Antenna element

Abstract

Antenna element 10, comprising: a radiator 20 on a substrate 14 (14a-14b e.g a ceramic substrate) and feed arrangement 21 comprising one or more feed (probe, coupling) elements 24 arranged on a surfac

Description

Technical field This invention relates to the field of antennas. In particular, but not exclusively, the invention relates to antenna elements and antennas configured for broadband performance. Background Various satellite systems are in use for navigational purposes, including the global navigation satellite system (GNSS), which provides global coverage and includes the global positioning system (GPS) and Galileo, among others, and also regional navigation satellite systems (RNSS). Global and regional systems may be referred to collectively as ‘G(R)NSS’ systems. Each system operates at one or more specific frequencies or frequency ranges, which may be referred to as ‘band allocations’, which in turn typically fall within the ‘L band’ that spans between 1GHz and 2GHz. For example, GPS includes band allocations such as the ‘LT frequency, at 1575.42MHz, and the ‘L2’ frequency, at 1227.60MHz. G(R)NSS signals are also typically right-hand circularly polarized. More generally, such systems operate at radio frequency (RF). Antennas used to transmit and receive signals for navigational systems may be configured for broadband performance, for example to cover a majority, or the entirety, of the L band, to enable the antenna to be used with a range of different systems and band allocations. It may be desirable to minimise the complexity, cost and / or size of a G(R)NSS antenna, for example to meet defined format standards, and to account for stringent operational and volumetric constraints of platforms on which the antenna may be employed. In this respect, such antennas are used in a range of applications, including on manned or unmanned vehicles of all types, as well as other portable applications. In these contexts, the packaging space available to accommodate an antenna is often limited. The antenna may also need to conform to, or otherwise minimise disruption to, existing exterior 1 surfaces. It is therefore desirable forG(R)NSS antennas to be compact, for example by being low profile and / or having a small footprint. However, reducing the size of an antenna typically impacts its performance, and so may hinder the ability of the antenna to provide multiband and / or multisystem coverage. As G(R)NSS signals are typically relatively weak, antennas for navigational systems may incorporate an array of antenna elements providing multiple received signals, which can be processed to remove interference and thereby maintain a link with a satellite, particularly in hostile or electromagnetically hazardous environments. An array of antenna elements may form a controlled reception pattern array (CRPA), for example. In such arrangements, compact elements may be particularly desirable. One type of antenna that is known for use with G(R)NSS systems is a microstrip patch antenna. Such antennas typically include a flat sheet, or ‘patch’, of conductive material such as metal, which is mounted over a conductive ground plane, which is again typically of metal. The patch is separated from the ground plane by a substrate layer that extends between the two conductive layers. The substrate layer may be of a similar type to those used in printed circuit boards (PCBs), for example. A basic microstrip patch antenna element may comprise a substrate layer with a metallised upper surface defining the patch and a metallised lower surface defining the ground plane to form a patch antenna element. Conveniently, such elements may be fabricated using processes similar to those used for PCB manufacture, and so may be referred to as ‘printed patches’. Printed patches may be probe fed or aperture fed, in which case compact high permittivity dielectrics may be used for the substrate to promote a compact configuration. However, this may lead to an undesirably large Q factor and limit the bandwidth achieved. In consequence, this approach may only succeed in addressing one or two sub-bands of a single system and therefore fail to provide full multiband, multisystem coverage. Stacked patch elements may be used to provide multi-band performance, allowing for the use of substrates of various dielectric properties. Such antennas may include two substrate layers stacked one on top of the other and fixed to one another and to a ground plane, each substrate carrying a patch that is tuned to resonate at a respective frequency. However, when a broadband respective coverage is achieved this entails 2 substrates of lower permittivity, and such elements tend to have a relatively large footprint and a tall profile, which can reduce their suitability for some applications, either as standalone elements or within an array. An alternative to a printed patch is a dielectric resonator, which is typically defined by a piece of dielectric material that itself acts as a resonator. Dielectric resonators can be configured for broadband performance with a relatively small footprint, but this typically requires multiple dielectric modes to be excited, leading to a tall profile that may be undesirable in some applications. Dielectric resonators may also be more prone to mutual coupling when placed within an array. In addition, if probes are used for excitation, the size or shaping of the probes may also contribute to an increased footprint and / or profile. Another option is a quadrifilar helical antenna, which is also seldom configurable as low profile. It is against this background that the present invention has been devised. Summary of the invention An aspect of the invention provides an antenna element, comprising: a substrate; a radiator arranged on the substrate; and a feed arrangement comprising at least one feed element arranged on a surface of the substrate. The surface of the substrate on which the feed element is arranged is suitably an external surface of the substrate. Arranging one or more feed elements on a surface of a substrate of an antenna element may allow the, or each, feed element to be configured to influence the performance of the antenna element, for example to contribute to impedance matching. This, in turn, may promote compactness of the antenna element, for example to enable the antenna element to have a low profile and / or a small footprint, while achieving broadband performance characteristics. This may be particularly advantageous in applications such as navigation systems, for example. Arranging the, or each, feed element on the surface of the substrate may also ease manufacture of the antenna element. The feed arrangement may be configured to interact electrically with the radiator, to exchange signals with the radiator. The feed arrangement may therefore be regarded as a coupling arrangement, and similarly the feed element may be regarded as a coupling element. The feed arrangement may be configured to interact with the radiator through capacitive coupling, for example capacitive coupling between the, or each, feed element and the radiator. The substrate may comprise a first surface, a second surface, and at least one side face extending between the first and second surfaces. In such embodiments, the feed element may be arranged on the side face of the substrate. The radiator may be arranged on the first surface. The second surface may define a mounting surface for mounting the element to a support structure and / or a ground plane. The, or