Effects of Ground Plane on a Square Graphene Ribbon Patch Antenna Designed on a High-Permittivity Substrate with PBG structures

In this paper, a wide bandwidth (20 GHz) and small sized (200×200×50 µm 3 ) terahertz (THz) square graphene ribbon antenna using Photonic Band Gap (PBG) substrate are investigated in the 0.6–0.8 THz band. The proposed antenna consists of graphene as radiating patch mounted on four types of ground plane. The important aim of this work is to eliminate the second resonance frequency by utilize a several types of ground plane and keep a single frequency by improving the performance of the proposed antenna. The effect of change ground plane on the performance of antenna is compared, in terms of re�ection coe�cient, bandwidth, directivity and radiation pattern. The antenna has achieved good gain (7.62 dB) and good directivity (6.38 dB) in comparison to work reported in the literature.


Introduction
Over the past few decades, a great interest of the research community has been attracted by the THz region of the electromagnetic spectrum [1], from 0.1 THz to 10 THz have been undergoing rapid development in recent years [2].This frequency range has an extension in imaging, networking, medical eld, wireless system, short distance, space science and remote systems [3].Characteristics of the THz band permit it to occupy a unique niche as other parts of the EM spectrum are already well-established [4].Moreover, THz wave is nonionizing, very safe technology as compared to the X-ray [5].The design of radiating elements of nanoscale represents a crucial factor for the reliable operation of networks.Metallic antennas exhibit excellentbehavioratmicrowavefrequenciesbuttheirradiation properties degrade considerably in the far infrared region, resulting in prohibitively complex power consumptions.Pertinent to the prior requisites at a wide frequency spectrum, including the far-infrared regime, is graphene, a planarallotrope of carbon atoms with high mechanical strength and bonded in a 2-D honeycomb crystal grid [6][7].The utilization of graphene as the radiating patch might be a pivotal breakthrough to the problem.Graphene possesses splendid electrical, optical, and mechanical properties [8].Complementary Metal Oxide Semiconductor (CMOS) compatible microwave graphene-based patch antennas, still at their infancy, as-sure extremely low noise and high-speed communications [9][10].Some techniques, include increasing the substrate thickness in [11] or using multiple substrates in [12] and the dielectric material, were employed to increase the bandwidth and to improve the radiation characteristics of the patch antenna [13].These techniques always proved useful and reduce de nitively surface wave losses.Other techniques are utilized to possess better performance as the insertion of the periodic structures.The Photonic Band Gap (PBG) surface is found to have the effect of reducing the patch resonant length, contribute to enhance the gain in TeraHertz patch antenna.The speci c electro-magnetic properties of PBG materials motivate the researchers to extensively study applying its band-gap phenomena for practical uses in microwave and sub-terahertz applications [14-15-16-17-18].The EBG structure is a special family of high impedance surface (HIS) structures, which prohibits the wave propagation at certain frequencies and certain incidence angles [19-20-21].Graphene-based PBG THz antenna is designed and simulated in [3] to improve gain, directivity and the radiation e ciency.But the criterion does not show a good match between the patch antenna and the transmission line (S 11 =-19dB at f r =1.04THz).
In this paper, we propose square ring patch antenna loaded with EBG's structures to achieve a multi-pole like radiation pattern and a good directivity but it generating two resonating frequencies.So, to recovery this problem it is proposed to load the antenna with different forms of ground plane.Various antenna parameters such as re exion coe cient, gain, directivity, and radiation patterns were also analyzed for the single band antenna operation.To our knowledge, elimination of a one resonance band by changing the size of ground plane, at THz regime has not been reported earlier.

Design Speci cations Of Graphene Ribbon Patch Antenna
The geometrical con guration of radiating square ribbon patch antenna is presented in gure1.The antenna is placed on the center of the substrate fed by a 50Ω microstrip transmission lines.The proposed antenna structure is designed to resonate in bandwidth of 0.5GHz to 0.8 THz.The dimensions of the antenna are summered in Table 1 Graphene behaves as a radiator in the THz range [3].The graphene's surface conductivity has been determined by Kubo's formula as given in [22-23-24-25-3].Graphene as a patch conductor has many astonishing properties, such as carrier mobility attaining 200,000 cm 2 V -1 s -1 at room temperature and Young's modulus [13].The following table present the electrical and non-electrical properties used to model under HFSS software the square ring patch antenna.
We investigate the effect of different dimensions of ground plane in the return loss characteristics, gain anddirectivity.

Design Approach
The SI unit for magnetic eld strength H is A/m.However, after the "et" in the Latin abbreviation "et al." (it is also italicized).The abbreviation "i.e.," means "that is," and the abbreviation "e.g.," means "for example" (these abbreviations are not italicized).
To overcome the problem of multi band as illustrated in gure 2, we apply a certain modi cation to it.
A. Proposed antenna (1) with the EBG structures and partial groundplane The proposed antenna (1) consists of a graphene ribbon patch antenna and a partial ground plane as presented in gure 4, separated by a dielectric substrate of the "Rogers TMM 10" type, with a permittivity equal to 9.2 and a thickness h=200 µm.This antenna is feed by a microstrip line of characteristic impedance equal to 50Ω.The idea is created a PBG structure on the other half of the remaining substrate an implantation a periodic arrangement of air cylindric square hole with a dimensions 20µm x 20µm, the distance between square cylinders is 30µm and height is 200µm.
B. Proposed antenna (2)with holey superstrate structure and partial ground plane Figure 5 illustrates the modi ed geometry of antenna ( 2) which covered with one layer of holey superstrate structure."RogersTMM dielectric (ε=9.2)with a dimension of 500x500x200 µm3 is employed to design the superstrate layer.The superstrate layer is kept parallel to the dielectric substrate and isolated by the air medium that formed the cavity with thickness h 1 .The superstrate is drilled with a square hole with a dimensions 20µm x 20µm to form a square periodic lattice around graphene.
C. Proposed antenna (3) with the PBG structures and partially defected ground plane In the proposed antenna, periodic arrangement of PBG structures is created on center of the graphene ring patch and crossed line gaps are etched on one side of the ground plane creating a partially defected ground structure.Figure 6 illustrates the modi ed geometry of antenna (3) with the PBG structures and partially defected ground plane.

