The present invention relates to antennas. The invention has particular although not exclusive relevance to a dual polarization patch antenna for use in a multiple input multiple output (MIMO) antenna array.

When using multiple input/output feeding ports on a single circular patch to achieve either simultaneous orthogonal polarization in MIMO applications or circular polarized radiation fields, the coupling between feeding ports limits the frequency of operation. The coupling is suppressed by physical distance between ports (that are placed at 90° angle to achieve orthogonal polarizations) and therefore the size of the antenna limits the minimum frequency of application and the maximum achievable bandwidth of operation. This effect cannot normally be reduced beyond 30% of the centre frequency of operation.

Patch antennas that do not make use of high dielectric constant materials in their fabrication normally require a side dimension of l/2 (where l is the wavelength of the radiation with which the antenna is used) to be efficient. If the patch antenna is used for dual polarization or with dual feed to achieve circular polarization, the band of operation and the effective bandwidth are limited by the coupling between the ports. This can generally be optimised across only a very limited bandwidth - failing that the power injected in one port may simply get coupled through the structure into the second port thereby reducing the radiated efficiency.

The present invention seeks to provide an improved antenna that addresses or at least partially mitigates the above issues.

The present invention provides an antenna, an antenna array, and a multiple input multiple output antenna array, as set out in the appended claims.

An antenna of the present invention comprises: a first antenna patch on a first antenna substrate, a second antenna patch on a second antenna substrate, and a ground plane on a ground substrate, wherein the first antenna substrate, second antenna substrate, and ground substrate are arranged to form a layered structure in which the first antenna substrate is located between the second antenna substrate, and ground substrates; and a surround extending from the second antenna substrate to the ground substrate, wherein the surround extends around a perimeter of the second antenna substrate to form a cavity in which the second antenna substrate is located; wherein the surround comprises a plurality of metallic fingers that extend from the second antenna substrate to the ground substrate and that are arranged at intervals around the perimeter of the second antenna substrate.

The plurality of metallic fingers of the antenna may be substantially equally spaced around the perimeter of the second antenna substrate.

The plurality of metallic fingers of the antenna may be substantially equally spaced around the perimeter of the second antenna substrate at intervals of between 1/35 and 1/45 (e.g. approximately 1/40) of a wavelength at a centre frequency at which the antenna is designed to operate.

The plurality of metallic fingers of the antenna may be of substantially equal width.

The plurality of metallic fingers of the antenna may be a width between 1/40 and 1/50 of a wavelength at a centre frequency at which the antenna is designed to operate.

The plurality of metallic fingers of the antenna may be electrically connected to the ground plane.

The ground plane of the antenna may be formed on a surface of the ground substrate that interfaces with the surround such that at least a major part of the ground plane is located in the cavity.

Matching circuitry for the antenna may be provided on a surface (e.g. an external surface) of the ground substrate, opposite a surface of the substrate on which the ground plane is formed, that interfaces with the surround such that at least a major part of the ground plane is located in the cavity.

The antenna may comprise a spacer layer within the cavity and arranged between the first antenna substrate and ground substrate in the layered structure.

The second antenna substrate and surround may form different parts of a single piece of material shaped to form a housing comprising the second antenna substrate and the surround.

At least one of the first and second antenna patch of the antenna may be generally circular in shape.

At least one of the first and second antenna patch of the antenna may be generally annular in shape. The surround of the antenna may comprise a plurality of side walls arranged to form a different respective side of the antenna.

The antenna may comprise at least one feed pin for providing an external connection to the first antenna patch.

The first antenna substrate and ground substrate may be each provided with at least one feed aperture through which the at least one feed pin extends, from the first antenna patch, through the first antenna substrate and ground substrate to provide the external connection to the first antenna patch.

The antenna may comprise a dual feed antenna.

The antenna may comprise a dual polarization patch antenna configured for use in a multiple input multiple output antenna array.

An antenna array of the present invention comprises a plurality of antennas of the present invention, wherein the antennas are arranged in a tiled configuration in which the metallic fingers of each antenna interface with the metallic fingers of at least one other antenna.

The metallic fingers of each antenna of the antenna array may be configured to form a support structure for supporting the antennas of the array.

A multiple input multiple output antenna array of the present invention comprises an array of antennas of the present invention.

An array of antennas, of the multiple input multiple output antenna array of the present invention, may comprise an antenna array of the present invention.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently (or in combination with) any other disclosed and/or illustrated features. In particular but without limitation the features of any of the claims dependent from a particular independent claim may be introduced into that independent claim in any combination or individually.

