Meta-surface (frequency selective surface) loaded high gain directional antenna systems for ultra-wideband applications

In this work, a meta-surface (frequency selective surface) loaded high gain directional antenna system is presented. The antenna system is developed using ultra-wideband (UWB) antenna element and meta-surface reflector. The UWB antenna element is designed and simulated without meta-surface reflector. The UWB antenna element has poor impedance bandwidth and directivity. A meta-surface is created using unit cell and equal in the size of the antenna substrate. The meta-surface is placed over the UWB antenna element at optimized height (H=30 mm). The impedance bandwidth, directivity and gain of the proposed antenna are improved by the meta-surface reflector. The proposed antenna is fabricated and experimentally validated by the comparison of the simulated and measured results. The antenna has 3 to 6 GHz wide impedance bandwidth, more than 5 dBi gain and maximum 4.6 dBi directivity at 3.5 GHz frequency. Performance of the proposed antenna is also compared with existing carried out work. Comparatively, the proposed antenna with high directivity is most suitable for IEEE 802.15.4a UWB wireless sensor network (WSN) security application.


INTRODUCTION
In the twenty first century, ultra-wideband-wireless sensor network (UWB-WSN) is an emerging technology for internet of thing (IoT) enabled medical and industrial applications such as: human physical activity monitoring systems, telepathy systems, drug delivery systems, Military surveillance systems, and remote monitoring systems for the industrial plant [1]- [4]. This technology has gained popularity due to its high data throughput at low power and cost. However, these IoT enabled WSN systems are vulnerable to the malicious attacks. Despite this, WSN is immune to threats using message encryption techniques, and authentication of the node [5]- [7]. The antenna technology is an alternative approach to reduce the chance of cyber physical attacks as well as simultaneously improve the link reliability and transmission range [8]. In this regard, a directional antenna system is a beneficial component that can be embedded with the sensor node. Directional antenna can protect the wireless sensor network (WSN) from such malicious attacks, narrow radiation range of the antenna cannot be tracked by attackers easily [9]- [12]. Therefore, the directional antenna is very helpful to mitigate the effect of eavesdropping, jamming and wormhole security risks. The WSN security which is provided by the directional antenna system is shown in Figure 1. Aforementioned favorable characteristics of the directional antenna, the designing of a novel and compact size directional antenna for UWB-WSN is a challenging task for the antenna developer. Because, usually

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FSS unit cell is developed and analyzed. In the next step, the meta-surface is created using unit cell. In the third step, an UWB antenna is designed and simulated. Finally, in the fourth step, meta-surface is placed over the UWB antenna at desired spacing. The reflection parameters of the proposed antenna are analyzed with and without meta-surface, to identify the desired frequency range of the UWB-WSN. Furthermore, the parametric analysis of the proposed antenna is carried out in terms of the spacing between the UWB antennas and meta-surface. Once, the desired performance criteria are attained in the simulated structure of the antenna, the prototype of the same is fabricated and experimentally validated. Whole simulation process is carried out through the high frequency computer simulation technology (CST). Proposed work is comprised in the following section as: the detailed discussion about the antenna development is described in section 2, and section 3 is covered experimental result study, the proposed work is concluded in section 4.

PROPOSED ANTENNA DEVELOPMENT METHOD
The proposed antenna structure is shown in Figure 2. The proposed antenna construct with the rectangular patch element, which act as a radiator. A meta-surface of the same substrate size of the patch element is placed at a certain height (H) over the patch element. Detailed design procedure of the development of the proposed antenna is described in the following sections.

FSS unit cell development
A FSS unit cell is created in CST design environment and simulated using frequency solver to analyze the behavior of transmission and reflection magnitude. Developed unit cell with optimized dimensions is shown in Figure 3(a). A square shaped unit cell is designed on the low cost FR4 substrate with dimension of 5.8 mm, thickness of 1.5 mm and dielectric constant 4.5. The bottom of the unit cell has a complete metallic ground, whereas, the top is etched with a rectangular shaped structure from the center and two inverted L-shaped structure are suspended both side of the etched plane of the top. To analyze the behavior of the unit cell, reflection and transmission coefficients of the unit cell is calculated as defined in [28], [29] and described by the (1) and (2): where, R=Z-1/Z+1, the Z is the impedance of the unit cell, n is the refractive index of the unit cell, k is the wavenumber, and d is the thickness of the substrate used in the unit cell. The boundary conditions of the simulated FSS unit cell is followed as utilized in [30]. The simulated results of the reflection and transmission coefficient for designed unit cell are shown in Figure 3(b). The reflection behavior of the unit cell is observed from the obtained result, the incident power in the unit cell is completely reflected back over the frequency 3-5 GHz. The transmission coefficient is 0 dB, which is indicated that, no power transmitted from the unit cell. The power reflection property of the developed unit cell is useful to develop a meta-surface reflector. The development of the FSS meta-surface reflector is carried out in the following section.

Development of FSS meta-surface
The meta-surface is developed from the unit cell and shown in Figure 4. An 8×8 array of the unit cell is perturbed on the top of 50×50 mm 2 substrate. The bottom side of the substrate fully loaded with conducting copper layer. The developed meta-surface is utilized in the final development of the proposed antenna. The meta-surface is act as reflector for the UWB antenna and can enhance the impedance bandwidth, gain and directivity of the proposed antenna.

