Impedance-matched microwave lens
  A microwave lens with highly reduced reflectance, as compared to conventional dielectric lenses, is proposed. The lens is based on two-dimensional or three-dimensional transmission-line networks that can be designed to have an effective refractive index larger than one, while having almost perfect impedance matching with free space. The design principles are presented and an example lens is studied using commercial simulation software. 
Pekka Alitalo, Olli Luukkonen, Joni Vehmas, Sergei Tretyakov