Embodiments of the invention will now be described by way of example only with reference to the attached figures in which:

Figure 1 is a simplified, exploded view of an antenna; Figure 2(a) is a plan view of the antenna of Figure 1 ;

Figures 2(b) to (c) are cross-sectional views through various sections of the antenna of Figure 1 ;

Figures 3(a) to (c) each comprise plan views of the top and bottom of a different respective component layer of the antenna;

Figure 4 is a circuit diagram of a simplified matching network for the antenna of Figure 1 ;

Figure 5 is an S-parameter, frequency plot for the proposed antenna of Figure 1 when used with the matching network of Figure 4;

Figure 6 is a circuit diagram of another simplified matching network for the antenna of Figure 1 ;

Figure 7 is an S-parameter, frequency plot for the proposed antenna of Figure 1 when used with the matching network of Figure 6;

Figure 8 is a realised gain vs frequency plot, for the example of Figure 6;

Figure 9 is a plot of Right Hand Circular Polarisation (RHCP) efficiency for a variety of sizes of the antenna of Figure 1 ;

Figure 10 is a simplified circuit diagram illustrating how the antenna of Figure 1 might be used with a hybrid coupler;

Figure 11 is a plot of Right Hand Circular Polarisation (RHCP) efficiency for the antenna of Figure 1 when used in the example of Figure 10;

Figure 12 is a simplified circuit diagram illustrating the use of the antenna with broadband radio transceivers; and

Figure 13 is a simplified drawing illustrating how the antenna of Figure 1 can be used in a multiple antenna application.

Overview

Referring to Figures 1 to 3, Figure 1 is an exploded view of an antenna 100, Figure 2 comprises plan and various cross-sectional views of the antenna 100 and Figure 3 comprises plan views of the top and bottom of various component of the antenna. As seen in Figures 1 and 2, the antenna 100 is a dual polarization patch antenna suitable for multiple-input multiple-output (MIMO) applications. The antenna 100 comprises a plurality of planar component layers 102, 104, 106 and a chassis or housing portion 108.

Each component layer 102, 104, 106 is generally square and comprises a respective substrate formed of a suitable electrically insulating material (e.g. a composite material, such as a glass woven epoxy used for conventional printed circuit boards (PCBs) or the like, for example an FR4 grade of material or similar). When the antenna is assembled, the component layers 102, 104, 106 are each arranged in axial alignment with one another along a respective central axes X-X’ (orthogonal to its respective major plane) with their major planes generally parallel to one another.

The substrate of a first of the component layers is coated, on one of its planar surfaces (an upper surface in the orientation of Figure 1), with an appropriate conductive material (e.g. copper, gold or silver) to form a ground plane 102 for the antenna (e.g. as seen in Figure 3(a), TOP view). The opposite surface (the lower surface in the orientation of Figure 1) effectively provides a feed layer where appropriate matching and other circuitry may be fabricated.

The substrate of a second of the component layers forms a spacer 104 for providing sufficient dielectric material, in addition to the thickness of the layer 106 (which is constrained by the maximum PCB thickness) to achieve the required distance required between a main patch fabricated on layer 106 and the ground plane 102. The substrate of a third of the component layers 106 has a first annular antenna metallisation 110 fabricated on one of its planar surfaces (referred to as an antenna surface), from an appropriate conductive material (e.g. copper, gold or silver), to form an antenna plane 106 (e.g. as seen in Figure 3(c), TOP view). This first annular antenna metallisation 110 effectively forms an active antenna element (i.e. the main patch).

The antenna 100 has a pair of connector pins 112 for providing respective external connections to the first annular antenna metallisation 110. To provide the means by which the connector pins 112 can be installed to make the connection, the ground plane 102, the spacer 104 and the antenna plane 106 are each provided with a respective pair of feed pin apertures 112-T, 112-2’, 112-3’ that extend through the corresponding component layer 102, 104, 106. The feed pin apertures 112-T, 112-2’, 112-3’ on a given component layer 102, 104, 106 are located on respective axes (p- p’, q-q’ in Figure 3(c), top) through the centre of (and parallel to the plane of) that component layer 102, 104, 106 orthogonally relative to one another.

