In many microwave and mmWave applications, there is a drive to improve the performance of the communication or sensing system, to either stay competitive or exceed the range, throughput, accuracy, frequency range, agility, or other desirable aspects of the system. As microwave/mmWave design and function dictates or otherwise heavily influences the performance of these systems, it is clear that improving antenna performance or introducing beamsteering enhancements can often provide the needed enhancement.
However, traditional metallic antenna typologies are limited by certain phenomena in terms of their size, shape, and performance. A way to overcome these limitations is to use metastructure techniques with arrangements of metallic conductive structures in three-dimensional (3D) space. Given the nature of metallic conductors as a material, there are also limitations to the size, shape, performance, and manufacturability of metallic metastructure antennas. Conveniently, microwave/mmWave metastructures can also be fabricated using dielectric materials, which in some cases, may be easier to machine or otherwise fabricate into complex 3D structures at the scale and resolution tolerances necessary to yield microwave/mmWave antennas or lenses. Recent advancements in 3D printing dielectric materials has also opened doors to the types, cost, degrees of freedom, and time-scales associated with fabricating dielectric metastructures for small-/large-scale manufacturing.
A notable result of these advancements and new capabilities/materials is the ability to cost-effectively fabricate even extremely complex gradient-refractive index (GRIN) type dielectric antennas and lenses. GRIN dielectric lens (or lens antenna) are a type of dielectric metastructure with a continuous spatially graded index of refraction, which allows for some control of the electromagnetic radiation passing through the structure. In short, GRIN dielectric lenses/antennas can be used to dramatically alter the performance of an antenna by modifying/augmenting the gain, directivity, antenna pattern, steering angle, bandwidth, and other key antenna parameters.
Hence, GRIN dielectric lenses/antennas have been a hot topic of research for virtually every microwave/mmWave application, from military/defense, aerospace/space, to commercial telecommunications. There have even been several companies, products, and government/DoD grants using GRIN dielectric lens/antenna technology. This article provides some discussion around recent research (since 2018) on GRIN dielectric lenses/antennas, and an overview of GRIN dielectric lenses/antennas being deployed or intended for practical use.
Research Literature Review Discussion
In general, a research review revealed that there are a few main authors/research groups publishing research on GRIN dielectric lense/antenna technology in recent years. Much of the research is focused on developing new types of GRIN lenses/antennas, augmenting horn antennas using GRIN dielectric lenses, augmenting phased array/array antennas, modeling and design methods of realizing GRIN dielectric lenses (including unit cell designs), and new GRIN dielectric lens designs that are intended to solve application specific challenges for 5G telecommunications, satellite communications, or various sensor applications.
Literature Review Categories
The following bullets list different categories of research and papers on GRIN dielectric Lens, metasurface lens, inhomogeneous dielectric lens, gradient refractive index antenna – which include:
- 3D Printed/Additive Manufacture –
- Illustrating this category is the research from  which discusses work from MIT Lincoln Laboratory on flat lens designs for antennas, matching networks, and filters using graded index composite materials.
MIT-LL dielectric deposition technology. a) Range of relative permittivity that can be generated. Measured data are the dot-dot curves while the solid lines represent the model used to fit the measured data b) Active mixing nozzle printing a graded filament. Source 
- Horn Antenna inserts, augmentations, new designs , , , , , , , , –
- Demonstrating this category is the research from  which describes the use of conformal transformation optics to enhance the directivity of an H-plane horn antenna using GRIN materials.
Simulated amplitude distribution of Ez for the two H-plane antennas: (a) horn lens, and (b) reference horn. The dimensions are LL = LH = 207 mm, WL = WH = 167 mm, HL = HH = 20 mm, and FL = 40 mm.” Source 
Photograph of (a) the H-plane horn lens prototype, and (b) the reference H-plane horn prototype.” Source 
- 5G , , , , , , –
- Space Applications , , , , , , , , , –
- Satellite , , –
- Phased Array/Array Antenna , , , , , , , , , , , , , , , –
- Luneburg Lens , , , , , , , –, , , , –, –, , , , , –
- An example of this type of research is  which discusses a 3D-printed wideband parallel-plate circularly polarized Luneburg lens antenna.
