Controllable beam scanning antenna

Abstract

A leaky-wave antenna device (101) is provided comprising: a partially reflective surface (2), a conductive ground plane (3), an actuator (6), and a feed array (155). The feed array (155) comprises a plurality of spaced apart feed sources (15). Each feed source (15) is arranged to couple a radio frequency input signal into the antenna (101). The partially reflective surface (2) and the conductive ground plane (3) are in a stacked arrangement and substantially parallel to one another, defining a cavity therebetween. The antenna device (101) is configured so that a beam formed by the antenna device (101) is steerable in a first direction by operation of the actuator, and in a second direction by a varying a relative phase of the radio frequency signal coupled to the cavity by each of the feed sources (15).

Description

[0001] CONTROLLABLE BEAM SCANNING ANTENNA

[0002] FIELD

[0003] The present invention relates to a leaky wave antenna with two dimensional beam steering and to a method of steering a beam in two dimensions.

[0004] BACKGROUND

[0005] Modern communication systems require antenna technologies which are characterised by high efficiency, increased data rates and beamsteering capabilities to cover the consumer demands for data. This can be achieved by the introduction of antenna systems that operate in higher frequency bands than the current infrastructure and components which can be integrated to allow the introduction of improved characteristics in the antennas. Recent progress has been made in beam scanning using Fabry-Perot antennas.

[0006] Earlier work by the present inventors (Rabbani, Churm & Feresidis; “Electro- Mechanically Tunable Meta-Surfaces for Beam-Steered Antennas from mm-Wave to THz”; Proceedings of the 50th European Microwave Conference; 12-14 January 2021; Utrecht; pp 416 to 419; (the full disclosure of which is hereby incorporated by reference) has developed a leaky-wave antenna (LWA) employing extremely low loss tuneable metasurfaces as a phase shifting material in order to obtain beam steering capabilities.

[0007] According to this previous work disclosure, beam scanning is only possible in one plane. Scanning in more than one plane is desirable.

[0008] SUMMARY

[0009] According to a first aspect, there is provided a leaky-wave antenna device comprising: a partially reflective surface; a conductive ground plane; an actuator; a plurality of feed sources; wherein the partially reflective surface and the conductive ground plane are in a stacked arrangement and substantially parallel to one another, defining a cavity therebetween; wherein the feed sources are spaced apart in an array, and each feed source is arranged to couple radio a frequency input signal into the cavity; wherein the antenna device is configured so that a beam formed by the antenna device is steerable in a first direction by operation of the actuator.

[0010] The beam may be steerable in a second direction by a varying a relative phase of the radio frequency signal coupled to the cavity by each of the feed sources.

[0011] The device may further comprise a high impedance surface in a stacked arrangement with the conductive ground plane and the partially reflective surface. The high impedance surface may be disposed between the partially reflective surface and the conductive ground plane.

[0012] Beam steering may comprise (or consist of) selecting a fixed orientation of the beam formed by the antenna (in the first direction and / or the second direction). Beam steering may comprise varying an orientation of the beam formed by the antenna in the first direction and / or the second direction.

[0013] The actuator may be configured to adjust a spacing between the ground plane and the partially reflective surface (PRS), and / or between the ground plane and a high impedance surface (HIS).

[0014] The actuator may be operable to adjust a spacing between the ground plane and the partially reflective surface, thereby steering the beam in the first direction. The actuator may move the partially reflective surface and / or the ground plane in order to beam steer in the first direction. The actuator may be configured to vary a distance between the high impedance surface and the conductive ground plane.

[0015] The feed sources may be arranged in a linear ID array. The feed sources may be arranged at an edge of the cavity. The feed sources may be arranged centrally within the cavity. Each feed source may comprise a dipole or any other radiating feed element, including an open ended waveguide or slot. Subsequent references to a “dipole” or “dipoles” can be replaced with any other suitable radiating feed element, such as an open ended waveguide or slot. The feed array may comprise an array of pairs of dipoles disposed on a feed substrate.

[0016] Each pair of dipoles may comprise a first dipole arranged to couple the radio frequency signal into a first side of the cavity, and a second dipole arranged to couple the radio frequency signal into the second side of the cavity.

[0017] Each first dipole may comprise a first dipole arm on a front surface of the feed substrate and a second dipole arm on a rear surface of the feed substrate. Each second dipole may comprise a first dipole arm on a front surface of the feed substrate and a second dipole arm on a rear surface of the feed substrate.

[0018] Each of the feed sources may comprise a feed port for receiving a radio frequency with a selected phase and / or amplitude.

[0019] The feed array may comprise control elements configured to control: i) which of the feed sources are energised by a radio frequency signal and / or ii) a relative phase and / or amplitude of the radio frequency signal coupled to each feed source. The feed array may be configured to produce a fixed phase delay to each feed source so as to produce a beam with a fixed orientation in the second direction. The feed array may comprise one or more transmission lines (or delay lines) of different configuration, so to fix the relative phase and amplitude of the signal feeding each feed source.

[0020] In embodiments that comprise a first dipole and a second dipole, the control elements may be operable to allow the first dipole and the second dipole to be energised together or separately.

[0021] The feed array may comprise one or more switching devices arranged to reconfigure the coupling of the radio frequency signal with the cavity.

[0022] The antenna device may further comprise a phase controller, configured to vary the relative phase of a radio frequency signal fed to each of the feed sources.

