Background of the Invention

[0001] The present invention relates to an electromagnetic lens arrangement, and in one particular example, to an electromagnetic lens arrangement suitable for use as a spherical or hemi-spherical Luneburg Lens.

Description of the Prior Art

[0002] 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 acknowledgement or admission or any form of suggestion that the 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.

[0003] Spherical Luneburg lenses are an electromagnetic lens that causes a parallel beam of electromagnetic radiation to converge to a focus on or near the spherical boundary of the lens. Such lenses are typically illuminated by a number of feeds to form a multibeam antenna, and an example of this is shown in Fig 9. With suitable placement of a feed, a beam can be directed in any direction. Multibeam antennas are used currently in telecommunications and should have increasing application in future more advanced systems. Current multibeam antennas often use arrays of networks such as Butler Matrices or ID lenses and typically require duplication of hardware to achieve dual polarization, and require a 2D array of radiating elements with resulting reflections that degrade performance.

[0004] A spherical Luneburg lens achieves the required bending of rays with a graded refractive index that decreases radially from the centre to the outer surface of the lens

[0005] The spherical Luneburg lens has a number of features that make it uniquely suitable for realizing multi-beam antennas. For example, the beams shapes do not vary with scan angle as a consequence of the lens’s spherical symmetry. No additional radiating elements are needed to form a narrow beam in 2D as the aperture forming the beam is the lens itself. No additional components are needed to radiate dual polarizations as the lens propagates orthogonal polarizations provided the dielectric is reasonably isotropic. The lens can be configured with a graded refractive index that can be 1 at the outer surface making for a low reflection. Very low loss is possible with losses of current multibeam antennas typically exceeding those of the spherical Luneburg lens by more than 2 dB at S-band. Such lenses often use artificial dielectrics consisting of composites of dielectrics or dielectric and conductors. This can be wide-band if the artificial dielectric is suitably designed. In designs at lower frequencies, for example below 15 GHz, the use of normal dielectrics such as plastics or plastics loaded with additives results in heavy structures so artificial dielectrics consisting of a light-weight low dielectric constant matrix such as plastic foam with conducting inserts is preferred.

[0006] Despite that in current cellular networks, such designs have generally only been used in specialized applications such as providing multicell coverage to handle extreme traffic density such as in sports stadiums or for portable base-stations at outdoor concert events. There is also a niche application for fixed rural stations where beams can be steered to villages and small communities for fixed wireless services. Reasons for the lack of acceptance include high cost, inconvenience in accommodating the spherical shape on site, and the like.

[0007] In practice, the larger, low-frequency Luneburg lenses normally have a composite structure, with layers of discrete concentric spherical shells, each of a different and graded refractive index. Such an arrangement can be difficult to manufacture and additionally does not always result in an ideal dielectric profile throughout the lens. For example, some configuration use wires or other conductors embedded in foam with random orientations, which can lead inconsistent refractive index, which can degrade antenna performance.

[0008] US21050002352 describes a manufacturing method for anew type of dielectric material having a predefined variable permittivity resulting from the manufacturing process. Main characteristic of the manufacturing method is that in a first step homogeneous dielectric material (100) is shaped in at least a direction and subsequently at least a part thereof is formed so that the resulting dielectric material body (102) has the predefined variation in permittivity in said at least one direction. Said forming step may advantageously comprise sub-steps of deforming at least a part of the shaped dielectric material body and fixing the so deformed dielectric material body. The manufacturing steps are adapted so to induce a predefined variation in permittivity corresponding but not limited to a certain law (e.g. Luneburg, Maxwell, ...). The invention further concerns a manufacturing method of an electromagnetic lens that can be used in a millimeter-waves multi-beam forming antenna system where said electromagnetic lens is composed of the new type of dielectric material.

[0009] US20100134368 describes an inhomogeneous lens with Maxwell's Fish-eye type gradient index (1), made in the shape of a hemisphere. The invention is characterized in that the lens comprises N hemispheric concentric shells (2 to 4), with different discrete dielectric constants and mutually interlaced without void between the two successive shells, with 3£N£20, the discrete dielectric constants of the N shells being such that they define a discrete distribution close to the theoretical distribution of the index inside the lens.

