Technical field

This invention relates to the field of antennas. In particular, the invention relates to omnidirectional antennas for moving platforms, and especially blade antennas for use in aviation.

Background

In communications systems for moving platforms such as aircraft, waterborne vessels and other vehicles, omnidirectional antennas are often used to accommodate the variable position and orientation of the platform with respect to remote entities with which a communication link is to be established, such as ground stations or other moving platforms. Omnidirectional antennas offer substantially uniform gain in a particular plane, typically a horizontal or ‘azimuth’ plane, and so are insensitive to the relative position of an entity at the other end of a communication link.

Steerable antennas are an alternative solution for moving platforms, but tend to be more complex and higher cost than omnidirectional antennas.

When used on an aircraft, omnidirectional antennas are typically mounted at the top or on the underside of the aircraft, to maximise performance in the azimuth plane. In this respect, for antennas mounted to the side of the body a shadowing effect can arise in the azimuth plane as signals are blocked by the aircraft body.

The performance of a given antenna is dictated by a range of factors, including the type of antenna and radiating element, the size and geometry of the antenna components and the context of use. For omnidirectional performance, ideally the antenna should be of a shape that does not bias performance for any particular direction. This generally entails an antenna with rotational symmetry and, ideally, a circular cross-section in the plane in which omnidirectional performance is desired. So, for example, a simple mast antenna may have excellent performance characteristics in this respect. However, the presence of a mast or other circularly symmetric-shaped antenna may be undesirable for moving platforms, in view of considerations relating to aerodynamics, vulnerability to damage or even aesthetics, among other constraints. This is of particular concern in avionics, where antennas are mounted on an exterior surface of an aircraft and can thus be exposed to extremes of air flow and mechanical vibration. Various other forms of antennas are therefore common for use on moving platforms, which offer a compromise between electromagnetic and mechanical performance.

Some avionic communication systems use vertically-polarised signals, which are typically received most effectively by radiating elements that are relatively tall in a vertical plane, and generally by elements that are taller than they are wide. Conversely, communication using horizontally-polarised signals typically entails the opposite geometry for the radiating elements, namely elements that are wide horizontally, which can be more challenging to implement in avionics.

Vertically-polarised signals can be handled by a simple mast antenna oriented vertically, where practical considerations allow. Otherwise, blade antennas that are vertically tall and horizontally thin are also well-suited for use with vertically-polarised signals, whilst offering superior mechanical characteristics in this context. Blade antennas are composed of antenna components housed within a blade radome having the general form of a fin. The blade radome is mounted to extend orthogonally from a surface of the aircraft in alignment with a longitudinal axis of the aircraft and therefore also with the primary direction of travel. The thin profile of the blade radome helps to optimise aerodynamics, while also being more mechanically robust than a mast antenna.

However, more advanced communications systems, including those implementing LTE (long-term evolution), Wi-Fi and 5G (fifth generation broadband cellular) standards, require dual-polarisation to maximize the channel capacity and to mitigate the effect of fading. Accordingly, there exists a need for antennas that offer omnidirectional performance whilst also being capable of handling orthogonally-polarised signals.

A particular challenge in this respect is that aircraft antennas are generally mounted to a conductive exterior surface of the aircraft, such as the fuselage exterior, and typically use that mounting surface as a ground plane. Any tangential electric field incident on the mounting surface therefore tends to be cancelled. This prevents signals that are polarised in a plane parallel to the mounting surface from being efficiently radiated in directions that are substantially parallel to the mounting surface. For antennas mounted to generally horizontal regions of the aircraft body, at the top of the body or on its underside, this means that horizontally-polarised signals are difficult to handle. This can be addressed by spacing horizontally-polarised radiating elements from the mounting surface, typically by at least a quarter of a wavelength (in air or vacuum) of the band for which the elements are tuned. However, this conflicts with mechanical performance considerations since aerodynamics and robustness to vibration favour a low-profile arrangement.

Figure 1 shows a known blade antenna 1 for an aircraft that addresses some of the above problems. The blade antenna 1 has a base 2 that mounts to a surface of the aircraft. An elongate plate-like fin 3 of generally rhomboidal cross-section extends from the base 2, orthogonally to the mounting surface. The fin 3 is surmounted by a centrally- mounted top disc 4 that extends substantially parallel to the mounting surface and therefore vertically. The fin 3 and the top disc 4 therefore together form a T-shaped configuration.

The diameter of the top disc 4 is similar to the height of the fin 3, typically being in the range of one third to one half of the intended wavelength (in air or vacuum), giving typical values of 75mm diameter for radiating elements tuned to transmit and receive at 2GHz, and a diameter of up to 200mm for radiating elements tuned to transmit and receive at 800MHz.