[0023] The partially reflective surface may comprise a metasurface comprising a periodically patterned conductive layer on a dielectric substrate. The partially reflective surface may be a fully metallic partially reflective surface (it is not essential that the PRS comprises a dielectric substrate). The partially reflective surface may comprise a metallic (or metal) sheet with patterned through holes. The patterned through holes may be an array and the array may be periodic or aperiodic. The antenna device may be a Fabry-Perot leaky-wave antenna.

[0024] The antenna device may comprise a high impedance surface arranged in the cavity between the partially reflective surface and the conductive ground plane. The high impedance surface may comprise a metasurface comprising a periodically patterned conductive layer on a dielectric substrate. The high impedance surface may be a fully metallic high impedance surface (it is not essential that the HIS comprises a dielectric substrate). The high impedance surface may comprise a metallic sheet with patterned through holes. The patterned through holes may be an array and the array may be periodic or aperiodic.

[0025] A first array of conductive elements may be formed on or in the dielectric substrate of the HIS and a second array of conductive elements may be formed on the dielectric substrate of the PRS. The conductive elements (or patterned holes) may be formed, for example, by way of etching, printing and / or micromachining.

[0026] The conductive elements of the HIS may all be substantially identical. The conductive elements of the first array may be square patches. The conductive elements of the HIS need not be square, but may have other shapes of any appropriate geometry that can be modified to allow to the required phase shift sensitivity. The conductive elements of the HIS may have a first size and a first periodicity. The actuator may be operable to move the ground plane relative to the HIS in order to steer the beam in the first direction. The actuator may move either the ground plane of the HIS to achieve this relative movement between the ground place and the HIS.

[0027] The conductive elements of the PRS may all be substantially identical. The conductive elements of the PRS may be square patches. The conductive elements of the PRS need not be square, but may have other shapes of any appropriate geometry that can be modified to allow to the required phase shift sensitivity. The conductive elements of the PRS may have a second size and a second periodicity. The first and second sizes and / or the first and second periodicities may be the same or different. The first size may be smaller than the second size. The first periodicity may be smaller than the second periodicity. In some embodiments, the first size is smaller than the second size and the first periodicity is smaller than the second periodicity.

[0028] In embodiments comprising dielectric substrates the conductive elements of the HIS and PRS may be formed on the surfaces of their respective dielectric substrates that face the ground plane. This arrangement provides the greatest control over the phase of the reflected wave. It is possible to form the conductive elements of one or both of HIS and PRS on the opposite surface of the respective dielectric substrate, but this may reduce the range of possible phase shifts and may thus reduce the range of beam steering that can be obtained.

[0029] The dielectric substrate of the HIS may have a thickness in a range of 0.01 to 2 wavelengths. The HIS dielectric substrate may have a dielectric constant ergreater than 1, optionally about 2.3. The conductive elements of the HIS may be any suitable thickness if the geometry of the patches are appropriately designed. The conductive elements of the HIS may be square patches of side lengths between 0.01 and 0.5 wavelengths; for operation at 25.5GHz, the side lengths may be about 1.5mm, optionally 1.53mm. The conductive elements of the HIS may have a periodicity between 0.01 and 0.5 wavelengths; for operation at 25.5GHz, the periodicity may be about 1.8mm, optionally 1.83mm.

[0030] The PRS dielectric substrate may have a thickness in a range of 0.01 to 2 wavelengths. The PRS dielectric substrate may have a dielectric constant ergreater than 1, optionally about 2.3. The conductive elements of the PRS array may be square patches of side lengths between 0.01 and 0.5 wavelengths, optionally about 3mm for operation at 25.5GHz. The conductive elements of the PRS array may have a periodicity between 0.01 and 1.0 wavelengths; for operation at 25.5GHz, the periodicity may be about 3.3mm, optionally 3.30mm.

[0031] The spacing between the HIS and the PRS is typically about equal to an operating half wavelength multiple, allowing for optimisation and dielectric effects. The spacing h2between the first metasurface and the conductive ground plane may be adjusted by the micro-actuator by some fraction of a wavelength, optionally between 0mm to 2mm, optionally from 0pm to 500pm. In some embodiments, the spacing h2may be adjusted to be from 25pm to 400pm. When the conductive elements of the first array on the HIS touch the conductive ground plane (h2= 0pm), the HIS becomes fully reflective, effectively removing the effect of the HIS.

[0032] The actuator may be configured to allow a substantially continuous adjustment of the spacing h2. Alternatively, the actuator may be configured to adjust the spacing h2in a step-wise manner.

[0033] The first and second arrays may take a wide range of array sizes. It is generally preferred for the arrays to be large from the perspective of the waves passing through the metasurfaces (relative to a wavelength X of the waves). In some embodiments, the arrays may be of the order of 10Z by 10X. In some embodiments, the arrays may be of the order of 100X by 100X or larger.

[0034] In some embodiments, beam steering through a range of at least -40° to +40° (relative to a line perpendicular to the PRS) is achievable.

[0035] Several LWAs could be used in an array, geometrically spaced so to provide sectors of, for example, ± 40°. In this way, five LWAs could be used to cover 360° around one radial plane.

[0036] Embodiments of the present disclosure provide a highly efficient antenna device, particularly well-suited for mm-wave and / or terahertz applications.

[0037] In some embodiments, good RF matching is obtained between 26GHz and 28GHz for h2values up to 200pm. This is a wide enough bandwidth for use with 5G infrastructure. Embodiments can be configured for operation at frequencies up to and / or exceeding 100GHz

[0038] The actuator may comprise a piezoelectric actuator. The actuator may be configured to move the partially reflective surface or the high impedance surface or the conductive ground plane. The actuator may be any other type of fast switching actuator, including but not limited to solenoid actuators, electroactive polymer actuators, microelectromechanical systems, magnetic drive actuators or micromotors. The conductive ground plane may be mounted on or connected to the micro-actuator so that the micro-actuator can move the conductive ground plane relative to the HIS and / or PRS.

