Comparison between Two Methods for Directivity Enhancement of Antennas through 2-D EBGs

Among the uses of Electromagnetic Band-Gap (EBG) materials to enhance the directivity of single radiators, thus avoiding the use of complex arrays relying on troublesome feeding networks, two methods are particularly effective. One realizes a resonator antenna by sandwiching the radiator in between a ground plane and an EBG material working in the bandgap, while the other embeds it inside an EBG structure working at the edge of the band-gap. In the former case a cavity is created and the periodically spaced elements act as a partially reflecting surface, while in the latter configuration, the EBG structure behaves like an epsilon near zero material and can generate equi-phase wavefronts at its boundaries. This paper pushes forward a benchmark between the two approaches by comparing their performance for a set of two-dimensional EBG structures, consisting of either triangular or square arrangements of dielectric cylinders, excited by a 8 GHz, TM-polarized, line source. Different finite-size configurations, obtained by changing the number of layers as well as the amount of cylinders per layer, are optimized with a full-wave solver. The resonator antenna generally gives the highest directivities, but there are operative conditions in which it is outperformed by the method with embedded source. In details, the former approach results more suitable in case of low-profile arrangements, i.e., very few layers with many rods, while the latter one better fits compact configurations, having more layers with few cylinders.The radiation patterns of the most attractive configurations are reported together with a discussion about the strengths and weaknesses of the methods.