The feed pin apertures 112-T, 112-2’, 112-3’ are arranged such that, when the antenna 100 is assembled, the pair of feed pin apertures 112-T, 112-2’, 112-3’ in one component layer 102, 104, 106 aligns with the respective pair of feed pin apertures 112-T, 112-2’, 112-3’ in each other component layer 102, 104, 106. The feed pin apertures 112-T, 112-2’, 112-3’ are configured in such a way that each connector pin 112 can be arranged to extend through a respective set of apertures 112-T, 112-2’, 112-3’, from the antenna plane 106, to and through (and insulated from) the ground plane 102, to provide a respective external connection to the first annular antenna metallisation 110.

The chassis portion 108 has a generally planar and square top section (the upper surface in the orientation of Figure 1) and four side walls arranged to form a surround for housing the antenna layer 106 and spacer 104. Each side wall is respectively connected along one (proximal) edge to a different side edge of the top section and extends, generally orthogonally away from the top section, to form a generally cuboid external shape. Whilst each of the side walls has a generally flat external surface, the side walls have profiled inner surfaces that, together, define an internal cavity for housing other components of the antenna.

The top section of the chassis portion 108 forms a substrate that has a second annular antenna metallisation 114 fabricated on a surface thereof (referred to as the chassis antenna surface) that is external to the antenna 100 when assembled. The annular antenna metallisation 114 on the chassis portion 108 is formed of an appropriate conductive material (e.g. copper, gold or silver). This second annular antenna metallisation 110 effectively forms a passive antenna element.

Each side wall of the chassis portion 108 comprises a plurality of metallic fingers 116 or strips that extend orthogonally from the proximal edge, adjacent the top section of the chassis portion 108, to a respective distal edge (opposite the proximal edge) of that side wall (i.e. the full height of the chassis portion in the orientation of Figure 1). The metallic fingers 116’ are periodically spaced (e.g. at equal distances) along each sidewall to form a periodic metal structure or‘cage’ 116 around the perimeter of the chassis portion 108. The width and spacing of the fingers 116’ may be optimised through simulations. Typically, for example, a beneficial spacing between fingers is in the order of 1/40 of the wavelength ( A ) (e.g. between 1/35 and 1/45 of the wavelength) at the centre frequency at which the antenna operates, while the width of the finger is typically 1/60 to 1/50 of the centre wavelength.

The chassis portion 108 together with its metallic finger cage 116 may, beneficially, be manufactured using relatively simple manufacturing techniques. For example, the chassis portion 108 together with its metallic finger cage 116 may be fabricated using a 3D printed or vacuum forming plastic with selective metallisation. The selective metallisation achieved through a dedicated metallisation process or simply with a metallised adhesive stuck onto the side walls of the chassis portion 108.

Moreover, the external metallic cage 116 and the passive antenna element 114 on the top section may be laser etch printed on plastic to make the full assembly even lighter.

When the antenna 100 is assembled, the component layers 102, 104, 106 are arranged in a layered configuration in which the spacer 104 is sandwiched between the coated surface of ground plane 102 and a surface of the antenna plane 106 opposite the antenna surface. The component layers 102, 104, 106 are also arranged with the feed pin apertures 112-1’, 112-2’, 112-3’ in each component layer 102, 104, 106 aligned with corresponding feed pin apertures 112-1’, 112-2’, 112-3’ in each other component layer 102, 104, 106. Each connector pin 112 is located in the antenna 100 such that it extends through a respective set of apertures 112-T, 112-2’, 112-3’, from the antenna plane 106 (and connected to the first annular antenna metallisation 110), to and through (and insulated from) the ground plane 102, to provide a respective external connection (or feed point) on the feed layer on layer 102.

When the antenna 100 is assembled, the chassis portion 108 is arranged in axial alignment along a central axis (orthogonal to its top section) with the central axes of the component layers (i.e. along axis X-X’ in Figure 1). The chassis portion 108 is also arranged with the distal edge of the side walls abutting the coated surface of the ground plane 102 (e.g. as seen in Figure 2(b), inset view D) to enclose the spacer 104 and antenna plane 106 in the internal cavity of the chassis portion 108. The metallic fingers 116’ of the metallic cage 116 connect, at the distal edge of a corresponding chassis portion 108 side wall, to form an electrical connection with the coated surface of the ground plane 102. Thus, the metallic finger 116’ extends orthogonally from the ground plane 102 to a side edge of an external surface of the top section of the chassis portion 108, generally adjacent the second annular antenna metallisation 114, to surround the antenna with the metallic finger cage 116.