[0039] The HIS may be designed to impart an engineered phase shift to RF signals when such signals are reflected. By altering or adjusting the spacing between the conductive ground plane and the HIS, it is possible to change the amount of phase shift applied to a reflected RF wave. Accordingly, by controlling the micro-actuator, it is possible dynamically to alter the reflective properties of the HIS. The HIS and PRS may be spaced from each other by a distance h±. The HIS may be spaced from the conductive ground plane by a distance h2The distance h may be fixed, while the distance h2adjustable across a predetermined range of distances by way of the actuator.

[0040] In embodiments that omit the HIS, an adjustment of the gap between the conductive ground plane and the PRS may alters the cavity depth of the Fabry-Perot leaky-wave antenna.

[0041] The conductive ground plane may comprise a plurality of ground plane sections, and the actuator may be configured to independently vary the spacing between the partially reflective surface and each of the ground plane sections.

[0042] A spacing between the feed sources may be less than a wavelength of an operating frequency of the antenna device.

[0043] According to a second aspect, there is provided a method, comprising: steering a beam from a leaky-wave antenna in a first direction by adjusting a gap between a ground plane and a partially reflective surface of the leaky-wave antenna; and steering a beam from the leaky-wave antenna in a second direction by varying a relative phase of a signal applied to a plurality of feed sources disposed to couple a radio frequency signal into the leaky-wave antenna. The method of the second aspect may employ any of the features described with reference to the first aspect.

[0044] Steering a beam may comprise selecting a fixed orientation for the beam. Varying a relative phase of a signal applied to a plurality of feed sources may comprise selecting a phase offset and / or amplitude offset of the signal applied to a plurality of feed sources to produce a fixed orientation for the beam.

[0045] An LWA is a type of travelling wave antenna, to be distinguished from a more conventional resonant antenna, such as a monopole or dipole. In an LWA, the radio frequency (RF) current that generates the transmitted radio signal travels along the antenna in one direction. This is in contrast to a resonant antenna, where RF currents travel in both directions along the antenna, bouncing between the ends. The travelling wave in an LWA is typically a fast wave, with a phase velocity greater than the speed of light.

[0046] A metasurface, in the context of the present application, is a material having a periodic array of scattering elements whose dimensions and periods are small compared with the operating wavelength. A typical metasurface may comprise a thin dielectric substrate of thickness <lmm with a two dimensional array of conductive metal elements printed or etched on the substrate, the conductive elements having a size of the order of millimetres.

[0047] BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Example embodiments of the invention will be described, with reference to the accompanying drawings, in which:

[0049] Figure la shows a unit cell of a prior art high impedance surface adjacent to a conductive ground plane;

[0050] Figure lb shows a prior art leaky wave antenna (LWA) comprising a high impedance surface disposed between a partially reflective surface and a conductive ground plane;

[0051] Figure 2 shows simulated magnitude response of the HIS from the LWA of Figure lb for different displacements of the ground plane; Figure 3 shows simulated Sn phase response of the HIS of Figure lb (relative phase shift between an incident free space wave and a free space wave reflected from the HIS) for different displacements of the ground plane;

[0052] Figure 4 shows beam scanning achieved by the LWA of Figure lb;

[0053] Figure 5 shows a further prior art LWA comprising a metallic partially reflective surface with circular apertures;

[0054] Figure 6 shows the simulated for field radiation pattern of the LWA of Figure 5;

[0055] Figure 7 shows the simulated Sn response of the LWA of Figure 5;

[0056] Figure 8 shows a LWA comprising a central feed source comprising a pair of switchable dipoles;

[0057] Figure 9 shows the simulated far-field gain (absolute) at (p = 90° of the LWA of Figure 8 for different displacements of the ground plane;

[0058] Figure 10 shows a) a front view of a two-way switchable printed dipole feed, b) a first detail of the dipole feed, and c) a second detail of the dipole feed;

[0059] Figure 1 1 shows a rear view of the dipole feed of Figure 10;

[0060] Figure 12 shows a leaky-wave antenna according to an embodiment comprising a feed array disposed at an edge of the antenna;

[0061] Figure 13 shows an example feed source printed dipole antenna;

[0062] Figure 14 shows the feed array of Figure 12 in isolation of the HIS and PRS;

[0063] Figure 15 illustrates beam steering from the antenna of Figure 12 on the y-z plane by adjustment of the distance h2 Figure 16 illustrates beam steering from the antenna of Figure 12 on the x-z plane by adjustment of the relative phase of a signal applied to each feed source;

[0064] Figure 17 shows a simulated radiation pattern from the antenna of Figure 12 arising from beam steering on both the y-z plane and the x-z plane;

[0065] Figure 18 shows a leaky-wave antenna according to an embodiment comprising a feed array disposed in the centre of the antenna;

[0066] Figures 19 to 21 show the central feed array in more detail;

[0067] Figure 22 beam steering from the antenna of Figure 18 on the y-z plane by adjustment of the distance h2

[0068] Figure 23 illustrates beam steering from the antenna of Figure 18 on the x-z plane by adjustment of the relative phase of a signal applied to each feed source;

[0069] Figure 24 shows a broadside beam formed from the antenna of Figure 18; and

[0070] Figure 25 shows a simulated radiation pattern from the antenna of Figure 18 arising from beam steering on both the y-z plane and the x-z plane.

