WO2024128953A1

WO2024128953A1 - Antenna array with filtering properties

Info

Publication number: WO2024128953A1
Application number: PCT/SE2023/051219
Authority: WO
Inventors: Matti KUOSMANEN; Tero Kiuru; Juha ALA-LAURINAHO; Ville Viikari; Jari Holopainen; Johan MALMSTRÖM; Sten Gunnarsson
Original assignee: Saab Ab
Priority date: 2022-12-13
Filing date: 2023-12-04
Publication date: 2024-06-20
Also published as: FI20226102A1

Abstract

An antenna array (1) comprising an antenna array base (2), at least two antenna sub-elements (3, 3'), each having a length axis (4, 4'), is described. The antenna sub-elements (3, 3') are arranged on the antenna array base (2), at a distance from each other and at a distance from a centre plane (13). Each antenna sub-element (3, 3') has an overall 3-dimensionally tapered shape, Each antenna sub-element (3, 3') comprises indentations (6, 6') at a plurality of different positions along the length axis (4, 4'), wherein the positions along the length axis (4, 4') are essentially the same on the different antenna sub-elements (3, 3'), and wherein the indentations (6, 6'), at each different position along the length axis (4, 4'), are shaped such that the distance between an antenna sub-element (3, 3') and the centre plane (13) is larger, in a plane through an indentation (6, 6') compared to the distance between said antenna sub-element (3, 3') and the centre plane (13) in planes adjacent to said indentation.

Description

ANTENNA ARRAY WITH FILTERING PROPERTIES

TECHNICAL FIELD

The present invention relates to an antenna array comprising an antenna array base and at least two antenna sub-elements arranged at a distance from each other on the antenna array base. Each antenna sub-element comprises an antenna sub-element body extending along a length axis from the antenna array base to a tip. Each antenna sub-element body has an overall tapered shape from the antenna array base to the tip.

BACKGROUND ART

Antenna arrays of the type comprising an antenna array base and at least two antenna sub-elements arranged at a distance from each other on an antenna array base are known from the prior art. Each antenna sub-element comprises an antenna subelement body extending along a length axis from the antenna array base to a tip. Each antenna sub-element body is typically rotationally symmetrical and has an overall tapered shape from the antenna array base to the tip. In electromagnetic sense, an antenna element is the slot between two antenna sub-elements.

In general, antenna arrays are designed to operate on a specific frequency range. On this frequency range, the antenna array has feasible performance, including active reflection coefficient and beam steering capability.

However, many antenna sub-elements, do not have an inherent limit for the highest operation frequency. In other words, there may be several passbands above the designed highest frequency. This might be a problem for a receiving radio system, if there is a strong interfering signal outside the designed frequency band. The signals above the operational frequency band may saturate the receiver if the system does not have a good filter. It is desirable to avoid said problem. It is also desirable that the antenna array has a good directivity, such that the electromagnetic radiation may be transmitted in the desired direction or received from the desired direction. Another implementation in which high-performance filters are needed is the transmission of a noisy or distorted signal, or a signal containing spurious emissions. Since realistic transmitters always generate some extra frequencies unintentionally, i.e., spurious signals, and also distort the intended transmission and corrupt it with noise, these unwanted components are also transmitted to air if the transmitting radio system does not include a proper filter. The out-of-band emissions may interfere other systems and radiate to unwanted directions.

Clearly, radio systems should have a good filter for both reception and transmission. Good filters have a steep transition band, low losses, and high attenuation. However, these requirements often mean bulky and expensive external filter that might not fit into modem active antenna array.

SUMMARY OF THE INVENTION

An objective of present invention is to provide an antenna array with an antenna array base and antenna sub-elements arranged on the antenna array base , wherein the antenna sub-elements have an overall tapered shape from the antenna array base to a tip, wherein the antenna array at least reduces some of the problems with the prior art.

Another objective of the present invention is to provide an antenna array with an antenna array base and antenna sub-elements arranged on the antenna array base, wherein the antenna sub-elements have an overall tapered shape from the antenna array base to a tip, which antenna array provides a stronger attenuation of signals above the designed frequency band than antenna arrays according to the prior art.

Another objective is to provide an antenna array, which can be manufactured with rational methods.

At least one of the above objectives is fulfilled with an antenna array according to the independent claim.

Further advantages are provided with the features of the dependent claims.

