the height of the substrate, respectively. The scattering parameters of the MTM unit cell and array can be analyzed by using the above equations. The S-parameters and the effective parameters of the proposed 7 × 7 I-shaped MTM array superstrate are shown in Figure 2. The simulated S-parameters for the 7 × 7 I-shaped MTM array are presented in Figure 2a

Design and Geometry of Rectangular MPA Using I-Shaped MTM Superstrate
The top view of the geometrical structure and the side view of the rectangular MPA are depicted in Figure 3a and

Design and Geometry of Rectangular MPA Using I-Shaped MTM Superstrate
The top view of the geometrical structure and the side view of the rectangular M are depicted in Figure Table   Table 2. Dimensional parameters of the rectangular MPA.

Parameter
Dimension ( The effect of the width of the patch ( ) and length of the patch ( ) on the reflec coefficient is presented in Figure 4a and Figure 4b, respectively. The is varied from mm to 20 mm. It was observed that at 10 mm the patch produced one resonance frequ at 11.5 GHz with a return loss of −12.78 dB. At 14 mm, two resonance frequencies w observed at 6.03 GHz and 10.4 GHz with a reflection coefficient of −13.4 dB and −34.97 respectively. When the patch dimension was increased to 20 mm it produced three quencies at 6.22 GHz, 10.6 GHz, and 11.5 GHz. The reflection coefficient at 6.22 GHz, GHz, and 11.5 GHz are −11.1 dB, −18 dB, and −17.3 dB, respectively. It can be obse that the rectangular MPA has better performance at 15.3 mm with three operating quencies at 6.18 GHz, 9.09 GHz, and 11.48 GHz, with a reflection coefficient of −17.9 −17.6 dB, and −34.4 dB, respectively. The presented antenna is suitable for C/X-band gain multi-band wireless communication applications. The operating frequencies GHz and 11.48 GHz are due to the higher order modes. Form Figure 4b, it can be obse that the resonant frequency decreases upon increasing the value of . The analysis o resonant frequencies for different modes is carried out using the Equation (7) [37]: The geometric parameters of the rectangular MPA, such as width and length, are calculated by using the transmission line model equations which can be found in [35,36]. The size of the ground plane and patch of the antenna are 0.50λ 0 × 0.42λ 0 and 0.31λ 0 × 0.23λ 0 , respectively (λ 0 = free space wavelength). The rectangular MPA configuration and simulation are done using the finite integration technique-based electromagnetic CST simulator. The dimensions of the rectangular MPA are presented in Table 2. The effect of the width of the patch (W p ) and length of the patch (L p ) on the reflection coefficient is presented in Figures 4a and 4b, respectively. The W p is varied from 10 mm to 20 mm. It was observed that at 10 mm the patch produced one resonance frequency at 11.5 GHz with a return loss of −12.78 dB. At 14 mm, two resonance frequencies were observed at 6.03 GHz and 10.4 GHz with a reflection coefficient of −13.4 dB and −34.97 dB, respectively. When the patch dimension was increased to 20 mm it produced three frequencies at 6.22 GHz, 10.6 GHz, and 11.5 GHz. The reflection coefficient at 6.22 GHz, 10.6 GHz, and 11.5 GHz are −11.1 dB, −18 dB, and −17.3 dB, respectively. It can be observed that the rectangular MPA has better performance at 15.3 mm with three operating frequencies at 6.18 GHz, 9.09 GHz, and 11.48 GHz, with a reflection coefficient of −17.9 dB, −17.6 dB, and −34.4 dB, respectively. The presented antenna is suitable for C/X-band high gain multi-band wireless communication applications. The operating frequencies 9.09 GHz and 11.48 GHz are due to the higher order modes. Form Figure 4b, it can be observed that the resonant frequency decreases upon increasing the value of L p . The analysis of the resonant frequencies for different modes is carried out using the Equation (7) [37]: where f r(m,n,p) is the resonant frequency of the TM mnp mode. For the computation of the resonant frequencies, the variation of the simulated values of the dielectric constant with the frequency is considered, as given in Table 3. The computed and simulated resonant frequencies of the rectangular MPA are given in Table 4. From Table 4, it can be observed that the analytical resonant frequencies and simulated resonant frequencies are closely matched. Hence, using the higher mode resonant frequencies analysis and optimization in CST microwave studio, the presented antenna can be designed for the other specified frequency bands. where ( , , ) is the resonant frequency of the mode. For the computation of the resonant frequencies, the variation of the simulated values of the dielectric constant with the frequency is considered, as given in Table 3. The computed and simulated resonant frequencies of the rectangular MPA are given in Table 4. From Table 4, it can be observed that the analytical resonant frequencies and simulated resonant frequencies are closely matched. Hence, using the higher mode resonant frequencies analysis and optimization in CST microwave studio, the presented antenna can be designed for the other specified frequency bands.