The fin 3 contains one or more vertically-polarised monopole elements. Correspondingly, the top disc 4 contains at least two horizontally-polarised dipole elements extending in a plane parallel to their polarisation. Together, the radiating elements of the fin 3 and the top disc 4 create a dual-polarised antenna arrangement, with spacing of the horizontally-polarised elements from the mounting surface to avoid the cancelling effect of the ground plane being provided for by the height of the fin 3.

The blade antenna 1 is mounted on the aircraft such that the fin 3 is aligned to a forward direction of travel of the aircraft, to minimise the surface area presented by the fin 3 to oncoming air and thereby optimise aerodynamics. However, to function correctly the top disc 4 requires a wide, disc-shaped upper radome section, which has a negative impact on drag. The top disc 4 also performs poorly with dynamic loads and vibration during flight, and so the blade antenna 1 of Figure 1 is particularly unsuited to use on helicopters and other rotary wing aircraft.

In extreme cases the top disc 4, which is expensive to manufacture, can detach from the fin 3 at high speeds.

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 omnidirectional blade antenna for a moving platform. The moving platform may be an aircraft, a waterborne vessel or a land vehicle, for example.

The antenna comprises a support structure configured for mounting to an exterior wall of the platform defining a mounting surface, and a set of low-profile radiators supported by the support structure so that the radiators are substantially parallel to each other. The set of radiators includes at least one radiator having a first polarisation, and at least one radiator having a second polarisation, the second polarisation being orthogonal to the first polarisation. The radiators also typically extend substantially orthogonally to the mounting surface, when the antenna is mounted.

Low-profile radiators have a low profile in one direction and therefore predominantly extend in a two-dimensional plane whilst being thin in a direction orthogonal to that plane, even if the radiator may have some limited curvature. Accordingly, the radiators being parallel to one another means that the respective planes in which the radiators predominantly extend are parallel to each other.

By using low-profile radiators and arranging the radiators parallel to one another, the overall profile of the antenna can be kept thin to maintain a generally blade or planar profile and thereby optimise aerodynamics. Meanwhile, providing at least one radiator having each polarisation entails that the antenna is equipped to handle, i.e. transmit and/or receive, signals with orthogonal polarisations. The support structure may include a base that is configured to mount to the wall of the moving platform, in which case the radiators are supported by the base, directly or indirectly.

The first polarisation may be in a plane that is substantially parallel to the mounting surface. The first polarisation may correspond to horizontal polarisation, for example, in which case the second polarisation corresponds to vertical polarisation.

The, or each, radiator having the second polarisation may be disposed between the, or each, radiator having the first polarisation and a base of the support structure that is configured to mount to the wall of the moving platform. In other words, the, or each, radiator having the second polarisation may be disposed between the moving platform and the, or each radiator having the first polarisation when the antenna is mounted. The radiators of the set may be aligned along an axis that is orthogonal to the mounting surface when the antenna is mounted. This arrangement beneficially spaces the radiator(s) having the first polarisation from the mounting surface, which avoids interference if the first polarisation corresponds to horizontal polarisation or is otherwise generally parallel to the mounting surface.

Each radiator of the set may comprise a respective plate element that is supported by the support structure to extend substantially orthogonally to the mounting surface, when mounted. The plate element may be a patch of a patch radiator, for example, or a ground plane having a slot defining a slot radiator in another example. The plate elements may be flat or slightly curved. Respective plate elements of the radiators of the set may comprise respective coplanar surfaces. For example, a radiator having the first polarisation may have a surface that is coplanar with a surface of a radiator having the second polarisation.

The set of radiators may comprise a first pair of radiators having the first polarisation, the radiators of the first pair being arranged back-to-back, in that the radiators are mutually-spaced and aligned and, typically, arranged symmetrically about a plane extending between them. Correspondingly, the set of radiators may also comprise a second pair of radiators having the second polarisation, the radiators of the second pair being arranged back-to-back. The radiators of the first pair may be fed together. Similarly, the radiators of the second pair may be fed together. The use of back-to-back radiators that are fed together is a convenient way to provide omnidirectional performance, in that each radiator of the pair covers a respective half of the azimuth plane. A back-to-back pair can also be configured as horizontally polarised even when the radiators extend vertically, as the spacing between the radiators of the pair provides a horizontal dimension. Each radiator of the first pair may be aligned with a respective radiator of the second pair. A gap between the radiators of a pair may be small compared to a height or width of the radiators, to enhance omnidirectional performance. For example, the gap between the radiators may be less than or equal to a tenth of the wavelength (in air or vacuum) that the radiators are configured to transmit and/or receive.

