Modeling and characterization of graphene for efficient millimeterwave and terahertz antennas

Abstract

The development of millimeterwave (MMW) and Terahertz (THz) technologies for a wide range of wireless sensing and communication applications have been rapid due to the unique characteristics of waves in these bands. Antennas are regarded as the core of wireless applications, and its development for efficient operation within these bands require the use of new carbon nanomaterials such as graphene. This is due to the conductivity deterioration of conventional metals with increasing frequency. This work focuses on improving the antenna performance parameters in the MMW and THz bands based on the utilization of graphene. The employment of graphene requires mathematical modelling and characterization of its surface impedance, which are frequency dependent. The chemical potential (𝜇c) (doping) indicated a more significant effect compared to other variables. Its values are influenced by electrical bias, magnetic bias, or chemical doping. The 𝜇c = 0eV value is used as the non-doped graphene (ND-G) model, while (𝜇c = 0.25eV and 𝜇c = 0.5eV) values are used for the doped graphene (D-G) models. D-Gs were found to exhibit higher surface conductivities than ND-G, while the increasing 𝜇c resulted in increased graphene conductivity. The material models of graphene (D-G and ND-G) are then integrated onto the substrate and the microwave solver software CST prior to simulations to determine the graphene based antenna (GBA) performance. These GBA antenna models indicated significant performance improvement at THz frequencies (1 THz, 1.29 THz, and 1.49 THz) compared to the copper-based antenna models. On the contrary, the use of ND-G does not always show such significant improvements at MMW frequencies (64GH), but is rather more concentrated on the bands beyond it. Two other new techniques (coating and adhesives) are proposed as alternative fabrication methods of graphene based antennas. These techniques indicated similar parameter improvements to that of the direct deposition technique in the MMW (70GHz) and THz (1.71 THz) bands. It was also discovered that antenna modeling using the adhesive technique performs better than the direct deposition technique at MMW frequencies. Next, an investigation to determine the critical frequency (CrFr) for the use of ND-G and copper at MMW and THz spectrum (30GHz-3THz) was performed. This study considers different topologies and performance parameters of ND-GBA to determine CrFr. The curves of the performance parameters are plotted against frequency to facilitate comparison between various GBA configurations. The CrFr is determined using the intersections of the performance curves for copper-based and graphene-based antenna. The investigation is also extended onto GBAs with graphene as ground or patch only. The comparison for different antenna topologies, configurations, and parameters reported different CrFrs, ranging from 0.130 - 0.240 THz, with an average of 0.147 THz. On the other hand, the critical frequencies for the patch and ground only graphene-based configurations ranged between 0.145 - 0.365 THz, with an average of 0.213 THz indicating that the CrFr of ND-GBA depends on the antenna topologies and configurations.