[0010] EP0464647 describes an optical spherical Luneburg lens which is also useful for millimeter wave and microwave operation, which can provide a multitude of simultaneously acting receiving beams over a hemispherical or spherical field of view

[0011] WO2018232325 describes a hollow light-weight, low-cost, and high-performance 3D Luneburg lens structure using partially-metalized thin film, string, threads, fiber or wire base metamaterial to implement the continuously varying relative permittivity profile, characteristic of Luneburg lens structures, is disclosed. The hollow light-weight lens structure is based on the effective medium approach and may be implemented by a number of means. Further, most of the volume of the lens structure is free-space, thus the weight of the lens is significantly less than conventional 3D Luneburg lens structures of the same dimensions.

[0012] "Non-Uniform Metasurface Luneburg Lens Antenna Design" by Marko Bosiljevac, Massimiliano Casaletti, Francesco Caminita, Zvonimir Sipus, and Stefano Maci (IEEE Transactions On Antennas And Propagation, Vol. 60, No. 9, September 2012) describes a using an array of size varying circular patches on a dielectric substrate inside a parallel plate waveguide (PPW) structure to obtain a variable surface impedance, which realizes an equivalent refraction index as that of a Luneburg lens.

[0013] "A Sliced Spherical Luneburg Lens" by Sebastien Rondineau, Mohamed Himdi, and Jacques Sorieux (IEEE Antennas and Wireless Propagation Letters, Vol. 2, 2003) describes a manufacturing process using stack of homogeneous dielectric coaxial cylinders which approximate a sphere.

Summary of the Present Invention

[0014] In one broad form an aspect of the present invention seeks to provide an electromagnetic lens arrangement including: a substantially spherical or hemi-spherical lens body; and, conductive members embedded within the lens body, the conductive members being positioned within the body in accordance with an equally spaced three dimensional orthogonal grid so that an effective refractive index of the lens body decreases from a centre to an outer surface of the lens body so that the lens acts as a Luneburg lens.

[0015] In one embodiment the lens body includes a plurality of substantially planar layers of supporting material, each layer having a generally circular shape, the plurality of layers including different diameters configured so that the layers are stacked to form the lens body.

[0016] In one embodiment the layers are made of foam.

[0017] In one embodiment the layers are bonded together.

[0018] In one embodiment each layer includes a number of recesses configured to receive the conductive members.

[0019] In one embodiment the recesses extend part way through each layer, the recesses having a depth so that conductive members are positioned substantially mid-way through each layer.

[0020] In one embodiment conducting members are positioned between adjacent layers.

[0021] In one embodiment the grid arrangement includes orthogonally arranged grid layers, rows and columns, wherein the grid layers, rows and columns are equally spaced.

[0022] In one embodiment each grid layer is provided in a respective layer of supporting material.

[0023] In one embodiment each layer of supporting material has a thickness equal to a grid layer spacing. [0024] In one embodiment the conductive members are progressively smaller from the centre to the outer surface of the lens body.

[0025] In one embodiment the conductive members at least one of: have at least three orthogonal planes of symmetry; are substantially cubic; and, are substantially spherical.

[0026] In one embodiment the planes of symmetry and arranged parallel to layers, rows and columns of the grid arrangement.

[0027] In one embodiment the conductive members include faces, and wherein faces of adjacent conductive members are substantially parallel.

[0028] In one embodiment the conductive members include at least one of: a conductive body; a metal body; a hollow metal body; a metal body including holes; a hollow metallic mesh; a conductive coating; a foam or plastic insert body with a conductive coating.

[0029] In one embodiment the lens is anisotropic.

[0030] In one embodiment a focus of the lens is external to the lens body so that in use feed locations are radially spaced from the lens body.

[0031] In one embodiment a focal length of the lens varies depending on a lens orientation relative to a feed location.

[0032] In one embodiment feed locations at different azimuthal and elevational positions are provided at different radial spacings from the lens body.

