Antenna System

Abstract

An antenna system (50, Figure 3) comprises: an antenna comprising an array 40 of tapered slot elements 10; and a feed arrangement (62, Figure 3) for feeding the elements of the array. At least one ele

Description

Technical field This invention relates to the field of antennas. In particular, but not exclusively, the invention relates to antennas and antenna systems for ground-penetrating radar systems. Background Ground-penetrating radar (GPR) systems are used in a variety of applications to image the subsurface. For example, a GPR system may be used for non-destructive testing, to investigate structures or to locate underground objects such as utility lines. GPR may also be used for detecting other subsurface objects such as mines, improvised explosive devices (lEDs) and electrical connections to such devices. A typical GPR system may operate using similar principles to other types of radar system. In this respect, a GPR system may transmit electromagnetic pulses in the form of short-duration radio-frequency (RF) signals into the ground, which are then reflected, refracted, scattered or otherwise altered by subsurface features, such as buried objects or interfaces between different materials. The signals are typically high-frequency, for example in the range of 10MHz to 3GHz (or higher), and distinctly polarised. Reflected signals returning to, and received by, the GPR system may then be analysed to identify and detect physical features within the scanned area. For example, as RF signals have a substantially constant speed, the time-of-flight of reflected signals may be indicative of the distance to the point at which the signal was reflected. Meanwhile, the amplitude of a reflected signal may be indicative of the reflectivity of the subsurface feature that caused the reflection, which may enable the nature of the reflecting feature to be derived. Tapered slot antennas or elements, an example of which is a ‘Vivaldi antenna’ or ‘Vivaldi element’, are known for use in GPR systems. A Vivaldi element typically includes a planar radiator having a tapered slot that produces a wide bandwidth. This wide bandwidth, together with the relatively simple structure of the element, makes Vivaldi antennas attractive for radar applications, as a wider bandwidth typically enables a higher resolution to be achieved. The slot of a Vivaldi element typically has curved edges each prescribed by an identical (but mirror image) exponential analytical function, so that Vivaldi elements may be categorised as non-linear tapered slot antennas. Other types of tapered slot antennas are also known that may be similarly useful, including other non-linear tapered slot elements and also elements having slots with straight edges, which may be referred to as linear tapered slot antennas. Another type of tapered slot antenna has a slot with a tapered portion and a portion of constant width. Various other slot shapes are also known following different analytical, numerical or staircase mathematical descriptions. It is against this background that the present invention has been devised. Summary of the invention According to an aspect of the present invention, there is provided an antenna system. The antenna system may be for a ground-penetrating radar system. The antenna system comprises an antenna comprising an array of tapered slot elements, and a feed arrangement for feeding the elements of the array. At least one element of the array is independently-operable, in that the element can be operated independently of the, or each, other element of the array. For example, the antenna system may be configured so that one element of the array can be operated in a different mode to another element. The tapered slot elements may have planar radiators and may be generally flat, for example with the general form of a board or sheet. Embodiments of the invention may use any type of tapered slot element, including non-linear tapered slot elements, linear tapered slot elements, constant width slot elements and other tapered slot elements having slots of various shapes. The elements may be Vivaldi elements, for example. Each element may have a pair of radiators, which may be on opposite sides of a substrate. It is also possible for one or more of the elements to have a single radiator. The tapered slot of each element may extend generally centrally through a radiator of the element. The feed arrangement may be configured to feed the elements of the array by interacting electrically with radiators of the elements, to transmit signals to and / or from the radiators, for example. In some embodiments, each element of the array is operable independently of the other elements of the array. The antenna system may be configured so that at least two elements of the array are operable in different modes simultaneously. The different modes may comprise a transmission mode and a receiver mode. Configuring the system so that at least one element can be operated independently of the other element(s) of the array beneficially enhances the flexibility and / or functionality of the array. This may enable the array to provide functionality that would otherwise require multiple separate antennas, for example, thereby promoting a compact arrangement that may also offer reduced complexity. For example, if the system is configured so that elements of the array are operable independently in transmission and reception modes, the antenna may be operable as an active sensor. More generally, the antenna system can be readily adapted to suit the requirements of each application. The elements of the array may be mutually similar. For example, the elements may all be of the same type, and / or may have similar geometry and / or dimensions to one another. The elements of the array may be substantially identical to one another. Alternatively, two or more elements of the array may be mutually dissimilar. At least some, and optionally all, of the elements of the array may be in contact with each other. For example, elements of the array may engage along respective edges. Alternatively, the elements of the array may be mutually spaced. The elements of the array may extend in respective vertical planes, in use. Respective slots of the elements of the array may extend along axes that are parallel to one another. Optionally, each element of the array is parallel to another element of the array. Each element of the array may be orthogonal to another element of the array. The array may be cube-shaped. In some embodiments, at least one element of the array is inclined relative to a central axis of the array and / or relative to another element of the array. Respective slots of two or more elements of the array may extend along respective axes that converge. Similarly, elements of the array may be inclined to extend towards one another, for example to extend in respective convergent planes. The array may be generally pyramidal, for example having the general form of a square-based pyramid and / or of a truncated pyramid. Configuring the array with inclined elements may enhance common illumination of the ground by the antenna, for example, and may reduce blockage in embodiments in which the antenna is used alongside a magnetic sensor. The array may comprise four elements arranged in a square formation, such that the elements collectively form a profile that is generally square in shape. The elements of the array may be arranged in a loop. The elements of the array may be arranged side-by-side. The elements of the array may define sides of the array. The antenna system may comprise a transmitter, in which case the feed arrangement may be configured to connect the transmitter selectively to at least one element of the array independently of the other elements of the array. In such embodiments, the feed arrangement may be configured to connect the transmitter selectively to any one or more elements of the array independently of the other elements of the array. The antenna system may comprise a receiver, in which case the feed arrangement may be configured to connect the receiver selectively to at least one element of the array independently of the other elements of the antenna. The feed arrangement may be configured to connect the receiver selectively to any one or more elements of the array independently of the other elements of the antenna. Accordingly, independent operation of the at least one element of the array may be provided for at least in part through the configuration of the feed arrangement. For example, the feed arrangement may include a switching arrangement, connecting network or other suitable means for selectively connecting the at least one element to a transmitter or to a receiver of the system. The antenna may comprise radiation absorbent material. For example, radiation absorbent material may be arranged in an interior of the array, between the elements. Radiation absorbent material may extend across the interior. Radiation absorbent material may extend diagonally across the interior, for example between corners of the array where adjacent elements meet. Radiation absorbent material may extend parallel to the elements, within the interior. Radiation absorbent material may form a cross formation within the interior. The antenna may comprise one or more sheet members comprising radiation absorbent material in the interior of the array, which sheet members may intersect one another, for example to form a cross formation. The antenna may comprise an absorber element in the interior of the array, the absorber element comprising radiation absorbent material. The absorber element may be in the form of an isolating body or one or more isolation barriers, for example, and may be configured to absorb electromagnetic radiation within the interior of the array. Radiation absorbent material may be arranged on at least one of the elements of the array, and optionally on each element of the array. For example, at least one of the elements of the array may be received in a housing or cover comprising radiation absorbent material. Respective housings of neighbouring elements may be in contact with one another, and may engage along respective edges for example. Radiation absorbent material may cover side edges of the or each element. The radiation absorbent material may be arranged so that a slot of the or each element is exposed. The radiation absorbent material may be arranged to resist interaction between elements of the array. For example, the radiation absorbent material may reduce side lobes from the elements and so improve the directionality of the elements. Reducing interaction between elements of the array beneficially enhances the ability of the system to support high integrity waveforms. The radiation absorbent material may be configured to reduce interference, for example internal interference or interference from external sources. The antenna system may be configured to support high integrity time domain waveforms. Each element of the array may be shaped to enhance compatibility of the antenna with a sensor. The sensor may be a magnetic sensor and may comprise a magnetic detector coil. The sensor may be a sensor of a metal detection system. In such embodiments, an outer profile of each element of the array may be shaped to enhance compatibility of the antenna with the sensor. For each element, an edge towards which a slot of the element tapers, which may define a rear edge of the element, may be shaped to enhance compatibility of the antenna with the sensor. The edge towards which a slot of the element tapers may be curved, for example. The shaping of the element may reduce a metal content of the element relative to a corresponding