[0071] Figure 1 shows a first LWA design, comprising a tuneable high impedance surface (HIS) 1 and a static partially reflective surface (PRS) 2, each formed as a metasurface and arranged substantially parallel to each other. The first LWA design is configured to operate at 37GHz. As shown in Figure lb), the HIS 1 and the PRS 2 are each formed by a two dimensional array of square conductive patch elements etched on a planar printed circuit board (PCB).

[0072] The HIS 1 and PRS 2 each comprise a Rogers RT / Duroid 5880 substrate with a dielectric constant er= 2.2 , a thickness h =0.787mm, with copper cladding of thickness t=0.035mm. The HIS 1 is etched to define a two dimensional array of square conductive patch elements 4, each having sides of length 1.53mm and a periodicity of 1.83mm, on a first dielectric substrate 100 of thickness h±. The PRS 2 is etched to define a two dimensional array of square conductive patch elements 5, each having sides of length 3mm and a periodicity of 3.3mm, on a second dielectric substrate 200, also of thickness hi.

[0073] The HIS 1 is disposed substantially parallel to and above a conductive ground plane 3, the HIS 1 being spaced a distance h2above the ground plane 3. The PRS 2 is disposed over the HIS 1, and is spaced a distance h3above the HIS 1. The distance h3defines a cavity height of the LWA, and determines a resonance frequency of the LWA. The overall area of the HIS / PRS assembly is about 3cm x 9cm. The ground plane 3 is mounted on a piezoelectric actuator 6, and is vertically displaceable by the piezoelectric actuator 6 so as to allow the distance h2to be varied. Figure la) shows an isolated unit cell of the HIS 1 of Figure lb), but upside down relative to Figure lb). The unit cell comprises a single square conductive patch element 4 on the first dielectric substrate 100. The conductive ground plane 3 is spaced a distance h2from the first dielectric substrate 100.

[0074] Corresponding edges of the HIS 1 and PRS 2 are joined at one side of the LWA by a reflector 7, and a printed dipole feeding antenna 8 is provided adjacent the reflector 7 and between the HIS 1 and PRS 2.

[0075] Figures 2 and 3 respectively show the simulated magnitude and phase response of the HIS reflection coefficients for various distances h2for the LWA of Figure lb) operating in a frequency band from 35GHz to 40GHz. The ground plane separation, h2, can be varied from 0pm to 400pm. It can be seen that a variation from 40pm to 400pm (Ah2= 360pm) produces a phase shift in reflection coefficients of the HIS 1 (<PHIS) °f about PHIS=142° at 37GHz while exhibiting negligibly low loss of about 0.06dB.

[0076] A flexure amplified piezoelectric actuator 6 may be employed to adjust the distance h2between the periodic array layer of the HIS 1 and the ground plane 3.

[0077] Similarly, the simulated absolute magnitude (R) and phase (<PQRS) °f the PRS 2 can be obtained, and these are 0.94 and 156° respectively at 37GHz. The directivity and main beam angle of the LWA will depend on the responses of both the HIS 1 and the PRS 2, according to the relation: where F(0) is the radiation power pattern the feeding element, and A is the operating wavelength. The phase shift provided by the HIS 1 is given by the relation: where (ptis the phase shift applied to a wave as it passes through the patterned metal surface.

[0078] It is predicted, using equations (1) and (2), that the LWA will exhibit a beam steering range of about 37° (14° to 51°) for an HIS phase shift of <[>HIS=142° at 37GHz, as shown in Figure 3.

[0079] The piezoelectric actuator 6 may comprise a linear piezoelectric actuator with flexure amplification, and may give a displacement of up to 0.5mm. The piezoelectric actuator 6, on which the ground plane 3 is disposed, can expand or contract depending on an applied DC biasing voltage, thus allowing the distance h2to be varied as required.

[0080] The printed dipole antenna 8 of the LWA is a half-wave antenna designed to operate at 37GHz. Directivity of the printed dipole antenna 8 is enhanced by placing a reflector 7 (see Figure lb)) at 0.9mm from a rear side of the antenna 8. Implementing the printed dipole antenna 8 in an LWA having 9 x 27 PRS elements and 16 x 49 HIS elements and running an appropriate simulation centred on an operating frequency of 37GHz gives rise to an response below -lOdB at around 37GHz for h2variations (8h2) from 40pm to 400pm. The maximum gain of the LWA is 23.9dBi with a maximum beam steering of 33° at 37GHz.

[0081] In embodiments, the HIS is not essential, and may be omitted.

[0082] A second LWA design is shown in Figure 5 (which may be useful for the low-THz band e.g. around 280GHz, but may also be applicable at other frequencies). This second LWA design comprises a circular aperture periodic PRS 10 and omits the HIS close to the ground plane 3. In this variation, the PRS 10 is displaced vertically relative to the ground plane 3 by way of a piezoelectric actuator 6, thus allowing the resonant cavity height h3to be adjusted. The PRS 10 takes the form of a metallic layer of thickness h = 0.3mm, with a two dimensional 11 x 31 array of circular apertures 11 each of diameter 0.6mm and periodicity 0.75mm. The metallic ground plane 3 has a thickness h2, and the PRS 10 is disposed at a distance of 0.46mm above the ground plane 3, with the cavity height h3being variable between 0.44mm to 0.5mm by applying different DC biasing currents to the piezoelectric actuator 6. The thickness h2is typically of the order of millimetres so as to provide a stable base and to provide enough anchorage for a flange of a standard waveguide feed. Preferably, the thickness h2is greater than a skin depth, ideally greater than two skin depths. The LWA is excited by a waveguide-fed slot element 12 formed towards one edge of the ground plane 3.

