TECHNICAL FIELD

The present disclosure relates to a Luneburg lens, and to an antenna device that comprises a Luneburg lens.

BACKGROUND

Luneburg lens (LL) antennas have several known advantages, such as being broadband in nature, and having high gain and multibeam capability. This is important in applications such as 5G communication systems. Since electromagnetic waves suffer higher atmospheric propagation loss and blockage at the high frequencies at which 5G equipment operates, antennas with high gain performance and wide scanning range are needed for point-to-point wireless applications, i.e. the Mobile Backhaul Transport Network, to achieve higher spatial coverage.

Compared with other multibeam antennas, such as phased array antennas, Luneburg lens antennas do not suffer from scanning loss, and do not require sophisticated and costly feeding networks, which make them a promising candidate for millimeter-wave communications.

The radial dielectric constant variation of a Luneburg lens obeys the following relation: where e _G is the relative permittivity, R is the radius of the lens and r is the distance from the center. With such material distributions, all focal points reside on the surface, which enables the Luneburg lens to originate collimated beams by transforming cylindrical or spherical incident waves into plane waves. In addition, the refractive index at the external surface of the lens (where r - R) is 1, such that a perfect refractive index matching is naturally achieved with free space.

The development of Luneburg lenses has, up until now, been impeded by the difficulty in making them, due to the lack of available varying-permittivity materials. Previous attempts to fabricate Luneburg lenses have used double or multiple shells of polystyrene, quartz or Teflon; exponential chamfering; hole-drilling; sliced spherical slabs; and customized foams. While these methods work, they require complex and costly manufacturing processes. Further, due to the limitations of known fabrication techniques and materials, antennas that use such Luneburg lenses become less effective at higher frequencies, e.g. in millimeter-wave (mm-W) bands. There have also been attempts to fabricate Luneburg lenses using additive manufacturing techniques. In these previous approaches, the lens is discretized into unit cells that are cubes or rings, and each unit cell is filled with a pre-calculated amount of material to vary the refractive index. However, these proposed structures become weak after the additive manufacturing process is completed, especially after the supports used during the additive manufacturing process are removed. This could easily cause deformation, and deterioration of the lens performance.

SUMMARY

The present invention relates to a Luneburg lens comprising a plurality of concentrically disposed spherically symmetric layers, each layer being formed from a plurality of triangular regions.

By using a spherically symmetric configuration for the lens, it is possible to ensure that incident electromagnetic waves pointing to the center of the lens will experience almost the same material environment regardless of the position of feeding. This is particularly beneficial for circularly polarized incident radiation.

The plurality of triangular regions of each layer may form a geodesic polyhedron, such as a regular icosahedron.

In some embodiments, each layer is composed of a substantially equal number of triangular regions.

The triangular regions of each layer may substantially radially align with the triangular regions of each adjacent said concentrically disposed layer. In this way, the tendency for excess material to be trapped during additive manufacturing of the lens is greatly reduced.

In some embodiments, each triangular region defines a plane having a normal that is aligned with a centre of the Luneburg lens.

In some embodiments, each triangular region comprises at least one void. The voids may be triangular. By forming the lens with void-containing triangular regions, it is possible to form the Luneburg lens with a single material while still being able to vary the refractive index as a function of radius.

In some embodiments, a number of triangular regions N in the plurality of triangular regions is determined by: N = 20 * n ² , where n is an order of the icosahedron. For example, the order n of the icosahedron may be 1. In some embodiments, each triangle is equilateral and has a side length L within a range of l/10 to l/4, where l is an expected wavelength of an electromagnetic wave received by the Luneburg lens.

In some embodiments, a thickness of each concentrically disposed layer is equal.

The present invention also relates to an antenna comprising: a Luneburg lens as disclosed herein; and one or more antenna feed structures arranged to transmit electromagnetic radiation to, or receive electromagnetic radiation from, the Luneburg lens.

