Low-profile directive quasi-planar antennas based on millimetre wave Fabry-Perot cavities

A new configuration of directive and quasi-planar antennas is presented at millimetre wave frequencies. It comprises a plano-convex Fabry-Perot (FP) resonator excited by an arbitrary printed primary source. The total thickness of these low-profile antennas does not exceed 4 mm (0.8×λ₀) at 60 GHz for directivities usually varying between 15.5 dB and 23.5 dB. By analogy with dielectric loaded open resonators, their theoretical radiation patterns are perfectly Gaussian. Experimental results obtained with single- or multi-layer planar feeds and with FP cavities of various radii of curvature and grid mirror reflectivities, confirm theoretical predictions. As examples, the patterns of two structures illuminated by the same aperture-coupled microstrip antenna array are compared in the 60 GHz band. They are rotationally symmetric and have low sidelobes (<-27 dB). No significant depolarisation effect is observed. The efficiency of these new radiating structures varies between 20% and 65%, depending on the quality factor of the FP cavities.