The internal profiles of the side walls (and the size of the spacer 104 and antenna plane 106) are configured such that, when the antenna 100 is assembled, the internal cavity of the of the chassis portion 106 has a generally square portion into which the spacer 104 and antenna plane 106 fit relatively closely. The internal profiles of the side walls are also configured such that, when the antenna 100 is assembled, the internal cavity of the of the chassis portion 108 is of the same general shape as layer 106 and is located between the first annular antenna metallisation 110 and an internal surface of the top section of the chassis portion 108. This internal cavity is coaxially aligned along the central axis X-X’ of the antenna 100 and the annular antenna elements 110, 114.

The ground plane 102 is slightly larger in area than the top section of the chassis portion 108 such that the ground plane extends a small distance beyond each side wall of the chassis portion 108 to form a flange around a perimeter of the antenna 100 (at its base in the orientation of Figure 1).

The antenna 100 is provide with fixings 118, 120 and 122 for securing the chassis portion 108 to the ground plane 102 and for ensuring that the correct internal spacing is maintained. Whilst those skilled in the art will appreciate that any suitable fixings may be used, in the present example the fixings comprise four sets of fixings each comprising a male-female standoff component 118, a screw 120 and a nut 122. The standoff component 118 has a female portion configured to receive the screw 120 for securing the chassis portion 108 to the standoff component 118 and a male portion for extending through the ground plane and receiving a corresponding nut for securing the ground plane to the standoff component 118.

The top section of the chassis portion 108 has four holes, one in each corner, each hole being configured to receive a screw 120 of a respective set of fixings. The ground plane 102 also has four holes 118-1’, one in each corner, each hole being configured to receive a male portion of a standoff component 118 of a respective set of fixings. The corners of the spacer 104 and of the corners of the antenna plane 106 each comprise a respective cut-away portion 118-2’ such that the spacer 104 and antenna plane 106 fit snugly between the four standoff components 118, when the antenna 100 is assembled, with each cut-away portion 118-2’ extending partly around a perimeter of a respective standoff. The design of the dual polarization patch antenna 100 described with reference to Figure 1 to 3, and in particular the a discontinuous metallic finger caging around the antenna chassis 108, allows the relative size of the antenna to be reduced potentially beyond what currently achievable (without needing to use high constant dielectric substrates (e.g. having a relative permittivity, e _G , greater than 10) and/or the useable bandwidth broadened which is particularly beneficial for Ml MO applications.

More specifically, the pitch and width for the metallic finger cage 116, around the edges of the chassis 108 of the patch antenna 100 can, beneficially, be optimised to allow tuning the re-distribution of the RF currents flowing normally through the ground plane. This makes possible both a reduction of the coupling effect that can occur between ports, and an improvement of the overall efficiency. Moreover, fringing effects developed between the passive antenna element 114 and the cage fingers 116’ allow a matching network to be used, as described later, that achieves a very broad usable bandwidth and reduces electrically the antenna dimensions.

The use of the periodic metallic structure 116 with an appropriate matching circuit thus allows an electrically small patch antenna, with a dual feed, to simultaneously and efficiently generate orthogonal electromagnetic fields over a broad range of frequencies. This allows creating patch antennas with a significant efficiency (>80%) within an area smaller than (l/4) ² . The proposed antenna also allows a nominal bandwidth of 47% of the centre frequency to be achieved and is scalable in size and frequency.

By way of comparison a state of the art ceramic antenna, making use of a ground plane of the same dimension as the proposed antenna might use, is the TAOGLASS GP.1575.25.4. A.02 that achieves 35% over only a very narrow matched band.

Matching Networks

Figure 4 is a circuit diagram of a simplified matching network for the antenna 100. The matching network of Figure 4 comprises two substantially identical matching circuits, each comprising just three components, which are configured to increase the matching bandwidth of the antenna at each port.

Figure 5 is an S-parameter, frequency plot for the proposed antenna 100 (with a 50mm x 50mm ground plane) using the simplified three component based matching network of Figure 4. Figure 5 illustrates how a matched bandwidth from 1480MHz to 2330MHz (45% relative bandwidth) may be achieved. Figure 6a is a circuit diagram of another matching network that may be used with the antenna 100. Figures 6b and 6c are Smith charts illustrating a transformation

The circuit of Figure 6a comprises a first low pass filter 602 followed by a second low pass filter 604 bonded together by a quarter wave microstrip transformer 606. The first low pass filter 602 (closer to the antenna ports) provides a transformation that can be used to shift the impedance of the antenna from the region highlighted in Figure 6b into the region highlighted in Figure 6c.