each, side face may be orthogonal to the first surface. The first and second surfaces may be parallel. For example, the substrate may be block-like or board-like in shape. The first surface may be curved or planar, the second surface may be curved or planar and the or each side face may be curved or planar. Alternatively, the radiator and the feed element may be arranged on a common surface of the substrate, which surface may be curved or planar. The substrate may be spherical, hemispherical, dome-shaped or otherwise curved, for example. The feed element may be arranged in a side region of the substrate, for example close to a peripheral edge of the substrate, and / or outwardly of the radiator. The feed element may be formed on the surface of the substrate, for example using a printing process, an etching process, a sputtering process, and / or a chemically-based forming process. It is also possible for the feed element to be formed separately and then attached to the surface of the substrate, for example using adhesive. The feed element may be planar, in that the feed element is thin and has planar major surfaces. The feed element may therefore be substantially coplanar with the surface of the substrate on which the element is arranged. The feed arrangement optionally comprises multiple feed elements. In such embodiments, two or more feed elements may be arranged on respective surfaces of 4 the substrate. If the substrate has side faces, multiple side faces may include respective feed elements. In other embodiments, the feed arrangement may comprise a single feed element, which element may optionally substantially define the feed arrangement. The feed arrangement may be configured for circularly polarised signals. The antenna element may comprise an axis of rotational symmetry, around which the element has multiple degrees of symmetry. The axis of rotational symmetry may intersect the radiator, and may be orthogonal to a plane in which the radiator extends. The feed element may be shaped to increase inductance within the feed element relative to a corresponding straight feed element. A corresponding straight feed element may mean an element having ends at similar positions to the feed element. In other words, the feed element may not follow a straight path between its ends, which would minimise the length of the feed element, but may instead follow a longer path. The feed element may comprise an elongate track. The track may comprise at least one bend. The track may comprise bends in opposed directions, and may comprise a series of bends in alternating directions. Portions of the feed element on each side of the, or each, bend may each intersect an axis that is orthogonal to a surface of the substrate on which the radiator is arranged. The track may have a uniform width along its length. The feed element may have an undulating, zigzag or wavy shape. The shape of the feed element may be periodic or aperiodic. The feed element may be shaped to create capacitance between portions of the feed element, and may be shaped to create capacitance between at least two pairs of portions of the feed element, which may provide distributed capacitance. For example, if the feed element includes bends, capacitance may be created between portions of the element on each side of a bend, and optionally between portions on each side of multiple bends. The feed arrangement may comprise a conductive element arranged on the surface of the substrate, adjacent to the feed element. The conductive element may be separate from both the feed element and the radiator. The conductive element may be configured 5 to interact with the feed element through capacitive coupling. The conductive element may define a parasitic element. The conductive element may be configured to connect to a ground plane. The feed arrangement may be configured to contribute to impedance matching. For example, the or each feed element of the feed arrangement may be shaped to contribute to impedance matching, for example by shaping the feed element for increased impedance and / or to incorporate capacitance. The feed element may be spaced from the radiator. The feed element may be spaced from an edge of a surface of the substrate on which the radiator is arranged. The antenna element may comprise a terminal for connecting to a port, in which case the feed element may comprise, or be connected to, the terminal. The feed element may act to transfer signals between the radiator and the terminal, in use. The radiator may comprise one or more slots. Each slot may be aligned with a respective feed element. The or each slot may advantageously cooperate with a corresponding feed element to enhance performance of the antenna element. The substrate may comprise a dielectric. The substrate may comprise a ceramic material. The antenna element may be configured to excite dielectric resonator modes. The antenna element may be configured to excite a patch mode. The substrate may comprise multiple layers, in which case each layer may be of a different material. The radiator may comprise a patch radiator. The radiator may comprise a conductive element, and may be of metal for example. Similarly, the feed element may be of conductive material such as a metal. The antenna element may have the form of a microstrip patch antenna element. The antenna element may comprise a single radiator. The radiator may be arranged centrally on the substrate. The feed element may at least partly define a feed probe. The invention also extends to an antenna assembly comprising the antenna element of the above aspect. The antenna element may be mounted to a support structure. The support structure may comprise a ground plane. The antenna assembly may comprise a port, and the feed element may be connected to the port. For example, a terminal of the antenna element may connect to the port, the terminal being connected to, or integral with, the feed element. The antenna assembly may comprise an array of antenna elements. The array may be configured as a controlled reception pattern array. Another aspect of the invention provides a communication system comprising the antenna element or the antenna assembly of the above aspects. The communication system may form part of a navigation system. The invention also extends to a navigation system comprising the antenna element, the antenna assembly or the communication system of the above aspects. Another aspect of the invention provides a method of fabricating an antenna element, the method comprising arranging a radiator on a substrate, and arranging at least one feed element on a surface of the substrate. The method may comprise forming the feed element on the surface of the substrate, or attaching the feed element to the surface of the substrate. The feed element may form part of a feed arrangement. It will be appreciated that preferred and / or optional features of each aspect of the invention may be incorporated alone or in appropriate combination in the other aspects of the invention also. Brief description of the drawings One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which like features are assigned like numerals, and in which: Figure 1 shows an antenna element according to an embodiment of the invention in isometric view; Figure 2 shows a cross-sectional view of the element of Figure 1; Figure 3 is a schematic diagram of an antenna system including the element of Figure 1; Figure 4 is a plot of gain for the element of Figure 1; Figure 5 is a cut radiation pattern for the element of Figure 