[0083] Embodiments with or without a HIS 1 close to the ground plane 3 can be designed for frequencies at least between 20GHz and ITHz.

[0084] Figures 6 and 7 show the simulation results for the second LWA design operating at 280GHz, with Figure 6 showing the FRP in the H-plane, and Figure 8 showing the response, in each case for different values of h3. It can be seen that the LWA has a maximum gain of 18dBi and produces beam steering over a range of about 18° within 3dB gain loss over a 0.06mm range of displacements. However, it can be seen that the S-L-L response rises up from -lOdB for some displacements.

[0085] Figure 8 shows a third LWA. Similarly to the LWA of Figure lb), the LWA comprises a tuneable high impedance surface (HIS) 1 and a static partially reflective surface (PRS) 2, each formed as a metasurface and arranged substantially parallel to each other. The illustrated LWA is configured to operate at 26GHz to 28GHz, but may be configured to operate at other GHz frequencies as required.

[0086] The HIS 1 and the PRS 2 are each formed by a two dimensional array of conductive patch elements etched on a planar dielectric PCB substrate 100, 200. The substrates 100, 200 may each comprise a Rogers RT / Duroid 5880 substrate with a dielectric constant er=2.2, a thickness of around 0.8mm, with copper cladding of thickness around 0.035mm, although these measurements are merely exemplary and non-limiting.

[0087] The HIS 1 is etched to define a first two dimensional array of conductive patch elements

[0088] 4. The PRS 2 is etched to define a second two dimensional array of conductive patch elements 5. The HIS 1 is disposed substantially parallel to and above a conductive ground plane 3, the HIS 1 being spaced a distance h2above the ground plane 3. The PRS 2 is disposed over the HIS 1, and is spaced a distance h±above the HIS 1. The distance h defines a cavity height of the LWA, and determines a resonance frequency of the LWA. The ground plane 3 is mounted on a piezoelectric actuator 6, and is vertically displaceable by the piezoelectric actuator 6 so as to allow the distance h2to be varied.

[0089] Instead of a feeding antenna being located adjacent a reflector at one edge of the LWA, a feed 150 comprising a pair of switchable dipoles is disposed between the HIS 1 and the PRS 2 at a location corresponding to a centre of the first and second arrays. By employing a pair of switchable dipoles at this location, an increase in the beam steering angle 0 is obtained, in this example ±40° relative to a perpendicular to the PRS 2.

[0090] The feed 150, shown in more detail in Figures 10 and 11 and further described hereinbelow, is configured to selectively energise the LWA structure so as to augment the available beam scanning range.

[0091] Metamaterial devices are capable of being designed so that they reflect waves with an engineered phase shift applied. This can be done using a high impedance surface, which consists of a sheet of periodic metal patches printed onto a dielectric substrate, suspended over a ground plane. By altering the separation between the periodic array and the ground plane it is possible to change the amount of phase shift imparted onto a reflected wave. Therefore using a piezoelectric actuator (or other fast switching microactuator), it is possible to dynamically alter the reflective properties of the high impedance surface.

[0092] If such an HIS 1 is built into a leaky wave antenna (using a engineered partially reflective surface (PRS) 2) and fed from the centre with switchable dipoles 150, as shown in Figure 8, a highly directive antenna is obtained that is capable of steering its main radiation beam with high efficiency at millimetre wave frequencies, where other competing technologies usually have increased losses.

[0093] This beam steering mechanism has no active elements within the RF path itself and offers best-in-class efficiencies. For example, good RF matching of the embodiment of Figure 8 between around 26GHz and 28GHz with radiation efficiencies above 90% has been demonstrated for separations h2up to 0.32mm. This is a wide enough bandwidth for the latest communication systems used in 5G infrastructure.

[0094] The switchable pair of dipoles 150 may be used to either energise the right or left portions of the antenna, to produce a beam steering performance that can be seen in Figure 9, which shows the far-field gain (absolute) (phi = 90°) for various separations h2from 0.0mm up to 0.32mm, and with the switchable dipole feed 150 energising either a right or a left portion of the antenna. Beam angles of between ±40° are possible, which enables 5G infrastructure to direct data toward individual consumers as they move around within the service area of a particular antenna system. These devices may be configured to match the electromagnetic requirements of 5G systems.

[0095] The table below summarises simulated results (peak gain, peak gain angle, half power beam width) from the antenna of Figure 8 at 26 GHz:

[0096] Figure 10a) shows a front view of an exemplary two-way switchable printed dipole feed 150. The feed 150 comprises conductive tracks printed on a front surface of a dielectric substrate 200a that includes an RF connector 201 at one end. In the illustrated arrangement, the dielectric substrate 200a is RT / Duroid 5880 substrate of thickness 0.254mm, although it will be appreciated that other appropriate dielectric substrates may be used. A feed line 202 extends from the RF connector 201, via a biasing network 203, towards front surface dipole arms 204a, 204b, shown here as pointing back towards the RF connector 201. The front surface dipole arms 204a, 204b are shown in more detail in Figure 10c). In the biasing network 203, shown in more detail in Figure 10b), a capacitor 205 is provided to prevent the DC biasing current, supplied at VI, from flowing back towards the RF connector 201. An inductor 206 is provided between VI and the feed line 202 to prevent RF currents flowing back to the DC biasing circuit. A resistor 208 is included in the biasing network 203 to control the DC biasing current to an appropriate level for the pin diodes 207a, 207b that connect the feedline 202 to the front surface diode arms 204a, 204b. The DC biasing current terminates, respectively, at V2 or V3, by way of inductors 209, 210 and 25 resistors 211, 212, as shown in Figure 10c).