[0033] In one embodiment a dielectric lens is provided between one or more feed locations and the lens body to adjust a radial position of the focus so that feed locations at different azimuthal and elevational positions are provided at the same radial spacing from the lens body.

[0034] In one embodiment vertical and horizontal polarizations have different focal lengths.

[0035] In one embodiment a dielectric lens including parallel spaced conducting members is provided for one of the polarisations so that the focal lengths for the horizontal and vertical polarizations are coincident. [0036] In one embodiment the lens has an operating frequency range of 0.6GHz to 15 GHz depending on configuration of the conducting inserts.

[0037] In one embodiment the lens body is hemispherical and the where the plane surface of the hemisphere is a reflecting surface.

[0038] In one embodiment the lens body is made from at least one of: foam; a low dielectric material; a low dielectric structure; and, a lattice structure.

[0039] In one embodiment the lens body has a dielectric constant that is less than 1.2.

[0040] In one broad form an aspect of the present invention seeks to provide a method of making an electromagnetic lens arrangement including: providing a substantially spherical or hemispherical lens body; and, embedding conductive members within the lens body , the conductive members being positioned within the body in accordance with an equally spaced three dimensional orthogonal grid so that an effective refractive index of the lens body decreases from a centre to an outer surface of the lens body so that the lens acts as a Luneburg lens.

[0041] It will be appreciated that the broad forms of the invention and their respective features can be used in conjunction and/or independently, and reference to separate broad forms is not intended to be limiting. Furthermore, it will be appreciated that features of the method can be performed using the system or apparatus and that features of the system or apparatus can be implemented using the method.

Brief Description of the Drawings

[0042] Various examples and embodiments of the present invention will now be described with reference to the accompanying drawings, in which: -

[0043] Figure 1A is a schematic side view of an example of a lens arrangement;

[0044] Figure IB is a schematic perspective top side view of the lens arrangement of Figure 1A; [0045] Figure 1C is a schematic plan view of the lens arrangement of Figure 1A;

[0046] Figure ID is a schematic perspective cut-away top side view of the lens arrangement of Figure 1A;

[0047] Figure 2A is a schematic plan view of an example of a tenth layer of the lens arrangement of Figure 1A;

[0048] Figure 2B is a schematic perspective view of the tenth layer of Figure 2A;

[0049] Figure 2C is a schematic cross sectional view along the line A-A of Figure 2A;

[0050] Figure 2D is a schematic plan view of an example of a sixth layer of the lens arrangement of Figure 1A;

[0051] Figure 2E is a schematic perspective view of the sixth layer of Figure 2D;

[0052] Figure 2F is a schematic cross sectional view along the line B-B of Figure 2D;

[0053] Figure 2G is a schematic plan view of an example of a second layer of the lens arrangement of Figure 1A;

[0054] Figure 2H is a schematic perspective view of the second layer of Figure 2G;

[0055] Figure 21 is a schematic cross sectional view along the line C-C of Figure 2G;

[0056] Figure 3 is a schematic perspective view of an example of a conductive member;

[0057] Figure 4A is a schematic diagram illustrating a Luneburg lens with a first example focal length coincident with the lens surface;

[0058] Figure 4B is a schematic diagram illustrating a Luneburg lens with a second example focal length radially spaced from the lens surface;

[0059] Figure 4C is a graph illustrating the relationship between refractive index and radial position for different focal lengths; [0060] Figure 4D is a schematic plan view of an example of feed locations relative to the lens arrangement of Figure 1A;

[0061] Figure 5A is a schematic top side perspective view of an example of a test mounting arrangement;

[0062] Figure 5B is a schematic side view of the test mounting arrangement of Figure 5 A;

[0063] Figure 6A is a schematic top side perspective view of an example of the feed arrangement of Figure 5A;

[0064] Figure 6B is a schematic end view of the feed arrangement of Figure 6A showing a crossed dipole configuration;

[0065] Figure 6C is a schematic perspective view of the feed arrangement of Figure 6A with an additional dielectric lens;

[0066] Figure 7 is a graph illustrating an example of the measured gain for respective ports at different frequencies for a test lens with feed;

[0067] Figure 8A is a graph illustrating an example of a measured azimuth radiation pattern for a first port for a test antenna operating at different frequencies with co- and cross-polarized patterns superimposed;

[0068] Figure 8B is a graph illustrating an example of a measured azimuth radiation pattern for a second port for a test antenna operating at different frequencies with co- and cross-polarized patterns superimposed; and,

[0069] Figure 9 is a schematic diagram showing an example of the principle of a Luneburg lens operating as a multibeam antenna.