Result and Discussion
The simulated reflection coefficient 11 (dB) for the rectangular MPA and I-shaped MTM-based MPA is illustrated in Figure 6. It is observed that the rectangular MPA resonates at 6.18 GHz, 9.14 GHz, and 11.48 GHz with a bandwidth of 330 MHz, 700 MHz, and 800 MHz, respectively. When the I-shaped MTM is integrated with the rectangular MPA,

Result and Discussion
The simulated reflection coefficient S11 (dB) for the rectangular MPA and I-shaped MTM-based MPA is illustrated in Figure 6. It is observed that the rectangular MPA resonates at 6.18 GHz, 9.14 GHz, and 11.48 GHz with a bandwidth of 330 MHz, 700 MHz, and 800 MHz, respectively. When the I-shaped MTM is integrated with the rectangular MPA, the resonant frequencies are at 6.18 GHz, 9.65 GHz, 11.5 GHz with a bandwidth of 240 MHz (3.88%), 850 MHz (8.8%), and 1010 MHz (8.82%), respectively. The bandwidth of the first resonant frequency became narrower when the rectangular MPA was loaded with the superstrate. However, a significant increase in the bandwidth, 21.43% for the second band and 26.25% for the third band, is observed.

Result and Discussion
The simulated reflection coefficient 11 (dB) for the rectangular MPA and I-shap MTM-based MPA is illustrated in Figure 6. It is observed that the rectangular MPA re nates at 6.18 GHz, 9.14 GHz, and 11.48 GHz with a bandwidth of 330 MHz, 700 MHz, a 800 MHz, respectively. When the I-shaped MTM is integrated with the rectangular MP the resonant frequencies are at 6.18 GHz, 9.65 GHz, 11.5 GHz with a bandwidth of MHz (3.88%), 850 MHz (8.8%), and 1010 MHz (8.82%), respectively. The bandwidth of first resonant frequency became narrower when the rectangular MPA was loaded w the superstrate. However, a significant increase in the bandwidth, 21.43% for the seco band and 26.25% for the third band, is observed.  The Agilent 8719ET network analyzer was used to measure the proposed I-shaped MTM-based rectangular MPA. Figure 7 depicts the simulated and measured S11 (dB) of the I-shaped MTM-based rectangular MPA. The measured resonant frequencies are 6.18 GHz,9.94 GHz, and 11.7 GHz. A slight disparity can be observed between the simulated and measured S-parameter, which can be attributed to fabrication tolerance, variation of material specification, calibration of the network analyzer, and impedance from the connector soldering. Figure 8 depicts the reflection coefficient of the proposed antenna at various heights (ag). The plastic spacers separate the superstrate and rectangular MPA. The spacers have no significant impact on the antenna's performance. The distance between the rectangular MPA and the I-shaped MTM superstrate is varied from 6 mm to 15 mm. The reflection coefficient characteristics for the four values of ag, i.e., ag = 6 mm, ag = 7 mm, ag = 10 mm, and ag = 15 mm are presented in Figure 8. It is observed that for the first and second bands, ag = 7 mm provides the minimum reflection coefficient along with the wide bandwidth. However, for the third band, ag = 15 mm provides the minimum reflection coefficient with almost same bandwidth as ag = 7 mm. Considering the overall performance for all the frequency bands, for ag = 7 mm, the antenna gives the optimum performance.
The reflection coefficient characteristics for the four values of ag, i.e., ag = 6 mm, ag = 7 mm, ag = 10 mm, and ag = 15 mm are presented in Figure 8. It is observed that for the first and second bands, ag = 7 mm provides the minimum reflection coefficient along with the wide bandwidth. However, for the third band, ag = 15 mm provides the minimum reflection coefficient with almost same bandwidth as ag = 7 mm. Considering the overall performance for all the frequency bands, for ag = 7 mm, the antenna gives the optimum performance.  The normalized simulated 2-D radiation patterns of the rectangular MPA and the proposed antenna at three operating frequencies are shown in Figure 9. The resonant frequency 6.18 GHz in E-plane (phi = 0 0 ) and H-plane (phi = 90 0 ) shows a broadside radiation pattern. In Figure 9b, an omnidirectional radiation pattern is observed at 9.65 GHz. In Figure 9c, the antenna shows a dipole-like pattern at E-plane and an omnidirectional pattern at the H-plane.
