Scanning characteristics of metamirror antennas with sub-wavelength   focal distance
  We investigate beam scanning by lateral feed displacement in novel metasurface based reflector antennas with extremely short focal distances. Electric field distributions of the waves reflected from the antenna are studied numerically and experimentally for defocusing angles up to 24 degree. The results show that despite their sub-wavelength focal distances, the scanning ability of metamirrors is similar to that of short-focus reflectarrays (focal distance about several wavelengths). In addition to offering a possibility to realize extremely small focal distances, metamirror antennas are practically penetrable and invisible for any radiation outside of the operating frequency range. 
Svetlana N. Tcvetkova, Viktar S. Asadchy, and Sergei A. Tretyakov
physics.optics