element having similar performance characteristics. In some embodiments, each element of the array has a curved outer edge towards which a slot of the element tapers. In some embodiments, for each element, one or more radiators of the element may include one or more apertures on each side of a slot of the element. Each element of the array may comprise a slot that tapers towards a rear of the array. Respective edges of the elements may align in a common plane at the rear and / or the front of the array. At least two elements of the array may have different polarisations. Another aspect of the invention provides a method of operating an antenna. The antenna may be an antenna of a ground-penetrating radar system. The antenna comprises an array of tapered slot elements. The method comprises operating elements of the array in different modes simultaneously. For example, the method may comprise operating at least one element in a transmitting mode and at least one other element in a receiving mode simultaneously. The antenna may be an antenna as defined in the above aspect, for example. The antenna may form part of an antenna system that also comprises a feed arrangement for feeding the elements of the array. The antenna or the antenna system may form part of a ground-penetrating radar system. The invention therefore also extends to a method of operating an antenna system and to a method of operating a ground-penetrating radar system. Another aspect of the invention provides an antenna. The antenna may be an antenna for a ground-penetrating radar system. The antenna comprises at least one tapered slot element, and radiation absorbent material. The radiation absorbent material may enhance the performance of the antenna in various ways, for example by reducing interference, either external or internal, or by improving the directionality of the, or each, element. The, or each, tapered slot element may have planar radiators and may be generally flat, for example with the general form of a board or sheet. Embodiments of the invention may use any type of tapered slot element, including non-linear tapered slot elements, linear tapered slot elements, constant width slot elements and other tapered slot elements having slots of various shapes. The elements may be Vivaldi elements, for example. Each element may have a pair of radiators, which may be on opposite sides of a substrate. It is also possible for each element to have a single radiator. The tapered slot of each element may extend generally centrally through a radiator of the element. The, or each, element may carry radiation absorbent material. The radiation absorbent material may enhance the performance of the, or each, element by, for example, reducing side lobes from the element and so improving the directionality of the element. The, or each, element may be received in a housing or cover comprising radiation absorbent material. If there are multiple elements, each element may be received in a respective housing, or alternatively multiple elements may be received in a common housing. The radiation absorbent material may cover side edges of the, or each, element. The radiation absorbent material may be arranged so that a slot of the, or each, element is exposed. The antenna may comprise an array of tapered slot elements. In such embodiments, radiation absorbent material may be arranged in an interior of the array, between the elements. Radiation absorbent material may extend across the interior. Radiation absorbent material may extend diagonally across the interior, for example between corners of the array where adjacent elements meet. Radiation absorbent material may extend parallel to the elements, within the interior. Radiation absorbent material may form a cross formation within the interior. The antenna may comprise one or more sheet members comprising radiation absorbent material in the interior, which sheet members may intersect one another, for example to form a cross formation. The antenna may comprise an absorber element in the interior of the array, the absorber element comprising radiation absorbent material. The absorber element may be in the form of an isolating body or one or more isolation barriers, for example, and may be configured to absorb electromagnetic radiation within the interior of the array. Radiation absorbent material may be arranged on at least one of the elements of the array, and optionally on each element of the array. The radiation absorbent material may be arranged to resist interaction between elements of the array. For example, the radiation absorbent material may reduce side lobes from the elements and so improve the directionality of the elements. Reducing interaction between elements of the array beneficially enhances the ability of the system to support high integrity waveforms. The radiation absorbent material may be configured to reduce interference, for example internal interference or interference from external sources. The, or each, element may have a curved outer edge towards which a slot of the element tapers, in which case radiation absorbent material may cover the curved edge. The invention also extends to an antenna system comprising the antenna of the above aspect. The antenna system of either of the above aspects may comprise a controller configured to operate the antenna to transmit and / or receive signals. In any of the above antenna systems, the antenna may be operable as an active sensor. The invention also extends to a ground-penetrating radar system comprising the antenna system of any of the above aspects. The invention also extends to a device, which may be a handheld device, comprising the antenna system or the ground-penetrating radar system of the above aspects. The device may also comprise a metal detection system. The metal detection system may include a sensor, for example a magnetic sensor such as a magnetic detector coil, which may be co-located with, for example in close proximity to, the antenna. The or each element of the antenna of the antenna system may be shaped to enhance compatibility of the antenna with the sensor. An outer profile of the or each element may be shaped to enhance compatibility of the antenna with the sensor. For each element, an edge towards which a slot of the element tapers, which may define a rear edge of the element, may be shaped to enhance compatibility of the antenna with the sensor. The edge towards which a slot of the element tapers may be curved, for example. The shaping of the element may reduce a metal content of the element relative to a corresponding element having similar performance characteristics. One or more radiators of the element may include one or more apertures on each side of a slot of the element. 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; Figure 2 shows an array of the antenna elements of Figure 1; Figure 3 shows an antenna system including the array of Figure 2; Figure 4 shows the array of Figure 2 in combination with radiation absorbent material; Figure 5 shows the element of Figure 1 with added radiation absorbent material; Figure 6 shows an array assembly including an array formed from elements as shown in Figure 5; Figure 7 is a graph representing performance characteristics of the arrays of Figures 2 and 6 and the element of Figure 4; Figure 8 shows a variant of the array assembly of Figure 6; Figure 9 shows a variant of the array assembly of Figure 8; Figure 10 shows an alternative antenna element in isolation; Figure 11 shows the element of Figure 10 including radiation absorbent material; Figure 12 shows an array of elements as shown in Figure 11; Figure 13 shows the array of Figure 12 in combination with a magnetic detector coil; Figure 14 shows another variant of the array assembly of Figure 6; and Figure 15 shows another variant of the array assembly of Figure 8. Detailed description In general terms, embodiments of the invention use antenna elements in the form of tapered slot elements, such as ‘Vivaldi elements’, to provide improved antennas. Such antennas may support high-discrimination ultra-wideband radar applications and may, for example, be used as antennas for GPR systems. In this context, ‘ultra-wideband’ may refer to a frequency of operation covering several octaves, and optionally up to a decade or more. In some embodiments, the antennas may also be configured as active sensors, namely antennas that are capable of both transmission and reception. An example of a tapered slot antenna element 10 in the form of a Vivaldi element is shown in Figure 1. In general terms, the element 10 offers broadband performance and linear polarisation, as is typical for Vivaldi elements and other tapered slot elements. As described later, the element 10 is used to provide antennas according to some embodiments of the invention. First, details of the element 10 itself will be described, although it is noted that the element 10 is generally similar to conventional elements and 10 so some details are omitted for clarity. The skilled reader will appreciate that the specific geometry and dimensions of the element 10 may be adjusted to account for various factors, including operational bandwidth, directivity of radiation and dielectric boards that may be used with the element. More generally, various other tapered slot elements may be used in embodiments of the invention. The element 10 includes a substrate 12, which in this example is a thin, rigid, flat, rectangular board having planar surfaces, and may be similar to a substrate of a conventional printed circuit board (PCB). The substrate 12 is of dielectric material in this example. The size of the substrate 12 may be determined according to the frequencies that the element 10 is configured to operate at. In this example, the substrate 12 is approximately 110mm in length and width, although this is purely illustrative. Figure 1 shows that one side of the substrate 12 carries a planar radiator 14. Although not visible in Figure 1, a second radiator 14 is carried on the opposite side of the substrate 12, so that the substrate 12 carries a radiator 14 on each side. The radiators 14 on each side of the substrate 12 are substantially identical to one another, and are arranged in mutual alignment. In this example, each radiator 14 comprises, and is substantially defined by, a layer of conductive material formed on, and covering a portion of, a respective surface of the substrate 12. The conductive material may be a metal such as copper, for example, and may be formed or otherwise arranged on the substrate 12 using conventional techniques, which may include etching, sputtering, adhesive attachment and / or chemically-based metallisation, for example. The radiators 14 are therefore very thin and so may be regarded as flat plate elements that each predominantly extends in a plane parallel to the surface of the substrate 12 on which it is formed. The radiators 14 are also electrically thin, in that they have a small thickness relative to the wavelengths that they are configured to operate at. The element 10 as a whole is therefore generally flat and sheet-like or board-like. Each radiator 14 is shaped to create a slot 16 having curved edges, through which the substrate 12 is exposed, that extends centrally through the radiator 14 and across the substrate 12, from left to right as viewed in Figure 1. The slot 16 extends along an axis 18 that is parallel to edges of the substrate 12 and that intersects a centre of the substrate 12. The curved edges of the slot 16 converge with one another so that the slot 16 tapers from an open end, shown to the left in Figure 1, towards the centre of the substrate 12, and hence the Vivaldi element 10 defines a tapered slot