Figure 5: Different quasi-optical systems for beam-steering from literature: (a) Rotman lens array for beam-steering on aircrafts [46]; (b) 3D-representation of a spherical Luneburg lens; (c) a graded-index lens [53]; (d) representation of beam-steering using lenses [67]; (e) working principle of pillbox antennas [84]; (f) Risley prism based design for 2D beam-steering [82].
Another interesting quasi-optical technique for beam-steering involves the use of a pair of identical Risley prisms. With the rotation of each prism, the radiated beam can be deflected in different directions on a 2-D plane and is highly suitable for applications in 5G terrestrial networks [80]–[82]. The limitation of such beam-steering mechanisms is the need for physical rotation and the large volume of the overall structure.
Continuous transverse stub (CTS) or pillbox antennas are another class of antennas popularly seen in ground station applications that can also be modified for terrestrial networks [83]–[85]. These are dual-layered structures where the bottom layer consists of multiple feed points and the top layer consists of an array of radiating elements such as an SIW antenna or a phased array. The two layers are connected with a convex-shaped reflector created with metallic vias, and based on the feed position, the beam direction can be reconfigured.
Quasi-optical systems for beam-steering are highly efficient and can produce wide angle steering. They are cost-effective and can easily be manufactured with simple techniques such as additive manufacturing. However, they tend to be 3D structures with large volume and may not be suitable for every environment. Such quasi-optical systems are of great interest to both antenna engineers and material scientists to further reduce their physical size and make them suitable for practical applications in terrestrial networks.