Detailed Description of the Preferred Embodiments [0070] An example of a lens arrangement will now be described. [0071] The lens arrangement includes a substantially spherical or hemispherical lens body and conductive members embedded within the lens body, the conductive members being positioned within the body in accordance with an equally spaced three dimensional orthogonal grid so that an effective refractive index of the lens body decreases from a centre to an outer surface of the lens body so that the lens acts as a Luneburg lens.

[0072] The nature of the conductive members and the lens body will vary depending on the preferred implementation, but typically the body is constructed of foam, but alternatively could be constructed from another lightweight low dielectric structure, such as a lattice structure or similar. The conductive members typically have at least three orthogonal planes of symmetry, and could be spherical, cubic, or other suitable convex polyhedral shapes. The conducting members are typically formed from plastic or foam coated with a metallic coating, although other arrangements are possible, such as using a conductive body, or solid or hollow metal shapes, metallic bodies with holes, metallic meshes or similar, with the sizes of the conducting members being arranged within the body to provide the desired refractive index.

[0073] The construction based on foam or a lattice structure with conducting inserts results in a low-weight, low-cost structure, which is easy to manufacture. Thus, in contrast to some previous arrangements, the use of foam reduces the overall weight, which has been a significant problem with earlier designs using solid dielectric materials. This is further improved by the use of light-weight conducting members made of plastic with a copper or silver coating, or other suitable arrangements. Furthermore, through suitable deterministic sizing and spacing of the conductive members, this allows the lens to operate as a spherical Luneburg lens with a high degree of accuracy over frequencies within the range 0.6 GHz to 15 GHz.

[0074] Whilst the above described lens can be constructed using any appropriate technique, a specific example construction will now be described with reference to Figures 1A to ID.

[0075] In this example, the lens 100 has a body including a plurality of layers 110 of supporting material, each layer including a number of recesses 111 configured to receive the conductive members. It will be appreciated however that conductive members could be mounted to the layers 110 using other suitable techniques, such as injecting or otherwise embedding the conductive members in the supporting material, or positioning the conductive members between adjacent layers of material. In this example, each of the layers has a generally circular shape, with the plurality of layers including different diameters so that when the layers are assembled together, the layers form the spherical shape of the lens body. It will be appreciated however, that a hemisphere can be created using half of the layers, with the plane surface of the hemisphere having a reflecting surface.

[0076] In the current example, there are twenty layers, with ten different layer configurations, denoted 110.1, 110.2, 110.3, 110.4, 110.5, 110.6, 110.7, 110.8, 110.9, 110.10, with the ten layers being used to construct a hemisphere, and with two hemispheres forming the overall sphere. Thus, in this example, each of the layers 110.1, 110.2, ... 110.10, progressively increases in diameter 110.1, 110.2, ... 110.10, so that first layers 110.1 form top and bottom layers of the sphere, whilst tenth layers 110.10 form the centre of the sphere. However, it will be appreciated that this configuration is for the purpose of illustration only and different numbers and/or arrangements of layers could be used. For example, greater or lesser numbers of layers could be used.

[0077] Typically the layers are made of foam, and could include closed or open cell foams, including but not limited to foams made from polystyrene, polypropylene, polyethylene, or the like. In one example, the foam is substantially air fdled and has a relative dielectric constant close to 1 and less than 1.2.

[0078] In one example, the layers are bonded together, for example using adhesive bonding, or similar, although this is not essential and other coupling arrangements can be used. For example, the layers may include mounting holes 112 that are continuous through all layers and which can be configured to receive fasteners to secure the layers together, or could be used to couple the lens arrangement to a mounting, as will be described in more detail below.