At least one of the radiators of the set may be a patch radiator. At least one of the radiators of the set may be a slot radiator. The set of radiators may comprise a mixture of different types of radiators, for example one or more patch radiators in combination with one or more slot radiators. Each radiator having the same polarisation may be of the same type.

The antenna may comprise a blade radome enclosing the radiators. The antenna may be configured to use the mounting surface as a ground plane.

The support structure may comprise a mounting face arranged to engage the mounting surface, when mounted, in which case the radiators of the set extend substantially orthogonally to the mounting face.

The antenna may comprise a stripline arrangement configured to feed the radiators of the set.

The antenna may comprise a printed circuit board that comprises the set of radiators. The printed circuit board may be arranged to extend orthogonally to the mounting surface, when mounted. Conveniently, a printed circuit board is inherently thin and planar, and therefore helps to achieve an aerodynamic blade antenna.

The printed circuit board may at least partially define the support structure, in which case the printed circuit board may substantially define the antenna. The printed circuit board may comprise multiple layers, and so may be embodied as a multi-layer printed circuit board. The set of radiators may comprise a first pair of back- to-back radiators having the first polarisation, the radiators of the first pair being arranged back-to-back on different layers of the printed circuit board. The set of radiators may comprise a second pair of radiators having the second polarisation, the radiators of the second pair being arranged back-to-back on different layers of the printed circuit board. Each radiator of the first pair may be on the same layer as a respective radiator of the second pair. The antenna may comprise a ground plane formed on a layer extending between the first pair of radiators, and optionally also between the second pair of radiators, if present, so that the ground plane is shared by the first and second pairs of radiators.

The antenna may comprise a feed arrangement on a layer of the printed circuit board extending between the first pair of radiators and between the second pair of radiators, the feed arrangement being configured to feed the first and second pairs of radiators. The feed arrangement may extend between two ground planes formed on respective layers of the PCB, in which case the feed arrangement and the ground planes may form a stripline arrangement. The feed arrangement is configured to feed the first pair of radiators in anti-phase and the second pair of radiators in-phase, particularly if the first polarisation corresponds to horizontal polarisation.

Each radiator of the set may be electrically thin.

A spacing between radiators of the set having different polarisations may be less than a width or height of the radiators, and for example less than half of the height or width. The spacing between the radiators may be of the order of a fifth of the wavelength (in air or vacuum) that the radiators are tuned to handle, for example. Correspondingly respective centres of the radiators may be separated by approximately half of the wavelength (in air or vacuum) that the radiators are configured to handle.

In some embodiments, at least one of the radiators is configured as a half-wave radiator. Optionally, all of the radiators of the set are configured as half-wave radiators. In such embodiments, the or each radiator has dimensions corresponding to approximately half of a wavelength of signals handled by the radiators when inside a dielectric supporting, or otherwise filling a space next to, the radiator. In some embodiments, at least one of the radiators is substantially square, and optionally all of the radiators of the set are substantially square.

The invention also extends to an aircraft comprising the antenna of the above aspect.

Another aspect of the invention provides a printed circuit board for an omnidirectional blade antenna for a moving platform. The printed circuit board comprises a set of low- profile radiators that are parallel to each other. The set of radiators includes at least one radiator having a first polarisation, and at least one radiator having a second polarisation, the second polarisation being orthogonal to the first polarisation.

Each radiator of the set may comprise a respective flat plate element, so that the respective plate elements of the radiators extend in parallel planes.

Another aspect of the invention provides a method of fabricating an omnidirectional blade antenna for a moving platform. The antenna comprises a support structure that is configured for mounting to an exterior wall of the platform defining a mounting surface. The method comprises arranging a set of low-profile radiators on the support structure so that the radiators are substantially parallel to each other, and configuring the radiators so that the set of radiators includes at least one radiator having a first polarisation, and at least one radiator having a second polarisation, the second polarisation being orthogonal to the first polarisation. The method may further comprise mounting the antenna so that the radiators extend substantially orthogonally to the mounting surface, and substantially parallel to a longitudinal axis of the platform and/or a primary direction of travel of the platform.

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

Reference has already been made to Figure 1, which shows a known dual-polarised antenna. One or more embodiments of the invention will now be described, by way of example only, with reference to the remaining accompanying drawings, in which like features are assigned like numerals, and in which:

Figure 2 shows an aircraft including a dual-polarised blade antenna according to an embodiment of the invention;

Figure 3 shows a partially exploded view of the antenna of Figure 2;

Figure 4 shows internal components of the antenna of Figure 2 in perspective view;

Figure 5 corresponds to Figure 4 but shows a front view;

Figure 6 shows an exploded view of a multilayer printed circuit board of the antenna of Figure 2;

Figure 7 is a side view of the printed circuit board of Figure 6, with some layers hidden;

Figure 8 shows a gain plot in an azimuth plane for vertically-polarised radiators of the antenna of Figure 2; and

Figure 9 shows a gain plot in an azimuth plane for horizontally-polarised radiators of the antenna of Figure 2.