D. Proposed antenna (4) with the PBG structures and holey ground plane
The defected ground structure (DGS) and the Electromagneticband gap structures (PBG) are the two different types of the genericstructures mostly used for the design of compact and high-performance microwave components [26].In the proposed antenna (4), a lattice of certain ower shapes is etched on the ground plane.These ower shapes are employed to reject unwanted frequency.Square photonic crystals implantation in substrate layer are used to create PBGs and the strategy preferred for layout of PBG is surrounding graphene ring patch to further modify the antenna design.

Simulation Result And Discussion And Submission
The THz antenna plays an important role in the ultra-broadband and secured data transfer in the wireless communication systems [27].The characteristics of the graphene ring patch is designed and investigated in the frequency range varying from 0.6 to 0.7 THz, using the HFSS simulation tool and the performance is analyzed in term of Coe cient re exion (S 11 ), Gain (dB), Directivity (dBi) and Bandwidth (GHz).The bandwidth of a graphene based nano patch antenna using PBG substrate is higher than of a microstrip patch antenna with conventional material and normal substrate [28].
In the present work, the performance of graphene of four types of antennas on photonics crystal substrate have been proposed and simulated.The gure 8 present coe cient re exion plot for the four proposed antenna with several types ground plane.
It is observed from Figure 8 that the modi cation of ground plane modi ed propagation of electromagnetic waves through dielectric substrate and can create a stop band at desired frequency.The operating frequencies the conventional antenna without PBG substrate and with whole ground plane vary in the range of 0.643-0.738THz and they have the same S 11 =-15dB.
According to the gure 8, we notice that the four proposed antennas (1,2,3 and 4) don't resonate at frequency at 0.738 THz and offers the coe cient re exion S11 ≤ −10 dB.
The electric eld radiation properties of a graphene ribbon patch antenna (1) with EBG substrate are illustrated in gure 9.
For the proposed antenna (1), the partial ground plane creates a stop band in the conventional antenna which results in deletion of second resonant frequency hence the adaptation of antenna is enhanced at S 11 =-51dB at 0.659 THz.
The S 11 con guration of the second patch antenna ( 2) is present in gure 8.It consists of a One-Dimensional Electromagnetic Band Gap (1-D EBG) structure with a partial PEC ground plane.
It is clear that coe cient re exion has been decreased from -15dB to -26.27dB but the resonance frequency has shifted from 0.643THz to 0.672 THz.
Figure 10 presents the simulated E-plane and H-plane radiation pattern of antenna (2).We observed that secondary lobes are virtually inexistent and the beam width is around 30°.
The ground plane of the graphene ribbon patch antenna is partial so there is some back radiation and the beam peak is directional.
According to the graph of the gure 8 it can be seen that the antenna (3) -with the PBG structures and partially defected ground planeresonates at 0.66 THz with S 11 =-41.76dB and it covers the wide bandwidth of 10 GHz.
In this case, along E-eld in Figure 11 which correspond to the radiation direct of defected ground plane, there is little power transmitted from the graphene ring patch antenna as comparedto the other antennas ( 1) and (2).The E-plane has an important role in this case because it's determining the point of strongest signal.
For the proposed antenna (4), the holey ground plane with the PBG structures creates a stop band in the conventional antenna.
The second resonant frequency has disappeared and we notice a good adaptation of antenna (4).The S 11 of this proposed patch antenna improves from -15 dB to -36.25 dB.
It can be observed that the 2D radiation pattern of the co-polarization has a null at θ=60°.The beam peak is directional   Analyzing the effects, the holey ground of antenna (4) causes a reduction in gain and directivity due to radiation loss.It can be observed from designs of antenna (1), antenna (2) and antenna (3) proposed that the gain and the directivity containing partial and defected ground plane improves.Maximum S 11 is improved up to -51.02 dB, which is a good coe cient in the THz range and better than the work reported by [29], [30], [31] and [33].

Conclusion
A novel THz square ribbon graphene antenna using PBG structures has been presented and investigated in this paper.The antenna radiation performance has been investigated using different type of ground plane.Moreover, performed investigation for antenna second resonance frequency cancellation with all four ground plane types validates the simulation approach in the frequency range from 0.6 to 0.8 THz.However, frequency adjustment has been observed only in the portion of 0.63 to 0.67 THz band.Finally, the projected antenna attained peak directivity and gain of 6.38 dB and 7.62 dB respectively.The square ribbon grapheme antenna on several ground plane also played a crucial role in increasing the value of bandwidth, which displays above 10 GHz.The performance of the proposed THz square ribbon graphene antenna for various parameters like antenna re ection coe cient, gain, directivity and patch shape are reported and compared with the other work carried out in the literature.

Declarations
Figures Radiation patterns at 0.660THz

Figure 1 Design
Figure 1

Figure 2 S
Figure 2
radiating from the center ofthe patch and there is back radiation characterized by the front-to-back ratio.The main directivity was directed at the horizontal plane, whichformed a directional signal level in the E-plane.

Table
IV summarized the radiation performance of existing THz graphene antennas with the four proposed structure.