Recongurable 2×1 CPW-Fed Rectangular Slot Antenna Array (RSAA) Based on Graphene For Wireless Communications

This article presents a 2×1 CPW ultra wideband rectangular slot antenna array (UWB-RSAA) with modified circular slot shape to support high data rate for wireless communications applications. The proposed antenna array dimensions are 0.7 λ o ×0.8 λ o ×0.064 λ o at the resonant frequency 1.8 GHz and it is fabricated on a commercially available rogers RO4003 substrate (with ε r = 3.5), which is fed by using coplanar waveguide (CPW). A graphene layer is added on the other side of the substrate to achieve frequency reconfigurable and improve the antenna array gain. The -10 dB impedance bandwidth of the RSAA extends from 1.7 GHz to 2.6 GHz, from 3.2 to 3.8 GHz and from 5.2 GHz to 7 GHz with peak gain of 7.5 dBi at 6.5 GHz at 0 Volt bias over the operating band with average gain of 4.5 dBi. When the graphene bias is increased to 20 Volt, the antenna bandwidth extend from 1 GHz to 4 GHz and from 5 to 7 GHz with array peak gain 14 dBi at 3.5 GHz and average gain 7.5 dBi. The proposed array achieved linear polarized behaviour over the operating bands to be suitable for short range UWB wireless communications and object detection. All simulation carried out using 3D high frequency structure simulator (HFSS) Ansys ver.


Introduction
Frequency tuning of the microwave systems has gained a great attention in the research field of antennas for wireless communications. Reconfigurable antenna array is one of the most important elements of the RF circuits system [1][2][3][4]. In addition, wide band antennas array with frequency reconfigurable could be served several selected frequency bands avoiding the unwanted frequencies. Reconfigurable antennas need less space in the RF system, resulting in multi-band integrated RF systems [6][7]. Several techniques have been used to achieve reconfigurable antennas array like using varactor diodes [8], PIN diodes [9], MEMS [10], and other techniques [11,12].
Graphene is considered to be one of the most important and revolutionary materials of the 21 st century, with its characteristics and widespread applications. Graphene sheet as a honeycomb lattice arrangement of carbon atoms is First time of investigated done in 2004 [13]. The application of RF antennas based on graphene has had positive implications for wireless communication applications. Graphene has superior electrical properties that enable it to be a very promising material in researchs in different applications. Many researches have been presented on graphene applications for radio frequency (RF), microwave (MW), millimetre wave (MM) and tera hertz (THz) applications [13][14][15]. One of the most effective research areas utilized graphene sheet layer based on its tunable conductivity [16]. This tunable function is based on changing the applied DC bias voltage to the graphene sheet layer, which varies the chemical potential and hence the conductivity/resistance of it layer [17]. Many researches on high-quality graphene have progressed rapidly due to improvements in laboratory procedures that allows the production of graphene with controlled properties [22]. So, a great effort has been done in this area for designed array antenna with frequency reconfigurablity [18][19][20][21] and the proposed 2×1 recongurable antenna array is shown in Fig.1.
In this paper, reconfigurable 2×1 CPW-fed RSAA with an acceptable gain is introduced. The array (a) (b) Fig. 1. (a) The structure geometry of the proposed 2×1 RSAA and (b) side view. is fed by 3 dB CPW power divider, which makes the structure easy to integrate with other microwave circuit. A graphene pad sheet is placed beneath the feeding network to achieve the frequency reconfigurable action.
This article is organized as follows; section one is introduced the paper idea. While, section two presented the proposed RSAA design geometry. Section three discussed the graphene layer structure and its properties, then the experimental setup and results are presented with the comparison using the software tool results in section four. Section five illustrates the group delay results and object detection application, finally section six concludes and summarizes the results of the paper.

Methods of Designing Antenna
The cpw-fed proposed antenna array is printed on a dielectric substrate of Roger RO4003 with dielectric constant ε r =3.5 and tan δ=0.0012. The substrate has dimensions (Lsub × Wsub), and thickness of substrate is 0.8 mm. CPW fed slot antennas are very puplar which exhebit broad bandwidth and aproperteate gain [23]. First design is a conventional 2×1 circular shape of monopole anttenna array, which is fed using 3-dB power divider cpw-fed network as shown in Fig. 2(a) [24]. The reflection coefficent |S11| of proposed design shown in Fig. 3 as dash red line, the array start to operate at 3 GHz at reflection impedance bandwidth ≤-10 dB. Then quarter of the circular shap is etched as shown in Fig. 2(b) to improve the bandwith and the resonant frequency is reduced as shown in Fig. 3 (black line) to be resonated at 2.5 GHz.
The rectrangular strip is added around the antenna circular shape as shown in Fig. 2 Table 1. In addition, the current distribution at different resonant frequencies of the RSAA are shown in Fig. 4. The highest magneitude of the current respresent the coresponding elements of the radiation.

|S 11 |
Step One Step Two Step Three . | versus frequency for the design steps of the proposed antenna array

Method of Graphene Antenna
The design of antenna arrays play a significant role in achieving high gain in a certain specified direction [13]. Moreover, Multiband designs offer an excellent opportunity to the system to serve more applications with the same physical components. The previous section described the proposed 2×1 antenna array. However, this section described the adding of two dimensions of graphene layer on the other side of the Roger substrate as a sandwich between the Roger substrate and silicon oxide substrate to reconfigurable the operating frequency.