According to a first aspect, an antenna array is provided. The antenna array comprises an antenna array base, and at least two antenna sub-elements, each having a length axis. The antenna sub-elements are arranged on the antenna array base, at a distance from each other and at a distance from a centre plane centred between their length axes. Each antenna sub-element extends along the length axis from the antenna array base to a tip, and has an overall 3-dimensionally tapered shape towards the tip. The antenna array is characterized in that each antenna sub-element comprises indentations at a plurality of different positions along the length axis, wherein the positions along the length axis are essentially the same on the different antenna subelements, and wherein the indentations are shaped such that the distance between an antenna sub-element and the centre plane is larger, in a plane, perpendicular to the length axis through an indentation compared to the distance between said antenna sub-element and the centre plane in planes, perpendicular to the length axis, adjacent to said indentation.

As described the antenna sub-element has an overall 3-dimensionally tapered shape. The indentations at each different position along the length axis have an extension, perpendicular to the length axis, in different non-parallel directions. In other words, the indentations are not only in a single plane together with the length axis.

The antenna array base is a structural support for the antenna sub-elements. The antenna array base may include the feeding structure and balanced to unbalanced transformers. The antenna array base may also function as a ground plane for the antenna sub-elements.

In an electromagnetic sense, one antenna element is the slot between two antenna sub-elements.

An antenna array according to the first aspect provides low pass filtering of emitted or received signals. The contribution to low-pass filtering is achieved mainly by the indentations. The lowest stopband frequency, known as the cut-off frequency, is mainly determined by the effective depth of the indentations.

An antenna array with indentations according to the first aspect, provide a stronger attenuation of signals above the designed frequency band than antenna arrays according to the prior art. The effective depth of the deepest indentations define the cut-off frequency of the antenna

Dielectric material may be configured in the indentations. By having a dielectric material in the indentations, the effective depth is increased. This may be used to lower the cut-off frequency or to enable the indentations to be made shallower. Thus, if the desired cut-off frequency requires deep indentations, the actual necessary depth may be decreased by adding the dielectric material in the secondary cut-outs.

Dielectric material may also be in the space between the antenna sub-elements. Thus, the dielectric material in one antenna sub-element may extend to the dielectric material in the adjacent antenna sub-element.

Each antenna sub-element may comprise a plurality of stacked metal sheets, wherein the shape and size of the metal sheets vary along the length axis according to the shape of the antenna sub-element. If the antenna sub-elements comprise a plurality of stacked metal sheets, the fabrication of the antenna element may be facilitated.

When each antenna sub-element comprises a plurality of stacked metal sheets, every other metal sheet may correspond to an indentation and every other metal sheet may correspond to the antenna sub-element between two indentations. This further facilitates the fabrication of the antenna element as the different metal sheets then may be fabricated using, e.g., punching.

Each antenna sub-element may comprise a plurality of non-conductive substrates, wherein on each substrate at least two conductive layers are attached on opposite sides of the substrate connected to each other by a plurality of electrical connections, and wherein the shape of the conductive layers on the substrates vary along the length axis according to the shape of the antenna sub-element. Arrangement of the antenna sub-elements in this way facilitates the fabrication of the antenna array, which in principle may be fabricated by using a plurality of standard substrates with at least two conductive layers on opposite sides of the substrate and standard fabrication techniques for the assembly of printed circuit boards, PCBs.

Every other substrate with its conductive layers may correspond to an indentation and every other substrate with its conductive layers may correspond to the antenna sub- element between two indentations. It is of course also possible to have a plurality of substrates and its conductive layers corresponding to an indentation and/or to the antenna sub-element between two indentations. This enables the indentations and the antenna sub-element between two indentations to have different dimensions.

The dielectric material in the indentations surrounds the antenna sub-element in each indentation. This facilitates the fabrication of the antenna element as the dielectric material then may be provided in continuous sheets, which may provide the dielectric material for all antenna sub-elements in the antenna array.

The antenna array may comprise at least four antenna sub-elements arranged in a 2x2 arrangement, wherein each antenna sub-element comprises two electrical feeding points, to enable dual-polarized operation.

The antenna array may comprise a waveguide surrounding each antenna subelement, wherein the waveguides are arranged in a diagonal cross pattern between the antenna sub-elements. The waveguide both improves the filtering performance of the low-pass filter and adds a high-pass filter function, which in combination realizes a band-pass filter function.

The indentation may have essentially the same dimension along the length axis as the distance between two adjacent indentations. This facilitates fabrication of the antenna array.