1 for a range of frequencies; Figure 6 shows an antenna element according to another embodiment of the invention in isometric view; Figure 7 shows an antenna element according to another embodiment of the invention in isometric view; Figure 8 shows an array formed from elements as shown in Figure 7; and Figures 9 to 13 show side views of respective alternative feed arrangements for the antenna element of Figure 7. Detailed description In general terms, embodiments of the invention provide antenna elements, and antenna arrangements including such elements. In some embodiments, an antenna element is configured to provide broadband performance such that the element is compatible with a wide range of systems, for example navigational systems, and associated band allocations. In this respect, a broadband performance may entail that the antenna element provides a non-zero gain ora minimum gain, and / or a generally consistent gain, over a wide band of frequencies, which may encompass at least half of the L band for the example of an antenna element for a navigational system, for example. In some embodiments, the antenna element supports all bands for GNSS and / or RNSS applications. If the element is used in an antenna array, such as a controlled reception pattern array (CRPA), it may be configured to enable a combined gain performance for the array that conforms to relevant standards, for example DO-228, DO-301 or EUROCAE GNSS MOPS. Antenna elements according to the invention may be used in various applications, including on moving platforms such as aircraft, waterborne vessels and land vehicles, for example. In some embodiments, the antenna element is compact and low profile, for example having a height of less than 15mm, and so is apt for use in space-constrained applications. Such applications may include unmanned aircraft systems (UAS), for example. The antenna element may also be relatively low cost to manufacture. The antenna element may include a feed arrangement that contributes to enabling the element to achieve a performance that would ordinarily require a physically larger element. Figures 1 and 2, which are now referred to collectively, show an example of an antenna element 10 according to an embodiment of the invention mounted to a metallic ground plane 12. In practice, the element 10 may alternatively be configured to be mounted within a cavity of a device, system or platform in which the element 10 is to be used, and may have a degree of conformance with the cavity and surrounding surface. It is also possible for an element to be configured as freestanding and with an individual ground plane. The antenna element 10 comprises a substrate 14 having the general form of a thin, rigid, flat, square board having planar surfaces, and is therefore a rectangular cuboid. The substrate may have a different shape in other embodiments, however. In general terms, the substrate 14 may be similar to a substrate of a conventional PCB. The substrate 14 is formed from dielectric materials and so defines a dielectric body. In this example, one or more ceramics with a controlled dielectric constant are used for the 9 substrate 14, the ceramics having a moderate or high value of permittivity, for example in the range of 6 to 12 F / m, and potentially 20 F / m or higher, although the precise value will be chosen according to the specific requirements of the application. Using a material with a high dielectric permittivity for the substrate 14 supports miniaturisation of the antenna element 10. In this respect, the size of the substrate 14 may be determined according to the frequencies that the element 10 is configured to operate at. In this example, the substrate 14 is approximately 35mm in length and width, although these dimensions may vary in practice. The substrate 14 comprises two layers, namely an upper layer 14a and a lower layer 14b, and so has a composite structure in this example. Optionally, the layers may be of different materials, which may enable the substrate 14 as a whole to exhibit dielectric properties that are not achievable with a single material. A lower surface of the substrate 14, in the orientation shown in the figures, defines a mounting surface 16 of the substrate 14. The mounting surface 16 engages, and is in direct contact with, the ground plane 12. In this respect, there is no metallisation on the mounting surface 16 in this embodiment, although in other embodiments metallisation may be applied, for example to enhance electrical contact to the ground plane 12. An upper surface of the substrate 14, in the orientation shown in the figures, defines a radiator surface 18 that supports a radiator 20, the radiator surface 18 being parallel to the mounting surface 16 and therefore also to the ground plane 12. The radiator 20 is arranged centrally on the substrate 14 in this example. The substrate 14 also has a set of four side faces 22 that extend between the mounting surface 16 and the radiator surface 18. Adjacent side faces 22 are orthogonal to one another, and each side face 22 is orthogonal to the radiator surface 18 and to the mounting surface 16. Each side face 22 shares one edge with the radiator surface 18, and shares another edge with the mounting surface 16. The remaining edges of the side faces 22 are much shorter, such that the side faces 22 are oblong. In this example, the side faces 22 are less than half as deep as they are wide. The side faces 22 are substantially identical to one another in this example. In this example, the radiator 20 comprises, and is substantially defined by, a layer of conductive material formed on, and covering a portion of, the radiator surface 18 of the substrate 14, to form a patch. The radiator 20 may therefore be regarded as a patch radiator. The radiator 20 may be of a metal such as copper, for example, and may be formed or otherwise arranged on the substrate 14 using conventional techniques, which may include etching, sputtering, adhesive attachment and / or chemically-based metallisation, for example. The radiator 20 is therefore very thin and so may be regarded as a flat plate element that predominantly extends in a plane parallel to the radiator surface 18 of the substrate 14. The radiator 20 is also electrically thin, in that it has a small thickness relative to the wavelengths in air that it is configured to handle. It is possible for the antenna element and the radiator to be slightly curved in other embodiments, in which case the curvature may be determined to maintain a small depth relative to the length and width of the plate. Curved elements may be used for conformance with another surface, for example. In this embodiment, the length and width of the radiator 20 is such that the radiator 20 is substantially square, although different shapes may be used in other embodiments. The radiator 20 therefore has the same shape as the radiator surface 18 of the substrate 14, and is slightly smaller than the radiator surface 18. The size of the radiator 20 is such that it covers a majority of the underlying radiator surface 18 of the substrate 14, leaving only a small portion of the radiator surface 18 extending beyond the edges of the radiator 20, to define a thin border around the perimeter of the radiator 20, the border being of uniform width in this example. The radiator 20 may completely cover the radiator surface 18 in other embodiments, however. The dimensions of the radiator 20, specifically its length and width, correspond to approximately half the wavelength of the signals that the radiator 20 is configured to handle when inside the dielectric material of the substrate 14, so that the radiator 20 is configured as a half-wave radiator in this example. In other embodiments, elements may be configured with radiators that are smaller, and may be configured as quarter-wave radiators for example. Conversely, the elements may be configured with radiators that are larger than the half-wave radiators of the present example. The radiator 20 has a patch face defined by an uninterrupted, generally planar and square surface facing outwardly from the substrate 14. 