Field of the invention

The present invention relates to an antenna source, a directional antenna, and methods for providing and controlling a directional antenna. Background of the invention

Directional antennas emit and/or receive electromagnetic beams in one or more specific directions. For example, dish- shaped antennas have a parabolic profile for focussing an emitted or received beam in the direction of the parabola's axis of rotational symmetry. As another example, arrays of planar antennas can be physically tilted and/or employ electronic phase shifters to adjust the beam direction. However, dish antennas in particular can be unsightly or the extent of their protrusion from, for example, a building external wall can be restricted by municipal regulations. While using planar antennas can assist to ameliorate this problem with dish antennas, many planar antennas with comparable directivity, such as arrays, have different problems, including increased losses relative to comparable dish antennas Such losses can lead to a decrease in antenna efficiency, a decrease in antenna gain, an increase in source power requirements, and increased challenges in heat management.

Electronic devices such as diodes in phase shifters tend to introduce non- linearity to the antenna system, producing signal distortion including intermodulation distortion. Non-electronic mechanical phase shifters have been developed for some mobile base-station antennas but are considered large for use in most other applications. Further, electronic devices in phase shifters have electrical requirements, such as DC biasing, power supplies to provide bias, and electric bias networks to apply appropriate bias voltage or current to each electronic device. These

requirements increase fabrication complexity. They also increase overall power consumption of the antenna system, reducing the time between charging in battery- powered devices. Significant variation of characteristics of electronic devices with temperature is another concern when the temperature of the antenna system is likely to vary over a large range, for example, on an airplane.

Summary of the invention

According to a first aspect of the invention, there is provided an antenna source, the source including:

an emitter or receiver for emitting or receiving a first electromagnetic radiation having non-uniform phase across lateral dimensions transverse to a propagating direction of the radiation; and

a delay compensator, including a delay-compensating metasurface, the delay compensator positioned in a near field of the emitter or receiver and configured to compensate propagation delays of the first electromagnetic radiation to thereby provide an interface between the first electromagnetic radiation and a second electromagnetic radiation having substantially uniform phase across lateral dimensions transverse to a propagating direction of the second electromagnetic radiation.

The delay compensator may be configured to introduce different compensating delays based on a lateral position at the delay compensator (a position within a plane transverse to the direction of the radiation) to equalise or substantially equalise the total delays along different respective paths between the emitter or receiver and the interface. For example, for the case of a single-input emitter or single-output receiver, the total delays are measured between the emitter input or receiver output and the interface. The delay-compensating metasurface may have spatially-varying delay characteristics. The delay-compensating metasurface may include a plurality of distributed cells, each encompassing a conductive material of a different size to effect the spatially-varying delay characteristics. The delay compensator may include a first interface for an incoming electromagnetic radiation and a second interface for an outgoing electromagnetic radiation and wherein the delay compensator includes conductive material positioned at either or both the first interface and the second interface, and/or in between the first interface and the second interface. The source may include both the emitter and the receiver for emitting and receiving the first electromagnetic radiation that is not uniform in phase across the lateral dimensions transverse to the propagating direction of the radiation. The emitter and receiver may be co-located. The delay compensator may be located within ¾ of a wavelength of the source.

The delay compensator may be located within ½ of a wavelength of the source.

The delay compensator may be located within ¼ of a wavelength of the source.

The source may be configured to have an operational frequency within a frequency range of approximately 0.1 to 1000 GHz.

The source may be configured to have an operational frequency within a frequency range of approximately 0.5 to 6.0 GHz. The source may be configured to have an operational frequency within a frequency range of approximately 60 to 66 GHz.

The source may be configured to have an operational frequency within a frequency range of approximately 10 to 30 GHz.

According to a second aspect of the invention, there is provided a directional antenna including:

a source configured to emit or receive an electromagnetic radiation having spatially substantially uniform phase across lateral dimensions transverse to a propagating direction of the electromagnetic radiation; and

at least one directional metasurface positioned in a near field of the source, the at least one directional metasurface moveable between a first position and a second position to control a direction of transmission or reception of the directional antenna between a first direction and a second direction, different from the first direction. The at least one directional metasurface may include a plurality of distributed cells, each encompassing a conductive material of a different size to effect spatially- varying delay characteristics. The directional antenna may include both an emitter and a receiver for emitting and receiving the electromagnetic radiation having spatially substantially uniform phase across lateral dimensions transverse to the propagating direction of the radiation. The emitter and receiver may be co-located.

The directional antenna may be configured so that each directional

metasurface is moveable by hand or by other means such as a stepper motor at least on installation of the directional antenna, to thereby controllably move the direction of transmission or reception of the directional antenna.

The at least one directional metasurfaces may include a first and a second directional metasurface, movable relative to both the source and each other.

At least one and optionally all of the directional metasurfaces may be located within ¾ of a wavelength of the source. At least one and optionally all of the directional metasurfaces may be located within ½ of a wavelength of the source.

At least one and optionally all of the directional metasurfaces may be located within ¼ of a wavelength of the source.

At least one and optionally all of the directional metasurfaces may be located within 1/10 of a wavelength of the source.

In one embodiment, the source of the directional antenna includes the source of the first aspect.

According to a third aspect of the invention, there is provided an antenna including: a source configured to emit or receive an electromagnetic radiation having spatially substantially uniform phase across lateral dimensions transverse to a direction of the electromagnetic wave; and first and second directional metasurfaces positioned in a near field of the source to steer the radiation by a steering angle, and each being rotatable about a respective axis, wherein rotation of the first and second directional metasurfaces facilitates adjustment of a direction of transmission or reception of the directional antenna over an elevation angle and an azimuthal angle.

Each of the first and second directional metasurfaces may include a plurality of distributed cells, each encompassing a conductive material of a different size to effect spatially-varying delay characteristics.

Each of the first and second of metasurfaces may include a spatially linear time delay profile for steering the radiation. The spatially linear time delay profile may be a spatially linearly wrapped or sawtooth time delay profile.