In some embodiments, at least one of the one or more antenna feed structures comprises a circularly polarizing antenna feed element.

In some embodiments, the circularly polarizing antenna feed element comprises: a waveguide cavity; and at least one septum dividing the waveguide cavity into two regions, and forming a respective port for each region, the septum concurrently producing both a right-hand circularly polarized wave and a left hand circularly polarized wave, for radiating from the circularly polarizing antenna feed element.

In some embodiments, the at least one septum is stepped. For example, the at least one septum may be stepped so that it tapers in a direction of propagation of a circularly polarized electromagnetic wave generated by the circularly polarizing antenna feed element.

In some embodiments, the circularly polarizing antenna feed element is one of a plurality of circularly polarizing antenna feed elements for radiating a circularly polarized electromagnetic wave into the Luneburg lens. For example, each circularly polarizing antenna feed element may be arranged, about an outer surface of the Luneburg lens, to introduce a respective circularly polarized electromagnetic wave radially towards a centre of the Luneburg lens.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of a Luneburg lens and a Luneburg lens antenna, in accordance with present teachings will now be described, by way of non-limiting example only, with reference to the accompanying drawings in which:

Figure 1 shows a Luneburg lens according to certain embodiments;

Figure 2 schematically depicts a concentric layer structure of the Luneburg lens of Figure l ;

Figure 3 is a further schematic depiction of the layer structure of the Luneburg lens; Figure 4 shows further detail of unit cells of a layer of the Luneburg lens;

Figure 5 is E-field distribution of the proposed LL structure fed by a dipole at 28 GFIz;

Figure 6 shows simulated radiation patterns for a Luneburg lens having ring-type composition, compared with simulated radiation patterns for a Luneburg lens according to embodiments of the present disclosure;

Figure 6 shows simulated radiation patterns for a prior art Luneburg lens having ring- type composition;

Figure 7 shows simulated radiation patterns for a Luneburg lens according to embodiments of the present disclosure;

Figure 8 shows results of impact testing of a Luneburg lens according to certain embodiments;

Figure 9 is a top plan view of an antenna according to certain embodiments;

Figure 10 is a front plan view of the antenna of Figure 9;

Figure 11 is a rear plan view of the antenna of Figure 9;

Figure 12 shows an electric field distribution of the antenna of Figure 9;

Figure 13(a) is a perspective view of a waveguide for an antenna feed element for an antenna according to certain embodiments;

Figure 13(b) is a front plan view of the waveguide of Figure 13(a);

Figure 13(c) is a cutaway side view of the waveguide of Figure 13(a);

Figure 14 is another perspective view of the waveguide of Figure 13(a), showing labelled dimensions of a septum structure of the waveguide;

Figure 15 is a perspective view of the waveguide in combination with a power combiner;

Figure 16 shows simulation results for an antenna according to certain embodiments;

Figure 17 shows simulation results for a dual-circularly polarizing feed according to certain embodiments;

Figure 18 shows simulated radiation patterns for a dual-circularly polarizing feed according to certain embodiments;

Figure 19 shows simulated and measured reflection coefficients for ports of a multi-beam antenna according to certain embodiments;

Figure 20 shows measured and simulated isolation of the multi-beam antenna; Figure 21 shows simulated and measured gain and axial ratios and simulated directivity of the multi-beam antenna; and

Figure 22 shows simulated and measured radiation patterns of the multi-beam antenna for LHCP and RFICP when different ports are excited.

DETAILED DESCRIPTION

Embodiments of the present disclosure generally relate to a Luneburg lens that comprises a plurality of concentrically disposed layers, each layer being formed from a plurality of triangular regions. The Luneburg lens may be used as part of a high-gain lens antenna with multi-beam radiation targeting for millimeter-wave wireless communication applications. An antenna according to certain embodiments has wide operating bandwidth, wide scanning angle and flexible polarization-matching ability.