11 The antenna element 10 therefore has the general form of a microstrip patch antenna element, albeit with a single metallisation layer and so representing a generalised patch, but also incorporates operating modes of a dielectric resonator by virtue of the high permittivity substrate 14. The substrate 14 may therefore be regarded as a dielectric puck. The antenna element 10 also includes a feed arrangement 21 for feeding the radiator 20, in that the feed arrangement 21 interacts electrically with the radiator 20 to transmit and / or receive signals in use. The feed arrangement 21 may alternatively be referred to as a coupling arrangement, in that it acts to couple electrically to the radiator 20 to transfer electromagnetic energy to and / or from the radiator 20. In general terms, the feed arrangement 21 is configured to provide for excitation of the desired modes for broadband operation of the radiator 20, and also to act as a match enabler to contribute to impedance matching with respect to antenna ports and associated circuitry to which the element 10 connects. While it may be possible to implement matching by dedicated circuitry after the antenna ports, this may incur losses and increase the complexity of the feed network. In this context, the antenna element 10 beneficially mitigates these potential issues by incorporating matching into the element 10 itself. As shall become clear from the following description, the feed arrangement 21 also enables the size of the element 10 to be reduced for a given performance, relative to an equivalent element with a different feed arrangement. More specifically, the feed arrangement 21 comprises a set of feed elements, or coupling elements, in the form of feed probes 24 that are formed on the side faces 22 of the substrate 14. In the example shown in Figure 1, each side face 22 carries a respective single feed probe 24, so that the element 10 has four feed probes 24 in total that collectively define the feed arrangement 21. The feed probes 24 are similar to one another in this embodiment, such that the element 10 has four degrees of rotational symmetry about an axis intersecting the centre of the radiator 20. Each feed probe 24 comprises a continuous track of conductive material that is arranged directly on the respective side face 22. The feed probes 24 may be of a metal such as copper, for example, and in this example are of the same material as the radiator 20, although this may vary. In this example the feed probes 24 are formed on the side faces 12 22 by a suitable process, for example any of the processes noted above for forming the radiator 20 on the radiator surface 18, including etching, sputtering, and / or chemically-based metallisation. The feed probes 24 could be arranged on the side faces 22 in other ways, for example by adhesive attachment. The feed probes 24 are planar elements in this example, and are generally coplanar with the side faces 22 on which they are arranged. Forming the feed probes 24 by a standard process, and / or using the same process as for the radiator 20, reduces the complexity involved in fabricating the element 10. Various other ways to form or arrange the feed probes 24 are possible, however. A lower end of each feed probe 24 defines a contactor, or terminal 26, that is positioned at a midpoint of an edge of the associated side face 22 that is shared with the mounting surface 16. The terminal 26 is therefore integral with the feed probe 24 in this example, but in other examples a separate terminal may be provided. As Figure 2 shows best, the terminal 26 of each feed probe 24 connects to a respective element port 28, which in this example is defined by a coaxial connector 30. More specifically, an inner conductor of the coaxial connector 30 extends through the ground plane 12 and connects to the terminal 26, for example by a soldered joint. Electrical signals are transferred to and / or from the feed probes 24 through the associated element port 28 in a manner that supports a polarisation forming interface, as described in more detail later with reference to Figure 3, such that the element ports 28 provide interfaces between the element 10 and a wider antenna system. It is noted, however, that various other connector options are possible for the element ports 28 instead of coaxial cables, including a printed microstrip ora stripline arrangement arranged on the underside of the ground plane 12, for example. Each feed probe 24 extends generally upwardly, in the orientation shown in Figure 1, from its terminal 26 towards the radiator 20. Each feed probe 24 has an upper end, at which the track defining the probe 24 terminates. The width of the track defining the feed probe 24 is substantially uniform between the lower and upper ends, although in other embodiments the width of a feed probe may vary along its length. The feed probes 24 do not extend all of the way to the radiator surface 18. Instead, the upper end of each feed probe 24 is spaced from the edge of the respective side face 22 that is shared with the radiator surface 18, and therefore below a plane of the underside of the radiator 20. So, the feed probes 24 do not connect directly to the radiator 20, but 13 are instead spaced from the radiator 20 and configured to feed the radiator 20 by proximity excitation through capacitive coupling, the spacing between the upper ends of the probes 24 and the radiator 20 being determined on this basis. More generally, the feed probes 24 may also be regarded as being ohmically discontinuous between the radiator 20 and the associated element ports 28, the discontinuity creating capacitive coupling. So, in an alternative, a discontinuity may be created along the length of a feed probe that connects to both the radiator 20 and to the associated element port 28. Such an arrangement may be regarded as a feed probe comprising two mutually spaced feed elements, one of which is connected to, and optionally integral with, the radiator. Alternatively, such an arrangement may be regarded as a pair of capacitively coupled feed probes, one connected to the radiator 20 and the other to the element port 28. The feed probes 24 do not follow straight paths between their respective lower and upper ends. Instead, in this example, the feed probes 24 are shaped to define a succession of bends in alternating directions, so that each probe 24 follows a regular periodic, zigzag path from the associated element port 28 towards the radiator 20. In this respect, Figure 1 shows an illustrative probe axis 32 that extends upwardly from the terminal 26 of one of the feed probes 24, orthogonally to the ground plane 12, towards the radiator 20. The associated feed probe 24, due to its shaping, crosses the probe axis 32 several times between its upper and lower ends. Shaping the feed probes 24 so that they are not straight, and instead have a zigzag shape defining a series of bends in this example, extends the length of the feed probes 24 