The first and second metasurfaces may have identical time delay profiles.

In one embodiment, co-rotation of the first and second metasurfaces facilitates an adjustment of the direction of transmission and reception of the radiation along the azimuth.

In one embodiment, counter-rotation of the first and second metasurfaces facilitates an adjustment of the direction of transmission and reception of the radiation along the elevation.

In one embodiment, the range of the elevation angle is twice the steering angle.

Either or both of the directional metasurfaces may be located within a quarter of a wavelength of the source.

According to a fourth aspect of the invention, there is provided a directional antenna including:

an emitter or receiver; and

at least one directional metasurface positioned in a near field of the emitter or receiver, the at least one directional metasurface positioned to control a direction of transmission or reception of the directional antenna. According to a fifth aspect of the invention, there is provided an antenna including:

an emitter or receiver; and

first and second directional metasurfaces positioned in a near field of the emitter or receiver to steer the radiation by a steering angle, and each being rotatably positioned about a respective axis to control a direction of transmission or reception of the directional antenna at an elevation angle and an azimuthal angle.

According to a sixth aspect of the invention, there is provided a method of providing a directional antenna, the method including:

providing a source configured to emit or receive an electromagnetic radiation having spatially substantially uniform phase across lateral dimensions transverse to an initial propagating direction of the electromagnetic radiation; and

providing a first directional metasurface and a second directional metasurface, each positioned in a near field of the source, each configured to steer the

electromagnetic radiation by a steering angle.

The method may further include installing the directional antenna and rotating the first and second metasurfaces relative to each other to provide a required steering angle.

Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the

accompanying drawings.

Brief description of the drawings

Figure 1 illustrates an arrangement of a source for a directional antenna. Figure 2A and 2B illustrate schematically a cross sectional view and a top view of a sample section of a metasurface.

Figures 2C and 2D illustrate the transmission and relative phase,

respectively, of a metamaterial having varying values of a and b. Figures 2E and 2F illustrate the transmission and relative phase,

respectively, of an all-dielectric material having different thickness and relative permittivity values.

Figure 3 illustrates a plan view of an example of a delay compensator. Figure 4 illustrates an arrangement of a directional antenna.

Figure 5 illustrates an example of a spatially linear (non-wrapped and wrapped) time delay profile.

Figure 6 illustrates a plan view of an example of a directional metasurface.

Figures 7-9 illustrate beam directions with a directional metasurface for various rotational angles of a pair of directional metasurface.

Figure 10A illustrates a schematic view of an example of a directional antenna.

Figure 10B illustrates a schematic view of another example of a directional antenna. Figure 11 illustrates a plan view of an example of a hybrid metasurface combining a delay compensating metasurface and directional metasurface.

Detailed description of embodiments

Described herein are embodiments of an antenna source, which may be used for a directional antenna. Also described are embodiments of a directional antenna and methods of providing and controlling a directional antenna with a variable beam direction. While the description hereinafter focusses on a directional antenna for emitting an electromagnetic radiation or beam, the antenna source, directional antenna and methods are also applicable to a directional antenna for receiving an

electromagnetic radiation or beam, in which case the antenna source is equivalent to a detector for detecting the received radiation or beam. Unless otherwise stated, the term "antenna source" is used to also describe such a detector. By appropriate physical scaling, embodiments in the present disclosure are applicable to electromagnetic radiation in at least telecommunication bands at radio and microwave frequencies, for example, at least any range between approximately 0.1 GHz to approximately 1000 GHz. For example, the range may be approximately 0.5 GHz to 6.0 GHz for mobile communication, wireless broadband and wireless networks (including Wi-Fi), approximately 60 to 66 GHz for Wi-Gig applications, in the C band, X-band, Ku band, K band, Ka band or another millimetre-wave band for systems including GEO, MEO, and LEO satellites providing various services including satellite TV, internet from satellite, the Internet of Things (IoT) and the Internet of Space, 30-300 GHz for other millimetre-wave wireless applications, or approximately 20 to 30 GHz for emerging millimetre-wave wireless communication methods including those proposed for 5G systems.

In this specification:

"Near field" in the context of an antenna, refers to a distance less than a wavelength for any frequencies that the antenna is configured to emit and/or receive.

A "metasurface" refers to a structure of metamaterials formed in a geometrical shape that extends substantially in two dimensions (e.g. is substantially planar), but does not preclude extending into a third dimension (e.g. having a thickness) or embedded into the third dimension (e.g. being an embedded layer). In other words, a metasurface includes metamaterials over at least two dimensions. A "metamaterial" refers to a non-homogenous material formed from composite materials (e.g. a dielectric (such as foam) and a conductor (such as metal)).

The present disclosure in at least one described arrangement includes a substantially planar antenna, where the beam direction can be adjusted by movement of antenna components forming one or more directional metasurface, for example rotation about an axis of one or more rotationally asymmetrical metasurfaces. This arrangement requires no physical tilting of the antenna nor active devices such as electronic phase shifters. Such beam direction adjustment may allow the antenna to remain oriented in substantial alignment (e.g. in parallel or flush) with a supporting surface, such as an external wall of a building, on which the antenna is installed, while transmitting or receiving at an angle other than normal to the supporting surface. In some applications, the described antenna is to be installed for fixed point- to-point communication. For example, the antenna may be installed on a fixed platform, such as an external wall of a building, for point-to-point wireless communication to another fixed point, such as may occur in backhaul links, synchronous satellite links, local multi-point distribution systems (LMDS), microwave links, and emerging millimetre-wave wireless links such as WiGig. In these applications, the each directional metasurface may be rotated, for example manually or by a respective stepper motor, until a desired beam direction is obtained. The rotational position of the directional metasurfaces may then be fixed until or unless another direction is desired.

