It was recently shown that large-aperture lens antennas can be designed by using gradient-index (GRIN) metamaterials and that higher directivity and gain can be obtained than with traditional dielectric lens antennas. This provides an effective method to design high-performance lens antennas.
A paper titled "Three-dimensional large-aperture lens antennas with gradient refractive index," published in SCIENCE CHINA Information Sciences, 2013, No. 12, investigates the principles and performance of GRIN lens antennas from the perspectives of design, fabrication, and experiment.
Lens antennas have traditionally been of two different compositions: either the dielectric delay or the metal accelerating lens. These two lens types are designed to focus electromagnetic waves by means of special hyperbolic or ellipsoidal geometric configurations, which can incur many difficulties in design and fabrication. Classical lenses made of homogeneous dielectrics use curved surfaces to compensate for phase differences, while metamaterial lenses make use of gradients in refractive index to realize the required phase changes.
GRIN lenses offer several advantages: (1) a simple flat GRIN lens can be used to focus the electromagnetic waves and hence is easier to design and realize; (2) the impedance of a GRIN lens is easy to match with air, and hence little reflection loss exists to affect the antenna's efficiency; and (3) a wide range of indices of refraction can be obtained using metamaterials, making possible GRIN lenses that are much thinner than traditional ones.
In this work, three-dimensional GRIN lenses are fabricated using multilayer inhomogeneous drilling holes or square ring resonators, which possess the desirable characteristics of high gain, broad bandwidth, and dual polarization. Two impedance-matching layers are proposed for the two sides of the GRIN lens, yielding very low reflection coefficients.
In addition, an intriguing development has recently emerged in GRIN metamaterials—GRIN metasurfaces. By employing an idea similar to the GRIN slab lens to generate the required phase distributions, a GRIN metasurface can also be used to manipulate wavefronts for high-gain antenna performance.