relative to equivalent straight probes occupying the same space on the side face 22 and having upper and lower ends at similar positions. In turn, lengthening the feed probes 24 increases their inductance. Portions of the feed probe 24 between each successive pair of bends may be regarded as defining arms of the feed probe 24. In the example shown in Figure 1 the feed probe 24 has ten arms, but this is purely illustrative. Each arm crosses the probe axis 32, and so the arms overlap in the direction of the probe axis 32. This overlap arising from the zigzag path of the probe 24 creates mutual inductance between the individual arms of the feed probe 24. In this respect, the arms may be regarded as inductors in close 14 proximity. The mutual inductance acts to fine tune the equivalent impedance that the feed probe 24 presents, and so tunes the overall inductance of the probe 24 to match the capacitance of the radiator 20 as may be desired. The zigzag shaping of the feed probe 24 also creates mutual capacitance, in that the overlapping arms create residual inter-arm capacitances that act in parallel to the incremental zigzag inductance. Moreover, as capacitance arises between each adjacent pair of arms, the feed probe 24 incorporates distributed capacitance. The inductance and capacitance within each feed probe 24, in turn, contributes to impedance matching throughout the frequency band of interest for the associated element port 28. In this respect, the distributed capacitance within the feed probe 24 enables virtual distributed parallel capacitive loading of that distributed capacitance, due to proximity effects of adjacent arms of the probe 24. Incorporating capacitance and added inductance into the feed probe 24 also enables the depth of the substrate 14 to be reduced and the probes 24 to be shortened, relative to a similar element having equivalent performance with straight feed probes. It is noted that the feed probes 24 are not configured to act as radiators in themselves, but are instead configured to cooperate with the radiator 20 to excite integrated printed patch and dielectric resonator modes, whilst also providing a matching function. Although the feed probes 24 may themselves radiate to some extent, this radiation is predominantly directed internally and so contributes to enabling the desired operating modes. Figure 3 shows, in simplified schematic form, a configuration of an antenna system 34 including the antenna element 10. This reveals that the element ports 28 provide an interface between a polarisation network 36 of the system 34 and the feed probes 24 of the antenna element 10. The polarisation network 36 is also connected to antenna system ports 38, which in turn connect to an RF receiver 42, in this example. In this example, the system 34 is configured for receiving only, but in other examples systems may be alternatively, or additionally, configured for transmission. In general terms, the antenna system 34 is configured to generate right-hand circular polarisation in the element 10, as may be used in navigation systems. This is facilitated by the presence of multiple feed probes 24 on the element 10. In this respect, a symmetrical simple patch element with a single feed probe may only be capable of linear polarisation, or elliptical polarisation with high ellipticity. In this example, right-handed circular polarisation is implemented through composite excitation of two crossed linear polarised signals, which are supplied to the receiver 42 through the antenna system ports 38 and then fed by the polarisation network 36 to the element ports 28. More specifically, the polarisation network 36 feeds the individual element ports 28 with orthogonally polarised signals with a phase offset sufficient to generate said right-hand polarised reception. The polarisation network 36 may also include circuitry configured to adjust the impedance of the element ports 28 to a level required by front end RF electronics connected to the antenna system 34, such as the receiver 42, as well as resistive termination to dump unwanted polarisation reception. Figure 4 shows a plot of gain for the element 10, with all of the feed probes 24 fed, over a range of frequencies extending from 1.16GHz to 1.64GHz, which covers a noticeable part of the L band and therefore many G(R)NSS band allocations. Although the gain falls at frequencies above 1.64 GHz, the element 10 nonetheless covers a sufficient portion of the L band to be regarded as broadband in performance. Indeed, it may be desirable for the element 10 not to cover certain portions of the L band, for example to avoid out-of-band interference vulnerability of a GNSS terminal that may arise from the increased sensitivity of a wider bandwidth, and to avoid covering bands intended for other service transmissions. In other embodiments, however, the element may be configured to cover the entire L band. A series of common band allocations are shown as dots on the plot. These include a first band allocation at 1164.45MHz, which corresponds to IRNSS / NAVIC L5. At this frequency, the plot indicates a gain of approximately 1 5dBi. The gain increases as the frequency rises, reaching 2dBi at approximately 1190MHz and then remaining above 2dBi up to approximately 1610MHz, which corresponds to the G1 GLONASS band allocation. Accordingly, the element 10 provides a useful gain for all band allocations within this range, including the L1 and L2 GPS bands for example, which are indicated 16 as T and ‘2’ in Figure 4, which mark points close to the respective centres of the L1 and L2 bands. The gain achieved for all band allocations may be similar, for each band allocation, to the gain of a narrowband patch antenna of similar dimensions tuned to that band allocation. Figure 5 shows a set of radiation patterns for the element 10 for the range of band allocations covered in Figure 4. This shows that the element 10 has a very similar radiation pattern for each of the band allocations, and thus provides similar performance at each of the tested frequencies. This demonstrates the broadband capability of the element 10. To summarise, the element 10 provides adequate gain throughout a frequency range covering all band allocations of interest, whilst being compact and low profile. In this respect, the element 10 has a smaller footprint, namely the area of its mounting surface 16, than a typical equivalent dual-band patch antenna. The compact nature of the element 10 is in part enabled by the feed arrangement 21, by shaping the feed probes 24 to incorporate inductance, thereby enabling the depth of the substrate 14 to be reduced. In this particular example, the element 10 may be entirely compatible with ease to the entire ARINC743 standard, both in terms of footprint and height. More generally, the feed arrangement 21 mitigates fundamental limitations relating to the relationship between the volume of an antenna and its electrical performance, such as the Q factor and Chu-Fano’s limit, to enable the element 10 to provide acceptable performance in a compact package. Figure 6 shows another antenna element 110, which represents a variant of the element 10 shown in Figure 1. Like the element 10 of Figure 1, the element 110 of Figure 6 has a substrate 114 with upper and lower surfaces, in the orientation shown in Figure 6, connected by four side faces. The upper surface defines a radiator surface 118 that supports a radiator 20. The lower surface of the substrate 114 defines a mounting surface 116, which is mounted to a ground plane 12. The