[0097] A grounded strip line 213 is provided on the rear surface of the dielectric substrate 200a, as shown in Figure 11. The grounded strip line 213 follows the path of the feed line 202 and includes rear surface dipole arms 214a, 214b, shown here as pointing away from the RF connector 201. The grounded strip line 213 is placed at a quarter wavelength distance from the feed line 202 and acts as a reflector to direct the dipole antenna radiation along the y-axis.

[0098] Respective front surface and rear surface dipole arms 204a, 214a on the one hand, and 204b, 214b on the other hand, define a pair of dipoles arranged one on either side of the feed line 202 and the grounded strip line 213. The dipoles can be energised separately or together, depending on operation of the pin diodes 207a, 207b. For example, if pin diode 207a is switched to allow current to pass to the front surface dipole arm 204a, then the dipole formed by front surface dipole arm 204a and rear surface dipole arm 214a is energised and can radiate. Likewise, if pin diode 207b is switched to allow current to pass to the front 10 surface dipole arm 204b, then the dipole formed by front surface dipole arm 204b and rear surface dipole arm 214b is energised and can radiate. If only one of the dipoles is energised and radiates, then the beam is steered towards the corresponding side of the LWA (positive or negative y-axis). If both of the dipoles are energised and radiate (with the same phase), then the beam will be a central beam.

[0099] The illustrated printed dipole feed 150 allows the return loss to be kept below lOdB, with the radiation main beam directed to the left, right or centre depending on whether one or other or both of the pin diodes 207a, 207b is switched on. Once an appropriate pin diode 207a, 207b state is set for a desired beam orientation, the piezoelectric actuator 6 biasing voltage is varied so as to adjust the spacing distance h2, thereby to tune the HIS 1 to steer the LWA main beam to a desired pointing angle (e.g. in elevation).

[0100] The approach described above enables beam steering in a direction (i.e. a selection of an angle in a particular plane). For example, beam steering in Figure 8 is limited to within the y-z plane. Adjustments of the beam angle in the x-z plane are not possible solely by adjusting the distance between the ground plane and the PRS 2.

[0101] In order to enable 2D beam steering (e.g. beam steering in two mutually orthogonal planes such as x-z and y-z), plurality of feed sources may be provided for coupling an RF signal into a leaky-wave antenna like any of those described above. Control of the relative phase of the RF signal at each of the plurality of feed sources enables beam steering in the x-z plane. In polar co-ordinates, the adjustment of beam angle resulting from moving the ground plane may be considered as an adjustment in elevation angle, and the adjustment of beam angle resulting from the relative phases of multiple feed sources may be considered as an adjustment in azimuth angle.

[0102] Figure 12 shows a leaky-wave antenna device 101 according to an embodiment comprising a high impedance surface (HIS) 1, a partially reflective surface (PRS) 2, feed array 155 and reflectors 153. Although not shown in Figure 12, the antenna device 101 also comprises a ground plane and an actuator configured to adjust a spacing between the ground plane and the high impedance surface 1 (as described above).

[0103] The HIS 1 may be similar to that described with reference to Figures lb) and Figure 8 and features described with reference to those examples are also applicable here. The width (in the x-direction) of the antenna is 105.6mm and the length (in the y-direction) is 95. 1mm (~10A X 10A for a design frequency of 26.5GHz), but other dimensions may also be used in other examples.

[0104] The feed array 155 is disposed at an edge of the antenna device 101, between the HIS 1 and PRS 2. The feed array 155 comprises a plurality of identical feed sources 15. In this example there are ten feed sources 15, but other numbers of feed sources may be used. The feed sources 15 are each identical, and are arranged in a linear array, spaced apart in the x-direction. The spacing between each adjacent feed source in this example is 10mm (other spacings may be used in other examples). An example feed source 15 is shown in more detail in Figure 13. The feed source 15 comprises a printed dipole antenna comprising a dielectric substrate (e.g. RT / Duroid 5880, with thickness 0.254mm) and conductive elements on either side thereof (e.g. copper, patterned by etching). On an edge of the feed source 15 there is disposed a radio frequency connector 201 (e.g. a miniature coaxial connector). On a first side of the substrate (shown in Figure 13b)), the conductive element comprises a strip line that couples a first dipole arm 204a to connector 201. On a second side of substrate the conductive element comprises a second dipole arm 214b. The first and second dipole arm point in opposite directions, with the first dipole arm 204a in the -x direction and the second dipole arm 214a in the +x direction). The printed dipole antenna is configured to radiate in a y-direction when fed with a suitable RF signal via connector 201.

[0105] By splitting an RF signal between the feed sources 15 of the feed array 155 and controlling the relative phase of the signal that is applied to each of the feed sources 15, a beam may be formed. According to linear phased array theory, the phase difference dp at adjacent feeding points in a linear phased array is given by:

[0106] Where d(p is the phase difference required between adjacent sources, 9 is the direction of the main beam, d is the distance between the feed sources, and A is the wavelength (c / freq). For a frequency of 26.5GHz and a spacing d of 10mm, the required phase offset dp between adjacent feed sources for difference beam angles 9 is:

[0107] Figure 14 shows the feed array 155 comprising ten printed dipole antennas assembled in isolation of the HIS 1 and PRS 2.

[0108] The selection of distance between the feed sources is significant, because it defines the direction of the beam resulting from phase offsets, and is often chosen to be A / 2. In this case, the spacing d between the feed sources 15 is 10mm, which is ~A, which is the minimum spacing that accommodates the printed dipole antenna shown in Figure 13. In the example of Figure 12, a phase controller (not shown) may be used to control a phase delay for each feed source 15.