The additional quarter wave transformer 606 achieves an impedance transformation from the higher impedance of the antenna to a nominal 50ohm required by the system. The track length, of the quarter wave transformer, is Ag/4 where Ag is the central wavelength corresponding to the centre frequency of the matching bandwidth to be achieved, calculated as per Ag = -^ (where e _G is the dielectric constant of the substrate used). The impedance of the track to achieve the required transformation is calculated as:

ZT=(Z _L *ZS) ^A 0.5

Where Z _s is the average impedance achieved inside the highlighted area on the Smith chart of Figure 6c and Z _L is the required system impedance (50ohm). The second low pass filter network 604 allows further optimisation and improvement of matching.

Figure 7 is a s-parameter, frequency plot for the proposed antenna 100 (but this time with a 40mm x 40mm ground plane) using the matching network of Figure 6. Figure 7 illustrates how a matched bandwidth from 1620MHz to 2930MHz (57% of relative bandwidth) may be achieved.

Figure 8 is a realised gain vs frequency plot, for the example of Figures 6 and 7. As seen in the use of the antenna 100 in this manner provides good gain at zenith and at low scanning angles across the band of use.

Scalability

Figure 9 is a plot of Right Hand Circular Polarisation (RHCP) efficiency for a variety of antenna sizes. As seen in Figure 9 good efficiency achieved reliably for a wide range of frequencies for each patch size.

Applications The proposed dual feed antenna 100 is particularly well suited for satellite communications where circular polarization is required.

For example, Figure 10 is a simplified circuit diagram illustrating how the proposed antenna 100 can be combined with a broadband hybrid power divider that provides a 90° shift between the ports to generate circular polarization. The antenna can, for example, be easily paired to an integrated coupler such as the Anaren 1 E0320-3 to achieve efficiently broadband right angle polarization across the full band as shown in Figure 11 which is a plot of Right Hand Circular Polarisation (RHCP) efficiency for the example of Figure 10.

Another possible application is for broadband (beam forming) phased arrays. Regular arrays with broad scanning angles require the element pitch to be smaller than 1/(1+sin0)*A to avoid the generation of undesired grating lobes (where Q scanning angle). With a typical size of l/2*l/2 dual polarization patch antennas working down to the frequency of 1.47GHz (as in this case) can only be spaced at 100mm. Such a pitch would limit the scanning angle to only 21 ° at top end of the frequency band. Moreover, the optimised gain across frequency for low scanning angles make the proposed antenna particularly suited for this type of application.

In this example, to allow digital electronic beam forming the antenna 100 could be integrated with fully digital flexible radios with each radio connected to an individual antenna port as shown in Figure 12 which is a simplified circuit diagram illustrating the use of the antenna with broadband radio transceivers.

Other applications of the antenna 100 include, for example, automotive (roof-top) for multi-standard applications, like LTE, Wfi, GPS, GNSS; satellite applications with extension to assisted-GPS and hybrid Earth and Satellite Connectivity (covering most of LTE terrestrial bands and L and S satellite bands); and aircraft applications (enabled in particular by the low antenna profile); and MIMO cellular base stations antennas.

Multiple Antenna Applications

Figure 13 is a simplified drawing illustrating how the antenna 100 of Figures 1 to 3 can be used beneficially, in a multiple antenna application.

As seen in Figure 13, the antennas 100 can be tiled together in an array structure in which the external metallic finger cage 116 is used as a support element. The additional advantage given by the metallic finger cage 116 for this type of application is the isolation generated to transversal modes of propagation between antenna elements reducing significantly the mutual coupling between them and therefore the ability to steer the beam generated without affecting too much the port impedance and the efficiency for different scanning angles.

Modifications and Alternatives

Detailed embodiments have been described above. As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. It will be appreciated, for example, that the antenna could be combined with a different tuneable matching network, to extend the usability of the antenna beyond the 45% relative bandwidth described.

Moreover, to reduce material and overall weight, the plastic support for the finger cage could be manufactured using vacuum forming to provide a very light and cost- effective, structure. This would be particularly beneficial for applications in which weight is important, such as for aircraft, satellites or the like. Moreover, the support for the top antenna element 114 may be omitted and the top antenna element 114 can be fabricated using a thin film PCB covering the whole area. Similarly, to reduce material and weight, the spacer may be omitted, albeit with a potential reduction of achievable relative bandwidth due to the dielectric loading of the main active patch 110.

It may also be possible to use of metamaterial high impedance ground plane to further increase the matching bandwidth with relatively simple matching networks.

Various other modifications will be apparent to those skilled in the art and will not be described in further detail here.