The number of indentations in each antenna sub-element may be at least 5. The number of indentations may be in the range 5-20. Such a number makes it possible to tune the antenna element to minimize the losses and to provide the desired filtering.

The dielectric material may surround at least a part of the antenna sub-element. The dielectric material may have perforations along the length axis. It might be preferable to tune the dielectric material in the indentations and/or in the space between the main curves to achieve the desired properties of the antenna element with regard to filtering frequencies, and losses and impedance matching. One way of doing this is to provide the dielectric material with said perforations. The mean dielectric constant of the dielectric material is decreased by such perforations. As an alternative to perforations, the dielectric constant of the dielectric material may be varying in any direction.

An antenna array as described above may be used in many different implementations. One example of an implementation is a transceiver comprising an antenna array according to the above description.

According to a second aspect, an antenna array is provided, comprising an antenna array base, at least two antenna sub-elements, each having a length axis, wherein the antenna sub-elements are arranged on the antenna array base, at a distance from each other and at a distance from a centre plane centred between their length axes. Each antenna sub-element extends along the length axis from the antenna array base to a tip, and has an overall 3-dimensionally tapered shape towards the tip. The antenna array is characterized in that each antenna sub-element comprises a plurality of non-conductive substrates, wherein on each substrate at least two conductive layers are attached on opposite sides of the substrate connected to each other by a plurality of electrical connections, and wherein the shape of the conductive layers on the substrates vary along the length axis according to the shape of the antenna subelement.

The antenna array according to the second aspect can be manufactured with rational methods, such as standard fabrication techniques for the assembly of printed circuit boards. No metalworking is necessary.

The antenna array may comprise waveguides surrounding each antenna sub-element, wherein the waveguides is arranged in a diagonal cross pattern between the antenna sub-elements. The waveguides add a high-pass filter function.

An antenna array according to the second aspect described above may be used in many different implementations. One example of an implementation is a transceiver comprising an antenna array according to the above description. In the following, preferred embodiments of the invention will be described with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a cross-section of an antenna array 1 according to an embodiment.

Figure 2 shows a cross-section along the length axes at A-A in Figure 1 .

Figure 3 shows an alternative cross-section along the length axes at A-A in Figure 1 .

Figure 4 shows an alternative cross-section along the length axes at A-A in Figure 1 .

Figure 5 shows a cross-section through the centre of the two adjacent antenna subelements, in an antenna array 1 according to an alternative embodiment.

Figure 6 shows in a perspective view an antenna array according to an alternative embodiment.

Figure 7 shows a cross-section of an antenna array 1 according to an alternative embodiment.

Figure 8 is a plan view of one substrate in Figure 7.

Figure 9 is a plan view of one substrate in Figure 7 according to an alternative embodiment.

Figure 10 is a plan view of one substrate similar to the substrate in Figure 8, in which waveguides have been implemented.

Figure 11 illustrates a transceiver comprising an antenna array.

Figure 12 is a perspective view of an antenna array similar to the antenna array shown in Figure 7, but the substrate material between the conductive layers is hidden.

Figure 13 shows a cross-section of an antenna array according to an alternative embodiment. DETAILED DESCRIPTION

In the following embodiments of the invention will be described with reference to the drawings. The same reference numeral is used for the same or similar feature in the different drawings. The drawings are not drawn to scale.

Figure 1 shows a cross-section of an antenna array 1 , comprising an antenna array base 2, and a first and a second antenna sub-element 3, 3’, each having a length axis 4, 4’. The cross-section is along a first symmetry plane P1 (Figures 2-4) through the centre of the two antenna sub-elements 3, 3’. In electromagnetic sense, an antenna element is the slot between two antenna sub-elements 3, 3’. The antenna subelements 3, 3’, are arranged on the antenna array base 2, at a distance from each other and with their length axes 4, 4’, essentially parallel to each other. A centre plane 13 is centred between the antenna sub-elements 3, 3’. Also shown in Figures 1 are a second symmetry plane 13’ and a third symmetry plane 13”, which each are parallel to the centre plane 13. The first antenna sub-element 3 is symmetrical in the first symmetry plane P1 and the second symmetry plane 13’ and the second antenna subelement 3’ is symmetrical in the first symmetry plane P1 and the third symmetry plane 13”. Each antenna sub-element extends along its length axis 4, 4’, from the antenna array base 2 to a tip 5, 5’, and has an overall tapered shape towards the tip 5, 5’. Each antenna sub-element comprises indentations 6, 6’, at a plurality of different positions along the length axis 4, 4’. The positions of the indentations are essentially the same on the two different antenna sub-elements 3, 3’, i.e. , at the position of an indentation 6 on the first antenna sub-element 3 there is a corresponding indentation 6’ on the second antenna sub-element. In Figure 1 , a cross section A-A is marked. The indentations 6, 6’, may be filled with a dielectric material as is indicated by the dashed lines 7, 7’.

