Novel compact antenna designs for future wireless communication systems

Abstract

Wireless and mobile devices are becoming packed with more applications that require an increasing number of communication technologies as well as a constant desire for size reduction. Ideally, every single communication technology should have a dedicated antenna system. This puts pressure on antenna engineers as the available space for the radiation elements is becoming very limited, particularly for small mobile phones and small cell base stations. Traditionally, reducing the size of the antenna was enough to accommodate these requirements. However, making antennas smaller strongly affects their performance. In order to further shrink a multi-radio transmitting system, antenna reconfiguration provides an option to converge multiple radiating elements into a single one, and hence, save space in wireless and mobile devices. The creation of novel materials such as carbon nanotubes, graphene and metamaterials, which present extraordinary electrical and mechanical properties, has opened new possibilities within the antenna field. The most interesting properties of these materials are the presence of plasmons in carbon nanotubes and graphene at much lower frequencies than in metals, the ability to tune the surface impedance of graphene by applying a DC voltage bias and the possibility of generating size-independent resonances in metamaterials. These materials are studied here as alternative methods to achieve antenna size reduction and reconfigurability at microwave frequencies. This thesis presents an initial study of the advantages and disadvantages of designing small and reconfigurable antennas fully made of carbon nanotubes and graphene at microwave, millimetre wave and terahertz frequencies. Here, the focus is on the trade-offs between the antenna performance and the achievable size reduction and reconfigurability at microwave frequencies. The results show that the resulting low antenna efficiencies do not compensate the small size reduction and reconfigurability of these antennas at such frequencies. This is mainly caused by the large losses suffered in carbon nanotubes and graphene and the low inductive behaviour of these materials at microwave frequencies. Furthermore, carbon nanotubes present extremely high input impedances, which make the antenna matching very difficult, with little reconfigurability due to not being able to actively tune their resistivity and the infeasibility of using plasmons at frequencies for commercial applications (up to 10 GHz). For this reason, three planar hybrid antennas made of a traditional conductor (i.e. copper) and graphene are presented as the main proposed solutions for antenna reconfigurability and size reduction at microwave frequencies. The first proposed design provides frequency reconfigurability by changing the electrical length of microstrip patch antennas using the variable surface impedance of graphene. However, the resulting antenna efficiencies are low compared to other reconfigurable antennas found in the literature. The second design provides polarization reconfigurability by adding graphene sheets to the truncated corners of a square patch antenna. The resulting antenna efficiencies are improved when compared to the first antenna design. This is achieved because the impact of graphene on the antenna efficiency is reduced due to the use of graphene sheets with smaller size. The final design combines size reduction and frequency reconfigurability. Size reduction is achieved by designing a zeroth order resonant (ZOR) antenna, while frequency reconfigurability is achieved by tuning the surface impedance of graphene. The variable surface impedance of graphene changes the inductive and capacitive behaviour of the ZOR antenna which in turns changes its resonant frequency. The resulting antenna efficiencies are better compared to the first design but worse than in the second design. Additional features presented by the first and third proposed antenna designs are the ability to tune the reflection coeficient and antenna bandwidth, which might help to reduce the complexity of the matching network; and to select any intermediate resonant frequency between two edge frequencies. The latter property which might be useful to compensate undesired effects in wearable antennas, by selecting appropriate values of the surface impedance of graphene. In addition, an analysis of the power consumed in the proposed reconfigurable antennas is also provided when switching between different values of the surface impedance of graphene. Finally, the proposed antenna designs are also evaluated as fully transparent and flexible reconf-gurable antennas which allows integration in scenarios where flexibility and transparency are a requirement or an advantage.