Embodiments of a Luneburg lens will now be described with reference to Figures 1 to 4. The Luneburg lens 10 comprises a plurality of concentrically disposed layers, labelled 12.1 to 12. N in Figure 2, which shows a succession of such layers going from left (innermost) to right (outer-most).

Each layer 12.1 to 12. N is formed from a plurality of triangular regions 14. The triangular regions 14 form a tessellation of an approximately spherical surface. For example, the plurality of triangular regions 14 of each layer may form a geodesic polyhedron, such as a regular icosahedron. The N concentric layers may comprise (N-l) shells 12.2 to 12. N that are disposed around a solid core 12.1, which is itself a geodesic polyhedron of the same type as layers 12.2 to 12. N.

In at least some embodiments, each of the triangular regions 14 defines a plane having a normal that is aligned with a centre of the lens 10. This helps alleviate the degradation of radiation patterns when the incident wave impinges from various polarization angles.

As used herein, a "polyhedron" may be a solid or hollow figure the surface of which is composed of elements that have a polygonal boundary. The elements of which the surface is composed may be simple polygons, or polygons that have one or more holes.

The layers 12.1 to 12. N may each have the same thickness, and each layer 12.1 to 12. N may have the same number of triangular regions 14.

As best shown in the inset of Figure 4, each triangular region 14 may comprise at least one void 18, which may itself be triangular, though other void shapes are also possible. In some embodiments, instead of a void 18, the triangular region 14 may comprise a material having a different refractive index than the remaining material of the layer of which the triangular region 14 forms a part. Flowever, it is generally beneficial for there to be a void 18 to simplify the manufacturing process, since a single material can then be used to form the Luneburg lens 10.

The Luneburg lens 10 may be designed by discretizing the desired shape into the plurality of layers 12 and 12.1 to 12. N and applying Effective Medium Theory (EMT). For a lens of radius R and comprising N concentric layers with equal thicknesses, the permittivity of the i-th layer, e _; , can be expressed as: where r _i+1 - = ^R / _N .

After each layer is divided into a desired number of triangular regions, EMT can be used to approximate the permittivity. The desired permittivity can be attained by controlling the filling ratio of the inclusion material within the host material. The asymmetric Bruggeman (A-BG) is applied (see Eq. 3 below), where e _ίh and s _ho are the permittivities of the inclusion and host materials; and e _eff is the effective permittivity of the composite material p represents the constant volume fraction/filling ratio. The A-BG theory is applied here by controlling the wall thickness of each triangle unit cell 14 (e.g., wall thickness d/2 of the wall 16 of unit cell 14 in Figure 4).

In the present disclosure, the host material for the purposes of the A-BG theory is air {s _h0 - l), and so the filling ratio p that is required for each triangular region 14 in each layer may be determined based on Eq. 3 after acquiring the approximate permittivity of respective shells, e _; , and the permittivity e _ίh of the filling material.

The Luneberg lens 10 may be formed by an additive manufacturing process. To this end, a 3D model of the Luneburg lens 10 may be generated in suitable software, such as CAD software, and exported as a file in a format suitable for input to an additive manufacturing device (e.g., STL format). The 3D model may be generated by designing the solid core 12.1 first, followed by applying the next layer 12.2, the following layer 12.3, and so on up to the outermost layer 12. N. The triangular regions 14 of each layer may substantially radially align with the triangular regions 14 of each adjacent said concentrically disposed layer such that the structure will be self-supporting during the printing process when the resultant STL file is used as input, i.e. no separate support 1 structure needs to be printed. In particular, the walls 16 of each triangular region 14 may align with those of a triangular region 14 in the layer below.

By forming the lens structure 10 based on geodesic polyhedron models, such as icosahedron models, warpage or deformation after the additive manufacturing process is substantially avoided, thereby ensuring high performance of the lens 10 when used as part of an antenna.

