Compact Multifunctional Gain Enhancing GRIN Lens for Widely Separated Frequency Band Shared Aperture Antennas

Abstract

these solutions, gradient index (GRIN) lenses have shown tremendous potential due to their ability to manipulate electromagnetic wave propagation in the desired way by exploiting their tailored spatiallyvarying permittivity profiles. GRIN lenses have been used to reduce the size of existing antennas, [15,16] reduce sidelobe levels, [17,18] and enhance gain. [19][20][21] To fabricate any GRIN lens, one must have the ability to tune the dielectric permittivity throughout the lens volume. To this end, perhaps the most common approach is to vary the volume fraction of two materials to create an effective permittivity. This is often achieved by mixing a base dielectric material and air and is realized through the introduction of negative inclusions (i.e., holes) whose relative size is used to tune the local permittivity value. [22,23] This can easily be accomplished with additive manufacturing techniques (i.e., 3D printing). [21,[24][25][26][27] Historically, 3D-printed GRIN lenses have been mainly created from plastics. However, ceramic materials offer a much larger range of relative permittivity values than plastics which designers can exploit to realize significant performance improvements over conventional GRIN lenses. A significant contribution of this work is the demonstration of a functionalized ceramic GRIN lens produced via a state-of-the-art additive manufacturing process. Alternatively, GRIN lenses have been realized with metamaterials consisting of engineered metal-dielectric unit cells. [28,29] Compared to those based on metamaterials, all-dielectric GRIN lenses can be bulky and heavy; however, all-dielectric GRIN lenses often operate over much broader bandwidths with lower losses than those based on metamaterials as they do not rely on resonant effects. In addition, all-dielectric lenses are often better suited for high power applications due to the complete absence of metal and the elimination of large local field enhancement hot-spots on the edges/corners and in the capacitive gaps of the metamaterial unit cells.Common GRIN lens solutions have been realized using analytical approaches (e.g., the Luneburg lens [30] and Maxwell fisheye lens [31] ), geometrical optics, [32,33] Transformation Optics (TO), [34][35][36] or the field transformation approach. [37] Among these approaches, TO has been the predominant method for its ability to recreate the performance of conventional solutions in more desirable geometries. For example, the flattened Luneburg lens has been demonstrated for a variety of beam steering and multibeam applications. [38][39][40][41] However, it is not Gradient index (GRIN) lenses embody a powerful technology that enables control of electromagnetic wave propagation over wide frequency bands. However, GRIN lenses that achieve multifunctional (i.e., dual broadband) performance have not been realized mainly due to the limitations of conventional techniques such as Transformation Optics (TO). This paper proposes a multiobjective inverse-design approach for the optimization of multi-band GRIN...