The dielectric material is preferably some sort of plastic, but it is possible to use another dielectric material such as e.g. porcelain, mica, or glass. The shortest distance D1 between the first antenna sub-element 3, and the centre plane 13, at the position of the specific indentations at the line A-A is shown by the double arrowed first line D1 . The shortest distance D2 between the antenna sub-element 3, and the centre plane 13, at a position adjacent to and above the indentations at the line A-A is shown by the double arrowed second line D2. The shortest distance D3 between the antenna sub- element 3, and the centre plane 13, at a position adjacent to and below the indentations at the line A-A, is shown by the double arrowed third line D3. The distance D1 between the antenna sub-element 3 and the centre plane 13, is larger in a plane through any one of the indentations 6, 6’, compared to the distances D2, D3, between said antenna sub-elements 3, 3’, in planes, along the length axes 4, 4’, adjacent to said indentation 6, 6’. In Figure 1 the antenna sub-elements have the same shape and the depth of an indentation may be defined either in relation to the size of the antenna sub-element 3, 3’ adjacent to and above the indentation 6, 6’, or in relation to the size of the antenna sub-element 3, 3’, below the indentation 6, 6’. In the first case, the depth of each one of the indentions at the line A-A is (D1-D2) and in the second case, the depth of each one of the indentations at the line A-A is (D1 -D3). The depths (D1 - D2) and (D1 -D3) can be made arbitrarily small, depending on the filter requirements. Without any dielectric material in the indentations 6, 6’, the material in the indentations is air, which have a relative permittivity or dielectric constant close to 1 . Plastics typically have a dielectric constant of 2-5. The effect of the dielectric material in the indentations is that the effective depth of the indentations is increased. This affects the filtering properties of the antenna sub-elements 3, 3’. As can be seen in Figure 1 the dielectric material in one antenna sub-element extends to the dielectric material in the adjacent antenna sub-element. Thus, the effective distance is increased also in the region between the antenna sub-elements but outside the indentations. It is possible to have a dielectric material in the entire space between the antenna sub-element and not only in the positions of the indentations. Thus, the space between the antenna subelements and between the dashed lines 7, 7’ in Figure 1 , may also be filled with a dielectric material.

As can be seen in Figure 1 the depth of the indentations 6, 6’, vary along the length axes 4, 4’. The deepest indentations are in the middle region along the length axes 4, 4’.

The antenna array 1 comprises at the antenna array base 2 an electrical feeding point 8, for the antenna formed by antenna sub-elements 3, 3’. An additional feeding point 8’ is shown for an antenna to the left formed partly by the first antenna sub-element 3. One feeding point 8, 8’, per sub-element is sufficient for single-polarized operation, while at least two feeding points 8, 8’, per sub-element are required for dual-polarized operation. Dual-polarized operation is possible when four or more antenna sub- elements are arranged in two dimensions such as in a 2x2 pattern. This will be described in further detail below with reference to Figure 4.

The antenna sub-elements 3, 3’ comprise an electrical contact from the antenna array base 2 to the tip 5. According to a first alternative, the antenna sub-elements may be of solid metal, or any other suitable conductive material. Such antenna sub-elements may be produced by common metalworking. According to another alternative, each antenna sub-element comprises a plurality of stacked metal sheets 9, 9’, of which only a few are shown in Figure 1 . The shape of the metal sheets vary along the length axis 4, 4’ according to the shape of the antenna sub-element 3, 3’. Every other metal sheet 9, 9’, corresponds to an indentation 6, 6’, and every other metal sheet 9, 9’, corresponds to the antenna sub-element 3, 3’, between two indentations 6, 6’. In the embodiment of Figure 1 , the indentation has essentially the same dimension L2 along the length axis as the distance L1 between two adjacent indentations, in the embodiment described above wherein each antenna sub-element comprises a plurality of stacked metal sheets, this means that each metal sheet 9, 9’, has the same thickness. The sum of L1 and L2 is the period of the indentations.