[0109] The reflectors 153 comprise a thin conductive element that is perpendicular to the substrate of the substrate of each feed source 15 (i.e. parallel with the x-z plane) disposed on either side of the feed sources 15 to reflect electromagnetic energy excited therefrom toward the cavity defined between the HIS 1 and the PRS 2. The electromagnetic waves propagate in the cavity and Teak’ from the PRS 2 in a direction in the y-z plane determined by the spacing between the HIS 1 and the ground plane (not shown) as explained above with reference to Figure 8, for example.

[0110] Figure 15 shows a simulated radiation pattern 250 from the antenna 101, illustrating that the beam can be steered on the y-z plane by movement of the ground plane 3 (relative to the HIS 1 and PRS 2 to vary d2), so that the peak gain direction 251 of the radiation pattern 250 from the antenna 101 is at a selectable angle 9 on the y-z plane. The ground plane 3 is shown in Figure 15 (the offset between the ground plane 3 and HIS 1 is not necessarily to scale).

[0111] Figure 16 shows a simulated radiation pattern 250 from the antenna 101, illustrating that the beam can be steered on the x-z plane by adjusting the relative phase of the RF signal applied to the feed sources 15 so that the peak gain direction 251 of the radiation pattern 250 from the antenna 101 is at a selectable angle 9 on the x-z plane. In Figure 16 the phase difference dp between adjacent feed sources 15 is illustrated by an arrow 252 in the x direction.

[0112] Figure 17 shows the results obtained when the direction of the beam is set to 10° on the x-z plane (by controlling the relative phase of the feed sources 15) and 20° on the y-z plane (by controlling the distance h2).

[0113] Full simulation results of beam scanning in both x-z and y-z planes are shown in the below table (which shows: frequency, f; beam steering angle 9 on the x-z plane; beam steering angle 9 on the y-z plane; displacement / variation of h2directivity; max realised gain; total efficiency, radiation efficiency, Side Lobe Level (SLL) on the x-z plane; and Side Lobe Level on the y-z plane. The results show that the performance of the antenna 101 performs well within the range ±48 ° on the y-z plane and ±25 ° on the x-z plane. The directivity is within 17.31 to 26dBi while the radiation efficiency remains at unprecedented levels since the structure does not suffer from high losses. The total efficiency starts to decrease when the beam is directed at extreme angles due to the mismatches that occur between the feeding dipoles and the resonant frequency of the rest of the structure which is shifted from 26.5GHz to 27.5GHz. One of the challenges for the antenna array 101 is the Side Lobe Level (SLL) which starts to deteriorate when the beam is steered over wide angles. Selectively tapering the magnitude of the RF signal that is introduced to different feed sources can be used to suppress the side lobes. With this antenna a broadside beam direction (i.e. parallel with the z direction, so 0° on the x-z and y-z plane) is not possible.

[0114] The antenna 101 employs a plurality of discrete feed sources 15, each disposed on a separate substrate. This is not essential, and in some embodiments a plurality of feed sources may be disposed on a single substrate, which may enable a smaller spacing between feed elements.

[0115] An alternative leaky-wave antenna 102 is shown in Figure 18. The antenna is similar to that of Figure 12, and features described with reference to Figure 12 are applicable to that of Figure 18. However, in antenna 102 a feed array 156 is placed in the centre of the antenna 102 and is bi-directional. The size of the antenna 102 is also increased to W X L= 79.4mm x 184.9mm (~7A X 162).

[0116] As is more clearly shown in Figure 19, the feed array 156 comprises a single feed substrate comprising a plurality of printed dipoles. Each of the printed dipoles is similar to that described with reference to Figures 10 and 11, and the features that are described there are generally also applicable to the dipoles of the feed array 156 and like features are given like reference numerals in Figures 19 to 22.

[0117] In contrast to the feed 150 shown in Figures 10 and 11, each feed source dipole 150a in the feed array 156 is feed by a discrete port 301. Each feed source dipole 150a has a signal port 301 on the front side of the substrate 200a and a corresponding ground port on the back side of the substrate 200a. The signals that are provided to each port 301 have their relative phase controlled by a phase controller (not shown). The ports 301 are numbered 1 to 11, starting from the bottom (-y) left (-x) corner and moving to the bottom (-y) right (+x) corner. The ports 301 are numbered 12 to 22 starting from the top (+y) right (+x) corner to the top (+y) left (-x) corner.

[0118] When ports 1 to 11 are energised, electromagnetic waves are introduced to the antenna to propagate along the -y direction. When ports 12 to 22 are energised, electromagnetic waves are introduced to the antenna to propagate along the +y direction. When all the ports 1-22 are energised, the beam scans in the broadside direction, which was not possible with the example of Figures 12 to 17. This approach allows independent application of magnitude and phase one each source, and the close spacing of the feed source dipoles 150a enable greater beam scanning angles.

[0119] A reflector 153 is provided, perpendicular to the substrate 200a of the feed array 156 which helps to focus the RF signal in the desired direction and rejects any unwanted reflections.

[0120] Arraying more than one feed source on a common substrate in this way enables the spacing between adjacent feed sources to be reduced (e.g. to a distance of 2 / 2 or even closer). Closer proximity between feed sources enhances the beam scanning range of the antenna 102.

[0121] Figures 22 and 23 respectively illustrate beam scanning in the x-z plane and the y-z plane by controlling a phase difference dp between the feed sources 150a and by varying the distance h2between the ground plane 3 and the HIS 1, in a very similar way as shown (and described) in Figures 15 and 16.