Detailed description

In general terms, embodiments of the invention provide dual-polarised blade antennas for moving platforms such as aircraft, the antennas being configured to handle orthogonally-polarised signals while offering substantially omnidirectional performance in a flat package. In this respect, omnidirectional performance entails 360° coverage in an azimuth plane. For example, embodiments of the invention may provide equivalent performance to the known antenna described above with reference to Figure 1 , but without requiring a top disc and therefore avoiding the associated drawbacks. In embodiments of the invention, a flat package is achieved through the use of low- profile radiators that are electrically thin and hence have a small thickness relative to the wavelengths that they are configured to handle. The skilled reader will appreciate that a low-profile radiator’ has a geometry that presents a low profile, i.e. a relatively small cross-sectional area, in a particular direction, for example a direction of travel. The radiator may therefore be generally tall and wide in one plane and thin in another, orthogonal plane. Low-profile radiators are therefore typically planar, or close to planar. Examples of low-profile radiator elements include thin, flat plates, or thin plates having a small degree of curvature to maintain a small depth relative to the height and width of the plate. Curved elements may be used for conformance with another surface, for example. Patch radiators and slot radiators are common examples of low-profile radiators.

As they have a low-profile, the radiators can be aligned to a plane that is: orthogonal to a surface on which the blade antenna is mounted; and parallel to a primary direction of travel. This minimises the lateral thickness of the blade antenna and therefore enhances aerodynamics.

The radiators therefore typically extend generally vertically, since the blade antenna is typically mounted on substantially horizontal surfaces of a moving platform. To achieve horizontal polarisation in vertically extending low-profile radiators, pairs of such radiators may be arranged back-to-back to provide for the required horizontal dimension.

The antenna of the embodiment described below is configured for use on an aircraft, although it is noted that in general terms antennas according to the invention may be used in any type of moving platform, including waterborne vessels and land vehicles for example.

To provide context for the invention, Figure 2 shows an aircraft 10 that fitted with a dualpolarised omnidirectional blade antenna 12 according to an embodiment of the invention.

The aircraft 10 is a helicopterof a known kind. As noted above, the known blade antenna 1 of Figure 1 is particularly unsuited to use with rotary wing aircraft such as a helicopter, in view of its poor ability to handle the dynamic loads and vibrations that are characteristic of such aircraft The biade antenna 12 of the present embodiment does not suffer from the same problems and therefore provides a particular benefit in this context of use. It is noted, however, that the blade antenna 12 may be used on any type of aircraft.

The antenna 12 is mounted to the underside of a body 14 of the aircraft 10 in a central position beneath a cockpit 16. As noted above, positioning the antenna 12 on the underside ensures that no part of the azimuth plane is blocked by the body 14 of the aircraft 10 and therefore allows the antenna 12 to achieve omnidirectional performance. This is entirely illustrative, however, and the antenna 12 may be positioned elsewhere on the aircraft 10, particularly on the top of the aircraft 10.

Figure 3 shows the blade antenna 12 in a partially exploded view. This shows that the blade antenna 12 comprises a base 18, a printed circuit board (PCB) 20 mounted to the base 18, and a blade radome 22 that fits over and encloses the PCB 20 and couples to the base 18. Figure 5 shows that the antenna 12 further includes a pair of feed terminals 24 that are mounted on the underside of the base 18, beneath the PCB 20, one of which is visible in Figure 5 with the second being hidden behind.

The base 18 is mounted to an exterior wall of the aircraft 10, with an underside of the base 18 engaging the underside of the aircraft 10 in this example. The underside of the base 18 therefore defines a mounting face 26 of the antenna 12 that engages a mounting surface of the aircraft 10 in this embodiment.

The base 18 is rigid and robust, to secure the antenna 12 to the aircraft 10 in a mechanically robust manner. The base 18 has the general form of an oblong plate with rounded corners, such that the mounting face 26 of the base 18 is planar. To absorb any relative differences in shape of the mounting face 26 and the mounting surface of the aircraft 10, mounting fairings may be interposed between the base 18 and the aircraft surface. In other embodiments, the mounting face 26 may be shaped to conform to the surface of the aircraft 10.

Mounting holes 28 are provided in the base 18 to provide for fixing of the base 18 to the mounting surface. A pair of parallel, elongate ribs 30 extend orthogonally from an upper surface of the base 18 to define a gap between them in which the PCB 20 is mounted and secured by suitable fixings, the ribs 30 therefore together defining a PCB mount 32. The ribs 30 are evenly spaced on each side of a central longitudinal axis 34 of the base 18, such that the PCB 20 is mounted centrally on the base 18.