element 110 of Figure 6 also includes a feed arrangement 121. In this respect, each side face 122 includes a feed probe 124 that is formed on the side face 122 and connected to a respective element port 28, the feed probes 124 collectively defining the feed arrangement 121. The feed probes 124 are generally similar to those of the element 10 of Figure 1 and may be formed by similar techniques, although include additional arms in this variant. In the element 110 of Figure 6, the substrate 114 has a single layer and is formed from a single material, the substrate 114 therefore being unitary and formed as a single piece in this variant. The substrate 118 is also proportionately deeper than the substrate 14 of the element 10 of Figure 1, being more than half as deep as it is wide. The substrate 114 is nonetheless of a depth that the element 110 is low profile, for example below 15mm. Relatedly, the feed probes 124, whilst longer than those of the element 10 of Figure 1, terminate at upper ends that are approximately midway between the radiator surface 118 and the mounting surface 116 of the substrate 114. The element 110 of Figure 6 is otherwise similar to the element 10 of Figure 1, and can be used in the system 34 of Figure 3 in a similar manner. More generally, the variants shown in Figures 1 and 6 are not limiting, but illustrate ways in which the proportions and composition of the substrate may be varied to tune the performance of an element as may be desired. Figure 7 shows another antenna element 210, which represents a further variant of the element 10 shown in Figure 1. The element 210 of Figure 7 has a substrate 214 of similar dimensions to the substrate 14 of the element 10 of Figure 1, although the substrate 214 of the element 210 shown in Figure 7 is formed as a single piece from one material, and so has a single layer. In the orientation shown in Figure 7, the substrate 214 has upper and lower surfaces connected by four side faces. The lower surface defines a mounting surface 216, which is mounted to a ground plane 12. The upper surface of the substrate 214 defines a radiator surface 218 that supports a radiator 220, which in this example entirely covers the radiator surface 218. Each side face 222 of the substrate 214 includes a feed probe 224 that is formed on the side face 222, each feed probe 224 being connected to a respective element port 28. The feed probes 224 collectively define a feed arrangement 221, and are generally similar to the feed probes of the element 10 of Figure 1 and may be formed using similar techniques, although include fewer arms in this variant, those arms being larger than the arms of the feed probes shown in Figure 1, such that the probes 224 shown in Figure 7 are slightly wider than those of Figure 1. In the element 210 shown in Figure 7, the radiator 210 is provided with slots 50. More specifically, four slots are formed in the radiator 220, each slot 50 being defined by a generally square aperture in the radiator 220, through which the underlying substrate 214 is exposed. Each slot 50 is aligned with a respective feed probe 224, in that the slot 50 is positioned adjacent to, and slightly offset from, a point on an edge of the radiator 220 towards which the associated probe 224 extends, and that is the closest point on the edge to the upper end of the probe 224. The slots 50 are configured to cooperate with the feed probes 124 to enhance the performance of the element 210, and may enable the element 210 to achieve similar performance to a similar element of greater depth. The slots 50 may therefore enable the depth of the element 210 to be reduced. The slots 50 may also contribute to impedance matching. Accordingly, the slots 50 provide for tuning of the performance of the element 210. The element 210 of Figure 7 is otherwise similar to the element 10 of Figure 1, and can be used in the system 34 of Figure 3 in a similar manner. Correspondingly, slots could be added to the radiators of the elements of Figures 1 or 6 in a similar manner, and so Figure 7 is illustrative of the option of adding slots. The slots can also have different shapes and / or proportions to those shown in Figure 7, and may also be positioned differently. In addition, the number of slots is not necessarily equal to the number of feed probes as in the example of Figure 7, although in general terms each slot may act in conjunction with a nearby probe. Turning now to Figure 8, an array 60 is shown that is formed from four of the elements 210 shown in Figure 7. The elements 210 of the array 60 are configured to act together as a single antenna, which may be referred to as an ‘array antenna’. For example, the array 60 may define a controlled reception pattern array (CRPA), which may offer enhanced security of navigation, for example by acting as an anti-jam antenna, when operating in hostile or electromagnetically hazardous environments. The elements 210 are arranged in a two-by-two square array, the elements 210 being mutually spaced and oriented so that opposed side faces 222 of neighbouring elements 210 are mutually parallel. The elements 210 are mounted to a common ground plane 12 in this example, and so the patch faces of the respective radiators 220 of the elements 19 210 are substantially coplanar. The ground plane also acts as a support structure for the elements 210 in this example. As the individual elements 210 are compact, the overall array 60 is correspondingly compact. In this example, the size of the array 60 is such that it can fit into an area that is three inches (76.2mm) in length and width. The array 60 shown in Figure 8 is illustrative, and in practice antenna elements according to the invention may be used as building blocks for forming a variety of different arrays. For example, an array of two or more elements, including any of the example elements described in this specification, can be used to produce an antenna array. The elements and their respective radiators do not have to be arranged in a two-by-two square, and pattern arrangements may vary depending on the end application. The elements may not share a common ground plane as in the example of Figure 8, and may instead have individual ground planes, for example. A variety of support structures are possible for supporting the elements in an array. The feed probes of the elements described above all have a similar, regular periodic shape, albeit of different sizes. However, in other embodiments feed probes may have entirely different shapes while still providing the dual functionality of radiator excitation and contributing to impedance matching. In turn, the feed probes enable enhanced performance of the associated antenna element, for example to achieve satisfactory performance with a reduced profile. Figures 9 to 13 provide some examples of alternative feed arrangements, although these are not exhaustive and many other arrangements are also possible. In each example, a portion of a feed arrangement is shown on the side face of an antenna element, which for the purposes of this description is the element 210 of Figure 7, but could be a different element in practice. It should be appreciated that in each example the feed arrangement may include further features, for example similar feed probes, on the side faces of the substrate that are not visible. More specifically, Figure 9 shows a feed probe 324 formed on the side face 222 of the element 210 and connected to a port 28, the probe 324 having an aperiodic zigzag shape, in that the probe 324 has two bends at