In certain embodiments, each layer has at least 20 equilateral triangular regions 14 to form an icosahedron. Each such triangular region 14 may be sub-divided into further equilateral triangle regions. For example, each region 14 of a layer may have n ² subdivisions, where n is referred to herein as the order of the geodesic polyhedron. Accordingly, each layer 12.1 to 12. N of an icosahedral lens will have 20 * n ² triangular regions 14.

In certain embodiments, the order of an icosahedron layer is the same or almost the same for each layer 12.1 to 12. N. This substantially avoids misalignment between triangular regions from those layers, and thus prevents formation of additional chambers in the model that can trap extraneous material during the additive manufacturing process. Such extraneous material will be contained inside the chambers and be cured together with the lens, which will cause the lens to have a permittivity distribution that deviates from the desired behavior. On the other hand, if all triangular regions 14 are aligned, it is much easier for uncured material to drain out of the part.

As noted above, each triangular region of each shell 12.1 to 12. N of the lens 10 may be subdivided into n unit cells. For a given radius r, n corresponds to a specific unit cell 14 side length L due to geometry. In practice, the side length L, and thus the order n, may be subject to some constraints in order to ensure optimal performance. For example, it is known that A-BG theory is only valid when the composition is evenly distributed and the lengths of unit cells (L) are within the range of l/10 ~ l/4. Side length L is also influenced by the material filling ratio p. For each unit cell (triangle) with wall thickness of d/2 and side length L, there exists the following relationship:

The range of possible wall thicknesses d is constrained by the printing ability of the additive manufacturing process. Once the range of p is determined by Eq. 3 for each concentric layer 12.1 to 12. N, there will also be a range for L. After these constraints are satisfied, the targeted L will again be translated back to the order n. It is possible that the order n will vary slightly for different layers. Flence, there will be a trade-off when choosing a specific value of the order n for all layers. In some embodiments, the order n derived from the middle layer can be used as the n for all layers.

Each unit cell is a triangle having a void which is itself triangle-shaped, such that a hollow triangle having a wall of thickness d/2 is defined. It will be appreciated that, in finished form, the lens 10 will not have hard boundaries between adjacent triangular unit cells 14. Accordingly, adjacent unit cells will have a shared wall.

The void does not need to be triangle-shaped, and may be circular, square, hexagonal, or other shapes. In some variants, more than one void can be formed. The important thing is that the volume fraction p of the inclusion material (being the material that is used to form the lens, with the host material being air) is chosen for each layer in a manner that produces an effective permittivity for that layer that is in accordance with the Luneburg relation (1). It will be advantageous to form the triangle regions with only a single void such that it is easier to ensure that the voids of adjacent layers align radially, for example so that it is easier for residual material to drain from the structure after an additive manufacturing process.

The electric-field distribution of an example Luneburg lens at 28 GHz when illuminated by a dipole (linearly polarized source) is shown in Figure 5. The lens used to generate the electric field in Figure 5 had a diameter of 48 mm and a layer thickness of 2.4 mm, and was formed from Photopolymer Resin FLGPCL02 {s _r = 2.85, tanS = 0.02).

As can be seen in Figure 5, the fabricated lens was proven to be effective since it can be seen that the spherical wave from the dipole is successfully transformed into plane waves.

Compared with existing Luneburg lenses which have unit cells in the form of rods, cubes or rings based on discretization in Cartesian coordinates, the Luneburg lens according to embodiments is based on discretization in spherical coordinates. The benefit of using a centrally symmetric design is to ensure that waves pointing to the center will experience almost the same material environment regardless of the position of feeding. The advantage of such a structure becomes more pronounced for circularly polarized radiation, since Luneburg lenses designed according to Cartesian coordinate models may destroy the optic paths for such sources.

