Broadband Reflecting Luneburg Lenses Based on Bed-of-Nails Metasurfaces

This communication describes the bandwidth (BW) characteristics of reflecting Luneburg lenses (RLLs) implemented by a bed of nails (BoN). RLLs' beamformers consist of two vertically stacked parallel-plate waveguides (PPWs) of circular shape. The bottom PPW contains a graded index (GRIN) medium with azimuthal symmetry to address the rays launched by a point source on a focal line along curvilinear paths up to a corner reflector. After reflection and coupling, the rays emerge collimated in the top PPW. The lens' symmetry allows for generating plane waves with arbitrary directions by just changing the azimuthal position of the source in the bottom layer. The use of BoN with higher symmetries helps to synthesize the refractive index profile in the RLL, which also features high values and increases its operational BW by mitigating frequency dispersion. A Ka-band RLL featuring higher symmetries has been fabricated and tested. Simulations and measurements are in good agreement, showing an overall fractional BW of {\approx } 39\% . This architecture constitutes a metal-only, low-profile beamformer with full azimuthal scanning from 26 to 40 GHz.