Screen Printed and Graphene and Carbon Nanotube Antennas at UHF and EHF - Design, Modelling and Measurement.

Abstract

Researchers seek to design small meander line antennas for radio frequency identification (RFID) applications, while taking into consideration the achievement of best performance parameters of the antenna: impedance, return loss, voltage standing wave ratio (VSWR), radiation efficiency, gain, directivity and etc. Various planar meander line antennas were proposed which have the lowest resonant frequency and maximum radiation efficiencies for a fixed area, using multi-parameter optimization routines. The design in planar form with maximum radiation efficiency was optimized with the inclusion of matching structure for complex impedance matching with RF transceiver chip CC1101 for 433 MHz RFID applications. The design was also optimized for impedance match with commonly used transmission lines (50 ohms characteristic impedance) for 2.45 GHz applications. The antennas were fabricated using circuit in plastic (CiP) technique with screen printed silver conductor on 3mm FR4 and plastic substrates respectively. The technique allows the creation of a waterproof antenna for underground (mining geophysics, environmental monitoring, etc.) and underwater (swimmer communications, tidal monitoring, etc.) applications, because the antenna unit is completely sealed in a thin plastic coating. The impedance, band width, return loss and radiation efficiency of these antennas are the main parameters reported. Since the discovery of graphene, its electrical, mechanical and electronic properties have been explored. The high conductivity, low fabrication cost, flexibility, controllable isotropic and anisotropic behavior are some of the main properties of graphene which make the technology fascinating for electrical engineers and antenna designers. It can be used for antenna applications in the Terahertz frequency band either as a two dimensional, infinitely thin, planar sheet or in the form of one dimensional rolled up graphene known as carbon nanotubes. A pole-zero analysis on the input impedance of carbon nanotube (CN) dipole antennas with different lengths was used to explain the damped plasmonic resonances of these dipoles with increasing length. We used model based parameter estimation to approximate the input impedance of the antennas with a rational function in the complex frequency domain. Despite the dispersive nature of CNs, the imaginary part of the poles and zeros are respectively the integer multiples and odd multiples of the imaginary part of the first pole and zero. However, the real parts of impedance at the poles are almost constant, while the pattern was not observed for the real part of zeros. Carbon nanotube dipoles operating between 43-53GHz are well matched if the source impedance is much higher than 50 ohms, and even higher than 12.9kΏ. The fundamental resonances (f0) of carbon nanotube dipoles plotted versus their inverse-half-length (1/L) are linearly related, but the intercept of the fitted straight line is non-zero unlike that for perfect electric conductor (PEC) dipoles. This leads to a non-linear variation in wavelength scaling of CN dipoles. The resonant CN antennas are relatively much shorter than PEC dipoles of equivalent size. The fundamental properties of carbon nanotube straight wire dipole and Yagi- Uda antennas were determined using theoretical and numerical modelling. The insertion loss, radiation efficiency and Q-factor have been reported using classical Hallén’s-type integral equation, based on quantum mechanical conductivity. Contrary to the properties of metal antennas with dimensions less than 0.01 m, the addition of parasitic elements in a Yagi-Uda antenna with carbon nanotubes does not lead to significant changes in the radiation characteristics as the induced current in the parasitic elements is very small. The radiation efficiency of the Yagi-Uda antenna shows little change. The radiation efficiency of these antennas when compared with the Pfeiffer’s maximum efficiency bound lies well below the maximum efficiency limit. The ����-factor of carbon nanotube antennas limited by their radiation efficiency was compared to the fundamental limits proposed by Chu and Thal and although the ���� -factor of the nanotube antennas is very small, it is still larger than the fundamental limits. This analysis was also extended to carbon nanotube loop antennas operating in transmitting mode. The extinction properties of carbon nanotube loop antennas were also investigated and compared with ordinary metallic loop antennas. Since graphene is an ideal candidate for THz antenna applications due to its excellent electrical and mechanical properties, the conductivity and scattering properties of monolayer graphene were explored over a wide frequency range. Two monolayer graphene samples having different dimensions printed on a plastic (PET) substrate were measured using horn waveguides at microwave frequencies. Cascade matrix theory was applied to calculate scattering parameters of monolayer graphene patch on PET substrate assuming a frequency independent surface conductivity ������������ (manufacture’s specification) and to deembed the surface conductivity of graphene from measured transmission parameter s21. De-embedding ������������ from measured reflection parameter s11 was performed using transmission line theory. The theoretically modelled Sparameters were in good agreement with measured S-parameters of graphene. The measured conductivity of graphene using proposed formulations was in good agreement with the theory. The results were compared with previously published conductivity measurements of mono and multi layered graphene. This work on graphene and carbon nanotube antenna properties is a contribution towards THz antenna applications and we intend to extend this work to explore the properties of graphene meander line antennas which have not been reported to date.