element. The tapering of the slot 16 acts to guide a travelling wave so that, among other things, the guide impedance matches that of free space for unhindered transition to radiation over broadband, with substantially flat group delay. The edges of the element 10 that are parallel to the slots 16 of the radiators 14 define side edges 17 of the element 10. Meanwhile, the edge of the element 10 corresponding to the open ends of the slots 16 defines a front edge 19 of the element 10, and the edge opposite to the front edge 19 defines a rear edge 21 of the element 10. In the orientation shown in Figure 1, the slot 16 may be regarded as dividing the radiator 14 into similar upper and lower segments 20a, 20b, which connect to one another through a bridge 22 of conductive material adjacent to the rear edge 21 of the element 10. The segments 20a, 20b mirror one another, and so the element 10 is symmetrical about an axis corresponding to the slot axis 18. Each segment 20a, 20b has a generally semi-circular curved portion at an end shown to the left in Figure 1, the curved portions being arranged side-by-side, such that an initial portion of the slot 16 is defined between converging curved edges of the curved portions. The curved portions are not perfectly semi-circular, and instead the converging sides straighten as they approach, and cross, the centre of the substrate 12. Accordingly, the taper of the slot 16 is sharpest in a region nearest to the front edge 19 of the element 10, which corresponds to the open end of the slot 16, and becomes gentler towards the centre of the substrate 12. In this example, the curvature of each edge of the slot 16 follows a generally exponential profile, although Vivaldi elements and other tapered slot elements may have slots of a different shape. The slot 16 continues past the centre of the substrate 12, to connect with a circular aperture 24 formed in the radiator 14, which is shown towards the right of the substrate 12 in Figure 1. The circular aperture 24 defines a cavity of the radiator 14. The respective edges of the segments 20a, 20b on each side of the slot 16 each merge with an edge of the circular aperture 24, such that the radiator 14 has a continuous edge extending around its perimeter and incorporating the slot 16 and the circular aperture 24. The circular aperture 24 may act to alter the short circuit path between opposed sides of the slot 16 in a broadband fashion, enabling the forward firing mode of the element 10, while at the same time can also be used as part of impedance matching of the element 10. An array of plated holes defining vias 26 extends around the continuous edge of the radiator 14, the vias 26 being regularly spaced at relatively short intervals around the entire edge of each radiator 14, including along the edges of the slots 16 and around the edge of the circular aperture 24. Each via 26 penetrates the radiator 14, the substrate 12 beneath and the radiator 14 on the opposite side of the substrate 12, and therefore creates an electrical link between the radiators 14. The vias 26 are plated with the same material as that used to form the radiators 14 in this example, and so also create continuity of the conductive material on each side of the element 10. Figure 1 also shows a feed for the element 10, which in this example includes a feed cable 28 in the form of a coaxial cable having an inner conductor and an outer conductor, the outer conductor being connected to ground. The feed cable 28 extends across the upper segment 20a of the visible radiator 14 in Figure 1, from a side edge 17 of the element 10 towards the slot 16. The outer conductor of the feed cable 28 is connected to the surface of the upper segment 20a of the radiator 14 at a position adjacent to the slot 16 and near the circular aperture 24. The inner conductor protrudes beyond the outer conductor, and extends across the slot 16 to connect to the lower segment 20b of the radiator 14, on the opposite side of the slot 16. A region around the feed connections defines a feed region 30 of the element 10, which is denoted by a dashed box in Figure 1. The element 10 of this example therefore incorporates direct gap feeding. This is purely illustrative, however, and other feed arrangements are possible for antenna elements in other examples. For example, another option is to use a dedicated microstrip line board, which may be attached to or spaced from the element. Turning now to Figure 2, an array 40 of antenna elements 10 as shown in Figure 1 is illustrated. The array 40 offers broadband performance by virtue of the broadband capability of the elements 10. The array 40 may also provide for a multi-polar system, in that different polarisations can be applied to the individual elements 10. The array 40 includes a set of four identical elements 10 arranged in a square formation to form the array 40, so that the array 40 is generally cube shaped. The elements 10 are arranged side-by-side in a loop that extends around a central axis 42 of the array 40 to surround a hollow interior of the array 40, so that the array 10 includes a pair of open sides that are spaced along the central axis 42 and surrounded by the elements 10. The open sides define a front 44 and a rear 46 of the array 40, the front 44 being shown foremost and to the left in Figure 2. The elements 10, correspondingly, define respective sides of the array 10. Each element 10 is therefore parallel to another element 10 on an opposite side of the array 40, and orthogonal to the remaining two elements 10. In this example, the elements 10 are slightly mutually spaced, such that they do not contact one another along their respective adjacent side edges 17. In other examples, the elements 10 may engage along their side edges 17. The elements 10 are arranged with similar orientations, such that their respective front edges 19 are aligned in a common plane at the front 44 of the array 40, so that the front edges 19 collectively form a border around the front 44 of the array 40. Correspondingly, the respective rear edges 21 form a border around the rear 46 of the array 40. The respective slot axes 18 along which the slots 16 extend are mutually parallel and are also parallel to the central axis 42 of the array 40. The elements 10 are also arranged with their respective feed cables 28 oriented similarly. Accordingly, the array 40 has four degrees of rotational symmetry about the central axis 42. The slots 16 of the elements 10 therefore all extend towards, and have open ends at, the front 44 of the array 40, and taper towards the rear 46 of the array 40. In use, signals are transmitted from the front 44 of the array 40 generally in a direction parallel to the central axis 42. The array 40 also receives signals travelling in generally the opposite direction. The array 40 is typically oriented to direct signals into the ground in operation, in which case the elements 10 of the array 40 extend in respective substantially vertical planes. The array 40 defines an antenna that forms part of an antenna system 50 that is shown in Figure 3, in schematic and simplified form. The array 40 may therefore alternatively be referred to as an antenna. The antenna system 50 is part of a GPR system 51 in this example, in which the array 40 acts as a sensor that serves to transmit electromagnetic pulses in the form of short-duration radio-frequency signals into the ground, and to receive returning signals that have been reflected, refracted, scattered or otherwise altered by subsurface features, such as buried objects or interfaces between different materials. The returning signals can be analysed by the system 50 to determine the presence of objects of interest, or ‘targets’, below the surface. It should be appreciated that the GPR system 51 may have various other components and features in addition to the antenna system 50, including a control module and a user interface, for example, but these are omitted for clarity. In the example shown in Figure 3, the GPR system 51 is implemented in a device 53. The device may also include a suitable housing containing or otherwise supporting the components of the GPR system 51, for example. The device 53 is configured as a handheld device in this example, which is facilitated by the compact nature of the array 40. The device 53 may also include other systems in addition to the GPR system 51, and in this example also includes a metal detection system 55. The device may include a controller (not shown) that coordinates operation of the GPR system 51 and the metal detection system 55. The GPR system 51 may also be implemented in a variety of other ways, however, for example in a different type of device or as a subsystem of another system. The antenna system 50 includes a transmitter 52 configured to generate RF pulses to be transmitted from the array 40, a receiver 54 for receiving returning RF signals collected by the array 40, and a connecting network 56 for connecting the transmitter 52 and the receiver 54 to each of the elements 10 of the array 40 individually and selectively. More specifically, the connecting network 56 connects to the respective feed cables 28 of the elements 10, through suitable ports. In this respect, the connecting network 56 includes a set of ports 58 for connecting to the transmitter 52, the receiver 54 and each of the elements 10, with interconnections along which signals are communicated, in use, indicated generally by dashed lines in Figure 3. The connecting network 56, the feed cables 28, the ports of the connecting network 56 and the elements 10 and the associated interconnections collectively form a feed arrangement 62 that is configured to feed the elements 10 of the array 40, in that the feed arrangement 62 interacts electrically with the radiators 14 of the elements 10 of the array 40 to transmit and / or receive signals in use. The feed arrangement 62 may alternatively be referred to as a coupling arrangement, in that it acts to couple electrically to the radiators 14 to transfer electromagnetic energy to and / or from the radiators 14. The antenna system 50 also includes a control and processing module 60, which is configured to operate the transmitter 52 to issue signals to be transmitted by the array 40 and to process signals received by the receiver 54, for example to identify objects of interest. The control and processing module 60 therefore acts as a controller that is configured to operate the array 40 and the antenna system 50 to transmit and / or receive signals in this example. Alternative controllers and control architectures may be provided in other examples. In general terms, the system 50 is configured to promote the preservation of system waveforms, namely the pulses transmitted and received by the array 40, so that the control and processing module 60 is able to analyse a range of characteristics of received waveforms for detection purposes, including the phase, power, frequency, amplitude and duration of the waveforms, for example. The array 40 and the associated feed arrangement 62 are configured so that each element 10 of the array 40 can be operated independently of the other elements 10 of the array 40. In particular, the elements 10 can be operated in different modes or states simultaneously. In this respect, the connecting network 56 of the antenna system 50 is configured so that each element 10 of the array 40 can be connected to the transmitter 52, to the receiver 54, or to neither the transmitter 52 nor the receiver 54, independently of the other elements 10. For example, the connecting network 56 may include a suitable switching arrangement for providing selective connections between the transmitter 52, the receiver 54 and each