As is conventional, the antenna 12 is configured to use the mounting surface and the wall of the aircraft 10 to which it is mounted as a ground plane. To provide for this, the base 18 is formed from conductive material such as steel, so that it is configured to act as an interface plate that connects electrically to the wall of the aircraft 10. Specifically, the ribs 30 provide electrical contact between ground planes of the PCB 20 and the main ground plane defined by the aircraft wall, as described in more detail later.

The blade radome 22 is of a fin-like shape that is similar to known blade antennas, being relatively tall and wide in side view, whilst being thin in a transverse plane. In general terms, the blade radome 22 is shaped for optimised aerodynamics and stability whilst providing sufficient interior space to accommodate the PCB 20, which in this example is approximately 100mm tall, 35mm wide and 5mm thick.

The antenna 12 is mounted such that the blade radome 22 is aligned to a longitudinal axis of the aircraft 10, and therefore to a forward direction of travel of the aircraft 10, to minimise the surface area presented by the antenna 12 to oncoming air and thereby optimise aerodynamics.

Although not shown in Figure 3, the blade radome 22 is provided with mounting holes that align with those of the base 18 to enable the radome 22 to be secured to the base 18 and to the mounting surface of the aircraft 10.

As Figures 4 and 5 show, the PCB 20 is a thin, rigid, flat rectangular board, as is conventional, having the dimensions noted above in this example. The two largest exterior surfaces of the PCB 20 on opposed sides of the PCB 20 are planar and define main surfaces 36 of the PCB 20. A front main surface 36 is visible in Figure 4, the opposite main surface 36 that is not visible therefore defining a rear main surface. The PCB 20 is thin in the sense that the spacing between the front and rear main surfaces 36 is small relative to the height and width of the main surfaces 36. The PCB 20 is a multilayer PCB 20, being composed of multiple laminated layers that are bonded together to form a unitary stack. The interfaces between neighbouring layers extend in planes parallel to the main surfaces 36 of the PCB 20.

Figure 5 shows best that the PCB 20 is mounted between the ribs 30 of the PCB mount 32 so that end regions of the main surfaces 36 engage planar inner faces of the ribs 30, the PCB 20 being mounted to extend upright from the base 18. The PCB 20 is therefore oriented with its main surfaces 36 parallel to the longitudinal axis 34 of the base 18 and orthogonal to the mounting face 26 of the base 18. In turn, the main surfaces 36 of the PCB 20 are orthogonal to the mounting surface of the aircraft 10.

As shown in Figures 6 and 7, end regions 38 of the PCB 20 that are received between the ribs 30 of the PCB mount 32 are enlarged in a direction parallel to the longitudinal axis 34 of the base 18, to match the shape of the ribs 30. These end regions 38 of the PCB 20 are in electrical contact with the ribs 30, as shall become clear later.

The PCB 20 comprises a set of radiators 40 in the form of patch radiators. Each radiator 40 comprises and is substantially defined by a layer of conductive material formed on, and covering a portion of, one of the main surfaces 36 of the PCB 20, to form a patch. The patches may be of copper, for example, and are formed on a substrate of the PCB 20 in the conventional manner.

The PCB 20 acts as a support structure for the radiators 40 in a broad sense. In turn, the base 18 supports the PCB 20, such that the PCB 20 and the base 18 may be regarded as collectively forming a support structure for the radiators 40 in this embodiment.

Each patch has a patch face 42 defined by a generally planar, uninterrupted rectangular surface facing outwardly from the PCB 20. As is conventional for conductive elements of a PCB 20, the patches are very thin, having been formed by an etching process, and so may be regarded as flat plate elements that predominantly extend in planes parallel to the main surfaces 36 of the PCB 20. It follows that each radiator 40 comprises, and is substantially defined by, a respective flat plate element that extends in a plane that is orthogonal to the mounting face 26 of the base 18, and therefore also to the mounting surface of the aircraft 10.

As the skilled reader will appreciate, the size and shape of each radiator 40 in part determines its resonant frequency and bandwidth, and so can be adjusted to suit the requirements of the system in which the antenna 12 is used. In general terms, the dimensions of the radiators 40 are typically small compared to the wavelengths in air that they are tuned to handle such that the radiators 40 act as point sources, which inherently enhances omnidirectional performance. For example, the height and width of each radiator element may be less than a third, and often less than a quarter, of the wavelength (in air or vacuum) that the element is configured to handle. In this embodiment, the height and width of the radiators 40 are such that the radiators 40 are substantially square, although different shapes may be used in other embodiments.

