Enabling RF/microwave devices using negative-refractive-index transmission-line metamaterials

Metamaterials are artificially engineered structures with unusual electromagnetic properties. In this article, we review the implementation of isotropic metamaterials that exhibit a negative permittivity and a negative permeability, thus leading to a negative index of refraction. Specifically, the article focuses on transmission-line metamaterials, which are planar structures comprising a network of distributed transmission lines loaded periodically in a dual (high-pass) configuration with lumped inductors, L, and capacitors, C. The periodic unit cell is much smaller than the wavelength, thus leading to an effective medium in which the lumped loading elements can be either chip or printed. Based on this transmission-line method, a number of RF/microwave devices are presented, including lenses that can overcome the diffraction limit, compact phase-shifting lines, small antennas, antenna feed networks and baluns, backward leaky-wave antennas and high-directivity coupled-line couplers.