The advantages of embodiments of the present Luneburg lens relative to a ring-type lens having cubic unit cells are apparent from Figures 6 and 7, for example. Figure 6 shows simulated radiation patterns for the ring-type Luneburg lens for an x-polarized feeding dipole (left) and a y-polarized feeding dipole (right). Figure 7 shows simulated radiation patterns for a Luneburg lens according to embodiments of the present invention. As shown in Figure 6, the radiation pattern of the ring-type LL deteriorates drastically when the x-polarized feed changes to y-polarized. On the other hand, as seen in Figure 7, the Luneburg lens according to embodiments manifests stable radiation pattern performance regardless of the feeding polarization angle. This therefore obviates the need to take into account feeding alignment. Further, the stable radiation pattern implies that the lens of embodiments of the invention is capable of transmitting circularly polarized waves, because orthogonal modes would not be distorted by the lens structure.

A further advantage of the structure of the lens 10 is that it is durable under application of external forces, which makes the lens 10 stronger and provide longer service time. To verify this, an impact test was conducted where the lens 10 was put into a Shimadzu® Universal Testing Machine to examine the structure displacements under a certain amount of force. The plot of force versus structure displacement is shown in Figure 8. For a lens 10 with diameter D « 50mm, forces causing 2%, 4%, 6% and 8% deformation are 1.19 kN, 2.17 kN, 3.21 kN and 4.50 kN, with no crack being observed in the lens 10. Taking 4.5 kN as the maximum force, the maximum weight such a lens can bear can be calculated as 459.18 kg, which is approximately the weight of a full-grown bull. The structure can bear this weight due to the cascaded shells 12.1 to 12. N, which neutralize the force layer by layer.

As discussed above, the Luneburg lens 10 comprises triangular regions 14 that support each other. This geometry makes the Luneburg lens 10 self-supporting, and thus no additional structure is required when 3D-printing the lens 10. In this way, not only is the effect of support structures on estimated permittivity minimized, but the fabrication success rate is enhanced because no post-processing is involved. For previously known Luneburg lenses based on cubic unit cells, support removal is usually inevitable, at the risk of damaging the lens structures.

Embodiments of a Luneburg lens antenna will now be described with reference to Figures 9 to 15.

Referring initially to Figures 9 to 11, an example of a Luneburg lens antenna 100 comprises a Luneberg lens 10 and one or more antenna feed structures arranged to transmit electromagnetic radiation to, or receive electromagnetic radiation from, the Luneburg lens 10. In this example, five antenna feed structures are provided, and are labelled 110, 120, 130, 140 and 150.

At least one, and in some embodiments all, of the antenna feed structures 110-150 may comprise a circularly polarizing antenna feed element. The lens 10 and the feed structures 110-150 are each supported on a support structure 102 such that the feed structures 110-150 are maintained in fixed alignment with the lens 10 in use. In some embodiments the lens 10 and/or the feed structures 110-150 may be permanently affixed to the support structure 102. The feed structures 110-150 may be attached by mounting brackets or similar, such as the mounting bracket 118 extending from a lower end of feed structure 110 as shown in Figure 15.

The feed elements 110-150 each comprises a waveguide (e.g. waveguide 112 of feed element 110 or waveguide 152 of feed element 150) that has an end that is arranged in close proximity to the surface of the Luneburg lens 10 so that electromagnetic radiation can be coupled from the waveguide (112, 152) into the lens 10, or from the lens 10 into the waveguide (112, 152). It will be appreciated that in some embodiments, nonwaveguide based feed elements may be used, such as horn antennas, patch antennas and the like.

An exemplary waveguide 112 of feed element 110 is depicted in Figures 13 and 14. The waveguide has an outer shell 202 that encloses a waveguide cavity of square cross- section. The cavity houses a ridged septum 204 that divides the cavity lengthwise into two regions, forming a respective port for each region (labelled Port 1 and Port 2 in Figure 14). For example, the ridged septum 204 may be a four-step septum (having steps 206.1, 206.2, 206.3, 206.4) that is inserted into the square waveguide 202 in order to transform part of the TE01 mode entering Port 1 or Port 2 to TE10 mode. As is understood by those skilled in the art, by selecting appropriate step sizes, the phase difference between the degenerate modes TE01 and TE10 can be made to be 90 degrees and thus a circularly polarized (CP) wave can be achieved as a result. The output CP waves excited by Port 1 and Port 2 are left-hand circularly polarized (LFICP) and right- hand circularly polarized (RFICP) respectively.