Conversely, as set out in more detail later, the radiators 40 may be configured as halfwave radiators, in that each radiator 40 has dimensions that are approximately half the wavelength of the signals that the radiator 40 is configured to handle when inside a material representing a dielectric filling a space adjacent to the radiator 40. Accordingly, the radiators 40 are relatively small in terms of the wavelength in air but around half of the wavelength inside the dielectric substrate supporting the radiators 40.

The flat plate elements defined by the patches of the radiators 40 are inherently low- profile and electrically thin. Other types of radiators may be used in other embodiments that have alternative low-profile elements, such as slot radiators in which plate elements include slots defining openings for a resonance cavity. It is noted that although convention is to designate the slots as the ‘radiators’ in such arrangements, in reality the ‘radiator’ may be the combination of any of: the slot; the plate in which the slot is formed; a resonance cavity to which the slot defines an entrance; and a ground plane. Such a combination defining a radiator can nonetheless be arranged to have a low profile.

It is also possible to have a mixture of different types of radiators. For example, the PCB may include one or more patch radiators having one polarisation, and one or more slot radiators having a different polarisation. More specifically, the set of radiators 40 of the PCB 20 comprises four radiators 40 arranged in two back-to-back pairs. Two of the radiators 40 are visible in Figure 4, each of which is paired with another radiator 40 at a corresponding position on the rear main surface 36 of the PCB 20, so that each radiator 40 is aligned with the other radiator 40 of the associated pair. Accordingly, the uppermost radiator 40 in Figure 4, which is furthest from the base 18, belongs to a first pair of radiators 40a, and correspondingly the radiator 40 closest to the base 18 belongs to a second pair of radiators 40b.

The radiators 40 of each pair are fed by a common feed line, so that the radiators 40 of each pair act together in combination.

The respective patch faces 42 of the two radiators 40 visible in Figure 4 are coplanar and are aligned along a radiator axis 44, which extends orthogonally to the base 18 and intersects axes passing through the respective centres of each pair of radiators 40. Respective side edges of the patch faces 42 are therefore colinear. Accordingly, the second pair of radiators 40b is disposed between the first pair of radiators 40a and the base 18.

A gap between the patch faces 42 of the radiators 40 of each pair is small, for example being of the order of a fifth of the wavelength (in air or vacuum) that the radiators 40 are tuned to handle. It follows that the separation between the axes passing through the respective centres of each pair of radiators 40 is of the order of half of the wavelength (in air or vacuum) that the radiators 40 are configured to handle in this example.

The first and second pairs of radiators 40 are arranged with orthogonal polarisations. In this embodiment, each radiator 40 of the first pair 40a is horizontally-polarised, and each radiator 40 of the second pair 40b is vertically-polarised. It follows that the horizontally- polarised radiators 40 are spaced from the base 18 and therefore from the wall of the aircraft 10 that acts as a ground plane. This addresses the problem of cancelling of tangential fields by the ground plane. As the antenna 12 is mounted to the underside of the aircraft 10, in a region of the aircraft body 14 in which the mounting surface is substantially horizontal when the aircraft 10 is level, horizontally-polarised signals will tend to be cancelled. It is therefore a benefit of this embodiment that the horizontally- polarised radiators 40 are positioned furthest from the base 18 and therefore outside a region close to the mounting surface in which horizontally-polarised signals are cancelled.

Correspondingly, locating the vertically-polarised radiators 40 between the base 18 and the horizontally-polarised radiators 40 utilises the space between the base 18 and the horizontally-polarised radiators 40 and thus minimises the height and width of the antenna 12 and so aids mechanical performance.

Signals do not pass effectively through the PCB 20, and so each radiator 40 effectively covers only half of the azimuth plane, the remaining half being blocked by the PCB 20 behind the radiator 40. Accordingly, arranging the radiators 40 in back-to-back cooperating pairs enables each pair to act in tandem to cover the entire azimuth plane and therefore provide omnidirectional performance in transmission and reception, for signals of each polarisation.

The omnidirectional performance of each pair of radiators 40 is enhanced by the small gap between the respective radiators 40, which corresponds to the thickness of the PCB 20, which is 5mm in this embodiment. In this respect, Figures 4 and 5 make clear that the gap between a pair of radiators 40 is much smaller than the height and width of each radiator 40, for example being less than half, or even less than a quarter, of the radiator height and/or width. More generally, the gap between a pair of radiators 40 is typically less than a tenth, and in this example approximately a thirtieth, of the wavelength (in air or vacuum) that the pair handles.

Placing the radiators 40 of each pair close together enables them to act together to enhance coverage in the side regions, enabling the pair of radiators 40 to transmit or receive signals travelling parallel, or close to parallel to the patch faces 42.