of the elements 10 of the array 40. There are various possible ways to configure the connecting network 56 to achieve this functionality, and so details of the configuration of the connecting network 56 are omitted for the sake of clarity. It follows that each element 10 of the array 40 can be placed in one of three operating modes, namely: a transmitting mode, in which the connecting network 56 connects the element 10 to the transmitter 52; a receiving mode, in which the connecting network 56 connects the element 10 to the receiver 54; and a disconnected mode, in which the element 10 is disconnected from both the transmitter 52 and the receiver 54. The set of modes in which the elements 10 of the array 40 are placed, in turn, defines a state of the array 40. The mode in which each element 10 is placed at any given time is independent of the modes of the other elements 10. So, for example, any one or more elements 10 can be connected to the transmitter 52 to transmit signals from corresponding sides of the array 40 individually, and correspondingly any one or more elements 10 can be connected to the receiver 54 to receive signals at corresponding sides of the array 40 individually. Each element 10 is capable of transmitting and receiving signals with one of two orthogonal polarisations, for example linear, mutually orthogonal polarisations of arbitrary inclination. In this example, the polarisations with which the elements 10 may transmit and receive correspond to ‘X’ and ‘Y’ shown in Figure 2. In turn, the transmitter 52, the receiver 54 and the connecting network 56 determine the details of the polarisation created in each element 10. The configuration of the system 50 enables different elements 10 to use different polarisations at the same time in this example. For example, it is possible for two elements 10 of the array 40 to transmit with different polarisations simultaneously, and similarly two elements 10 may receive with different polarisations simultaneously. Similarly, one element 10 may transmit with one polarisation while another element 10 receives with a different polarisation simultaneously. If two elements 10 on adjacent sides ofthe array 40 are excited simultaneously, although individually transmit with different polarisations, a net wave may be produced that has an intermediate polarisation corresponding to ‘45a’ or ‘45b’ as indicated in Figure 2. As elements 10 on opposite sides ofthe array 40 have similar polarisations, but opposite orientations relative to one another in the sense that they are rotated by 180° relative to one another, swapping one element 10 of an adjacent pair with the opposite element 10 of the array 40 has the effect of altering the polarisation of the net wave produced by the pair by 90°, to change between ‘45a’ and ‘45b’ in this example. It is noted that, in principle, mutually orthogonal circular or elliptical polarisations are also possible if the excitation network contains suitable provisions, although this is omitted in the present example for simplicity. 5 The below table summarises eighteen possible states of the array 40: State Element Polarisation A B C D 1 TX nc RX nc Polarisation Y for TX &RX 2 RX nc TX nc Polarisation Y for TX &RX 3 nc TX nc RX Polarisation X for TX &RX 4 nc RX nc TX Polarisation X for TX &RX 5 TX RX nc nc Polarisation Y for TX; Polarisation X for RX 6 RX TX nc nc Polarisation X for TX; Polarisation Y for RX 7 TX nc nc RX Polarisation Y for TX; Polarisation X for RX 8 RX nc nc TX Polarisation X for TX; Polarisation Y for RX 9 nc RX TX nc Polarisation Y for TX; Polarisation X for RX 10 nc TX RX nc Polarisation X for TX; Polarisation Y for RX 11 TX TX RX RX Polarisation 45b for TX; Polarisation 45b for RX 12 RX RX TX TX Polarisation 45b for TX; Polarisation 45b for RX 13 TX RX RX TX Polarisation 45a for TX; Polarisation 45b for RX 14 RX TX TX RX Polarisation 45a for TX; Polarisation 45b for RX 15 TX RX -TX -RX Polarisation Y for TX; Polarisation X for RX 16 RX TX -RX -TX Polarisation X for TX; Polarisation Y for RX 17 TX RX TX RX Polarisation Y for TX; Polarisation X for RX; null on axis 18 RX TX RX TX Polarisation X for TX; Polarisation Y for RX; null on axis In the above table, elements ‘A’, ‘B’, ‘C’ and ‘D’ are arranged sequentially around the array 40, with element ‘A’ being arbitrarily assigned. ‘TX’ means that the element 10 is 10 in the transmitting mode, ‘RX’ means that the element 10 is in the receiving mode, and ‘nc’ means that the element 10 is in the disconnected mode. The polarisation directions referred to in the above table are indicated in Figure 2. For each state, if two elements 10 are indicated to be in the same mode, those elements are combined with zero time delay, except for in states 15 and 16 as indicated by signs that denote a 180° difference in phase, which may be a real phase shift or an equivalent field reversal based on the element orientations. It follows that, in some states, the elements 10 are effectively placed in sub-groups, in which each element 10 of the sub-group acts in a similar manner. For example, in state ‘11’ elements A and B belong to a first sub-group that acts in transmission, and elements C and D belong to a second sub-group that acts in reception. States 15 to 18 in the above table represent cross-polarised detection capability for the array 40, which may be particularly useful for scenarios in which targets have predominantly high aspect ratios, for example wires or tubes. These states may also be useful for targets that are otherwise predominantly symmetrical, but which under illumination may have an apparent shape exhibiting a high aspect ratio when a centre of illumination is offset from the centre of the target. Operating the array 40 in one of these states may reduce sensitivity to predominantly symmetrical characteristics of spurious features such as from ground reflection, which may hinder target detection by generating strong unwanted clutter. Arranging the array 40 in the antenna system 50 so that different elements 10 may transmit and receive simultaneously means that the array 40 defines a complete transmission and reception sensor in a compact package. So, for example, the array 40 does not need to be arranged alongside other antenna elements or arrays to form a functioning sensor. For example, as the array 40 includes multiple elements 10 that can be operated in a receiving mode, multiple received signals can be collected and analysed, either simultaneously or in fast succession, to determine the bearing of a target. The arrangement of the array 40 also means that, for a given state of the array 40, a receiving element 10 is located in close proximity to a transmitting element 10, which improves the performance of the array 40 and the system 50 in which the array 40 is used. In this respect, closer proximity of the transmitting and receiving elements 10 enables the projected footprint to fields of search to have an enhanced common overlap, which translates to enhanced detection potential for a search object residing in the searched location. In addition, the ability to operate each element 10 of the array 40 independently enhances the flexibility of the array 40 and the associated antenna system 50, in that the state of the array 40 can be adapted to suit the requirements of each application. Typically, an equal number of elements 10 operate in the transmitting and receiving modes at any given time, although this is not essential. So, for example, one element 10 may operate in a transmitting mode while another element 10 operates in the receiving mode and the two remaining elements 10 are disconnected. If two elements 10 operate in the same mode simultaneously, they may be operated as a sub-group to transmit or receive in unison with substantially zero time delay. The ability to operate the array 40 in a range of states may enable the polarisation state of an object under investigation to be assessed, for example, by enabling states with different polarisations and analysing the response. In addition, configuring the array 40 for a range of operating states may enable the sensor and associated circuitry to be simplified whilst achieving the desired functionality. The precise elements 10 chosen to operate in each mode may also vary depending on operational circumstances. For example, in some cases it is beneficial for neighbouring elements 10 to operate in opposite modes, whereas in other situations opposed elements 10 operate in the same mode. Operating opposed elements in a receiving mode, for example, may aid in determining the direction from which received signals originated, for example by comparing the phases of the received signals. The elements 10 selected for each state may also depend on the polarisation of each element 10, for example. In this respect, the connecting network 56 may be configured such that each element 10 has a different polarisation, for example. This may enable flexible polarisation for illuminating targets when the array 40 is used in a GPR system, in that the elements 10 may be operated to transmit pulses towards a target sequentially, so that the resulting series of pulses have different polarisations. Arranging the array 40 such that individual elements 10 perform different functions simultaneously and in close proximity could lead to unpredictable distortion to the excited waveforms due to parasitic radiative coupling and other interaction between the elements 10, for example interaction arising due to residual internal reflection mechanisms and internal propagation properties. For example, an element 10 operating in a receiving mode could receive part of a signal transmitted by an element 10 operating in a transmitting mode, which could interfere with signals that the receiving element 10 is intended to receive, for example signals returning from underground. Vivaldi antenna elements may exhibit radiation patterns having relatively poor directionality, for example in that the radiation pattern has relatively large side lobes. This may exacerbate interference between the elements 10 of the array 40, or otherwise hinder the performance of the array 40. To mitigate these potential effects, in some embodiments radiation absorption means such as radiation absorbent material (RAM) is used on and / or in the array 40, to reduce mutual coupling and other intra-element interaction and any resulting interference, and / or to resist transmission or reception of signals propagating in unwanted directions, and thereby improve the directionality, gain, return loss and efficiency of the elements 10 and / or the array 40. In this way, stray interactions within the antenna system 50 may be mitigated and controlled, which in turn enhances the ability of the system 50 to support high integrity waveforms. In this respect, a high-integrity pulse waveform may be defined as a waveform that has high-fidelity in the time domain, for example a well-defined and well-behaved signature in the time domain. Such waveforms may support high discrimination in the time domain, for example by means of real or synthesised system pulses generated by the control and processing module 60 of the system 50, for example using step frequency postprocessing. Such signals may also have a well-defined and short time domain window of activity, with a low time residual side lobe level, for example -35 dB or below, outside of that window. Figure 4 shows one option for incorporating RAM into an array, in that a modified antenna is shown in which an isolating body 64 is arranged in combination with the array 40 of Figure 2 to form an array assembly 150 defining the antenna. The isolating body 64 may be entirely of RAM, or the body 64 may comprise RAM alongside other