[0079] Thus, the above describes a method of manufacturing a Luneburg lens. This involves, embedding conductive members within a lens body, with the conductive members being positioned within the body in accordance with an equally spaced three dimensional orthogonal grid so that an effective refractive index of the lens body decreases from a centre to an outer surface of the lens body so that the lens acts as a Luneburg lens. Specifically, in one example, this involves placing conductive members within layers of foam material, so that the conductive members are arranged in a grid arrangement having an equal centre to centre spacing, but with decreasing sizes towards an outer edge of the foam layer. The foam layers can then be bonded together, providing an easy method for constructing the lens.

[0080] However, it will be appreciated that other construction techniques could also be used. For example, the lens body and conducting members could be created using additive manufacturing techniques, such as 3D printing, with this being used to construct the lens body, and also be used to embed the conducting inserts, using additive manufacturing techniques that employ multiple different materials. Examples of such techniques are described in "Charge- programmed three-dimensional printing for multi-material electronic devices" by Ryan Hensleigh, Huachen Cui, Zhenpeng Xu, Jeffrey Massman, Desheng Yao, John Berrigan and Xiaoyu Zheng in Nature Electronics | VOL 3 | 216 April 2020 | 216-224.

[0081] The conductive members are configured so that the effective refractive index n of the lens body follows a prescribed profile. The prescribed profile could be achieved in a variety of manners depending on the preferred implementation. For example, the conductive members typically have different sizes so that the dielectric constant of the lens body decreases radially outwardly, although it will be appreciated that a similar effect could be achieved using dielectric inserts or a combination conducting inserts and dielectric inserts.

[0082] As mentioned above, the conductive members are provided in a three-dimensional orthogonal grid arrangement, with a plurality of layers including rows, columns, and layers with the rows, columns and layers being equally spaced.

[0083] An example of this is shown in Figures 2A to 21, which show examples of the tenth, sixth and second layers 110.10, 110.6, 110.2, respectively.

[0084] Specifically, in this example, columns and rows of recesses 111 that receive the conductive members are equally spaced, so that a centre spacing between the conductive members is constant throughout each layer and between layers. Furthermore, the recesses typically extend part way through each layer, the recesses having a depth so that conductive members are positioned substantially mid-way through each layer. [0085] In this example, the conductive members are progressively smaller from the centre to the outer surface of the lens body, so that the effective refractive index decreases radially outwardly. As a result of the differences in size of the conductive members, the depth of the recesses decreases from a centre to the outer surface of the lens body, and hence each layer, thereby ensuring centres of the conductive members are co-planar within each layer. However, this is not essential and in other examples, the conductive members could be surface mounted on each layer.

[0086] By virtue of the layered arrangement, and by selecting a layer width that corresponds to the spacing between adjacent rows or columns, the conducting members can be arranged in a three dimensional grid, including grid layers, each grid layer including rows and columns, and the grid layers, rows and columns being equally spaced. Whilst a single grid layer is shown in each support material layer in this example, it will be appreciated that alternative arrangements could be used, such as providing multiple grid layers in each material layer.

[0087] In any event, it will be appreciated that providing recesses 111 in the foam layers provides an easy mechanism to control the spacing between the conductive members in the grid, allowing these to be accurately positioned to thereby ensure the desired dielectric profde is obtained. Additionally, the conductive members are easily positioned in the recesses, making the antenna arrangement easy to construct.

[0088] The conductive members typically include a foam or plastic insert body with a conductive coating, such as a copper, silver, or similar. This helps minimise the overall weight of the antenna arrangement, although this is not essential and other configurations could be used, such as solid, porous or hollow metal bodies, as described above.

[0089] In this example, the conductive members are substantially cubic, and may have rounded comers and/or edges, and an example of this is shown in Figure 3. It will however be appreciated that other shapes could be used, such as spherical conductive members, or the like.