Although the spacing between the radiators 40 of each pair is small, it is nonetheless sufficient to provide the horizontal dimension necessary to enable the first pair of radiators 40a to capture horizontally polarised signals, albeit within a relatively narrow bandwidth that is limited by the spacing of the radiators 40. In this respect, the bandwidth of each radiator 40 is related to a depth of a patch cavity defined by a space between the patch radiator 40 and a corresponding ground plane within the PCB 20, which is shown in Figure 6 and is described later. The thickness of the patch cavities is kept small to obtain good omnidirectional performance, and also to maintain a thin overall profile for the antenna 12.

As Figure 5 shows, the feed terminals 24 are positioned on the mounting face 26 of the base 18 directly beneath the PCB 20, and are configured to allow feed connections to be made to the radiators 40. Specifically, a first feed terminal 24, which is visible in Figure 5, feeds the first pair of radiators 40a, while a second feed terminal 24 that is identical to and hidden behind the first feed terminal 24 in Figure 5 feeds the second pair of radiators 40b. Each feed terminal 24 is arranged to be received inside an opening in the wall of the aircraft 10, so that the feed terminals 24 protrude into the interior of the aircraft 10 where electrical connections can be made. The feed terminals 24 may be of any suitable type, including SMA or TNG type connectors, for example.

Moving on to Figure 6, the PCB 20 is shown in an exploded view to reveal the features of individual layers, making clear that the PCB 20 is composed of four layers 46 in this embodiment. In horizontal succession from right to left in Figure 6, the PCB 20 comprises: a first layer 46a, which includes the front main surface 36 of the PCB 20 and two of the four radiators 40 of the set; a second layer 46b, which includes a first ground plane 48 on a front face; a third layer 46c, which includes a feed arrangement on a front face and a second ground plane 48 on a rear face; and a fourth layer 46d, which includes the rear main surface 36 of the PCB 20 and the remaining two radiators 40 of the set.

The first and second ground planes 48 are each defined by continuous conductive layers that substantially cover the surfaces of their respective layers 46, aside from interruptions caused by plated holes defining vias that penetrate the second and third layers 46b, 46c to provide interconnections between layers 46, as shall become clear. Each ground plane 48 therefore extends over the entire cross-section of the PCB 20 and so is shared by radiators 40 of the first and second pairs.

The first and second ground planes 48 are parallel to each other and are electrically connected to each other by one or more vias, to form an effective common ground plane that is shared by all of the radiators 40.

In turn, at least one of the first and second ground planes 48 is electrically connected to the base 18 and therefore electrically connected to the wall of the aircraft 10 as noted above. Accordingly, the effective common ground plane of the PCB 20 is electrically continuous with the main ground plane defined by the wall of the aircraft 10.

To connect the first and/or second ground plane 48 to the base 18, the PCB 20 exposes the ground plane 48 on its external sides in the end regions 38, below the second pair of radiators 40b. The ribs 30 of the base 18 are attached to the end regions 38 of the PCB 20 with metallic features that ensure a pressure contact on the exposed conductive material of the ground plane 48, thereby providing grounding of the antenna 12.

As shown more clearly in Figure 7, in which the first and second layers 46a, 46b of the PCB 20 are hidden, the feed arrangement comprises a pair of conductive traces 50 formed on the third layer 46c of the PCB 20. These conductive traces 50 extend between the first and second ground planes 48, being separated from each ground plane 48 by substrate material. Accordingly, the conductive traces 50 of the feed arrangement define stripline networks 52 that are configured to feed the radiators 40, in which the traces 50 act as transmission lines and the substrate material between the traces 50 and the first and second ground planes 48 represents a dielectric. Although not shown in the figures, stripline matching studs may be added to expand the bandwidths of the stripline networks 52.

It is noted that signals that the radiators 40 handle have a different wavelength when inside the dielectric represented by the substrate material compared to when in air. The ratio of the wavelength in air to the wavelength in the dielectric represented by the substrate is the square root of the applicable dielectric constant. This ratio may have a value that is typically 1.5 to 2 times larger or more, for example. It follows from the above that the radiators 40 may be configured as half-wave radiators, in that the radiators 40 have dimensions that are approximately half the wavelength of the signals inside the dielectric represented by the substrate filling the cavity formed between each pair of radiators 40.

More specifically, the third layer 46c includes a first stripline network 52a that feeds the first pair of radiators 40a towards the top of the PCB 20, and a second stripline network 52b that feeds the second pair of radiators 40b near the base 18, the first and second stripline networks 52a, 52b being electrically isolated from each other. Correspondingly, the first pair of radiators 40a are electrically isolated from the second pair of radiators 40b. Accordingly, the first and second pairs of radiators 40 are fed individually.

