Active and Reconfigurable Millimetre-Wave Antennas and Systems
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The millimetre-wave (mm-wave) spectrum offers considerable advantages in terms of antenna form factor and spectrum availability. However, use of this region often requires reconfigurable antennas and systems. Initially, a review of the various applications which are taking hold in the lower regions of the mm-wave spectrum (30 to 100 GHz) is undertaken. Specifically, reconfigurable reflectarray technologies are selected for further research, and critical analysis of the reconfiguration techniques for including these in antennas is considered. Silicon as an optically activated semiconductor is chosen as the reconfiguration mechanism due to its low cost and the scope for improvement in this area. A new form of illumination is used, replacing traditional infra-red (IR) lasers with high power IR-LEDs enclosed in a cavity, increasing the efficiency of the silicon illumination. However, to make use of this novel illumination source, and subsequently integrate it into an antenna, the silicon response has to be characterised within Ka-band. This is done through measurements in a waveguide-based characterisation test cell, from which the complex electromagnetic properties of silicon under IR-LED illumination are retrieved with the aid of full-wave simulations. Using the measured conductivity properties of the illuminated silicon, reflectarrays with non-uniform amplitude distributions can be designed. Through variation of illumination intensities of IR-LEDs throughout the array, it is shown through measurements and full-wave simulations that unit cell reflections can be modified while phases are kept relatively constant. This theoretically allows switching between, for example a low side-lobe pattern binomial array, or a narrow beamwidth pattern Chebyshev array. To implement this, a novel multilayer unit-cell is designed, integrating the IR-LED. This is then used in a full reflectarray design which is measured. The key contributions of this work include the novel illumination mechanism and its integration into a reflectarray antenna, and the use of reconfigurable photoconductive materials to provide a mechanism for beam shaping and pattern synthesis at Ka-band.
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