Graphene antenna design and characterisation for fifth generation applications

Abstract

The incoming fifth generation (5G) technology requires antennas with a greater capacity, wider wireless spectrum utilisation, high gain, and beam steering ability. This is due to the cramped spectrum utilisation in the previous generation. As a matter of fact, conventional antennas are unable to serve the new frequency due to the limitations in fabrication and installation mainly for smaller sizes. The use of graphene material promises antennas with smaller sizes and thinner dimensions, yet capable of emitting higher frequencies. Graphene is a unique material that can display tuning characteristics. This characteristic originates from its surface complex conductivity, which is controlled by a chemical potential. Most characteristics of tunable graphene antenna have been studied on terahertz frequency range, thus making it difficult to be realised practically. Besides, the standard antenna that uses switching components may have trouble during installation, and size consuming as it can be seen in the reconfigurable antenna. Due to that, another study to produce graphene with excellent properties is vital for the advancement of wireless communication system. In this thesis, graphene antennas for fifth generation applications are conducted in three parts of studies. In the first part, the graphene antenna properties are studied in different curing temperatures and times. The curing temperatures are 250°C, 300°C, and 350°C, then each temperature is set with curing times of 20 minutes, 30 minutes, 1 hour, 2 hours, and 3 hours to manufacture graphene based antenna with different properties. The proposed graphene based antenna properties are then respectively investigated using performance network analyser (PNA), vector network analyser (VNA), field-emission scanning electron microscope (FESEM), and Raman spectroscopy. From analyses on the dielectric, conductivity and characterisation on graphene’s physique, the antenna properties exhibit a tunability through its resonance frequency and main beam direction of the radiation pattern by the variation obtained in curing temperature and time. In the same time, the gain of the antennas can also be varied. The second part is the study of graphene antennas at a frequency of 15 GHz in both single and array elements. The high-frequency antenna contributes to a large bandwidth and is excited by coplanar waveguide for easy fabrication on one surface via screen printing method. The defected ground structure is applied in an array element to improve the radiation and increase the gain. The results show that the printed graphene antenna for single element produces an impedance bandwidth, gain, and efficiency of 48.63%, 2.99 dBi, and 67.44%, respectively. Meanwhile, the array element produces slightly better efficiency (72.98%), approximately the same impedance bandwidth as the single element (48.98%), but higher gain (8.41 dBi). Moreover, it provides a beam width of 21.2° with scanning beam capability from 0° up to 39.05°. The last part is a tunable antenna based on graphene operating at microwave frequency range is proposed. The antenna is designed and fabricated at 15 GHz with a gate electrode placed behind it. They are connected to external direct current (DC) bias during the measurement. The biasing is applied from 0 V to 30 V. The result shows that the resonance frequency is tuned to 20 MHz and reflection coefficient magnitude improves by 1.24 dB. Following this, an analytical calculation on chemical potential is also derived to enhance the graphene tunability. It is shown that at least 2.85 kV of the gate voltage is needed to vary the chemical potential and less than 0.29 µm of dielectric thickness is suitable for tuning purpose with a given condition. Based on the three parts of studies on antenna design and characterisation, graphene can be a good alternative material for future communication. It is due to the exhibited performances are comparable with conventional material and could act beyond the common antenna properties under the influence of tunability, which is owned by graphene.