E-field distribution for Ex components at 28 GHz for a LL illuminated by an open-ended waveguide (a) before (b) after applying EMT.” Source 
(a) Luneburg lens constructed by sweeping the 2D cross-section along the y-direction where darker color representing increasing permittivity. (b) LL’s structure constructed by dielectric posts. (c) & (d) i-th layer before and after inserting inclusion material into the unit cells (insets: unit cell).” Source 
- Remote Sensing, Radar, or Imaging , , , , , , , , –
- Flat, Low-profile, Compact, or Collapsible , , , –, , , , , , , , –, , , , , , –, , , –
- Terahertz (THz) , , , , 
Earlier research into GRIN lenses were often of the Luneberg type or inserts/augments for waveguide horn antennas. The results of much of this research were proof-of-concept type 3D objects, or demonstrations of gradient refractive index synthesis or manufacturing techniques. More recent research appears to be focusing on providing more practical dielectric lens designs in more compact form factors, some even collapsible, such as for satellite communications. Another large set of recent research focuses on improving the algorithms and design tools to yield GRIN and dielectric metasurface lenses that are more computationally efficient, accurate, or enable more straightforward unit cell design. Additionally, there has been a significant amount of research into integrating other types of structures alongside or including GRIN dielectric regions, such as absorbers/anti-reflective layers, reflectors, and complex conductive antenna structures. Other areas of interest in the body of GRIN dielectric research include the use of various nontraditional materials to realize GRIN structures, including fused-filament deposition (FDM) printable polymers, cold sintered ceramics/composites, and various other 3D printable materials. There is even a small but seemingly growing body of research focused on using GRIN dielectric structures for Terahertz (THz) communication experiments.
GRIN Dielectric Lenses In Action –
Advancement of dielectric lens/antenna for enhancement of antenna arrays and antenna structures housed in confined spaces has been of particular interest to the US Department of Defense (DoD). Dielectric lenses/antennas capable of ultra-wideband (UWB) and high power operation while maintaining a compact profile (often flat) have appeared in small business innovation research (SBIR) grant abstracts [104,105,107-109]. The DoD applications appear to be predominately focused on enhanced radar and sensing technology. Given the relatively recent advancements of GRIN dielectric lens/antenna, there may be many additional applications in the process of development or even deployment where information regarding these GRIN applications isn’t publicly available.
There have been a few commercial products created using GRIN and Luneburg lens/antenna technology. The solutions that have publicly available information include passive radar, automotive radar, satellite communication arrays (space), satellite communication ground terminals, and multi-beam antennas for 5G telecommunication applications. The majority of these applications cover microwave/mmWave frequencies, which is likely due to the frequency dependent size of dielectric lens and antenna structures, which become practical at microwave frequencies.
GRIN dielectric lens/antenna technology is being heavily pursued in order to overcome the constraints of traditional antenna design and enhance microwave/mmWave antenna performance beyond what is achievable with purely metallic conductive antennas. Upper microwave and mmWave communication and sensing applications tend to suffer from higher RF and atmospheric losses, and GRIN dielectric lens/antenna technology may provide a way to offset some of these losses by providing enhanced gain/directivity while minimizing the relatively size, weight, power, and cost associated with traditional higher gain antenna topologies. Another major area of possibility is the use of GRIN lenses/antennas to provide enhanced beamsteering capability for electronically steered array antennas including reduced loss at wide scanning angles. It appears likely that within the next few years that there will be many more examples of GRIN dielectric lens/antenna technology deployed in a variety of applications, including military/aerospace, drone/Unmanned Aerial Vehicle (UAV) communication, satellite communications, and automotive radar.
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