Graphene Conductivity Variation
Graphene oxide (GO) has become important material as a result of its low cost, simple with various preparation techniques to be easily transformed to commercial graphene as RGO (reduced graphene oxide) [25][26]. 2D Graphene sheet is a material composed of highly conductive films with high mobility of 0.2 Mcm 2 v -1 sec -1 to enhance the antenna performance [27][28], [30].  [31][32][33][34]. In this method of reconfigurabilty the frequency where the antenna exhibits the best input matching and the maximum gain could be adjusted electronically by modifying the voltage applied to graphene sheet.
ȟ is reduced Planck's constant, KB is Boltzmann's constant, T is the temperature, e is the electron charge, and μc is the chemical potential. T=300K and τ =0.1Ps. Zs is the surface impedance of the graphene, σ is the conductivity of the graphene.

Reconfigurable Graphene Antenna Array
The radio technology of ultra wideband (UWB) is that uses transmitted very low energy for high data rate over a large portion of the radio spectrum. The other definitions of UWB is the antenna achieved bandwidth greater than 25% from the resonant frequency. There are many wireless applications cover these spectrum as four/five generation of wireless communications, so, the reconfigurable between these applications are very useful to cover and protected from channel fading and other losses. Since UWB offers higher data rates with lesser multipath interference at lower power levels as compared to the narrow band communication systems as shannon theorem [35]. So, it is therefore the preferred band for implementation for many 4G and 5G wireless communication services. The operating band of the proposed antenna is extended by applying the DC voltage bias on the graphene layer as shown in Fig. 7.

Experimental of Antenna and Results
The proposed array is designed and all simulations are done by using finite element three dimensions electromagnetic simulator, HFSS Ansys ver. 15 [29].  respectively. These figures indicate that good agreements between measured and simulated. Fig. 9 shows the performance at 0 Volt and the |S11|<-10dB array impedance bandwidth extend from 1.  Table 2.      Table 3. It can be observed that the proposed array exhibits the ability to provide sufficient range of reconfigurable in most of wireless communication 4G and lower band of 5G frequency bands with increased gain and wide operating bandwidth by using single substrate in addition to the simplicity in the fabrication process.

Group Delay
Group delay is the series parameter of design UWB antenna for wireless communications. This it could be control the distortion of the transmitted pulses. The definition of the group delay is as the derivative with respect to frequency of the phase coupling between two identical UWB antennas |S21| [34][35][36][37][38]: The group delay should be close to a constant within the operation bands for perfect pulse transmission to obtain the phase linearity of the transmitted signals in the far field. The two antennas were aligned, and placed face to face orientation with separation distance between them greater than far field of the lowering operation bands as shown in Fig.11(a). The comparison results of |S21| for both measured and simulated HFSS results shown in Fig. 11. Fig.11(b) shows that the proposed antenna has perfect performance in this aspect and the group delay over the operating UWB band is less than 1 ns and the group delay and the magnitude transfer function are very flat in the whole band except in the notched bands which makes it quite suitable for UWB wireless applications.  Fig. 11. Comparison between measured and simulated group delay of proposed 2×1 array antenna.

Object Detection
By using two face to face orientation as shown in Fig. 12(a) and placed different object with same size and different materials as perfect conductor, polyethylene and water in the far field region.
The magnitude and the convolution angle could be changed of the transmission coefficient |S21| between both rectangular slot antenna arrays. Fig. 12(b) and 12(c) show that the maximum transmission obtain when the perfect conductor placed, however it gives less commutative angle.
On the other side the water produces highest transmission at 4.5 GHz and less transmission at frequency start from 6.5 GHz with largest commutative angle. Fig. 13 shows the measured transmission coefficient magnitude and phase with three different objectives (perfect conductor, polyethylene and water) and compared with free space result.

Conclusion
A CPW rectangular slot 2×1 antenna array is presented in this paper with frequency range extend from 1.7 to 2.5 GHz, 3.3 to 3.5 GHz and from 5.5 to 5.8 GHz with average gain about 7.5 dBi over the operating bands. Graphene sheet layer was added on the other substrate side under the 2×1 rectangular slot array feeding network to realize frequency reconfigurablility by applied DC bias.
The reconfigurable frequency extend of lower band of 5G and other wireless communication bands. The operating frequency range extend from 1.5 to 3.5 GHz to 4G bands and from 5 to 7 GHz with lower band of 5G. The proposed antenna array with omni-radiation pattern and the array gain improved by applying different DC voltages bias of the graphene sheet layer to be 15 dBi at 20 volt. Experimental results have shown a range of reconfigurable and good agreement with the simulated results. The group delay of the proposed array is also studied, there was very good agreement between simulated and measured results and achieved less than 1ns over the operating band. Moreover, the proposed array is used for object detection and obtained good results to differentiate between different materials objectives.

Declarations
-Availability of our proposed data and materials and all authors (Dalia Elsheakh and Osama Dardeer) must include an "Availability of Data and Materials" section in their manuscript detailing where the data supporting their findings can be found. Authors who do not wish to share their data must state that data will not be shared, and give the reason. If this is not applicable, please write: "Data sharing not applicable to this article as no datasets were generated or analysed during the current study.