The realized gain for the rectangular MPA and the proposed antenna is presented in Table 5. It can be observed that there is significant improvement in the gain of the antenna integrated with the I-shaped MTM array. The gain of the rectangular MPA increased significantly at all the frequencies for the antenna integrated with the I-shaped MTM array  The normalized simulated 2-D radiation patterns of the rectangular MPA and the proposed antenna at three operating frequencies are shown in Figure 9. The resonant frequency 6.18 GHz in E-plane (phi = 0 0 ) and H-plane (phi = 90 0 ) shows a broadside radiation pattern. In Figure 9b, an omnidirectional radiation pattern is observed at 9.65 GHz. In Figure 9c, the antenna shows a dipole-like pattern at E-plane and an omnidirectional pattern at the H-plane. The angular 3 dB beamwidth, main lobe direction, total and radiating efficiency, maximum gain, and directivity of the proposed antenna are presented in Table 6. It can be observed that as the operating frequency of the antenna increases, the total and radiating efficiency of the antenna decreases. The comparison of the existing works with the Ishaped MTM-based rectangular MPA is presented in Table 7. From this comparison, it can be observed that the proposed low-cost antenna provides multiband operation, high gain, and high gain enhancement. Various antenna parameters confirm the suitability of the proposed antenna for multiband C/X-band wireless systems such as Wi-Fi devices, radar systems, and short-and long-range tracking systems.  The realized gain for the rectangular MPA and the proposed antenna is presented in Table 5. It can be observed that there is significant improvement in the gain of the antenna integrated with the I-shaped MTM array. The gain of the rectangular MPA increased significantly at all the frequencies for the antenna integrated with the I-shaped MTM array superstrate. At 6.18 GHz, the gain increased from 2 dBi to 4.18 dBi, while at 9.14 GHz a gain of 0.09 dBi was obtained by the rectangular MPA. As the I-shaped MTM array is integrated with the rectangular MPA, a gain of 2.39 dBi is achieved. A gain increase from 3.22 dBi to 5.63 dBi was experienced at 11.48 GHz. The gain enhancement in % for various frequencies is given in Table 5. It can be observed that the gain enhancement of 74.28% at 11.48 GHz is achieved. The simulated and measured radiation patterns of the proposed I-shaped MTM-based rectangular MPA at 6.18 GHz, 9.09 GHz, and 11.5 GHz are shown in Figure 10a, Figure 10b, and Figure 10c, respectively. It can be observed that the simulated and measured patterns are in good agreement.    [19] 50 mm × --3.51 GHz FR-4 ---- The angular 3 dB beamwidth, main lobe direction, total and radiating efficiency, maximum gain, and directivity of the proposed antenna are presented in Table 6. It can be observed that as the operating frequency of the antenna increases, the total and radiating efficiency of the antenna decreases. The comparison of the existing works with the I-shaped MTM-based rectangular MPA is presented in Table 7. From this comparison, it can be observed that the proposed low-cost antenna provides multiband operation, high gain, and high gain enhancement. Various antenna parameters confirm the suitability of the proposed antenna for multiband C/X-band wireless systems such as Wi-Fi devices, radar systems, and short-and long-range tracking systems.

Conclusions
A multiband rectangular MPA integrated with an I-shaped MTM superstrate array has been presented. The proposed antenna utilized a seven × seven I-shaped MTM array to improve the performance of the antenna. The rectangular MPA and the I-shaped MTM array are designed, fabricated, and etched on an FR4 substrate. The proposed antenna produces three resonance frequencies at 6.18 GHz, 9.65 GHz, and 11.45 GHz. The gain of the antenna has been improved significantly. A gain enhancement up to 74.28% has been achieved. The proposed low-cost antenna has a high gain and enhancement in both bandwidth and radiation efficiency. These properties make the proposed antenna suitable for multiband wireless communications systems such as Wi-Fi devices, radar systems, short-and long-range tracking systems, etc.