different angles creating three unevenly 20 spaced arms, such that first and second arms are closer together than the second and third arms. Overlap between the arms creates inductance within the probe 324 in a similar manner to the earlier examples. Figure 9 therefore illustrates one way in which the shape of a feed probe may vary. In other embodiments, feed probes may have entirely arbitrary shapes whilst still being shaped in a manner that creates inductance within the probe. Figures 10 to 13 show examples of feed arrangements that incorporate additional conductive elements alongside a feed probe to cooperate with the feed probe to provide altered behaviour. In this respect, in these examples one or more parasitic elements are added adjacent to the feed probe, the parasitic elements being configured to interact with the feed probe by capacitive coupling to alter the performance of the feed arrangement and so providing a means for further tuning of the performance of the antenna elements in which the feed probes are used. The parasitic elements may therefore be regarded as virtual circuit elements that provide additional degrees of freedom for broadband matching. In principle, adding one or more parasitic elements alongside a straight feed probe may achieve similar effects to using shaped feed probes, such as enabling the probe to be shortened and the profile of the associated element to be reduced. The additional elements, whether parasitic elements or otherwise, may be formed on the side face of the antenna element in a similar manner to the feed probe, and so may also be planar elements defined by a thin layer of conductive material. In Figure 10, a feed probe arrangement 70 representing part of an overall feed arrangement includes a feed probe 224 disposed between a pair of parasitic elements 72. Similarly to the earlier example, in the example shown in Figure 10 each side face 222 of the substrate 214 carries a respective feed probe arrangement 70, the feed probe arrangements 70 being similar to one another and collectively defining the feed arrangement. The feed probe 224 visible in Figure 10 is the same as that of the antenna element 210 of Figure 7 and is connected to a port 28 in a similar manner. Each parasitic element 72 is generally oblong but shaped with one or more curved recesses that each receive a corresponding bend of the feed probe 224. The parasitic elements 72 are a similar size to one another, having a depth that is slightly over half the depth of the feed probe 224 and a width that is slightly less than the width of the feed probe 224. The parasitic 21 elements 72 are entirely separate from the feed probe 224 and are not directly electrically connected to the feed probe 224 or to the port 28. Instead, the parasitic elements 72 are electrically connected to the ground plane 12 and so are shorted. Figure 11 shows a feed probe arrangement 74 representing a variant of the arrangement shown in Figure 10, in which the parasitic elements 172 are widened to reach the edges of the side face 222 of the element 210, so that the parasitic elements 172 substantially fill a lower portion of the side face 222. The antenna element 210 and feed probe arrangement 74 shown in Figure 11 are otherwise the same as in Figure 10. Figure 12 shows another approach, in which a feed probe arrangement 76 includes a feed probe 224 connected to a port 28 and a parasitic element 78 having a similar, complementary shape to the feed probe 224 extending beside the feed probe 224. In this respect, like the feed probe 224, the parasitic element 78 is elongate, being defined by a track of conductive material, and follows a path having a succession of alternating bends so that the parasitic element 78 has a wavy shape that is regular and periodic. The positions of the bends of the parasitic element 78 correspond to positions of bends of the feed probe 224, so that the parasitic element 78 weaves in and out of spaces between adjacent arms of the feed probe 224. The parasitic element 78 extends from a lower edge of the side face 222 of the antenna element 210 to an upper end that is adjacent to an uppermost bend of the feed probe 224. The parasitic element 78 therefore has a similar depth to, but is slightly shorter than, the feed probe 224. The parasitic element 78 is also substantially thinner than the feed probe 224, although like the feed probe 224 has a uniform width along its length. As for the parasitic elements of Figures 10 and 11, the parasitic element 78 shown in Figure 12 is shorted to the ground plane 12. Finally, Figure 13 shows a feed probe arrangement 80 representing a variant of the arrangement shown in Figure 12, in which the feed probe 224 and the antenna element 210 are generally unchanged, but a different parasitic element 82 is arranged beside the feed probe 224. The parasitic element 82 is shorter than that of Figure 12, and is shaped to extend into only one of the spaces between adjacent arms of the feed probe 224. The parasitic element 82 is also spaced from the lower edge of the side face 222 of the antenna element 210 in this variant and so is floating, in that the parasitic element 82 is 22 not directly electrically connected to any other part of the arrangement. A floating parasitic element may offer the benefit of adding a parasitically coupled parallel resonance circuitry, which may aid in broad banding. The arrangements shown in Figures 10 to 13 are examples only, and various other ways to incorporate parasitic elements into feed arrangements are possible. The parasitic elements may also be shaped differently to the examples provided in Figures 10 to 13. In general terms, the elements of a feed arrangement, including a feed probe and any additional elements, can be shaped, sized and positioned as required to tune the performance of the antenna element to meet the objectives of each application. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application. The substrate of the antenna element does not have to be cuboidal as in the above example. For example, the substrate may have any number of side faces, and so may have an octagonal cross section for example. The substrate may also be cylindrical and so have a single curved side face extending around the entire circumference of the element, in which case rectangular features of the embodiments described above may be transformed to circular geometry. The substrate may otherwise have one or more curved side faces, and a partly or wholly curved cross section. Various canonical shapes may be used for the substrate. It is also possible for the substrate to have no distinct side face at all, for example if the substrate is hemispherical, dome-shaped or otherwise curved. In such embodiments, feed probes may be formed on a surface of the substrate that also supports a radiator, in which case the radiator may reside in a central region of that surface while each feed probe is positioned outwardly of the radiator, in a side region, or edge region, of the substrate. More generally, antenna elements according to the invention may be configured for a range of applications, and not necessarily for GNSS or RNSS applications. Relatedly, while equipping antenna elements with multiple feed probes is useful for some applications, such as GNSS applications, in other embodiments an antenna element may have a feed arrangement that has a single feed probe. Such a feed arrangement 23 may also include other features such as a parasitic element, or the feed arrangement may be defined by the feed probe.

