Resonant-Cavity Antennas with Tapered and Wideband EBG Superstrates

Resonant-cavity antennas are obtained by applying an Electromagnetic-Band Gap (EBG) superstrate at resonating distance from a primary feed backed by a metallic plate. The use of a traditional EBG makes the frequency response of the antenna extremely narrow bandwidth, unless an optimized EBG layout is designed for a high reflectivity over a wide frequency interval. A wideband EBG can be implemented as a one-dimensional multilayer, by alternating dielectric layers having a permittivity contrast, and including an air layer in between, thus conferring multiple resonances to the final structure. However, a limiting issue in this class of antennas is in the high sidelobe level, that is due to the small in-plane extension of the EBG, typically smaller or limited to two wavelengths, which leads to high edge diffraction. A grid realization of the EBG layers is proposed, through a tapered in-plane geometry of non-uniform holes. With this realization, the reflectivity of the EBG is reduced from the center to the edges, when the multilayer is used as superstrate of a truncated rectangular waveguide chosen as primary source. The fabrication of customized EBG layouts can be facilitated though 3D printing technique, in particular in the case of low-permittivity layers made in plastic materials, as PLA. Experimental measurements on the antenna prototype are reported for an RCA with a superstrate made of vetronite and PLA.