Each stripline network 52 comprises a conductive trace 50, for example of copper, that extends between a respective base terminal 54 at the bottom of the PCB 20, within the PCB mount 32, to one or more radiator terminals 56 that connect to the associated radiators 40.

The first stripline network 52a is shown to the left in Figure 7, and the second stripline network 52b is shown to the right.

The first stripline network 52a comprises a first base terminal 54a located at a lower end of the third layer 46c, and between the ribs 30 of the PCB mount 32, when mounted. The first base terminal 54a is matched to 50ohm impedance and is connected to the first feed terminal 24.

A first trace portion 50a of the first stripline network 52a extends vertically upwardly from the first base terminal 54a, to a level approximately halfway up the PCB 20. The trace 50 then turns through a corner to define a second trace portion 50b that extends horizontally a short distance, before turning again to define a third trace portion 50c that extends vertically in alignment with the radiator axis 44. The third trace portion 50c terminates at a location slightly above the centres of the first pair of radiators 40a, and horizontal branches 58 of equal length extend in opposite directions from the upper end of the third trace portion 50c. Each branch 58 terminates in a respective radiator terminal 56, such that the radiator terminals 56 are equally spaced to each side of the radiator axis 44. Each radiator terminal 56 connects to a respective one of the radiators 40 of the first pair, through respective vias extending through the layers 46 of the PCB 20.

The second stripline network 52b comprises a conductive trace 50 having a series of horizontal and vertical portions that extend between a second base terminal 54b, which is at a location towards the right of the PCB mount 32 as shown in Figure 7, and a radiator terminal 56 that is positioned close to the respective centres of the radiators 40 of the second pair and in alignment with the radiator axis 44. Both radiators 40 of the second pair therefore connect to the same terminal of the second stripline network 52b, through a common via. Similarly to the first base terminal 54a, the second base terminal 54b is matched to 50ohm impedance and is connected to the second feed terminal 24.

Each of the first and second stripline networks 52a, 52b splits electrical power between the respective pair of radiators 40. The second stripline network 52b feeds the second pair of radiators 40b in-phase through the common radiator terminal 56 located in alignment with the centres of the radiators 40. In contrast, the two radiator terminals 56 of the first stripline network 52a on either side of the radiator axis 44 feed the first pair of radiators 40a in anti-phase.

The first and second feed terminals 24 provide separate RF inputs to the first and second pairs of radiators 40, therefore keeping the vertically-polarised and horizontally- polarised signals separate.

Each connection between the base terminal 54 of a stripline network 52 and its respective feed terminal 24 is implemented by exposing the copper of the stripline trace 50 at the lower edge of the PCB 20, by removing the substrate locally above the trace 50 where terminal pins will be soldered. Other embodiments use via holes instead of cut-outs on the substrates, to allow access to the stripline traces 50, which can be beneficial if SMT connectors or direct connections to cables are used.

Turning finally to Figures 8 and 9, these show azimuth patterns for the second pair of radiators 40b and the first pair of radiators 40a respectively. Accordingly, Figure 8 shows the performance of the antenna 12 with vertically-polarised signals in the azimuth plane, and Figure 9 shows the corresponding performance with horizontally-polarised signals.

Figure 8 shows an azimuth pattern that is close to circular, indicating near uniform gain for all angles in the azimuth plane. Figure 8 shows a slight reduction in gain around plus and minus 90°, although the reduction is small and so the performance nonetheless represents omnidirectional performance with vertically-polarised signals.

Figure 9 shows that the antenna 12 performs similarly with horizontally-polarised signals, in this case exhibiting slight reductions in gain at 0° and 180°, although again the performance is sufficiently uniform to qualify as omnidirectional. 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 other embodiments the set of radiators may not be provided by a single multilayer PCB, but instead by multiple discrete components.

Also, some embodiments require only two radiators, having orthogonal polarisations.

Various alternative feed arrangements to that described above are possible. For example, instead of via holes connecting the stripline traces to the radiators, slots could be formed on the ground planes, just below the stripline traces, to couple into the patch cavities. Slot feeding would potentially allow for air-filled suspended patches rather than printed patches as in the above example, which could offer advantages in terms of both bandwidth and cost. Capacitive coupling feeding is also possible.

In further variants, radiators or back-to-back pairs of radiators may be stacked, so that each radiator, or pair of radiators, covers a distinct frequency band, to provide for dualband or multi-band operation.

As noted above, slot radiators may be used instead of patch radiators. In one implementation, in a variant of the arrangement shown in Figure 6 the first and fourth layers carrying the patch elements may be dispensed with and back-to-back slots may be formed into the ground planes of the second and third layers. The feed arrangement can then be updated accordingly.