In other applications, the described antenna is to be installed for communication between two relatively moveable points. For example, the antenna may be installed on a mobile platform, such as a train or a bus, for satellite or mobile communication, or installed on a stationary or mobile platform to track movement of an asynchronous (low earth orbit (LEO) or medium earth orbit (MEO)) or geosynchronous earth orbit (GEO) satellite. In these applications, each directional metasurface may be rotated, for example by a respective stepper motor, which regularly or continuously optimises the beam direction as the mobile platform moves or the asynchronous satellite moves. Unlike satellite-tracking using dish-based antennas, the rotation of the directional metasurfaces do not substantially change the orientation or the profile (e.g. flatness or height) of the described antenna. This tracking by rotation of the directional metasurfaces does not require multi-axis joints, as in the case of tracking in some reflector antennas and passive antenna arrays achieved by physically orienting the antenna towards a moving satellite. A major advantage of the embodiments of the disclosed antenna is that Radio

Frequency (RF) transmission lines (or waveguides) are not connected to the moving parts of the antenna, which are the directional metasurfaces. They are only connected to the emitter or receiver, which are stationary parts of the directional antenna.

Therefore, unlike in some arrangements where radiating elements of the antenna are rotated to steer a beam, the embodiments of the disclosed antenna do not require expensive rotary RF joints or their equivalents. Source

Embodiments of the disclosed directional antenna include a source with substantially spatially uniform phase. Herein, references to spatial uniformity in the context of phase profile refer to uniformity across a plane. While such a source exists in various physical forms (e.g. a passive microstrip patch array, and a slot array), it may not always be the most appropriate choice given the requirements (e.g. loss performance and/or footprint) of specific applications. Accordingly, in one aspect, the present disclosure provides a source with spatially substantially uniform phase, based on an emitter that does not have a uniform phase (e.g. a single resonant cavity antennas (RCA) element). The present disclosure also provides a method producing such a source.

Figure 1 illustrates an arrangement of a source for providing a substantially uniform phase output. The illustrated source 100 includes an emitter 102 for emitting an electromagnetic radiation 104 and a delay compensator 106 for compensating propagation delays of the electromagnetic radiation 104. The emitter 102 may be a single RCA element. The emitter 102 includes an emitter input 103, which is connected to a RF transmission line or waveguide for receiving electromagnetic energy generated by an oscillator (not shown) through the RF transmission line or waveguide. The delay compensator 106 has a first interface 108 for an incoming electromagnetic radiation (e.g. the emitted electromagnetic radiation) and a second interface 110 for an outgoing electromagnetic radiation. The delay compensator 106 is positioned in the near field of the emitter 102.

The lines of electromagnetic energy flow illustrated in the accompanying figures (e.g. the lines representing radiation 114), including Figures 1, 4 and 10 are not drawn to scale and are not intended to reflect the actual distance in number of wavelengths, such as the distance between the emitter 102 and the delay compensator 106. In some arrangements, the delay compensator 106 is positioned less than a three- quarters of a wavelength, less than half a wavelength, less than quarter of a wavelength, or less than one-tenth of a wavelength of the electromagnetic field away from the emitter 102. The delay compensator 106 is configured to compensate energy propagation delays of the emitted electromagnetic radiation 104 along different propagation paths between the emitter 102 and the first interface 108 (e.g. path segments 112a, 112b, and 112c) to thereby provide at the second interface 110 an electromagnetic radiation 114 whose field is spatially substantially uniform in phase across lateral dimensions transverse to a propagating direction of the radiation 114. In one arrangement, the field is additionally spatially substantially uniform in amplitude.

In one arrangement, the delay compensator 106 may equalise or

substantially equalise the total delays along a selection of different propagating paths between the emitter input 103 of the emitter 102 and the second interface 110, so as to provide a field with the substantially uniform phase. For example, this may be achieved by the delay compensator 106 introducing compensating delays based on a lateral position at the delay compensator 106 (e.g. associated with path segments 116a, 116b and 116c) in addition to the respective propagation delays (e.g. associated with path segments 112a, 112b and 112c) between the emitter input 103 of the emitter 102 and the compensator 106. That is, in this example, the combined energy propagation delay of path segments 116a and 112a is made equal or substantially equal to the combined energy propagation delay of path segments 116b and 112b, which is in turn also made equal or substantially equal to the combined energy propagation delay of path segments 116c and 112c. The selection of different energy propagating paths (and therefore path segments) may be based on dividing the antenna aperture into a hypothetical grid of square cells, each approximately one quarter to one third of a wavelength long and considering delay of the electromagnetic energy to the centre of each cell from the emitter input 103 of the emitter 102. In one arrangement, with the delay compensator 106, the difference in (phase) delay across the antenna aperture is less than ± 45 degrees. In another arrangement, with the delay compensator 106, the difference in (phase) delay across the antenna aperture is less than ± 22.5 degrees.

To facilitate delay equalisation (or substantial delay equalisation), the delay compensator 106 includes a delay-compensating metasurface 300 (see e.g. Figure 3) having spatially-varying delay characteristics. The spatially-varying delay

characteristics may be specifically tailored, designed or otherwise chosen such that the compensating delays along path segments 116a, 116b and 116c, when respectively added to the propagation delays along path segments 112a, 112b and 112c, are made equal or substantially equal, so as to provide an electromagnetic radiation 114 whose phase is spatially substantially uniform across lateral dimensions. Details of a metasurface in general are first described. Details of the delay- compensator 106 including a delay-compensating metasurface 300 will be described thereafter.

Metasurface

While the metasurface may be of any thickness, the metasurface is preferably less than a quarter of a wavelength thick for compactness and reduction of propagation loss. In the following description, a metasurface includes a structure of alternating dielectric (e.g. foam) and a conductor (metal). However, for some metasurfaces, the dielectric may be omitted.