The waveguide 112 has an output 210 that is positioned proximate the lens 10 in use as shown in Figure 9. The output 210 is opposite an input 208 that receives input electromagnetic radiation that is used to generate the output CP waves. It will be appreciated that in some cases, the roles of output 210 and input 208 may be reversed.

In the example of Figures 13 and 14, the septum 204 is stepped so that it tapers in a direction of propagation of a circularly polarized electromagnetic wave generated by the circularly polarizing antenna feed element 110. That is, the steps 206.4, 206.3, 206.2 and 206.1 "descend" in a direction from input 208 to output 210.

An example set of dimensions for the ridged septum 204, with labels ai to as (step lengths extending along an axis of the waveguide) and bi to bs (step heights) corresponding to those shown in Figure 14, is as follows:

Simulation results for the ridged waveguide 112 are shown in Figure 16. It can be seen that the ridged waveguide 112 has a wide bandwidth up to 35% (27 GHz - 38.5 GHz) where reflection coefficients are below -10 dB, the coupling is lower -20 dB and axial ratio is below 3 dB. A wider bandwidth could be achieved by reasonably lengthening the ridged waveguide 112.

In some embodiments, a power combiner 111 may be provided as part of the feed structure 110, the power combiner 111 comprising two bended waveguides 115a, 115b that are attached to respective ports of the ridged waveguide polarizer 112 to facilitate the connection between the ridged waveguide polarizer 112 and waveguide feeds. In some embodiments, the power combiner 111 may be fabricated by an additive manufacturing process such as Direct Metal Laser Sintering (DMLS). Compared with CNC which requires assembling and might lead to air gaps, a power combiner 111 manufactured by DMLS possesses a lower insertion loss because power leakage is avoided. In some embodiments, the mounting bracket 118 extending from the lower end of power combiner 111 may also be fabricated by DMLS, in the same process as used to fabricate the power combiner 111. The bended waveguides 115a, 115b and square waveguide 202 of the ridged waveguide polarizer 112 may be printed together as a single part.

In the example shown in Figures 9 to 11, the Luneburg lens antenna 100 comprises five feed structures 110-150 that are essentially identical in construction (i.e., each feed structure is in accordance with feed structure 110 of Figures 13 to 15). However, it will be appreciated that the feed structures 110-150 may vary. For example, some feed structures may provide a linearly polarized source of radiation while others, such as feed structure 110, provide a circularly polarized source of radiation as described above.

Circularly polarized (CP) LL antennas, such as the example antenna 100, have several advantages in that they are capable of minimizing polarization mismatch, suppressing multi-path interference, and widening communication capacity. Further, for the antenna 100, a dedicated CP source does not need to be placed at a certain position with a certain orientation, has a relatively small footprint, and is a wide-band source.

The simulated reflection coefficients and the gain of a dual CP feed structure 110 with and without the lens 10 are depicted in Figure 17. In this case, Port 1 (LHCP) is excited. It could be observed that after adding the lens 10, the gain increased by approximately 12 dB without affecting the reflection coefficients. Hence, the design of the lens 10 is proven to be effective since the gain is improved significantly; and the impedance matching is still maintained where the | SI 11 is under -10 dB over the frequency range of 26.5 GHz to 40 GHz that covers the entire Ka band.

The radiation patterns for 26.5 GHz, 32 GHz and 36 GHz are shown in Figure 18, in which Figure 18(a) shows the pattern in the E-plane, and Figure 18(b) shows the pattern in the H-plane. It can be seen that the the radiation patterns are almost identical in both E and H planes; as such, the feed structure 110 produces a symmetric radiation beam.