In Figure 1 , the depth of an indentation 6 is D1 -D2. The period L1 +L2 is essentially the same along the length axes 4, 4’. It is, however, possible to have a varying period along the length axis 4, 4’.

The period L1 +L2 of the indentations is much less than a wavelength at the highest desired operational frequency. In practical antenna sub-elements, the period L1 +L2 is less than half a wavelength, preferably 0.05-0.25 wavelengths at the highest stopband frequency. That is, if the pass band is at 6-18 GHz and the stop-band is at 18.5 - 40 GHz, the highest stopband frequency is 40 GHz. The cut-off frequency, in turn, is about 18 GHz.

The deepest of the indentations 6 essentially defines the filtering properties of the antenna element. The depth DD of the deepest indentations is 0.15-0.35 wavelengths, preferably 0.20-0.30 wavelengths. The wavelength is defined at the desired cut-off frequency of the filter. In practice, the above limitations results in that the depth D1-D2 of the deepest of the indentations 6, shown as DD in Figure 1 , preferably is longer than the period L1 +L2 of the indentations.

Other partitions of the antenna sub-element are of course possible. In another alternative embodiment, the antenna sub-element may comprise conductive layers supported on non-conductive substrates. Such an embodiment is described below with reference to Figure 7.

The number of indentations in the embodiment of Figure 1 is 17. It is, however, possible to have fewer indentations. The number of indentations in each antenna subelement is at least 5, and preferably in the range of 5-20.

Figure 2 shows a cross-section along the length axes 4, 4’, at A-A. In Figure 2 the parts below the dashed line B in Figure 1 , are not shown. In the embodiment with the antenna sub-elements comprising a plurality of sheets the squares 10, 10’, in Figure 2 correspond to indentations 6, 6’, while the circles 11 , 1 T, correspond to the antenna sub-element between indentations 6, 6’. As can be seen clearly in Figure 2, the distance D1 between the antenna sub-element 3, and the centre plane at the position of the indentations 6, 6’, is larger than the distance D3 between the antenna subelement 3, and the centre plane at the position between the indentations.

Figure 3 shows an alternative cross-section. In Figure 3 the squares 10, 10’, corresponding to the indentations 6, 6’, are considerably larger than in Figure 2. However, as can be seen clearly in Figure 3, the distance D1 between the antenna sub-element 3, and the centre plane at the position of the indentations 6, 6’, is still larger than the distance D3 between the antenna sub-element 3, and the centre plane at the position between the indentations.

Figure 4 shows an alternative cross-section. In Figure 4 the circles 12, 12’, correspond to the indentations 6, 6’, in Figure 1 . An antenna sub-element 1 with a cross section as shown in Figure 4 may be easier to manufacture in one piece as it may be done using turning as the only metal working process.

Also shown in Figures 2-4 are a second symmetry plane 13’ and a third symmetry plane 13”, which each are parallel to the centre plane 13. The first antenna subelement 3 is symmetrical in the first symmetry plane P1 and the second symmetry plane 13’ and the second antenna sub-element 3’ is symmetrical in the first symmetry plane P1 and the third symmetry plane 13”.

In Figures 2 and 3, the antenna sub-elements 3, 3’, are depicted as having a quadratic shape 10, 10’, at the indentations 6, 6'. In Figure 4, the antenna sub-elements 3, 3’, are depicted as having a circular shape 12, 12’, at the indentations 6, 6'. The antenna sub-elements may have other shapes at the indentations 6, 6’. It is preferable that the first antenna sub-element at the indentations 6, have a shape, which is simultaneously symmetrical in the first symmetry plane P1 and the second symmetry plane 13’ and that the second antenna sub-element at the indentations 6’, have a shape, which is simultaneously symmetrical in the first symmetry plane P1 and the third symmetry plane 13”. Other shapes may be envisaged. The cross-section of the antenna subelements 3, 3’ between indentations 6, 6’ are shown as having a circular shape 11 , 11 '. Preferably, the cross-section of the first antenna sub-element 3 between indentations 6 has a shape, which is simultaneously symmetrical in the first symmetry plane P1 and the second symmetry plane 13’ and that the second antenna subelement 3’ between indentations 6’, has a shape, which is simultaneously symmetrical in the first symmetry plane P1 and the third symmetry plane 13”. Other shapes may be envisaged.