[0090] When cubic conductive members are used, the conductive members and are typically arranged with faces of the conductive members substantially parallel and with orthogonal planes of symmetry aligned with the three dimensional grid, so that faces are parallel to the rows, columns and layers of the grid, which can help maximise capacitive coupling between the conductive members, which in turn allows larger refractive indices to be realised for a particular spacing.

[0091] The size and spacing for the conductive elements will vary depending on a range of factors, including the nature of construction of the conductive elements, the dielectric constant of the supporting foam layers, and the intended operating frequency of the lens. In one example, the conducting inserts have dimensions of between 3.5mm to 13.5mm per side, and are spaced with a centre to centre spacing of 20mm.

[0092] As a result of the conductive member positioning on a rectangular grid, the lens is somewhat anisotropic. The refractive index for normal incidence on the rows is larger than, for example at an angle of incidence of 45 degrees. This moves the focus further from the lens at an angle of incidence of 45 degrees compared with normal incidence. This is a consequence of equal spacing of the conducting inserts which is a choice made to allow construction in layers and simplification of the manufacturing process.

[0093] Irrespective, the focus of the lens is external to the lens body so that in use the antenna feed locations are radially spaced from the lens body. An example of this will now be described with reference to Figures 4A and 4B.

[0094] Specifically, in the example of Figure 4A a focal length of the lens is equal to the lens radius so that the feed location must be positioned coincident with the lens surface to generate a parallel beam of radiation. In contrast, in the example of Figure 4B a focal length of the lens is greater than the lens radius so that the feed location must be positioned radially spaced from the lens surface to generate a parallel beam of radiation.

[0095] An example of the refractive index profile within the lens that results in a focal length coincident or spaced from the lens surface is shown in Figure 4C. This highlights how the refractive index of the lens tends towards 1 at the lens surface, with the refractive index increasing towards a centre of the lens. With a refractive index of just over 1.4 at the centre of the lens, this results in a focal length coincident with the lens surface, whereas a lower refractive index moves the focal length further away from the lens specification. [0096] In the current example, the anisotropic nature of the grid arrangement with respect to azimuthal and elevational positions means the focal length of the lens varies depending on a lens orientation so that feed locations at different azimuthal and elevational positions are provided at different spacings from the lens body. An example of this will now be described with reference to Figure 4D.

[0097] In this example, feed locations 451, 452, 453, 454, 455 are shown, with feed location 451 corresponding to an orientation perpendicular to a direction of the rows of conductive members, and feed locations 452, 453, 454, 455 provided clockwise and counterclockwise at angles, which in this example are 18°, 36°, 54° and 72° respectively for the purpose of illustration only. Each feed location is used to illuminate the lens, with each feed being used to generate a respective beam. In communications systems normally dual slant polarization is used.

[0098] In this instance, the feed location is offset from the surface of the sphere by ch for feed location 451. cl· for feed locations 452, 455 and d for feed locations 453, 454, where di<d2 < d . Thus, the feed location can be located closer to or further away from the lens’s surface to follow the movement of the focus, depending on the circumferential position of the antenna feed.

[0099] As an alternative to altering the feed location, a lens can be positioned between the feed location and the lens body 100 to alter, and in particular shorten, the focal length. Thus, in one example, a dielectric lens is provided within or in front of each feed so that the focus is returned to an original position corresponding to normal incidence. The principal effect of the lens anisotropy that occurs at azimuths away from the lines of inserts is that the focal length for vertical polarization becomes longer than the focal length for horizontal polarization. Where signals are polarised at angles offset to vertical, this results in horizontal and vertical components of the polarised signals having different focal lengths. This can be accommodated by positioning a feed at a position that is a compromise for the vertical and horizontal components, but this inevitable leads to signal degradation as the feed is not ideally positioned for either component. Alternatively, it is possible to correct this effect with a lens that corrects the focal position for one of the horizontal or vertical polarisation. For example, this can be achieved using an artificial dielectric with vertical arranged wires or tracks on substrate that provides a shortening of the focal length for vertical polarization only.

[0100] An example test antenna mounting is shown in Figures 5A and 5B. It will be appreciated that this is illustrative of a stand used for testing only, described in more detail below and is not intended to reflect a practical deployment of the antenna.