In some arrangements, the metasurface is a physically non-homogeneous meta-medium. In some arrangements, the metamaterial unit cell in a metasurface may be electrically small compared to the wavelength. In some arrangements, the metamaterial or a unit cell thereof may behave approximately as a quasi- homogeneous medium at some or all operating frequencies of the antenna.

In at least one arrangement, a metasurface includes a plurality of cells evenly distributed across the plane of the metasurface. Each cell encompasses a conductive material (hereinafter a conductive patch) so that neighbouring patches are electrically insulated from one another. The metasurface may be constructed, for example, by printing, depositing or otherwise positioning a patch of conductive material (e.g. square- shaped, circularly shaped, square-ring- shaped, or annular-ring- shaped metal) within each cell on a dielectric slab (e.g. a foam slab). In another example, some surfaces of the metasurface may be constructed by depositing metal over the whole surface and etching away or removing metal in some metal in some areas to leave patches of conductive material. In some examples, metal is removed to create slot-like shapes instead of patch-like shapes. In another example, the metasurface may be constructed by embedding dielectric or conductive materials, such as metal particulates, in one or more layers within each cell.

Figures 2A and 2B illustrate a schematic example of a section of a metasurface 200. As illustrated in Figure 2A, in this example, the metasurface 200 includes bonding two dielectric slabs 202 and 204 of thickness and t ₂ respectively. The dielectric slabs 202 and 204 each includes a respective outer layer (208 and 210) of conductive patches (208a and 210a) of respective dimensions a and c, and together sandwich an inner layer 212 of conductive patches (212a) of dimension b. The two outer layers of patches are positioned at the first and the second interfaces, while the inner layer of patches is positioned at the interface of the two slabs. The dielectric slab thicknesses provide respective separation between the inner layer 212 and the two outer layers 208a and 210a. The metasurface 200 includes a plurality of cells (such as 206) of width As (and therefore spatial periodicity of also As) each including a conductive patch at each of the inner layer (i.e. patch 212a) and the two outer layers (i.e. patches 208a and 210a). The spatial periodicity As of the cells is based on the wavelength content of the electromagnetic radiation. The spatial periodicity As of the cells nay be selected to achieve a balance between complexity of fabrication and time- delay compensation resolution. For example, in one arrangement, the spatial periodicity As of the cells is equal to a third of the centre wavelength λ ₀ of the electromagnetic radiation (i.e. As = λ ₀ /3). As illustrated in Figure 2B, the size of a conductive patch(es) may be different for different cells, imposing a spatially- dependent time delay on an electromagnetic radiation propagating across the metasurface 200. In the arrangement shown in Figures 2A and 2B, the conductive patches are square (where the dimensions a, b and c indicate the length of each side of the respective squares, in a specific case of a = c). In other arrangements, the conductive patches may be of other geometric shapes, such as circular (where the dimensions a, b and c indicate the diameter of the respective circles). In general, a conductive patch responds to an incoming field, with the magnitude of the reaction depending on the size of the conductive patch. Since the interaction in turn affects the delay, a desired delay may be achieved by selection of the patch size. The conducting patches in the metasurfaces in the illustrated embodiments are non-resonant and non- radiating. The thickness of the conductive patches is significantly less than a, b and c. In one arrangement, the thickness of the conductive patches is approximately 1000 times smaller than a, b and c. In some cases, the conductive patch thickness is limited by commercial dielectric slabs with pre-printed metal having standard thickness values. In one example, the metal thickness is 0.017 mm. The slab thickness is limited by the required profile (e.g. the total required height of antenna 400). Similarly it may not have any arbitrary values as one may prefer to use commercial laminates and they come in standard thickness. In one example, the slabs are each approximately 1.5 mm thick, resulting in a metasurface of approximately 3.0 mm thick. In one embodiment, to achieve a target or desired delay profile, the dimensions a, b, and c of each cell are obtained by way of multi-dimensional numerical simulation. In comparison, conventional analysis approaches rely on ray optics or geometric optics, and thus apply to the far-field regime. The present disclosure relates to solutions in the near- field regime and would therefore require a non-conventional analysis approach. One such non-conventional analysis approach is described in section "III. THEORY OF PHASE CORRECTING STRUCTURE" in an IEEE Transactions on Antennas and Propagation publication titled "Dielectric Phase-Correcting Structures for

Electromagnetic Band Gap Resonator Antennas" (Volume:63 , Issue: 8), the entirety of which is fully incorporated herein. In one arrangement, the simulation of each cell requires solving full-wave vector Maxwell equations with periodic boundary conditions on the two interfaces, using for example commercial software packages. The simulation is repeated and the compensating delay profile is optimised through any one or more of trial-and-error, iterative methods, parameter sweeping, and evolutionary optimisation methods such as Particle Swarm Optimisation (PSO). For example, an initial combination of dimensions a, b, and c of a cell are used to compute the resulting compensating delay. At each iteration of simulation, one or more of the dimensions a, b, and c are varied and to provide a different delay until the target delay is reached within an error tolerance. The same procedure is repeated for all other cells in the metasurface. Alternatively or additionally, the delays caused by different combinations of dimensions a, b, and c are computed and stored in a database, from which

approximate dimensions a, b, and c may be looked up based on a desired delay. For example, Table 1 shows an example of stored simulated results of time delays (and the corresponding wrapped delays which are 2π phase-wrapped) introduced by conductive patches within a cell 206 of As = 9 mm with the indicated values of a, b and c (where a = c) for a range of lateral offsets between the cell centre and the emitter input 103 of the emitter 102:

Table 1

The presence of a conductive material in a metasurface allows wider control over transmission and phase, compared to, for example, an all-dielectric material. As illustrated in Table 1, the control over the phase of the metasurface (which manifests as delay of the radiation) is achieved by varying the size of the conductive material within each cell of a metasurface. Further, Figures 2C and 2D illustrate the transmission and relative phase, respectively, of the metamaterial having varying values of a and b (but otherwise the same parameters as the structure whose calculated delays are summarised in Table 1). As shown in Figures 2C and 2D, high transmission may be maintained while achieving a large range of phase by varying the values of a and b. For example, Figure 2C shows a region 214 marked by the first contour line at the lower left of the contour plot where transmission is at -ldB or higher (i.e. -80% or above transmission). Within the same region, Figure 2D shows that the phase delay can vary close to 360 degrees.