A prototype in accordance with the Luneburg lens antenna 100 was fabricated by 3D printing of a Luneburg lens 10 using photopolymer resin FLGCPL02 as above, and DMLS fabrication of five dual CP feed structures 110-150. Support structure 102 was 3D- printed using ABS material. The multibeam antenna 100 was measured in a compact- range anechoic chamber.

Due to limitations of the measurement system, the upper limit of measured frequency for the gains and radiation patterns was 37.5 GHz. All the unused ports were connected with WR-28 waveguide loads. Under the consideration of the symmetry of the array and for simplicity of explanation, only results for Ports 1-6 (as labelled in Figure 11) are presented herein. All simulations were conducted within CST Studio Suite 2019.

A. Reflection and Isolation

The measured and simulated reflection coefficients of the antenna 100 are shown in Figure 19. It can be seen that the measured results agree well with the simulation. The impedance bandwidth for reflection coefficients under -10 dB covers the entire Ka band, i.e. 40.6% (26.5 GHz - 40 GHz).

Referring to Figure 20, it can be seen that Port 5 and 6 have the strongest mutual coupling as expected in the simulation, and cause the overlapped bandwidth to be 34.6% (27 GHz - 38.3 GHz) when also considering the isolation greater than 20 dB.

B. CP Gain and AR

The simulated and measured CP gain and axial ratios (AR), and simulated directivity when Ports 1-6 are excited sequentially, are shown in Figure 21. As results for the port pairs (Port 3, Port 4) and (Port 5, Port 6) were almost identical to those of the port pair (Port 1, Port 2), only results for Port 1 and Port 2 are shown. It can be seen that the measured gain agrees well with the simulation and ranges from 19 dBic to 21.2 dBic with 2.2 dB variation over the 3dB AR operating band from 26.5 GHz to 37 GHz (bandwidth of 33.1%). C. Radiation Pattern

Due to the symmetry of the antenna 100, only Ports 1-6 were excited and measured, since Ports 7-10 are expected to behave similarly according to the simulation results. The simulated and measured radiation patterns of the xoz-plane at 26.5 GHz, 32 GHz and 36 GHz are shown in Figure 22, and high coincidence could be observed. Nearly no scanning loss appears when the beam deviates ±44 degrees from the main beam direction. Dual CP radiation, i.e. LHCP and RHCP beams, are realized by exciting Port 1, 3, 5, 7, 9 and Port 2, 4, 6, 8, 10 respectively.

A description of some exemplary embodiments of the present disclosure is contained in one or more of the following numbered statements:

Statement 1. A Luneburg lens comprising a plurality of concentrically disposed spherically symmetric layers, each layer being formed from a plurality of triangular regions.

Statement 2. The Luneburg lens according to Statement 1, wherein the plurality of triangular regions of each layer form a geodesic polyhedron.

Statement 3. The Luneburg lens according to Statement 2, wherein the geodesic polyhedron is a regular icosahedron.

Statement 4. The Luneburg lens according to any one of Statements 1 to 3, wherein each layer is composed of a substantially equal number of triangular regions.

Statement 5. The Luneburg lens according to any one of Statements 1 to 4, wherein the triangular regions of each layer substantially radially align with the triangular regions of each adjacent said concentrically disposed layer.

Statement 6. The Luneburg lens according to any one of Statements 1 to 5, wherein each triangular region defines a plane having a normal that is aligned with a centre of the Luneburg lens.

Statement 7. The Luneburg lens according to any one of Statements 1 to 6, wherein each triangular region comprises at least one void.

Statement 8. The Luneburg lens according to Statement 7, wherein the voids are triangular.

Statement 9. The Luneburg lens according to any one of Statements 3 to 8, wherein a number of triangular regions N in the plurality of triangular regions is determined by: N = 20 * n ² , where n is an order of the icosahedron. Statement 10. The Luneburg lens according to Statement 9, wherein the order n of the icosahedron is 1.