Figure 5 shows a cross-section of an antenna array 1 according to an alternative embodiment. The cross-section is along a first symmetry plane P1 (Figures 2-4) through the centre of the two antenna sub-elements 3, 3’. The antenna array 1 comprises an antenna array base 2, and a first and a second antenna sub-element 3, 3’, each having a length axis 4, 4’. The antenna sub-elements 3, 3’, both have the same shape. In Figure 5 the centre plane 13 between the antenna sub-elements is a symmetry plane. The cross sectional shape 14, 14’ of the two adjacent antenna subelements define a tapered notch with indentations. The tapered notch 14, 14’, may have the shape of an exponential curve. The antenna array 1 comprises dielectric material 15, which surrounds the antenna sub-elements. The dielectric constant of the dielectric material 15 may vary along the length axes 4, 4’. As an alternative or additionally the dielectric material may have perforations 16 along the length axes 4, 4’. In operation, if electromagnetic radiation is to be transmitted at least one of the electrical feeding points 8, 8’, of each antenna sub element 3, 3’, is provided with an electromagnetic signal. Due to the configuration of the antenna array according to the present invention, the need for an external filter is not as high as for an antenna array according to the prior art. The indentations provide for a steeper attenuation of signals just above the cut-off frequency.

Figure 6 shows in a perspective view an antenna array 1 according to an alternative embodiment. The antenna array comprises nine antenna sub-elements 3 according to the embodiments described above. The antenna sub-elements 3 are arranged centred along row symmetry lines 17 and column symmetry lines 18. Each one of the antenna sub-elements 3 may comprise two electrical feeding points. This enables dualpolarized operation. All antenna sub-elements 3 have the same shape. As can be seen in Figure 6 the indentations are filled with dielectric material, in the form of dielectric sheets 19. There are large openings 28 in each one of the sheets such that dielectric material is present only between pairs of antenna sub-elements 3 forming antennas. The dielectric sheets 19 surrounds the antenna sub-elements 3, 3’, at the positions of the indentations along the length axes 4, 4’. The antenna sub-elements at the positions between the indentations along the length axes 4, 4’, (Figure 1 and Figure 5) are not surrounded by dielectric material.

Figure 7 shows a cross-section of an antenna array 1 according to an alternative embodiment. In Figure 7 each antenna sub-element 3 comprises a plurality of non- conductive substrates 20, wherein each substrate 20 comprise at least two conductive layers 21 , 22 attached on opposite sides of the substrate 20 connected to each other by a plurality of electrical connections 23, in the form of so called via fences. In the embodiment of Figure 7, the substrates 20 are common for all the antenna subelements 3. The shape of the conductive layers 21 , 22, on the substrates vary along the length axis 4 according to the shape of the antenna sub-element 3. The different substrates 20 with the conductive layers 21 , 22, are pressed together towards the antenna array base 2 by means of fasteners 24, such as screws, bolts or rivets. Also shown in Figure 7 are electrical feeding points 8, 8’, through which an electromagnetic signal may be fed to the antenna sub-elements 3, 3’. Figure 8 is a plan view of one substrate 20 in Figure 7. The substrate 20 shown in Figure 8 comprises nine different circular conductive layers 21 , corresponding to nine different antenna sub-elements 3 (Figure 7). The conductive layers 21 may have other shapes, but it is preferred that each conductive layer 21 is simultaneously symmetrical in a row symmetry line 17 and a column symmetry line 18. Also shown in Figure 8 are the electrical connections 23, in the form of via fences, which are arranged around the conductive layer 21 . The electrical connections 23 are arranged sufficiently close to each other so that the via fences act as conductive wall for the electromagnetic signal that is fed to the antenna sub-element 3.