In comparison, Figures 2E and 2F illustrate the transmission and relative phase, respectively, of an all-dielectric material has a particular thickness (x-axis) and a particular dielectric relative permittivity (y-axis). Figure 2E shows regions of high transmission (indicated by 216a-216d) in between regions of low transmission. For example, the region 216a has a unity or near-unity transmission peak following a curved line starting from around 2mm thickness and relative permittivity of 40 to around 10mm thickness and relative permittivity of 2. The region 216b has a unity or near-unity transmission peak following another curved line starting from around 4mm thickness and relative permittivity of 40 to around 10mm thickness and relative permittivity of 7.5. In between the regions 216a and 216b, transmission can drop to approximately 0.5. As an example, to achieve a phase variation of 360 degrees, Figure 2F indicates that a dielectric relative permittivity variation from ~2 to -15 is required for a fixed thickness of 9.8 mm. This required range of dielectric relative permittivity at this thickness is represented in Figure 2G, which illustrates the phase shift 218 and the normalised phase shift 220 (which offsets to zero phase at relative permittivity of 1) as a function of dielectric relative permittivity between 1 and 18. Figure 2H illustrates that, for a range of dielectric relative permittivity between ~2 to -15, the corresponding transmission can however drop to approximately 50%. Similarly, while varying the thickness of the all-dielectric material may achieve a variation in phase, doing so not only affects the profile (e.g. height) of the antenna but also lowers the transmission. Accordingly, in comparison to an all-dielectric material, a metasurface can, via selecting appropriate paired values of a and b, obtain high transmission (e.g. between around 0.8 and 1) while introducing a large variation of phase (e.g. 0 to 360 degrees) which then manifest in a large range in delay.

Delay-compensator

The delay compensator alters the electromagnetic radiation emitted by an emitter with a spatially non-uniform phase across a plane to result in a radiation with a spatially uniform phase across a plane. For example, referring to Figure 1, at the first interface 108, the propagation delay of electromagnetic energy along path 112b, representing the shortest distance from the emitter 102 to the first interface 108, may be smaller than that along the paths 112a and 112c, representing a longer distance from the emitter 102 to the first interface 108. In general, the spatial profile of the propagation delay increases at locations of the first interface 108 further away or more laterally offset from the emitter 102. In order to compensate for the difference in the propagation delays across these paths, so that the total propagation delay at the second interface 110 is equalised or substantially equalised (i.e. an outgoing radiation from the second interface 110 can exhibit a uniform field in phase), the spatial profile of compensating delays introduced by the delay compensator 106 is the inverse of the spatial profile of the propagation delays. For example, if the propagation delay increases linearly as a function of lateral offset from the centre of the emitter 102, then the compensating delay decreases linearly as a function of lateral offset from the centre of the emitter 102.

Figure 3 illustrates a plan view (i.e. one side) of an example of a delay compensator 300. The delay compensator 300 includes a delay-compensating metasurface with a plurality of distributed cells, which may be evenly or substantially evenly distributed across the metasurface. The description above on "metasurfaces" is generally applicable.

In at least one arrangement, the size of the delay-compensating metasurface is approximately equal to or larger than the antenna aperture of the directional antenna. The 'size' of the metasurface refers to the area over which it is effective to control delay. As illustrated in Figure 3, the delay compensator 300 on one side includes an inner region 302 with differently sized conductive patches and an outer region 304 with uniformly sized conductive patches. Within the inner region 302, the conductive patch size decreases from the centre to the periphery of the inner region 302. The larger sized conductive patches in the centre cause more delay to the radiation propagating through the centre, whereas the smaller sized conductive patches at the periphery causes less delay to the radiation propagating through the centre. In one embodiment, as shown in Figure 3, the delay-compensating metasurface is substantially radially symmetrical. The spatial profile of compensating delays of this example therefore represents an inverse of the spatial profile of the propagation delays from the emitter. The exact delays may be simulated using the procedure mentioned in the section under the heading "Metasurfaces" by, for example, solving the full-wave vector Maxwell equations.

Directional antenna with variable beam direction In another aspect, the present disclosure provides a directional antenna with a variable radiation beam direction. As illustrated in Figure 4, an embodiment of the disclosed directional antenna 400 includes (i) a source 402 and (ii) a pair of directional metasurfaces 406 and 408, both directional metasurfaces being positioned in a near field of the source 402. In some arrangements, the pair of directional metasurfaces 406 and 408 is positioned less than a quarter wavelength of the source 402 away from the source 402 (measured from the opposed input/output side of the source 402 and the directional metasurface closer to the source 402). In other words, in an embodiment with an air- gap between the source and the directional metasurface closer to the source 402, the air-gap is less than a quarter wavelength. In some arrangements, the separation of the two directional metasurfaces 406 and 408 is no greater than a tenth of the wavelength.

In some arrangements, the source 402 is configured to emit a source radiation 404 whose field is spatially substantially uniform in phase across lateral dimensions transverse to an initial direction 405 of the source radiation 404. The source 402 may be a passive microstrip patch array, a slot array, a leaky-wave antenna array or an array of wideband resonant cavity antennas (RCAs). Alternatively, a source can be the one described above (or an array of sources each described above) in the section under the heading "Source".

