Analysis of graphene based optically transparent patch antenna for terahertz communications
Graphical abstract
By shorting the microstrip line and ground plane of the antenna with a MWCNT, the return loss of the antenna is improved. It is observed that the return loss of −48.75dB is obtained for a MWCNT loaded antenna as compared to −39.37dB obtained for that without a MWCNT loaded antenna. Return loss improvement of 9.38dB is achieved for antenna loaded with MWCNT. Thus by optimizing the position of the MWCNT short return loss of the antenna is significantly improved for the same impedance bandwidth. Both the graphene based transparent antennas achieved the −10dB impedance bandwidth of 12.83%. The graphene based transparent antennas have broad bandwidth (12.83%), high directivity (7.56dB) and high gain (≥2dB).
Introduction
In recent years, transparent conducting material (TCM) has been a critical component in many photoelectronic devices such as liquid crystal displays (LCD) [1], solar cells [2], organic light emitting diodes (OLED) [3] and antennas [4], [5], [6]. Traditionally, this role has been well served by ITO (Indium Tin Oxide) material. But ITO is becoming very expensive due to the limited supply of element indium. In the race to overcome the ITO limitations, monolayer and multi-layered transparent carbon based conductors such as graphene, carbon nanotubes (CNTs) have emerged as the viable alternatives which satisfy the future requirements of TCM for optoelectronic applications [7]. Recent advances in graphene film synthesis and its characterization indicate that it is suitable for photoelectronic applications including its use as a flexible transparent conductor [8], [9]. The electrical properties of graphene are analyzed theoretically and experimentally in [10], [11], [12], [13]. The challenge in using TCM as the antenna patch is retaining the optical transmittance of the material while optimizing its electrical conductivity [14]. In [15], optical transmittance greater than 90% was observed in graphene films with a thickness of tens of nanometres in the 400–1800 nm wavelength range. Integration of optically transparent antennas with solar cell can reduce the overall system size, weight, cost and visual disturbances [16]. In space applications like satellites, size and weight of the on-board payload are the two critical parameters. By designing optically transparent antennas on solar cell panel displays such that the display itself can act as antennas, the overall size and weight of the satellite system can be reduced [17].
Increasing the carrier frequency of the communication system results in numerous advantages such as broad bandwidth for high data rate transmission, improved spatial directivity and resolution, reduced transmission power and system size. Most of the aforementioned advantages are achieved by operating the communication system in the terahertz spectrum [18], [19]. It is of particular interest to service providers and system designers, because it offers broad bandwidth and free spectrum. The microstrip patch antenna is widely used in military and commercial wireless communication systems because of its low profile, planar and robust structure [20]. Owing to these properties, it is best suited for system miniaturization for terahertz frequency applications [21], [22]. Conventional TCM patch antennas have limitations in terms of −10dB impedance bandwidth (<5%), gain (<2dB) and poor radiation efficiency [23]. The slight deformation in the patch made from conventional TCMs like indium tin oxide (ITO), titanium-doped indium oxide (TIO), etc. can cause its impedance to alter. This results in an impedance mismatch [24]. But the impedance of graphene undergoes negligible change under deformation [25]. The return loss of the antenna can be reduced by shorting microstrip line and the ground plane with the conducting via [21]. The transparent multiwalled carbon nanotube (MWCNTs) is widely used for designing the interconnects in integrated circuits [26]. The performance of CNT based device undergoes minimal changes under bending on a polyimide substrate [27]. These are the considerations behind the motivation to investigate radiation performance of graphene based transparent patch antenna. In [28], graphene based terahertz frequency-reconfigurable antenna is designed using electromagnetic simulator-Ansys HFSS.
Using the Kubo formalism, the surface conductivity of graphene sheet is described as a function of frequency, chemical potential, scattering ratio, temperature and reduced Plank′s constant [29], [30]. At terahertz frequencies, substantial change in temperature results in a minimal shift in resonant frequency for graphene based patch antenna. In [31], the effect of temperature on the graphene based terahertz patch antenna is studied. For 100°K change in the temperature from 250 to 350°K, resonant frequency of the antenna undergoes a shift of 0.15 THz. Graphene exhibits nonlinear elastic properties viz. third-order elastic stiffness and third-order elastic constant of 2 TPa and 690 N/m, respectively and a breaking strength of 42 N/m corresponding to Young′s modulus of 1 TPa [32], [33]. The graphene deposited on polyimide substrate is reported to have very good thermal stability in the range 4.4–400°K and its resistance is stable up to a bending radius of 1 mm, suggesting excellent mechanical flexibility [34].
In this work, with and without MWCNT loaded graphene based microstrip patch antennas are designed on a 2.5 μm thick optically transparent polyimide substrate. The antenna characteristics are studied by shorting the microstrip line and ground plane with transparent MWCNT. The entire structure is optically transparent in the visible spectrum region and designed to resonate at 6 THz. Organization of the paper is as follows. Section 2 discusses the design of graphene based optically transparent microstrip patch antenna with and without shorting MWCNT. Section 3 investigates −10dB impedance bandwidth and radiation characteristics of with and without MWCNT loaded antennas and the results are compared with conventional transparent antennas and graphene based non-transparent antennas. Conclusions are made in Section 4.
Section snippets
Graphene based transparent patch antenna
The graphene based optically transparent microstrip patch antennas with and without shorting MWCNT are designed to resonate at 6 THz. Their dimensions are same and are of the order of micrometres as listed in Table 1. As shown in Fig. 1, the antennas have transparent conducting patch and a ground plane separated by 2.5 μm thick optically transparent polyimide substrate. The rectangular shaped transparent graphene sheet (patch thickness t‹‹λ0, where λ0 is the free space wavelength) is used as the
Results and discussion
The proposed antenna structures are designed and simulated using Ansys HFSS, a finite element method (FEM) based electromagnetic solver. The radiation characteristics of the antennas are analyzed in 5.66–6.43 THz band. Graphene is a sp2-bonded monolayer of carbon atoms arranged in a hexagonal structure. The incident high frequency radiations generate surface plasmon polaritons (SPPs) wave along the metal-dielectric interface. Graphene provides best conditions for propagation of SPPs and supports
Conclusions
Application of graphene as a transparent conducting material for the microstrip patch antennas with and without MWCNT short is proposed and the radiation characteristics of the antennas are investigated in the 5.66–6.43 THz band. Transparent MWCNT is used as a conducting via to short the microstrip line and the ground plane of the graphene based transparent antenna. Return loss improvement of 9.38dB is achieved for antenna loaded with MWCNT. Thus by optimizing the position of the MWCNT short
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