materials, for example metal-backed RAM. A variety of options may be suitable for the RAM that is used for the isolating body 64. The array assembly 150 is otherwise identical to the array 40 of Figure 2. The isolating body 64 has an inner portion 66 that extends into the interior of the array 40, between the elements 10, and an outer portion 68 that extends across a rear 146 of the array assembly 150, which corresponds to the rear 46 of the array 40. The isolating body 64 is therefore configured to absorb radiation in the space between the elements 10 and at the rear 146 of the array assembly 150, and so acts as an absorber element. The outer portion 68 of the isolating body 64 is square and generally planar and sheetlike, and is sized to cover the rear 146 of the array assembly 150 entirely. The inner portion 66 of the isolating body 64 comprises a pair of generally oblong members 70 that extend orthogonally from a centre of the outer portion 68, the members 70 being arranged orthogonally to one another in a cross formation, and parallel to sides of the array 40. The members 70 of the inner portion 66 intersect one another along an axis corresponding to a central axis 142 of the array assembly 150. The inner portion 66 of the isolating body 64 is therefore generally X-shaped in axial cross-section, and is of a length such that the inner portion 66 extends to the front 144 of the array assembly 150, which corresponds to the front 44 of the array 40. Each member 70 of the inner portion 66 has a triangular cross-section, such that the member 70 tapers towards the front 144 of the array assembly 150. The members 70 are therefore wedge-shaped in this example. In this example, the inner and outer portions 66, 68 are formed integrally with one another, such that the isolating body 64 is a single unitary piece. In other examples, an isolating body may be formed from an assembly of parts, and may also have a different shape or configuration. The isolating body 64 defines an isolation barrier that acts to resist interaction between the elements 10 and to reduce internal or external interference for the array assembly 150, both in transmission and in reception, thereby supporting independent operation of the elements 10 of the array 40 and also the ability of the array assembly 150 to handle high integrity waveforms. In this respect, the isolation body 64 acts to absorb radiation within the interior of the array 40, thereby reducing the proportion of that radiation that reaches the elements 10 of the array 40 or that propagates from the array 40 in an unwanted direction. The array assembly 150 shown in Figure 4 is otherwise the same as the array 40 of Figure 2 and can be used in the system 50 of Figure 3 in the same way as the array 40 of Figure 2. Figure 5 shows that RAM may also be added to an individual element 10, for example to improve the directionality of the element 10, to form an assembly defining a loaded element 74. In this example, the loaded element 74 includes RAM that extends substantially continuously along, and encases, three of the four edges of the element 10, namely the side edges 17 and the rear edge 21, leaving only the front edge 19 and a central portion of the element 10 including the slot 16 exposed, the front edge 19 being exposed at a front edge 75 of the loaded element 74. In this example, the RAM is formed as a one-piece housing 76, or cover, the housing 76 including slots 78 for receiving the side edges 17 of the element 10, so that the element 10 slides into, and is thus received in, the housing 76, with the rear edge 21 of the element 10 abutting a rear portion of the housing 76. RAM may be added to an element 10 in other ways in other examples, however. For example, in other examples RAM may be added only to the side edges 17 of an element 10. RAM could also be applied to an element 10 as a coating. In the example shown in Figure 5, the RAM housing 76 is shaped such that the element 10 is loaded more heavily along its side edges 17, in that the housing 76 extends inwardly from the side edges 17 of the element 10, towards the slots 16 of the element 10, to a greater extent than the housing 76 extends inwardly from the rear edge 21. This loading of RAM along the side edges 17 acts to suppress side lobes of the element 10, both in transmission and in reception. The RAM housing 76 therefore promotes reception and transmission of signals predominantly along a path corresponding to the axes 18 of the slots 16, while suppressing side lobes and reducing interference emanating from the sides and the rear of the element 10. The RAM housing 76 is also shaped to increase the bandwidth of the element 10, and in particular to reduce the lower limit of the operating band of the element 10. This may be achieved by configuring the RAM housing 76 to provide dielectric loading, for example through the selection of the specific material of the housing 76 and the thickness of the material. Figure 6 shows an array 240 formed from four loaded elements 74 such as shown in Figure 5. The loaded elements 74 are arranged in a similar manner to the elements 10 of the array 40 shown in Figure 2, and so the array 240 of Figure 6 has the same general layout and shape as the array 40 of Figure 2, and may similarly define an antenna. In particular, in the example shown in Figure 6, the loaded elements 74 are arranged side-by-side in a square formation to form a loop around a central axis 242 to define sides of the array 240, while the array 240 has opposed open sides defining a front 244 and a rear 246 of the array 240, as in the example of Figure 2. Whereas the elements 10 are arranged with gaps between their side edges in the array 40 of Figure 2, those gaps are filled by the RAM housings 76 of the loaded elements 74 in the array 240 of Figure 6. Accordingly, the loaded elements 74 engage one another along adjacent side edges in the array 240 of Figure 6. The array 240 is shown in Figure 6 as part of an array assembly 250 that also includes an isolation barrier that is similar to that used in the array assembly 150 shown in Figure 4, namely an isolating body 64 comprising RAM arranged with its inner portion 66 extending into the interior of the array 240 and an outer portion 68 extending across the rear of the array 240. This demonstrates that these features can be combined to enhance performance further. In this respect, the loaded elements 74 provide for reduced side lobes from the elements 10, for example, while the isolating body 64 provides further refinement by resisting interaction between the elements 10, for example interference arising from any residual side lobes of the elements 10, and / or external interference. The array assembly 250 defines an antenna in this example. The array assembly 250 shown in Figure 6 is otherwise the same as the array 40 of Figure 2 and can be used in the system 50 of Figure 3, as a direct substitute for the array of Figure 2. Indeed, the array assembly 250 shown in Figure 6 may alternatively be regarded as an assembly of the array 40 of Figure 2 with the RAM housings 76 and the isolating body 64. Figure 7 provides a graphical illustration of how the array 40 of Figure 2, the array assembly 250 of Figure 6, and the loaded element 74 of Figure 4 respond to an excitation when in a transmitting mode. An excitation signal 80 is shown to the left of the graph, and respective pulses that are generated by the element 74, the array 40 and the array assembly 250 in response to the excitation signal are shown to the right of the graph and overlaid with one another. Specifically, Figure 7 shows a first pulse 82 corresponding to the array 40 of Figure 2, a second pulse 84 corresponding to the array assembly 250 of Figure 6, and a third pulse 86 corresponding to the loaded element 74 of Figure 4. This reveals that the loaded element 74 of Figure 4 and the array assembly 250 of Figure 6 respond similarly to the excitation signal, in that the second and third pulses 84, 86 closely match the excitation signal 80. In contrast, the array 40 of Figure 2, which lacks RAM or isolation barriers, produces a pulse 82 having more significant residual energy representing time side lobes following the excitation. Regarding other performance characteristics, compared with the array 40 of Figure 2, the array assembly 250 of Figure 6 exhibits generally higher radiation efficiency over the entire operating range of frequencies of the array 240. The array assembly 250 of Figure 6 also provides a generally higher return loss and a significantly higher gain across the bandwidth of the array 240 compared to the array 40 of Figure 2. The performance characteristics shown in Figure 7 demonstrate that the addition of RAM, in addition to controlling reverberation of stray energy, can also act as residual dielectric loading. This has beneficial effects, including reducing the low operational frequency limit of an otherwise compact antenna assembly. More generally, the array assembly 250 of Figure 6, through a combination of the configuration of the elements 10 within the array 240 and the isolation provided by the RAM, both on the elements 10 and within the array assembly 250, is capable of supporting high-integrity time-domain waveforms. High-integrity time-domain waveforms may be supported in transmission and also in transition to radiation, namely the transition of guided signals in the feed cables 28 or alternative transmission lines or feed components such as microstrip lines, as well as corresponding fields guided along the antenna structure prior to these escaping to the surrounding open space as radiation. This is achieved in part by enabling a wide bandwidth, and also by reducing residual internal reflection mechanisms and internal propagation properties, to avoid unpredictable distortion to excited waveforms. In this way, signals escaping to the air can preserve the desired time domain attributes. To support this behaviour, the antenna system 50 may have a substantially flat amplitude and group delay behaviour throughout the designated bandwidth, with minimal deviation from flatness and minimal ripples on top. In turn, the antenna system 50 may be able to handle real or synthetic time-domain signals, with the ability to create time pulses that are: of arbitrarily short duration; sharp; real or synthetic; and with low time side lobes. More generally, the antenna system is configured to support wideband, but real or synthetic direct baseband time-domain signals. A practical effect of the performance capabilities of the array assembly 250 is that the system 50 in which the array assembly 250 is used may be able to detect weak targets in the presence of strong targets, for example. Figure 8 shows another array assembly 350, representing a variant of the array assembly 250 shown in Figure 6, having different isolation barriers. The array assembly 350 shown in Figure 8 has the same underlying structure as the arrays and array assemblies of Figures 2, 4 and 6, and, like the array assembly of Figure 6, includes an array 240 that is formed from loaded elements 74 as shown in Figure 5. The array 350 of Figure 8 is also similarly suitable for use in the system 50 of Figure 3 and offers similar performance characteristics to the array assembly 250 of Figure 6. The array assembly 350 shown in Figure 8 does not include the isolating body 64 of Figure 6. Instead, isolation barriers in the form of a pair of inner RAM sheets 88 extend diagonally across the interior of the array 240, each inner RAM sheet 88 extending from an interface between side edges of adjacent loaded elements 74 at a corner of the array 240 to a corresponding interface between other loaded elements 74 in a diametrically