Negative refraction and focusing in hyperbolic transmission-line periodic grids

We propose a class of anisotropic periodic structures with spatial hyperbolic dispersion characteristics, synthesized by arranging unloaded transmission lines (TLs) in a two-dimensional planar grid. These planar periodic grids support the formation of sharp beams, called resonance cones. Analogous resonance-cone effects are observed in highly anisotropic resonant plasmas. By interfacing transposed versions of these anisotropic periodic grids, negative refraction and focusing of resonance cones can be achieved. Simulation results are presented for ideal TL grids and are verified experimentally using planar microstrip grids. These grids are easy to fabricate at a low cost. Moreover, since the TLs are unloaded and the periodicity is comparable to the wavelength, the structures are scalable from microwave to millimeter-wave frequencies and could be utilized for spatial filtering, multiplexing, and demultiplexing.