Each of the pair of directional metasurfaces 406 and 408 is configured to tilt a normally incident radiation away from the antenna broadside direction by a steering angle 1. The beam direction 422 of the outgoing beam 421 (i.e. the main beam) may be adjusted by relative movement of the metasurfaces 406 and 408 to each other and to the source 402. In one embodiment, each of the directional metasurfaces 406 and 408 is rotatable. In particular, the directional metasurface 406 is rotatable about a first rotational axis 414 extending in the same direction as an initial direction 405 of the source radiation 404. The directional metasurface 408 is rotatable about a second rotational axis 416 (which coincides with the first rotational axis 414), extending in the same direction as the initial direction 405 of the source radiation 404. As described in further detail below, rotation (414 and 416) of the pair of directional metasurfaces 406 and 408 facilitates adjustment of the beam direction 422 of the outgoing beam 421 over an elevation angle 410 and an azimuthal angle 412.

In one arrangement, the directional metasurfaces 406 and 408 are

substantially planar to facilitate a low-profile directional antenna. Further, directional metasurfaces 406 and 408 are both positioned in the near field of the source to facilitate a compact directional antenna.

The beam direction 422, which refers to both the elevation angle 410 and the azimuthal angle 412, may be defined in one of several ways. In one arrangement, the beam direction 422 is a direction of peak directivity of the antenna 400. In another arrangement, the direction is a centre direction within a certain beamwidth, defined relative to the peak directivity (e.g. 3dB or 6dB beamwidth) or defined by an absolute directivity (e.g. beamwidth above 15 dBi). In yet another arrangement, the direction includes a direction provided by weighting the energy density profile of the electromagnetic beam. In some arrangements, only the main lobe is taken into account when weighting the energy density profile, whereas in others, any side lobes are also taken into account. If reduced directional control is required, then only one of the metasurfaces 406 and 408 may be required.

Directional metasurfaces

In one arrangement, controlling the beam direction 422 using the directional metasurfaces 406 and 408 is achieved by a spatially linear time delay profile of the directional metasurfaces. As illustrated in Figure 5, a spatially linear time delay profile 500 (indicated as a solid line) is desired to introduce a linear phase on the source radiation for tilting at a steering angle δ. The steering angle δ is proportional to the gradient of the spatially linear time delay profile 500. To achieve the effect of the linear time delay with the directional metasurfaces 406 and 408, the conductive patches of each cell may be sized according to the desired delay (such as relying on Table 1). To limit the required range of the conductive patch size, however, delay corresponding to every period T=l/f can be wrapped to result in the equivalent linear delay. Figure 5 illustrates the spatially linearly wrapped or sawtooth time delay profile 502 (indicated as a dashed line) to result in the equivalent linear delay for a steering angle δ of 20°. The wrapped delay profile 502 is also provided in Table 1. Figure 6 illustrates a plan view (i.e. one side) of an example of a directional metasurface 600. The description above on "metasurfaces" is generally applicable. The size of the directional metasurface is approximately equal to or larger than the antenna aperture. The directional metasurface 600 includes a plurality of evenly distributed cells, with conductive patch size varying according to the spatially linear wrapped or sawtooth time delay profile 502. The delay increases in the direction of the sawtooth axis 604, but is relatively unchanged in the orthogonal direction.

Specific regions 602a, 602b and 602c of the directional metasurface 600 can be seen to correspond to the wrapped delays 504a, 504b and 504c of the sawtooth time delay profile 502. While the directional metasurfaces 406 and 408 can be configured with separate and different delay profiles, in the following examples, the directional metasurfaces 406 and 408 are assumed identical in their delay profiles and take the form of the directional metasurface 600.

In Figure 6, the directional metasurface 600 includes a total of 18x18 cells. In one embodiment the number of cells is chosen to completely cover the physical aperture of the source 402. To direct the outgoing beam 421 by an angle a away from broadside direction, the relative time delay (Δτ) between two adjacent cells is given by Δτ = dlc ₀ sin(oc), where d is the length of a cell and c ₀ is the speed of light in free space. For example, at an operational frequency of 11 GHz (corresponding to a wavelength of approximately 27 mm), to achieve a desired beam tilt a of 20° by a directional metasurface where d = the relative time delay Δτ is 10.36 ps.

Beam direction

As mentioned, the pair of directional metasurfaces 406 and 408 are rotatable and facilitates adjustment of an outgoing beam direction 422 of the outgoing beam 421 over an elevation angle 410 (Θ) and an azimuthal angle 412 (φ). Figures 7 to 9 illustrate a number of examples and procedures in adjusting the outgoing beam direction 422. Figure 7 illustrates outgoing beam directions 422, where a first directional metasurface 406 is fixed and a second directional metasurfaces 408 is rotated to various angles, and assuming that each directional metasurface 406 and 408 has a steering angle 1 of 20°. ψ ₁ is the angle 414 of first directional metasurface 406 rotated about the axis 418, and ψ ₂ is the angle 416 of the second direction metasurface 408 rotated about the axis 420. In Figure 7(a), ψ ₁ = ψ ₂ = 0° represents the case where their sawtooth axes 604 are aligned with the x-axis. The outgoing beam 421 is

consequently steered by both directional metasurfaces 406 and 408 in the same direction. In this case, the resultant beam is directed at Θ = 2δ = 40° and φ = 0°.

Figures 7(b) to (g) illustrate other values of ψ ₂ , increasing in steps of 30°, i.e. ψ ₂ = 30°, 60° ... ,180°, which result in gradually decreasing elevation of the outgoing beam 421. When ψ ₁ = 0° and ψ ₂ = 180°, the outgoing beam 421 is directed in the broadside direction, as shown in Figure 7(g).

The beam direction 422 of the of outgoing beam 421 (θ,φ) for all cases shown in Figure 7 is tabulated in Table 2. In addition, the beam position is tracked with a trace in Figure 7(h); it shows that beam follows a circular arc and moves between the two extreme positions, i.e. (θ, φ) = (40°, 0°) and (θ, φ) = (0°, 0° ).