opposed corner of the array. The inner RAM sheets 88 cross one another along a central axis 342 of the array assembly 350, which corresponds to the central axis 242 of the array 240, so that the sheets 88 together form an X-shape. The isolation barriers defined by the RAM sheets 88 act, both collectively and individually, as absorber elements in a similar manner to the isolating body of Figure 6. By arranging the RAM sheets 88 diagonally the length of the sheets 88 can be increased, thereby providing more isolation. This arrangement may also enhance flexibility for integration with associated electronics. Conversely, the arrangement shown in Figure 6 may offer an enhanced ability to dump multiple bounces between ground-antenna reflections (similarly to a conventional pyramidal absorber) compared to the arrangement shown in Figure 8. The array assembly 350 of Figure 8 includes an outer RAM sheet 88 that extends across a rear 346 of the array assembly 350, in a similar manner to the outer portion of the isolating body 64 of Figure 6, to act as another isolation barrier. The array assembly 350 shown in Figure 8 is otherwise the same as the array 40 of Figure 2 and defines an antenna that can be used in the system 50 of Figure 3, as a direct substitute for the array 40 of Figure 2. The isolation barriers of the array assembly shown in Figure 8, namely the inner and outer RAM sheets, may be purely of RAM or may comprise a mixture of materials. For example, the RAM sheets may be formed from metal-backed RAM, namely metal sheets that are partially or fully covered with RAM. Figure 9 shows another array assembly 450, which is based on the array assembly 350 of Figure 8 and incorporates RF front-end electronics for the elements 10. Specifically, Figure 9 shows boxes 90 supported on the RAM sheets 88, which boxes 90 house electronics for operating the elements 10 of the array assembly 450. The electronics may include, for example, subsystems such as low noise amplifiers, voltage-controlled oscillators for transmission, downconverters and filters, among other components. The boxes 90 are of metal in this example, although may be of other materials. Figure 9 depicts the boxes with bare faces, but it is also possible for the boxes to be covered by RAM. The array assembly 450 of Figure 9 is otherwise the same as that of Figure 8, and may be implemented in the system 50 of Figure 3 in the same way as the array assembly of Figure 8. Integrating front-end electronics into the envelope of the array assembly 450 means that the array assembly 450 represents a compact antenna unit, with a reduced packaging requirement in the system 50 in which the antenna is hosted. This arrangement also reduces cabling relative to if the front-end electronics were provided separately from the array. It is noted that front-end electronics may be incorporated into other arrays and array assemblies, including those shown in Figures 2, 4 and 6, in a similar manner. Turning now to Figure 10, an alternative antenna element 110 is shown that is shaped to provide similar performance characteristics to the element 10 of Figure 1, but with enhanced compatibility for use with another sensor. The element 110 of Figure 10 is generally similar in structure and configuration to the element 10 of Figure 1, and so the following description concentrates on the differences for clarity. To note some similarities briefly, however, like the element 10 of Figure 1, the element 110 shown in Figure 10 is a tapered slot element, and more specifically a Vivaldi element in this example, having a pair of similar conductive planar radiators 114 formed on opposed sides of a flat substrate 112, the radiators 114 each having a central slot 116 extending from a midpoint of a front edge 119 of the element 110 towards a centre of the element 110, which divides the radiator 114 into upper and lower segments 120a, 120b. Each slot 116 merges with a circular aperture 124 formed in the radiator 114. The element 110 is also symmetrical about an axis corresponding to an axis 118 of the slot 116. The element 110 includes a feed cable 128 extending across one of the radiators 114 to the slot 116 of that radiator 114, as for the element 10 of Figure 1. Although not shown in Figure 10, the element 110 also includes a series of vias around its perimeter as in the example of Figure 1, to connect the radiators 114 on each side of the element 110 together electrically. The element 110 of Figure 10 may also be of a similar size to that of Figure 1. In this example, the element 110 has a width of approximately 98mm and a length of approximately 94mm, although this is purely illustrative. In this respect, the width of the element 110 corresponds to a dimension between side edges 117 of the element 110 that extend parallel to the slots 116, while the length of the element 110 corresponds to a dimension between the front edge 119 and a rear edge 121 of the element 110. It follows that, unlike the element 10 of Figure 1, the element 110 of Figure 10 is not square but instead is wider than it is long. More specifically, an outer profile of the element 110 of Figure 10 has a different shape to that of the element 10 of Figure 1. In this respect, the rear edge 121 of the element 110 is curved and merges with the side edges 117. This may be regarded as effectively rounding off the corners at each end of the rear edge of the element 10 of Figure 1. Accordingly, the slot 116 of the element 110 tapers towards a curved outer edge of the element 110, namely the rear edge 121. The curvature of the rear edge 121 may be determined to enable the aperture 124 to remain circular. In another difference, in the element 110 of Figure 10 additional discrete apertures 130 are added in each of the segments 120a, 120b of the radiators 114, one on each side of the slot 116. The additional apertures 130 occupy a substantial portion of the associated segments 120a, 120b and are generally oblong, but with rounded corners so that the short ends of the slots are generally semi-circular. The precise shape and geometry of the element 110 may vary, to tune the performance characteristics of the element 110 as may be desired. For example, the curvature of the rear and side edges, and / or the size and shape of the apertures formed in the radiators, can be adjusted to tune performance. The configuration of the apertures may impact impedance matching, for example. The edges may have an entirely different shape, for example a sawtooth shape instead of curved as in the present example. The shaping of the rear edge of the element 110 and the addition of the apertures 130 has an overall effect of reducing the total mass of metal in the element 110 compared to the element 10 of Figure 1, without significantly compromising the performance of the element 110. In this respect, although the element 110 may have a slightly different response to the element 10 of Figure 1, the overall quality of performance may be similar. In turn, reducing the mass of metal in the element 110 enhances the compatibility of the element 110 for use alongside another sensor, such as a magnetic detector coil as discussed later with reference to Figure 13. In other words, the shaping of the element 110 enhances compatibility with a sensor by judiciously reducing the metal content of the element 110, while minimising any resulting degradation in performance of the element 110, such that the degradation may be negligible. As Figure 11 shows, the element 110 of Figure 10 can be loaded with RAM to form a loaded element 174 in a similar manner to the element 10 of Figure 1. In Figure 11, as in Figure 5, the RAM is provided in the form of a housing 176 having slots to receive the element 110. The housing 176 is shaped to complement the shape of the element 110 and to cover the side edges of the element 110 while leaving the front and the centre of the element 110 more exposed. Again, the RAM acts to improve the directionality of the element 110, in particular by suppressing side lobes in its radiation pattern. Figure 12 shows that loaded elements 174 as shown in Figure 11 can be formed into an array 540, in a similar manner to the array 240 of Figure 6. The array 540 of Figure 12 is configured in a similar manner to the array 240 of Figure 6, but offers enhanced compatibility with other sensors due to the shaping of the elements 110. Like the array 240 of Figure 6, the array 540 of Figure 12 defines an antenna, which can be used in the antenna system 50 shown in Figure 3, for example, as a direct substitute for the array of Figure 2. In the array 540 shown in Figure 12, the individual RAM housings of the loaded elements 174 are combined into a single, integrated housing 178 that receives all four elements 110 of the array 540. The integrated housing 178 also includes a rear face 180 that covers the rear of the array 540 and that extends between the respective rear ends of the elements 110. The array 540 may therefore alternatively be regarded as an array assembly comprising an array of elements 110 in combination with the integrated housing 178. It is noted that a similar rear face of RAM may be incorporated into other examples. Such rear faces may cover the rear of the array exactly, or may overhang the array to form side extensions around the periphery of the array. Although not shown in Figure 12, additional isolation barriers may also be added to the array 540, as in the examples of Figures 6 and 8. Similarly, the array 540 of Figure 12 may incorporate front-end electronics in a similar manner to the array shown in Figure 9. Turning to Figure 13, the array 540 of Figure 12 is shown in combination with, and colocated with, a magnetic detector (MD) coil 182, the coil 182 forming part of a metal detection system within a device in which the array 540 is used. For example, the metal detection system may correspond to the metal detection system 55 of the device 53 shown in Figure 3, while the array 540 may be used in the antenna system 50 of Figure 3. The array 540 and the MD coil 182 each represent sensors, and so operate alongside one another. More specifically, the array 540 acts as a radar sensor while the MD coil 182 acts as a metal detector sensor. In this context, the shaping of the elements 110 of the array 540 is configured to reduce electromagnetic impacts of the array 540 on the metal detection system, without significantly compromising the performance of the array 540 itself. The MD coil 182 shown in Figure 13 corresponds to a housing of a known multi-arm coil that is typically employed by magnetic detectors. The MD coil 182 is depicted as a toroidal element with a circular cross section in Figure 13, although the coil may have a different shape in practice whilst having an internal envelope that is compatible with the array 540 that acts as a radar sensor. Alternative shapes for the MD coil 182 may include elliptical, rectangular or a ‘figure of 8’ shape, with curved or straight arms, for example. The array 540 is positioned with its front end within the MD coil 182, so that the MD coil 182 encircles and surrounds the front of the array 540, with respective central axes of the array 540 and the MD coil 182 being aligned. Accordingly, the MD coil 182 generally extends in a plane that is orthogonal to the respective planes in which the elements 110 of the array 540 extend. In this example, as noted above the shaping of the elements 110 of the array 540 is configured to enhance compatibility between the array 540 and the MD coil 182, for example by reducing the metal content of the elements 110, to minimise any impact on operation of the MD coil 182 caused by the array 540. There is therefore a synergy between the array 540 and