Statement 11. The Luneburg lens according to any one of Statements 1 to 10, wherein each triangle is equilateral and has a side length L within a range of l/10 to l/4, where l is an expected wavelength of an electromagnetic wave received by the Luneburg lens.

Statement 12. The Luneburg lens according to any one of Statements 1 to 11, wherein a thickness of each concentrically disposed layer is equal.

Statement 13. A circularly polarizing source for producing a circularly polarized electromagnetic wave, comprising: a waveguide cavity; and at least one septum dividing the waveguide cavity into two regions, and forming a respective port for each region, the septum concurrently producing both a right-hand circularly polarized wave and a left had circularly polarized wave, for radiating from the circularly polarizing source.

Statement 14. A circularly polarizing source according to Statement 13, wherein the at least one septum is stepped.

Statement 15. A circularly polarizing source according to Statement 14, wherein the at least one septum is stepped so that it tapers in a direction of propagation of the circularly polarized electromagnetic wave.

Statement 16. A Luneburg lens assembly, comprising: a Luneburg lens assembly according to any one of Statements 1 to 12; and a circularly polarizing (CP) source for radiating a circularly polarized electromagnetic wave into the Luneburg lens.

Statement 17. A Luneburg lens assembly according to Statement 16, wherein the

CP source comprises: a waveguide cavity; and at least one septum dividing the waveguide cavity into two regions, and forming a respective port for each region, the septum concurrently producing both a right-hand circularly polarized wave and a left hand circularly polarized wave, for radiating from the CP source.

Statement 18. A Luneburg lens assembly according to Statement 17, wherein the

CP source wherein the at least one septum is stepped.

Statement 19. A Luneburg lens assembly according to Statement 18, wherein the at least one septum is stepped so that it tapers in a direction of propagation of the circularly polarized electromagnetic wave.

Statement 20. A Luneburg lens assembly according to any one of Statements 16 to 18, wherein the CP source is one of a plurality of CP sources for radiating a circularly polarized electromagnetic wave into the Luneburg lens. Statement 21. A Luneburg lens assembly according to Statement 20, wherein each CP source is arranged, about an outer surface of the Luneburg lens, to introduce a respective circularly polarized electromagnetic wave radially towards a centre of the Luneburg lens.

Statement 22. An additive manufacturing process for forming a Luneburg lens, comprising : obtaining a 3D model comprising a spherically symmetric core layer and a succession of concentric spherically symmetric layers that are disposed around the core layer; and printing the Luneburg lens according to the 3D model; wherein the core layer and each concentrically disposed layer is formed from a plurality of triangular regions; and wherein triangular regions of respective layers are aligned with each other.

Statement 23. The process according to Statement 22, wherein the plurality of triangular regions of each layer form a geodesic polyhedron.

Statement 24. The process according to Statement 23, wherein the geodesic polyhedron is a regular icosahedron.

Statement 25. The process according to any one of Statements 22 to 24, wherein each layer is composed of a substantially equal number of triangular regions.

Statement 26. The process according to any one of Statements 22 to 25, wherein each triangular region defines a plane having a normal that is aligned with a centre of the Luneburg lens.

Statement 27. The process according to any one of Statements 22 to 26, wherein each triangular region comprises at least one void.

Statement 28. The process according to Statement 27, wherein the voids are triangular.

Statement 29. The process according to any one of Statements 22 to 28, wherein a number of triangular regions N in the plurality of triangular regions is determined by: N = 20 * n ² , where n is an order of the icosahedron.

Statement 30. The process according to Statement 29, wherein the order n of the icosahedron is 1.

Statement 31. The process according to any one of Statements 22 to 30, wherein each triangle is equilateral and has a side length L within a range of l/10 to l/4, where l is an expected wavelength of an electromagnetic wave received by the Luneburg lens.

Statement 32. The process according to any one of Statements 22 to 31, wherein a thickness of each concentrically disposed layer is equal. Many modifications will be apparent to those skilled in the art without departing from the scope of the present invention.

Throughout this specification, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.