Claims

1. An antenna element, comprising:a substrate;a radiator arranged on the substrate; anda feed arrangement comprising at least one feed element arranged on a surface of the substrate.

2. The antenna element of claim 1, wherein the substrate comprises a first surface, a second surface, and at least one side face extending between the first and second surfaces.

3. The antenna element of claim 2, wherein the feed element is arranged on the side face of the substrate.

4. The antenna element of claim 3, wherein the radiator is arranged on the first surface.

5. The antenna element of any of claims 2 to 4, wherein the side face is orthogonal to the first surface.

6. The antenna element of any preceding claim, wherein the feed element is arranged in a side region of the substrate.

7. The antenna element of any preceding claim, wherein the feed element is formed on the surface of the substrate.

8. The antenna element of any preceding claim, wherein the feed element is planar.

9. The antenna element of any preceding claim, wherein the feed arrangementcomprises multiple feed elements.

10. The antenna element of claim 9, wherein at least two feed elements are arranged on respective surfaces of the substrate.

11. The antenna element of any preceding claim, wherein the feed arrangement is configured for circularly polarised signals.

12. The antenna element of any preceding claim, comprising an axis of rotational symmetry.

13. The antenna element of claim 12, wherein the axis of rotational symmetry intersects the radiator.

14. The antenna element of any preceding claim, wherein the feed element is shaped to increase inductance within the feed element relative to a corresponding straight feed element.

15. The antenna element of any preceding claim, wherein the feed element comprises an elongate track.

16. The antenna element of claim 15, wherein the track comprises at least one bend.

17. The antenna element of claim 16, wherein the track comprises bends in opposed directions.

18. The antenna element of claim 17, wherein the track comprises a series of bends in alternating directions.

19. The antenna element of any of claims 16 to 18, wherein portions of the feed element on each side of the bend each intersect an axis that is orthogonal to a surface of the substrate on which the radiator is arranged.

20. The antenna element of any of claims 15 to 19, wherein the track has a uniform width along its length.

21. The antenna element of any preceding claim, wherein the feed element has an undulating, zigzag or wavy shape.

22. The antenna element of any preceding claim, wherein the feed element is shaped to create capacitance between portions of the feed element.

23. The antenna element of any preceding claim, wherein the feed arrangement comprises a conductive element arranged on the surface of the substrate, adjacent to the feed element.

24. The antenna element of claim 23, wherein the conductive element is configured to interact with the feed element through capacitive coupling.

25. The antenna element of claim 23 or claim 24, wherein the conductive element defines a parasitic element.

26. The antenna element of any of claims 23 to 25, wherein the conductive element is configured to connect to a ground plane.

27. The antenna element of any preceding claim, wherein the feed arrangement is configured to contribute to impedance matching.

28. The antenna element of any preceding claim, wherein the feed element is spaced from the radiator.

29. The antenna element of any preceding claim, wherein the feed element is spaced from an edge of a surface of the substrate on which the radiator is arranged.

30. The antenna element of any preceding claim, comprising a terminal for connecting to a port.

31. The antenna element of claim 30, wherein the feed element comprises, or is connected to, the terminal.

32. The antenna element of claim 31, wherein the feed element is configured to transfer signals between the radiator and the terminal, in use.

33. The antenna element of any preceding claim, wherein the radiator comprises one or more slots.

34. The antenna element of claim 33, wherein each slot is aligned with a respective feed element.

35. The antenna element of any preceding claim, wherein the substrate comprises a dielectric.

36. The antenna element of any preceding claim, wherein the substrate comprises a ceramic material.

37. The antenna element of any preceding claim, configured to excite dielectric resonator modes.

38. The antenna element of any preceding claim, configured to excite a patch mode.

39. The antenna element of any preceding claim, wherein the substrate comprises multiple layers.

40. The antenna element of claim 39, wherein each layer is of a different material.

41. The antenna element of any preceding claim, wherein the radiator comprises a patch radiator.

42. The antenna element of any preceding claim, having the form of a microstrip patch antenna element.

43. The antenna element of any preceding claim, comprising a single radiator.

44. The antenna element of any preceding claim, wherein the radiator is arranged centrally on the substrate.

45. The antenna element of any preceding claim, wherein the feed arrangement is configured to interact with the radiator through capacitive coupling.

46. The antenna element of any preceding claim, wherein the feed element at least partly defines a feed probe.

47. An antenna assembly, comprising the antenna element of any preceding claim.

48. The antenna assembly of claim 47, wherein the antenna element is mounted to a support structure.

49. The antenna assembly of claim 48, wherein the support structure comprises a ground plane.

50. The antenna assembly of any of claims 47 to 49, comprising a port, wherein the feed element is connected to the port.

51. The antenna assembly of any of claims 47 to 50, comprising an array of antenna elements according to any of claims 1 to 46.

52. The antenna assembly of claim 51, wherein the array is configured as a controlled reception pattern array.

53. A communication system comprising the antenna element of any of claims 1 to 46, or the antenna assembly of any of claims 47 to 52.

54. A method of fabricating an antenna element, the method comprising arranging a radiator on a substrate, and arranging a feed element on a surface of the substrate.

55. The method of claim 54, comprising forming the feed element on the surface of the substrate.

56. The method of claim 54, comprising attaching the feed element to the surface of the substrate.31

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