the MD coil 182. The shaping of the elements 110 or of the array 540 may alter in various ways to satisfy the relevant operational objectives of the system. For example, the shaping of the elements 110, by reducing side effects arising from induced current excited during operation of the MD coil 182, may support alterations to the shape of the overall envelope of the array 540 and so enhance flexibility to accommodate the MD coil 182. In this respect, the sensitivity of the MD coil 182 to any nearby metallic objects is accounted for when incorporating the array 540, to minimise stray coupling. This may entail, for example, minimising currents that are induced on the metallic features of the array 540. Gaps may also be introduced to offer discontinuity obstacles, for example to form loops of similar shape to those primarily on the MD coil 182. Such gaps could be added to the radiators 114 of the array 540, for example by forming the radiators 114 with a grid-like construction. The separation between and placement of the sensors, namely the array 540 and the MD coil 182, may also be configured to reduce the impact that either sensor has on operation of the other. In addition, the metal thickness of the elements 110 of the array 540 influences its impact on the MD coil 182, and so may be minimised. In principle, however, any of the arrays described above can be used in combination with an MD coil in a similar way. Turning finally to Figures 14 and 15, these show further variants of arrays and array assemblies defining antennas that can, for example, be used as sensors in the system 50 of Figure 3, as a direct substitute for the array of Figure 2. In these variants, the individual elements are not parallel to a central axis of the array as in the above examples, but are instead inclined so that respective planes in which the elements extend, and therefore the sides of the array, converge with one another. In these examples, the sides of the arrays converge forwardly, from the rear of the array to the front of the array, so that the front of the array has a smaller area than the rear of the array. The arrays are therefore generally pyramidal in form, albeit truncated pyramids. Configuring an array with inclined sides may support enhanced common illumination of the ground, for example if the array baseline increases flexibly to allocate more space to larger elements, to improve broadband coverage towards lower operating frequencies. In addition, inclining the sides of an array may help to reduce potential blockage if the array is used in combination with an MD coil, as in the example of Figure 13. More specifically, Figure 14 shows an array 640 that may be regarded as representing a variant of the array 240 shown in Figure 6, the array 640 being formed from four of the loaded elements 74 of Figure 5 arranged in a square formation. As for the array 240 shown in Figure 6, in the array 640 of Figure 14 the loaded elements 74 are arranged side-by-side in a loop to define sides of the array 640 that surround a hollow interior, while the array 640 has opposed open sides defining a front 644 and a rear 646 of the array 240. So, the array 640 of Figure 14 has a similar general layout and shape as the array 240 of Figure 6. However, differently to the array 240 of Figure 6, in the array 640 of Figure 14 the elements 74 are oriented such that the sides of the array 640 are inclined relative to a central axis 642 of the array 640, so that the sides converge from the rear 646 to the front 644. Each element 74 is inclined towards the central axis 642 at a similar angle relative to the central axis 642. Thus, a square opening defining the front 644 has a smaller area than a corresponding generally square opening defining the rear 646, and the central axis 642 intersects respective centres of the front 644 and the rear 646 of the array 640. Correspondingly, respective slot axes 18 of the elements 74 converge with one another, and with the central axis 642 of the array 640, from the rear 646 to the front 644. The array 640 therefore has the general form of a truncated square pyramid and so is pyramidal. The elements 74 are positioned so that they contact one another at ends of their respective front edges 75. Due to the inclination of the elements 74, the corners of the elements 74 are spaced apart at the rear 646 of the array 640, and triangular gaps 77 are defined between each neighbouring pair of elements 74. These gaps 77 are open in this example but could alternatively be filled with RAM, which RAM could be separate from or integral with the RAM carried by the individual elements 74. Like the array of Figure 6, the array 640 of Figure 14 forms part of an array assembly 650 that also includes an isolation barrier that is similar to that used in the array assembly 150 shown in Figure 4, namely an isolating body 664 comprising RAM, the isolating body 664 having an inner portion 666 that extends into the interior of the array 640 and an outer portion 668 extending across the rear 646 of the array 640. In this example, the inner portion 666 of the isolating body 664 is generally similar to that of the example shown in Figure 4, whereas the outer portion 668 is enlarged and shaped to correspond to the size and shape of the rear 646 of the array 640. As for the earlier examples, the loaded elements 74 provide for reduced side lobes from the elements 10, for example, while the isolating body 664 provides further refinement by resisting interaction between the elements 10, for example interference arising from any residual side lobes of the elements 10, and / or external interference. The array 640 shown in Figure 14 is otherwise similar to the arrays 40, 240 of Figures 2 and 6. Similarly, the array assembly 650 shown in Figure 14 is similar to the array assemblies 150, 250 of Figures 4 and 6, and defines an antenna. Finally, Figure 15 shows an array 740 forming part of an array assembly 750 that may be regarded as representing a variant of the array assembly 350 shown in Figure 8, or alternatively a variant of the array assembly 650 shown in Figure 14. More specifically, the array 740 shown in Figure 15 is formed from four of the loaded elements 74 of Figure 5, which are arranged in a similar manner as in the array 640 of Figure 14. Accordingly, the elements 74 define the sides of the array 740 of Figure 15 and are inclined with respect to a central axis 742 of the array 740 to converge towards a front 744 of the array 740. Correspondingly, respective slot axes 18 of the elements 74 converge with one another and with the central axis 742. As for the array 640 of Figure 14, the array 740 of Figure 15 therefore has the general form of a truncated square pyramid and so is pyramidal. The array assembly 750 of Figure 15 differs from that of Figure 14 in that the isolating body 664 is replaced with isolation barriers in the form of an isolating body 764 comprising a pair of inner RAM sheet members 788a that extend diagonally across the interior of the array 740, and a rear RAM sheet member 788b that extends across the rear 746 of the array 740. In this example, the inner and rear RAM sheet members 788 are formed integrally with one another to define the isolating body 764. In other examples the RAM sheet members 788 could be separate parts. The array 740 of Figure 15 differs from the array 640 of Figure 14 in that the elements 74 are spaced apart to create gaps between them at the front 744 of the array 740, which gaps accommodate ends of the inner RAM sheet members 788a of the isolating body 764. The RAM sheet members 788 of the isolating body 764 are positioned similarly to the RAM sheets of the array assembly of Figure 8. In this respect, each inner RAM sheet member 788a extends between, and into, the gaps at diametrically opposed corners of the array 740. The inner RAM sheet members 788a cross one another along the central axis 742 of the array 740, so that the inner RAM sheet members 788a together form an X-shape. As for the example of Figure 8, arranging the inner RAM sheet members 788a diagonally enables the length of the sheet members 788a to be increased relative to the corresponding features of the isolating body 664 of Figure 14, thereby providing more isolation, and potentially also enhancing flexibility for integration with associated electronics. The rear RAM sheet member 788b extends across the rear 746 of the array 740 and is shaped and sized to complement the rear 746 of the array 740, in a similar manner to the outer portion of the isolating body 664 of Figure 14, to provide further isolation. In this example, the isolating body 764 further includes wedge portions 94 at the ends of the inner RAM sheets 788, the wedge portions 94 being shaped to fill the gaps between neighbouring elements 74. To reiterate, the arrays 640, 740 and corresponding array assemblies 650, 750 of Figures 14 and 15 are otherwise generally similar to the earlier examples, for example the array assemblies of Figures 6 and 8, and can be used as sensors in an antenna system such as that represented in Figure 3. Also, the array assemblies 650, 750 of Figures 14 and 15 can be used in combination with a MD coil in a similar way to the array of Figure 13. Arrays with inclined sides like those shown in Figures 14 and 15 could be formed from different elements, for example the loaded element 174 shown in Figure 11 or elements without RAM loading such as those of Figures 1 and 10. Arrays with inclined sides may also have various alternative shapes and can be sized to suit the requirements of the application in which they are to be used. 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. For example, in some embodiments one or more elements of an array may be configured with a permanent mode or state, for example by hardwiring the element, while the remaining elements remain operable in multiple modes. It is also possible to configure an array with a different number of elements, instead of four elements in a square array as in the above examples. In such embodiments, suitably shaped isolating features may be included that are adapted to the shape of the array. An array may have as few as two elements, for example, three elements, or more than four elements. In one example, an array such as shown in Figure 6 may be modified so that an opposed pair of elements are each replaced by respective walls of RAM, so that the array has only two elements, arranged in parallel. Similarly, arrays may have a different shape to the square formations of the above examples. For example, an array of four elements could have a rhombus shape. Arrays could have a variety of other shapes, including a star shape for example, or more generally a range of regular and irregular polygonal shapes. In some embodiments, an array may be formed from dissimilar elements. In one example, an oblong array could be formed from four elements comprising two longer elements and two shorter elements. Various alternative tapered slot elements may be used to form antennas in other embodiments. These include other non-linear tapered slot elements, linear tapered slot antennas, constant width slot elements, and others. In addition, a Vivaldi element or another type of tapered slot element may be formed in a different way to the above examples, for example as a PCB or as a solid piece of sheet metal. Each element may therefore have a single radiator instead of a pair of radiators as in the above examples. Although the arrays, array assemblies and the antennas that they define described above are configured for ground-penetrating radar systems, in other embodiments an antenna or antenna system may be configured for, or otherwise applicable to, different applications.