Table 2

Figure 8 illustrates an example where both the directional metasurfaces 406 and 408 are co-rotated, i.e. by the same angle in the same direction (or ψ _! = ψ ₂ ). It can be seen that the azimuthal angle 412 (φ) of the beam changes but its elevation angle 410 (Θ) remains unchanged. The beam is directed on the periphery of cone for several azimuth angles with a step size of 90°. In all the cases shown in Figure 8, the orientations of sawtooth axes 604 of the directional metasurfaces 406 and 408 differ by 30°, which keeps the elevation angle 410 (Θ) fixed while the azimuthal angle 412 (φ) varies.

In practice, one can first rotate only the first directional metasurface 406 or the second directional metasurface 408 to tilt the beam to the required elevation angle 410 (Θ) as described above, then both the directional metasurface 406 and 408 can be co-rotated to direct the beam to the required azimuthal angle 412 (φ). Hence, by combining the movements of both the directional metasurface 406 and 408, the outgoing beam 421 can be directed to any direction within the cone with a vertex angle of 4δ. Similarly, the elevation angle 410 (Θ) can be changed without changing the azimuthal angle 412 (φ) by counter-rotating the directional metasurfaces 406 and 408, i.e. in opposite directions but by the same angle. Doing so requires both directional metasurfaces 406 and 408 to have identical steering angles δ. In this case, the angles of rotation of the metasurfaces 406 and 408 are given by ψ ₂ = Φ ₀ + ξ and ψι = Φ ₀ - ξ where Φ ₀ is the initial orientation and ξ is the angle of rotation. When ξ = 0°, the sawtooth axes 604 of both directional metasurfaces 406 and 408 are aligned, producing the maximum tilt of Θ = 2δ. On the other hand, when ξ = 90° the sawtooth axes 604 are 180° apart, resulting in broadside radiation (Θ = 0°). If ξ is increased beyond 90°, the beam tilts again to the opposite side. Figure 9 illustrates the radiation pattern and hence the beam direction 422 for several combinations of ψι and ψ ₂ to demonstrate the effect of counter-rotation. In Figure 9(a), ψι = ψ ₂ = 0° and beam direction 422 is Θ = 40° and φ = 0°. In all of the subsequent cases, from Figure 9(b) to 9(g), ψι is increased by 15° while ψ ₂ is decreased by 15° in each step. It can be seen that the outgoing beam 421 stays in the xz -plane. The movement of beam from Figure 9(a) to 9(g) is traced with a straight line in Figure 9(h). The elevation angle 410 (Θ) varies but the azimuthal angle 412 is the same (φ = 0°). The beam directions for ψ ₂ = 15°, 30°, . . . ,90° and ψι = -15°, -30°, . . . , -90°, respectively, are given in Table 3. ψι (deg.) ψ2 (deg.) Φ (deg.) Θ (deg.)

-15 15 0 38

-30 30 0 34.6

-45 45 0 28.3

-60 60 0 20

-75 75 0 10.4

-90 90 0 0

Table 3

In practice, one can first co-rotate both the directional metasurfaces 406 and 408 to direct the beam to the required azimuthal angle 412 (φ) as described above, and then counter-rotate them to direct the beam to the required elevational angle 410 (Θ), without affecting its azimuthal angle. Again, the outgoing beam 421 can be directed to any direction 422 within the cone with a vertex angle of 4δ.

Source and directional antenna

In one arrangement, as illustrated in Figure 10A, the source in Figure 1 is configured as the source of the directional antenna in Figure 4. In the example of Figure 10A, the directional antenna 1000 includes an emitter 102 fed by a single emitter input 103, a delay compensator 106, and a pair of directional metasurfaces 406 and 408. In another arrangement, as illustrated in Figure 10B, the source of Figure 1 is modified to take the form of an array and includes multiple emitter inputs and is configured as the source in the directional antenna in Figure 4. In the example of

Figure 10B, the directional antenna 1001 includes an emitter 102 fed by emitter inputs 103 A, 103B and 103C, a delay compensator 106, and a pair of directional

metasurfaces 406 and 408. In this example, the emitter inputs 103 A, 103B and 103C have a fixed phase relationship and fixed amplitude relationship with respect to one another. The phase and amplitude relationships may remain fixed once arranged for a desirable beam profile, with the beam direction controlled by the pair of directional metasurfaces 406. The example of Figure 10B is therefore unlike a phased array, where the beam direction is varied by tuning the relative phase difference between element inputs of the phased array. The description hereinbefore on various components equally applies to those having the same and like reference numerals in the arrangements exemplified in Figures 10A and 10B. Furthermore, in these arrangements, the delay compensator 106 may be combined with one of the pair of directional metasurfaces 406 and 408 as a single hybrid metasurface. Figure 11 illustrates such an example of such a hybrid metasurface 1100, serving both functions of the delay compensator 106 (for creating a radiation whose field is spatially substantially uniform in phase) and a directional metasurface (for introducing a sawtooth time delay spatial profile). Like the directional metasurface 600 in Figure 6, the hybrid metasurface 1100 has a sawtooth axis 1102 in which the delay increases or decreases. Further, like the delay-compensating metasurface 300 in Figure 3, the hybrid metasurface 1100 includes a first region 1104, equivalent to inner region 302, with differently sized conductive patches and a region 1106, equivalent to outer region 304, with uniformly sized conductive patches.

Providing a directional antenna

The present disclosure also provides a method of providing a directional antenna. Referring to the description of the described source 100 and the described directional antenna 400, the method follows the general steps of (with some indicated as optional):

I. providing a low-profile (e.g. planar and near-field) source 100 or 402 - In some arrangements, the source has a sufficiently large lateral dimensions (i.e. large aperture) and rotational symmetry (at least approximately). In some arrangements, the source has a substantially uniform amplitude across the antenna aperture.