[0101] In this example, the antenna mounting 520 includes body 521 having spaced apart substantially parallel upper and lower arms 522, 523 positioned vertically spaced, so that the lens body 100 can be positioned between the upper and lower arms 522, 523. The upper and lower arms 522, 523 support upper and lower mounting plates 526, 527 attached to respective pairs of upper and lower shafts 524, 525, which are in turn coupled to the arms 522, 523, via respective movable mountings 528, 529, to allow an orientation of the antenna to be adjusted in a vertical plane. This allows gain, patterns and focal length of the antenna to be measured at different inclinations of the feed from the arrays of inserts.

[0102] Mounting rods 530 are provided attached to the upper and lower mounting plates 526, 527, with the rods 530 extending through the mounting holes 112 in the lens body 110, thereby allowing the lens body to supported by the mounting plates 526, 527.

[0103] The lens is excited by a single feed 540 to provide a single beam or by an array of feeds to provide multiple beams is mounted to the fixture 521 to position the feed on one of two positions relative to the lens. Specifically, the test mounting is designed to allow the lens to be rotated azimuthally and tilted so that different feed locations can be tested.

[0104] As shown in Figure 6A and 6B, the feed includes a short length of square waveguide 641 fed in turn by a crossed dipole to provide dual slant polarization as used in cellular systems, although it will be appreciated that other antenna arrangements could be used. The feeds used with this design are optimised for use in the band 3.3 to 3.8 GHz. Each feed has two connectors 642, 643, such as SMA (SubMiniature version A) connectors, or the like, on the rear for +/- 45 deg polarization.

[0105] An example of a feed including an integrated lens that corrects the focal position for one of the horizontal or vertical polarisations is shown in Figure 6C. In this example, the lens includes a number of focusing members 644 attached to an end of the waveguide 641 via mounting brackets 645. The focusing members 644 can include a dielectric material with conductive elements extending across an end of the waveguide, which can be used to shorten the focal length for the vertical polarization, thereby ensuring the focal lengths are coincident for the vertical and horizontal polarizations.

[0106] However, as previously mentioned, it will be appreciated that this mounting arrangement is for the purpose of example only and is not intended to be limiting.

[0107] Testing performed using the above described mounting and lens arrangement was used in conjunction with a far-field antenna test range to characterise the performance of the lens. In this example, the lens was 400mm in diameter, and includes cubic conducting members having the following X, Y, Z dimensions (with the number of each conducting member also provided) and a 20mm centre to centre spacing between conducting inserts:

[0108] Figure 7 is a graph showing the measured gains at the two connector ports of the feed corresponding to +45 and -45 degree polarization across the band 3.3 to 3.8 GHz. The gains obtained are less than 0.2 dB lower than the calculated directivity indicating that the efficiency of the lens and feed to be about T 94%, whilst Figures 8A and 8B show co-polarized and cross- polarized azimuth patterns superimposed for ports 1 and 2 respectively. The patterns are symmetrical with balanced sidelobes indicating the uniform behaviour of the lens. The cross polarization level of about -22 dB is mainly caused by imperfection in the feed due to coupling between the dipoles. [0109] Accordingly, the above described lens arrangement provides a mechanism for cheaply and easily constructing a lightweight Luneburg lens. The above described technique overcomes the weight and cost problem of traditional low frequency Luneburg lens arrangements without significantly sacrificing performance. The minor variation in sidelobe level and degradation in cross-polarization performance caused by the anisotropy can be improved by locating the feeds at distances from the lens that depend on azimuth angle or including small lenses in front of the feeds. This allows the lens arrangement to provide a cheap alternative to massive MIMO antennas currently employed by 5G systems. Compared with such base -station array antennas Luneburg lenses can be economical, flexible and efficient.

[0110] Throughout this specification and claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers or steps but not the exclusion of any other integer or group of integers.

[0111] Persons skilled in the art will appreciate that numerous variations and modifications will become apparent. All such variations and modifications which become apparent to persons skilled in the art, should be considered to fall within the spirit and scope that the invention broadly appearing before described.