Claims

1. An antenna system, comprising:an antenna comprising an array of tapered slot elements; anda feed arrangement for feeding the elements of the array;wherein at least one element of the array is independently-operable.

2. The antenna system of claim 1, wherein each element of the array is operable independently of the other elements of the array.

3. The antenna system of claim 1 or claim 2, configured so that at least two elements of the array are operable in different modes simultaneously.

4. The antenna system of claim 3, wherein the different modes comprise a transmission mode and a receiver mode.

5. The antenna system of any preceding claim, wherein the elements of the array are substantially identical to one another.

6. The antenna system of any preceding claim, wherein the elements of the array extend in respective vertical planes, in use.

7. The antenna system of any preceding claim, wherein respective slots of the elements of the array extend along axes that are parallel to one another.

8. The antenna system of any preceding claim, wherein each element of the array is parallel to another element of the array.

9. The antenna system of any preceding claim, wherein each element of the array is orthogonal to another element of the array.

10. The antenna system of any preceding claim, wherein the array is cube-shaped.

11. The antenna system of any of claims 1 to 5, wherein at least one element of the array is inclined relative to a central axis of the array and / or relative to another element of the array.

12. The antenna system of any of claims 1 to 5 or claim 11, wherein respective slots of two or more elements of the array extend along respective axes that converge.

13. The antenna system of any of claims 1 to 5, 11 or 12, wherein the array is pyramidal.

14. The antenna system of any preceding claim, wherein the array comprises four elements arranged in a square formation.

15. The antenna system of any preceding claim, wherein the elements of the array are arranged in a loop.

16. The antenna system of any preceding claim, wherein the elements of the array are arranged side-by-side.

17. The antenna system of any preceding claim, comprising a transmitter.

18. The antenna system of claim 17, wherein the feed arrangement is configured to connect the transmitter selectively to at least one element of the array independently of the other elements of the array.

19. The antenna system of claim 18, wherein the feed arrangement is configured to connect the transmitter selectively to any one or more elements of the array independently of the other elements of the array.

20. The antenna system of any preceding claim, comprising a receiver.

21. The antenna system of claim 20, wherein the feed arrangement is configured to connect the receiver selectively to at least one element of the array independently of the other elements of the antenna.

22. The antenna system of claim 21, wherein the feed arrangement is configured to connect the receiver selectively to any one or more elements of the array independently of the other elements of the antenna.

23. The antenna system of any preceding claim, wherein the antenna comprises radiation absorbent material.

24. The antenna system of claim 23, wherein radiation absorbent material is arranged in an interior of the array, between the elements.

25. The antenna system of claim 24, wherein radiation absorbent material extends across the interior.

26. The antenna system of claim 25, wherein radiation absorbent material extends diagonally across the interior.

27. The antenna system of claim 25 or claim 26, wherein radiation absorbent material forms a cross formation within the interior.

28. The antenna system of any of claims 24 to 27, wherein the antenna comprises one or more sheet members comprising radiation absorbent material in the interior of the array.

29. The antenna system of any of claims 24 to 28, wherein the antenna comprises an absorber element in the interior of the array, the absorber element comprising radiation absorbent material.

30. The antenna system of any of claims 23 to 29, wherein radiation absorbent material is arranged on at least one of the elements of the array.

31. The antenna system of claim 30, wherein at least one of the elements of the array is received in a housing comprising radiation absorbent material.

32. The antenna system of claim 30 or claim 31, wherein the radiation absorbent material covers side edges of the or each element.

33. The antenna system of any of claims 30 to 32, wherein the radiation absorbent material is arranged so that a slot of the or each element is exposed.

34. The antenna system of any of claims 23 to 33, wherein the radiation absorbent material is arranged to resist interaction between elements of the array.

35. The antenna system of any preceding claim, configured to support high integrity time domain waveforms.

36. The antenna system of any preceding claim, wherein each element of the array is shaped to enhance compatibility of the antenna with a sensor.

37. The antenna system of claim 36, wherein an outer profile of each element of the array is shaped to enhance compatibility of the antenna with the sensor.

38. The antenna system of claim 37, wherein, for each element, an edge towards which a slot of the element tapers is shaped to enhance compatibility of the antenna with the sensor.

39. The antenna system of claim 38, wherein the edge is curved.

40. The antenna system of any of claims 36 to 39, wherein the sensor comprises a magnetic sensor.

41. The antenna system of any preceding claim, wherein each element of the array has a curved outer edge towards which a slot of the element tapers.

42. The antenna system of any preceding claim, wherein each element of the array comprises a slot that tapers towards a rear of the array.

43. The antenna system of any preceding claim, wherein at least two elements of the array have different polarisations.

44. The antenna system of any preceding claim, wherein, for each element, one or more radiators of the element comprises one or more apertures on each side of a slot of the element.

45. The antenna system of any preceding claim, wherein the antenna system is for a ground-penetrating radar system.

46. A method of operating an antenna, the antenna comprising an array of tapered slot elements, the method comprising operating elements of the array in different modes simultaneously.

47. The method of claim 46, comprising operating at least one element in a transmitting mode and at least one other element in a receiving mode simultaneously.

48. The method of claim 46 or claim 47, performed on an antenna of a groundpenetrating radar system.

49. An antenna for a ground-penetrating radar system, the antenna comprising at least one tapered slot element, and radiation absorbent material.

50. The antenna of claim 49, wherein the or each element carries radiation absorbent material.

51. The antenna of claim 49 or claim 50, wherein the or each element is received in a housing comprising radiation absorbent material.

52. The antenna of any of claims 49 to 51, wherein the radiation absorbent material covers side edges of the or each element.

53. The antenna of any of claims 49 to 52, wherein the radiation absorbent material is arranged so that a slot of the or each element is exposed.

54. The antenna of any of claims 49 to 53, comprising an array of tapered slot elements.

55. The antenna of claim 54, wherein radiation absorbent material is arranged in an interior of the array, between the elements.

56. The antenna of claim 55, wherein radiation absorbent material extends across the interior.

57. The antenna of claim 56, wherein radiation absorbent material extends diagonally across the interior.

58. The antenna of claim 56 or claim 57, wherein radiation absorbent material forms a cross formation within the interior.

59. The antenna of any of claims 55 to 58, comprising one or more sheet members comprising radiation absorbent material in the interior of the array.

60. The antenna of any of claims 55 to 59, comprising an absorber element in the interior, the absorber element comprising radiation absorbent material.

61. The antenna of any of claims 54 to 60, wherein radiation absorbent material is arranged on at least one of the elements of the array.

62. The antenna of any of claims 54 to 61, wherein the radiation absorbent material is arranged to resist interaction between elements of the array.

63. The antenna of any of claims 49 to 62, wherein the or each element has a curved outer edge towards which a slot of the element tapers.

64. The antenna of claim 63, wherein radiation absorbent material covers the curved edge.

65. An antenna system comprising the antenna of any of claims 49 to 64.

66. The antenna system of any of claims 1 to 46 or claim 65, comprising a controller configured to operate the antenna to transmit and / or receive signals.

67. The antenna system of any of claims 1 to 46, claim 65 or claim 66, wherein the antenna is operable as an active sensor.5 68. A ground-penetrating radar system comprising the antenna system of any ofclaims 1 to 46 or claims 65 to 67.

69. A device comprising the antenna system of any of claims 1 to 45 or claims 65 to 67, or the ground-penetrating radar system of claim 68.1070. The device of claim 69, configured as a handheld device.

71. The device of claim 69 or claim 70, comprising a metal detection system.15 72. The device of claim 71, wherein the or each element of the antenna is shaped toenhance compatibility of the antenna with a sensor of the metal detection system.

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