UWB antenna with FSS meta-surface and their simulation results
To further enhance the impedance bandwidth the UWB antenna, the antenna is loaded with metasurface which is created in the previous section 2.2. The simulated reflection coefficient results of the UWB antenna with and without meta-surface is shown in Figure 6. The UWB antenna element without metasurface has impedance bandwidth (range of frequency for which 11 < −10 dB) is 3-4.2 GHz only. However, the antenna loaded with meta-surface is offered the impedance bandwidth 3-6 GHz. The antenna with meta-surface is offered the good resonance match and reduced the mismatch loss across the antenna port and radiating element. The reflection coefficient and antenna gain results are not sufficient to finalize the proposed antenna structure. Therefore, antenna directivity and surface current distribution are also included in the analysis of the proposed antenna at the spacing = 30 mm. To notice the effect of the meta-surface on the antenna directivity, the 3D results of the directivity with and without meta-surface is simulated and analyzed. The 3D directivity results with and without meta-surface at the frequency 3.5, 4.5, and 5.5 GHz are shown in Figures 8(a) and 8(b). Results are indicated that antenna with meta-surface is more directional comparatively antenna without meta-surface. For clear understating of the effect of meta-surface on the antenna directivity, the maximum directivity values and improved directivity with meta-surface are collected in Table 1. Here results are indicated that, the directivity of the proposed antenna with meta-surface is improved by the value of 2.8, 1.39, and 1.44 dBi at the frequency of 3.5, 4.5, and 5.5 GHz respectively.  The surface current density represents the current distribution across the radiator, ground plane and substrate. Efficient antenna systems must be showing the maximum current distribution across the radiator. The current density is evaluated at the frequency 3.5, 4.5, and 5.5 GHz for the antenna with and without meta-surface and results are shown in Figure 9. The antenna without meta-surface has shown more current coupling across the ground plane along with the radiator as shown in Figure 9(a). However, the antenna with meta-surface reflector has shown maximum current distribution across the radiator and less current distribution across the ground as depicted in Figure 9(b). Therefore, the meta-surface reflector not only enhances the directivity of the antenna as well as improves the gain, impedance bandwidth. The performance characteristics of the antenna with meta-surface is sufficient to meet the criteria of the UWB-WSN application. Finally, the antenna with meta-surface with spacing of = 30 mm is known as the proposed antenna. Further, experimental validation of the proposed antenna is carried out in the next section.

EXPERIMENTAL RESULTS VALIDATION OF THE PROPOSED ANTENNA
The designed proposed antenna is fabricated and experimentally validated by the comparison of the simulated and measured results. The UWB antenna element is fabricated first and then meta-surface is fabricated. The fabrication process is carried out using printed circuit board (PCB) micro-machine. The reflection coefficient is evaluated using vector network analyzer (VNA). Comparison of the simulated and measured reflection coefficient and gain are shown in Figures 10(a) and 10(b) respectively. The antenna has shown good matching between simulated and measured reflection coefficient results. The offered impedance bandwidth by the antenna is 3 GHz with reflection coefficient 11 < −10 for the frequency range 3 − 6 GHz. Measured gain of the antenna is shown in Figure 10(b), and these results are also well matched with simulated results. The proposed antenna is offered peak gain of 5 dBi and mostly stable gain within the interested frequency range. However, antenna without meta-surface has poor gain less than the < 2 dBi only as previously discussed in the section 2.4. The more details about gain and efficiency with and without FSS meta-surface is shown in Figure 11. Comparatively, the antenna with FSS meta-surface is offered efficiency more than antenna without FSS meta-surface. The antenna with FSS meta-surface has peak efficiency more than 90% at 4 GHz frequency, however antenna without FSS meta-surface can provide efficiency lower than 20% only. The antenna gain with FSS meta-surface is also enhanced by 3 dBi as compared to antenna without meta-surface. The measured radiation patterns of the proposed antenna are evaluated in the anechoic chamber. The test antenna is steered using automatic control stepper motor. The test antenna is rotated with step size of 30 degree with respect to reference antenna. The reference antenna is fixed at a certain distance in front of the test antenna. On the basis of received power by the test antenna the radiation pattern is drawn. The radiation patterns are defined the graphical representation of the radiated or received power by the antenna. The patterns are evaluated at the frequency 3.5 and 5.5 GHz and results are shown in Figure 12. Figure 12(a) shows the E-plane results at 3.5 and 5.5 GHz frequency and H-plane results are shown in Figure 12

COMPARATIVE STUDY WITH EXISTING WORK
The proposed work is compared with existing work. The comparative analysis is done in terms of employed directivity enhancement method, operating frequency range, gain of the antenna, and applications. The compared parameters are collected in Table 2.

CONCLUSION
In this study, a directional UWB antenna system with meta-surface is designed, tested and validated. The antenna directivity, gain and impedance bandwidth performance is enhanced by loaded the antenna with FSS reflector at the height (H=30 mm). The FSS meta-surface is comprised with 6×6 unit cell array. Before the development of the FSS meta-surface, the unit cell performance characteristic is verified by reflection and transmission coefficient results. The FSS meta-surface and antenna element is fabricated separately, then assembled FSS reflector above the antenna. The fabricated prototype is tested using VNA and anechoic radiation chamber. The proposed antenna directivity is observed 4.6, 2.9, and 3.58 dBi at the frequency 3.5, 4.5, and 5.5 GHz frequency respectively. The peak gain offered by the proposed antenna is 5 dBi. The antenna is offered wide impedance bandwidth 3 − 6 GHz. The proposed antenna bandwidth, gain, efficiency, and directivity performance is suitable for UWB-WSN application. The directivity achieved in this work is most useful and provide good security to UWB wireless sensor networks.