[0122] Figure 24 illustrates a broadside beam, in which the direction of the beam is 0° in the x-z plane and 0° in the y-z plane.

[0123] Figure 25 shows a simulated radiation pattern 250 from the leaky-wave Fabry Perot antenna shown in Figures 18 to 21 with a beam direction of 10° on the x-z plane and 20° on the y-z plane.

[0124] The design of the antenna shown in Figures 18 to 21 has been simulated to determine its performance, and results are shown in the table below:

[0125] This antenna design can scan on the x-z plane at angles of 0=±3O° (a slightly larger range than that shown for the previous design). The scanning on the y-z plane is again produced by displacement between the ground plane and the HIS (e.g. by a piezoelectric actuator) and the range of scan angles on the y-z plane remains the same at 0=±42°. The performance metrics in this design are improved significantly. The SLL is reduced within the range of -6.9 and -16 dB and can be improved further for extremely wide angles by tapering the amplitudes of signals provided to different feed sources.

[0126] Another advantage of this approach is that it is possible to activate as many (or as few) sources as desired and change the beamwidth of the main lobe (in some cases and applications a wider beam is required).

[0127] In some embodiments, RF feed cables may be connected directly on the bottom side of the feed array (which may, in some embodiments, protrude through the HIS 1 or PRS 2).

[0128] In some embodiments, a single point feeding mechanism may be used for the leaky- wave antenna and the activation of the feed source dipoles and control over phase difference may be achieved by varactors and diodes connected on the feed array substrate. A biasing network may also be provided on the feed array substrate.

[0129] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Although example embodiments have been described, further variations are possible, and the scope of the invention should be determined with reference to the appended claims.

Claims

CLAIMS1. A leaky-wave antenna device comprising: a partially reflective surface; a conductive ground plane; an actuator; a feed array comprising a plurality of spaced apart feed sources, each feed source arranged to couple a radio frequency input signal into the antenna; wherein the partially reflective surface and the conductive ground plane are in a stacked arrangement and substantially parallel to one another, defining a cavity therebetween; wherein the antenna device is configured so that a beam formed by the antenna device is steerable in a first direction by operation of the actuator, and in a second direction by a varying a relative phase of the radio frequency signal coupled to the cavity by each of the feed sources.

2. The antenna device of claim 1, wherein the feed sources are arranged in a linear ID array.

3. The antenna device of claim 2, wherein the feed sources are arranged at an edge of the cavity.

4. The antenna device of claim 2, wherein the feed sources are arranged centrally within the cavity.

5. The antenna device of any preceding claim, wherein each feed source comprises a dipole.

6. The antenna device of claim 5, wherein the feed array comprises an array of pairs of radiating feed elements disposed on a feed substrate.

7. The antenna device of claim 6, wherein each pair of radiating feed elements comprises a first radiating feed element arranged to couple the radio frequency signal into a first side of the cavity, and a second radiating feed element arranged to couple the radio frequency signal into the second side of the cavity.

8. The antenna device of claim 7, wherein each first radiating feed element comprises a first dipole arm on a front surface of the feed substrate and a second dipole arm on a rear surface of the feed substrate, and each second radiating feed element comprises a first dipole arm on a front surface of the feed substrate and a second dipole arm on a rear surface of the feed substrate.

9. The antenna device of any preceding claim, wherein the each of the feed sources comprises a feed port for receiving a radio frequency with a selected phase and / or amplitude10. The antenna device of any preceding claim wherein the feed array comprises control elements configured to control: i) which of the feed sources are energised by a radio frequency signal and / or ii) a relative phase and / or amplitude of the radio frequency signal coupled to each feed source.

11. The antenna device of claim 10, wherein the feed array is configured to fix the relative phase and amplitude of the signal feeding each feed source.

12. The antenna device as claimed in claim 10, including the subject matter of claim 7, wherein the control elements are operable to allow the first dipole and the second dipole to be energised together or separately.

13. The antenna device of claim 11, wherein the feed array comprises one or more switching devices arranged to reconfigure the coupling of the radio frequency signal with the cavity.

14. The antenna device of any preceding claim, further comprising a phase shifter, configured to vary the relative phase of a radio frequency signal fed to each of the feed sources.

15. The antenna device of any preceding claim, wherein the partially reflective surface comprises a metasurface comprising a periodically patterned conductive layer, and the antenna device is a Fabry-Perot leaky-wave antenna.

16. The antenna device of any preceding claim, further comprising a high impedance surface arranged in the cavity between the partially reflective surface and the conductive ground plane.

17. The antenna device of claim 16, wherein the high impedance surface comprises a periodically patterned conductive layer.

18. The antenna device of any preceding claim, wherein the actuator comprises a piezoelectric actuator.

19. The antenna device of any preceding claim, wherein the actuator is configured to move the partially reflective surface or the conductive ground plane.

20. The antenna device of any preceding claim, wherein the conductive ground plane comprises a plurality of ground plane sections, and the actuator is configured to independently vary the spacing between the partially reflective surface and each of the ground plane sections.

21. The antenna device of any preceding claim, wherein a spacing between the feed sources is less than a wavelength of an operating frequency of the antenna device.

22. The antenna device of any preceding claim, wherein the actuator is configured to adjust a spacing between the conductive ground plane and the partially reflective surface, or the antenna device of claim 16 wherein the actuator is configured to adjust a spacing between the conductive ground plane and the partially reflective surface and / or between the conductive ground plane and the high impedance surface.

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