Directive antennas based on two-dimensional dielectric EBG crystals
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The aim of this work is the design and analysis of novel antennas realised with electromagnetic bandgap (EBG) structures based on simple two-dimensional cylindrical, triangular and square lattices of dielectric rods. In particular, we focused our attention on designing antennas with high directivity and front-to-back-ratio (FTBR) on the azimuthal plane. Several EBG structures have been investigated, divided in two main categories: multilayer EBG structures with an angular defect window and EBG corner reflectors. The former are based on a feeding source excited within a cavity: fields at badgap frequencies are trapped inside the cavity and opening an angular defect window allows propagation in that privileged directions leading to directive radiation patterns. The latter are based on a source placed in front of an EBG corner reflector: at bandgap frequencies, the excited fields are reflected toward the corner aperture (in a similar fashion to metallic corners of analogous dimensions) enhancing radiation patterns’ directivity. The analysed structures have been also modified to host multiple sources to create multiple-feed antenna structures with the ability of rotating the radiation patterns on the azimuthal plane. Antennas have been modelled using an in-house developed Finite-Difference Time- Domain solver and the commercial Finite Element Method solver Ansoft HFSS, focusing on structures designed to operate in the X-band frequency region (8.2GHz-12.4GHz) in order to take advantage of the available equipment and facilities at Heriot-Watt University for prototypes testing. The proposed structures can be nevertheless scaled up or down in size in order to respectively scale down or up of the same factor the frequency of operation. The main achievements of the analysed multilayer EBG structures and corner EBG reflectors are the large impedance bandwidth (greater than 30%) with stable radiation patterns within, high gain (>12dBi) and high FTBR (greater than 25dB) accomplished using EBG structures made with a small number (10-20) of low-loss ceramic rods arranged in very simple two-dimensional crystals. EBG corner reflectors have been also found basically equivalent (at badgap frequencies) to metal reflectors in terms of achieved gain and radiation patterns, suggesting them as possible substitutes for high frequencies applications where dielectric losses would be smaller than metal losses.