II. if it is necessary to determine or confirm whether the source 100 or 402 produce sufficiently uniform phase at a near field, obtaining a spatial time delay profile of the source 100 or 402 at a near field region - The time delay may be the delay of the co-polar tangential electromagnetic near-field of the low-profile source in the plane in which a first metasurface is to be placed, measured from the emitter/source input(s). In some arrangements, this plane is perpendicular to the radiation 114 of the low-profile source 100 or 402 and is often placed at a distance no greater than a quarter of a wavelength. In some arrangements, this time delay profile can be obtained through full-wave simulations or experiments,

if the time delay profile is identified at step II as non-uniform in phase (such as in the worst case of 180 degree variation between any two pints in the observation area of the plane), providing a delay compensator 106 to compensate this non-uniform time delay profile to a more uniform time delay profile at the second interface of delay compensator 106, otherwise a delay compensator 106 or steps IV and V are not required.

optionally, re-evaluating the time-delay profile of the co-polar tangential electromagnetic near field phase at the second interface of the delay compensator for uniformity - If it is not desirably uniform (such as in some cases within ±45 degrees, or in other cases within ±22.5 degrees), optimise the delay compensator 106 to achieve an even more uniform near-field time-delay profile. For example, this optimisation can be done by trial-and-error, iterative methods, parameter sweeping, or evolutionary optimisation methods such as Particle Swarm Optimisation (PSO).

optionally, through experiments or simulations, obtain the beam direction 422 of the combination of the source 100 or 402 with the delay compensator 106 positioned in the near field of the source 100. In some arrangements, the beam direction 422 should be along the axis of the combination, i.e. perpendicular to the source 100 or 402 and delay compensator (i.e. broadside) and there should not be any significant side lobes. Otherwise, re-optimise the delay compensator 106. For example, this optimisation can be done using the methods mentioned before.

providing a first directional metasurface 406 with a spatially linear time delay profile (wrapped 502 or non- wrapped 500) positioned within a near field of the source 100 or 402 - Doing so steers the beam in the elevation plane, from broadside direction to a direction that is δ degrees away from the broadside direction. The angle δ is dependent and thus adjustable by the sawtooth slope in the time delay profile 500 or 502 of the first directional metasurface 406. VII. optionally, obtaining the beam direction 422 - The step may be

performed through experiments or simulations. In some arrangement, the beam quality of the combination of the source 100, the delay compensator 106 and the first directional metasurface 406 (or a hybrid metasurface 1100) positioned in the near field of the source 100 is obtained. As appropriate, the metasurfaces are optimised until any design criteria are met, using for example techniques mentioned in step IV. In cases where the profile (e.g. total height) of the directional antenna should be minimised, the delay-compensating metasurface 300 and the first directional metasurface 406 may be combined as a single hybrid metasurface 1100.

VIII. providing the second directional metasurface 408 with a spatially linear time delay profile (wrapped 502 or non-wrapped 500) positioned within a near field of the source 100 or 402 - In most cases, the second directional metasurface 408 may be identical to the first directional metasurface 406. In some arrangements, the second directional metasurface 408 are spaced apart from the first directional metasurface 406 by approximately one tenth of a wavelength.

IX. optionally, obtaining the beam direction and beam quality after the second directional metasurface 408 is aligned with the first directional metasurface 406, i.e. their sawtooth axes 406 are in the same direction, where the beam direction will vary in the elevation plane. The beam will be steered to a direction that is 2δ degrees away from the broadside direction. As appropriate, the metasurfaces are optimised until any design criteria are met, using for example techniques mentioned in step IV.

To adjust the direction of the beam of the directional antenna 400:

(i) by counter-rotating the directional metasurfaces 406 and 408 by the same angle β about the axis of the antenna, the beam direction will change in the elevation plane, i.e. beam steer will reduce gradually from 2δ degrees. When β is 90 degrees so that the sawtooth axis 406 is 180 degrees, beam steer becomes zero, i.e. beam is in the broadside direction along the axis of the antenna; and (ii) by co-rotating the directional metasurfaces 406 and 408 by the same angle γ about the axis of the antenna, the beam direction will change in the azimuth plane by γ, i.e. beam will maintain the same elevation and rotate azimuthally about the axis of the antenna.

Accordingly, by combining an adjustment of counter-rotation and an adjustment of co-rotation of the directional metasurfaces 406 and 408, the beam can be directed to any direction within a cone that has a cone angle of 2δ (or apex angle of 4δ ) degrees.

As previously described, the antenna source, directional antenna and methods are also applicable to a directional antenna for receiving an electromagnetic radiation. In this case, the emitter 102 is a receiver.

Embodiments of the disclosed antenna are passive, requiring no active RF electronic devices such as diodes or transistors to adjust its direction. These embodiments may thereby improve antenna performance (such as efficiency and gain), or facilitate similar performance at a reduced cost in comparison to active devices. Due to the absence of RF electronic devices, embodiments of the disclosed antenna do not require biasing, DC bias power supplies, or electrical bias networks. Further, compared to a phased array where the relative phase between the array elements are tuned to steer the beam direction, implementation of a significant feed distribution network is not required when the emitter has only one or few inputs. Hence, significant RF losses typically associated with large feed distribution networks are also avoided in the embodiments of the antennas with few emitter inputs. Due to the absence of RF electronic devices, the embodiments of the disclosed antenna do not introduce significant non-linearity effects including intermodulation distortion in communication systems. For the same reason, they can handle higher power levels than phased arrays with RF electronic devices or switches, produce less heat, and are much less sensitive to temperature variations. In another embodiment of the disclosed antenna, the source 402 of Figure 10B is modified. In particular, the multiple-input emitter 102 is replaced by a conventional two-dimensional phased array with multiple inputs and the compensator 106 is omitted. The initial beam direction of the phased array itself is steered by controlling the relative phase difference between its multiple inputs. This beam can be further steered by turning the directional metasurfaces 406 and 408, such as following steps (i) and (ii) described above. The directional metasurfaces 406 and 408 are located in the near field of the phased array. Hence, by combining the conventional phase array beam steering method and near-field turning metasurface method, the beam direction is steered further away from the boresight direction.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.