1 .1 INTRODUCTION
Development of the next generation wireless communication network requires system with high broadband capabilities in high mobility environment9. For such applications, range from personal communications to home, car and office networking, higher data rates, higher spectral efficiencies and stronger fading mitigation have to be achieved. Due to its particular role, the microstrip patch antennas simultaneously satisfy stringent requirements concerning geometrical characteristics (small-size, light-weight, adaptability to any platform and non-obstructive to user), electrical performance (wide-bandwidth, radiation properties, higher-efficiency, reconfigurability and suitability for diversity) and manufacturing constraints (low-cost, reliability, packaging capabilities) as well as the performance should not be degraded by environment and design must satisfy radiation safety standards. For the terahertz frequency , the microstrip patch antenna(A microstrip antenna is used to process ultra-high frequency signals. It is often used as a satellite radio or cell phone receiver or it is mounted on an aircraft or spacecraft)is an essential device because of its compatibility for miniaturization9. The microstrip patch antennas are broadly used in satellite and missile applications due to its advantages of conformal to planar and non-planar surfaces5 . The conformal patch antenna consists of parallel electric conductors, separated from the ?exible dielectric material and can be fabricated using thin ?lm deposition and nano lithography techniques. The ?exible dielectric material is polyimide (? r = 3.5) is used here. The low dielectric permittivity material is needed for improving the radiation ef?ciency in the desired direction5.
1.2 PROJECT OBJECTIVE
The main objective is to design an antenna for terahertz frequency. THz technology has become attractive due to the low energy content and non ionizing nature of the signal. Properties exhibited by THz radiation are penetration, high-resolution imaging, spectroscopy, low scattering, intensity, non-ionising nature. For the terahertz frequency system, the microstrip patch antenna is an essential device because of its compatibility for miniaturization.
Graphene is a promising material because of its excellent electrical conductivity, electro-magnetism and electro-mechanical properties for the next generation molecular electronics 1. Graphene nano ribbon has wide applications of high frequency transistors 2, modulator3, wireless nano systems 4, organic electronics 5 and devices operating in terahertz frequency band 6. Graphene nanoribbon polymer composites based conformal antennas are needed for a metallic thin ?lm and is prone to fail due to micro cracks 7,8. For the next generation wireless communication system requires ultra-broadband antenna with very low transmitting power in a high mobility environment. For the terahertz frequency system, the microstrip patch antenna is an important device due to its compatibility in miniaturization 9. The microstrip patch antennas are broadly used in satellite and missile applications for its advantages of conformal to planar and non-planar surfaces 1. The microstrip patch antenna consists of parallel electrical conductors, separated from the ?exible dielectric material and can be fabricated using thin ?lm deposition and nano lithographic techniques 2. The ?exible dielectric material is polyimide (?r = 3.5) with tan ? = 0.008 has been used here. The low dielectric permittivity material is needed for improving the radiation ef?ciency in the designed direction 1. Some researchers in this topic are unable to achieve high gain ( 2dB), broad impedance bandwidth (>5%) and better radiation ef?ciency3.
The work discusses the radiation characteristics and design of a graphene nano ribbon (GNR) based microstrip patch antenna on the polyimide substrate in terahertz frequency band. In ref1, patch antenna based on graphene has been designed and the surface conductivity of graphene sheet has been analyzed.
The effects of the graphene will affect the electromagnetic(EM) radiation when the lateral dimensions of graphene sheet is less than 100 nm 1. Calculation on surface conductivity of in?nite Graphene sheet by using the Kubo formalism(KF) is available in the previous literatures 2,4,5,7. conductivity of Graphene sheet based on fre quency has been presented in terms of inter band and intra band contributions1.
The inter band conductivity of the dielectric substrate (h) must be less than the free space wavelength (0.003 h0 h 0.05 h0) 7.The fringed ?eld is the main resources of electromagnetic radiation. The excited electric ?elds at the edges of the patch undergoes fringing. The substrate having smaller values of dielectric permittivity is preferred, because it enhances the fringing ?elds. The microstrip line feed technique has been used for impedance transistors.
The imaginary parts of both total conductivity and intra band are overlapping. Intra band conductivity is very high compared to inter band conductivity. At higher frequencies ,the intra band conductivity is dominating the inter band conductivity in total. In this region, it is, therefore approximated that the conductivity of graphene sheet depends only on the intra band conductivity as an assumption 4.
The graphene nano ribbon (GNR) based microstrip patch antennas are designed and has a dimensions in the order of few micro metres. The basic structure of the microstrip patch antenna consists of a conducting patch on it, a ground plane separated by a thin dielectric substrate material. The rectangular and square shapes of radiating patch antenna are preferred primarily because of easy analysis and fabrication. As GNR a radiating patch (h0 is very much greater than the patch thickness (t)) and also ground plane. The ground plane and radiating patch are separated by a height of 20 micrometer thin polyimide substrate. The microstrip line feed techniques are used for impedance transition between coaxial line and rectangular patch. The antenna has been fed by a 50 K microstrip transmission line. The width(W) of the respective microstrip line is balanced to match the antenna impedance with zero mismatch loss. The proposed antenna structure is designed to resonate at 0.750 THz.
The THz antenna is required for achieving the secured data transfer and ultra-broadband in the future wireless communication uses. The proposed antenna in ref5 is designed and its radiation characteristics are investigated in the band of 0.725–0.775 THz, using the Ansys – HFSS simulation tool.
The S11 parameter of the GNR based patch antenna is obtained. The antenna has achieved a -10dB impedance bandwidth of 6.67% in the band of operation. The conventional ?exible antennas have an impedance mismatch under deformation, due to formation of electrical resistance in the conducting patch 9. But the GNR based ?exible antenna has negligible resistance even under deformation 2.
Radiation performances of the Graphene based patch antenna.
Frequency in GHz 725 740 750 770
Gain in dB 4.93 5.05 5.09 5.07
Directivity in dB 5.54 5.67 5.71 5.70
Radiation ef?ciency in % 86.85 86.69 86.58 86.43
The table shows the antenna radiation ef?ciency in relation to the frequency. The peak radiation ef?ciency of the proposed antenna in ref5 is 86.85% at 0.725 THz. It is also given that the proposed antenna5 has better radiation ef?ciency than the conventional patch antennas 4.
The H and E planes far-?eld radiation patterns of the antenna proposed in 5 at 0.725 THz, 0.740 THz, 0.750 THz and 0.770 THz are shown in the respective papers. It is observed that graphene nano ribbon based patch antenna has resulted a peak gain of 5.71dB at 0.750 THz. This is the result as compared to conventional patch antenna gain which resulted a peak gain of 2dB 3. The radiation characteristics of the proposed antenna 5 are compared with that of the previously refered graphene based antennas and conventional antennas operating in terahertz frequency.The graphene nano ribbon based patch antenna is not optically transparent. In future, graphene based optically transparent antenna should be tried for GHz and THz applications.
The radiation characteristics of microstrip patch antenna have been analysed in the 0.725–0.775 THz band. Graphene nanoribbon based patch antenna for terahertz frequency is simulated by using the ?nite element method based simulator Ansys – HFSS. The antenna achieved the -10dB impedance bandwidth. The proposed antenna is shown to produce the maximum gain of 5.71dB at 0.750 THz and the 5.54dB is the minimum gain observed at 0.725 THz. Since the antenna has minimum back lobe in the desired band, there is better front to back lobe ratio. The radiation characteristics of the proposed antenna is listed above, and it is compared with that of the previously reported graphene based antennas and also with conventional patch antennas5.