The electrical connections 23, in the form of via fences, connects the conductive layers 21 , 22, on opposite sides of the substrate 20. The distance between the length axis 4 and electrical connections 23 determined the size of the antenna sub-element 3 for that substrate 20. As can be seen in Figure 7 the distance between the length axis 4 and the electrical connections 23, varies along the length axis such some of the substrates 20 and its conductive layers 21 , 22, corresponds to an indentation 6 and other substrates 20 and their conductive layers corresponds to the antenna subelement 3 between two indentations 6. The conductive layers 21 , 22, on one substrate are in contact with the conductive layers 21 , 22 on the adjacent substrates. The dielectric material in the substrate 20 between two adjacent antenna sub-elements 3 may be provided with cavities or perforations. Such cavities or perforations alter the effective dielectric constant/permittivity of the dielectric material. Figure 9 is a plan view of one substrate in Figure 7 according to an alternative embodiment. The difference between the substrate shown in Figure 8 is that the substrate also comprises electromagnetic bandgap, EBG, structures 25. The EBG structures may be implemented as metallized cavities, which extend from the first conductive layer 21 , to the second conductive layers 22. The purpose of the EBG structures 25 is to prevent the leakage of radio-frequency power through the air-gap that may be between two adjacent conductive layers 21 , 22, on two different substrates 20. The gap 27 between two adjacent conductive layers 21 is the gap defining an antenna element.

Figure 10 is a plan view of one substrate similar to the substrate in Figure 8, in which waveguides have been implemented. As can be seen in Figure 10 conductive strips 26 are arranged diagonally between the antenna sub-elements 3. When the sub-elements are stacked, these conductive strips 26 form a waveguide along the length axis 4 (Figure 7). The conductive strips 26 may be implemented on every PCB, or only some of the PCBs. In the resulting antenna array, a waveguide is formed surrounding each antenna element, wherein the waveguides are arranged in a diagonal cross pattern between the antenna sub-elements 3.

Such waveguides may of course be implemented also in an antenna array according to Figure 6.

In Figure 11 , a transceiver 30 is shown schematically. The transceiver comprises an antenna array 1 according to the above description. Perforations can be arranged layer-by-layer or completely throughout the stack of dielectric sheets as a through holes to change the effective dielectric constant/permittivity of the dielectric material.

Figure 12 is a perspective view of an antenna array 1 similar to the antenna array shown in Figure 7. In Figure 12, none of the non-conductive substrates is shown. In Figure 12 only the conductive layers 21 , 22 are shown. The shape of the conductive layers 21 , 22, on the substrates vary along the length axis 4 according to the shape of the antenna sub-element 3, but all conductive layers are essentially cross-shaped. Each conductive layers 21 , 22, is simultaneously symmetrical in a row symmetry line 17 and a column symmetry line 18. As described above, an antenna element is the slot between two antenna sub-elements 3. In the embodiment of Figure 12, the antenna elements are arranged along the row symmetry lines 17 and the column symmetry lines 18.

Figure 13 shows a cross-section of an antenna array 1 according to an alternative embodiment. In Figure 13 each antenna sub-element 3 comprises a plurality of non- conductive substrates 20, wherein each substrate 20 comprise at least two conductive layers 21 , 22 attached on opposite sides of the substrate 20 connected to each other by a plurality of electrical connections 23, in the form of so called via fences. In the embodiment of Figure 13, the substrates 20 are common for all the antenna subelements 3. The shape of the conductive layers 21 , 22, on the substrates vary along the length axis 4 according to the shape of the antenna sub-element 3. The different substrates 20 with the conductive layers 21 , 22, are pressed together towards the antenna array base 2 by means of fasteners 24, such as screws, bolts or rivets. Also shown in Figure 7 are electrical feeding points 8, 8’, through which an electromagnetic signal may be fed to the antenna sub-elements 3, 3’. The antenna array of Figure 13 is similar to the antenna array of Figure 7. However, in Figure 13 the sizes, parallel with the antenna array base, of the conductive layers are increased for every substrate in a direction towards the antenna array base 2. This means that no indentations are formed by the substrates 20, their conductive layers 21 , 22, and their electrical connections 23. In the embodiment of Figure 13 the conductive layers 21 , 22, on a substrate extends slightly farther from the respective length axes 4, 4’, that the electrical connections 23 between the conductive layers 21 , 22. However, the electrical connections 23 may extend as far as the conductive layers 21 , 22, from the respective length axes 4, 4’. The antenna sub-elements 4, 4’, may completely lack any indentations.

The antenna sub-elements 3, 3’, as described in Figure 13 may be arranged in configurations as shown in Figures 8-10.

An antenna array according to the embodiment of Figure 13 may be used in a transceiver 30 according to the embodiment of Figure 11 . The permittivity of the dielectric material in an indentation may be different from elsewhere in the antenna sub-element.

The embodiments described above may be modified in many ways without departing from the scope of the invention, which is limited only by the appended claims.

Claims

1. An antenna array (1 ), comprising

- an antenna array base (2),

- at least two antenna sub-elements (3, 3’), each having a length axis (4, 4’), wherein the antenna sub-elements (3, 3’) are arranged on the antenna array base (2), at a distance from each other and at a distance from a centre plane (13) centred between their length axes (4, 4’) , and wherein each antenna sub-element (3, 3’) extends along the length axis (4, 4’) from the antenna array base (2) to a tip (5, 5’), and has an overall 3-dimensionally tapered shape towards the tip (5, 5’), characterized in that each antenna sub-element (3, 3’) comprises indentations (6, 6’) at a plurality of different positions along the length axis (4, 4’), wherein the positions along the length axis (4, 4’) are essentially the same on the different antenna subelements (3, 3’), and wherein the indentations (6, 6’), at each different position along the length axis (4, 4’), are shaped such that the distance between an antenna subelement (3, 3’) and the centre plane (13) is larger, in a plane, perpendicular to the length axis (4, 4’) through an indentation (6, 6’) compared to the distance between said antenna sub-element (3, 3’) and the centre plane (13) in planes, perpendicular to the length axis (4, 4’), adjacent to said indentation.

2. The antenna array (1 ) according to claim 1 , wherein the indentations (6, 6’), at each different position along the length axis (4, 4’) have an extension, perpendicular to the length axis, in different non-parallel directions.

3. The antenna array (1) according to claim 1 or 2, wherein dielectric material is configured in the indentations (6, 6’).

4. The antenna array (1) according to claim 3, wherein the dielectric material in one antenna sub-element (3, 3’) extends to the dielectric material in the adjacent antenna sub-element (3, 3’).

5. The antenna array (1 ) according to any one of the preceding claims, wherein each antenna sub-element (3, 3’) comprises a plurality of stacked metal sheets (9, 9’), wherein the shape and size of the metal sheets (9, 9’) vary along the length axis (4, 4’) according to the shape of the antenna sub-element (3, 3’).

6. The antenna array (1 ) according to claim 5, wherein every other metal sheet (9, 9’) corresponds to an indentation (6, 6’) and every other metal sheet (9, 9’) corresponds to the antenna sub-element (3, 3’) between two indentations (6, 6’).

7. The antenna array (1 ) according to any one of claims 1 -4, wherein each antenna subelement (3, 3’) comprises a plurality of non-conductive substrates (20), wherein on each substrate (20) at least two conductive layers (21 , 22) are attached on opposite sides of the substrate (20) connected to each other by a plurality of electrical connections (23), and wherein the shape of the conductive layers (21 , 22) on the substrates (20) vary along the length axis (4, 4’) according to the shape of the antenna sub-element (3, 3’).

8. The antenna array (1 ) according to claim 7, wherein every other substrate (20) and its conductive layers (21 , 22) corresponds to an indentation (6, 6’) and every other substrate (20) and its conductive layers (21 , 22) corresponds to the antenna subelement (3, 3’) between two indentations (6, 6’).

9. The antenna array (1 ) according to any one of the preceding claims, wherein the dielectric material in the indentations (6, 6’) surround the antenna sub-element (3, 3’) in each indentation.

10. The antenna array (1 ) according to anyone of the preceding claims, comprising at least four antenna sub-elements (3, 3’) arranged in a 2x2 arrangement, wherein each antenna sub-element (3, 3’) comprises two electrical feeding points , to enable dualpolarized operation.

11. The antenna array (1 ) according to claim 10, comprising a waveguide surrounding each antenna sub-element (3, 3’), wherein the waveguides is arranged in a diagonal cross pattern between the antenna sub-elements (3, 3’).

12. The antenna array (1 ) according to anyone of the preceding claims, wherein the indentations (6, 6’) have essentially the same dimension (L2) along the length axis (4, 4’) as the distance (L1 ) between two adjacent indentations (6, 6’).

13. The antenna array (1 ) according to anyone of the preceding claims, wherein the number of indentations (6, 6’) in each antenna sub-element (3, 3’) is at least 5, preferably

14. The antenna array (1 ) according to anyone of the preceding claims, wherein the dielectric material surrounds the antenna sub-element (3, 3’).

15. The antenna array (1 ) according to anyone of the preceding claims, wherein the dielectric material has perforations along the length axis.

16. The antenna array (1 ) according to anyone of the preceding claims, wherein the dielectric constant of the dielectric material is varying.

17. A transceiver comprising an antenna array (1 ) according to any one of the preceding claims.