Abstract:
A planar Vivaldi antenna array, and method of forming such an array, the array comprising: a plurality of slots at an end of the antenna array, the slots extending through the whole thickness of the planar structure of the antenna array; and a plurality of grooves extending from the slots; wherein: the grooves do not extend through the whole thickness of the planar structure of the antenna array; and the cross-sectional shape of the grooves is complementary to the cross-sectional shape of the slots.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to antennas. In particular, the present invention relates to, but is not limited to, arrays of Vivaldi antenna elements, for example dual-polarised Vivaldi antenna arrays. 
       BACKGROUND 
       [0002]    Tapered-slot, or Vivaldi, antenna elements are known. A Vivaldi antenna element is a co-planar broadband-antenna. The Vivaldi antenna element  2  comprises a conductive layer  4  disposed on a substrate  6 . The conductive layer  4  is disposed on the substrate  6  such that a space  8  in the conductive layer  4  is formed. The distance between the sides of the space  8  widens from a minimum at a narrow end  10  of the space  8  to a maximum at an open end  12  of the space  8 . At the narrow end  10 , a square, or circular, or other shaped region  13  without conductor is provided. In conventional Vivaldi antenna elements, the space  8  is symmetrical about a central axis  14 . Further conductor material  15  extends beyond the narrow end  10  and the region  13 . 
         [0003]    It is known to form Vivaldi antenna element into arrays.  FIG. 2  is a schematic illustration of an example of a Vivaldi antenna array comprising four Vivaldi antenna elements, indicated in  FIG. 2  by the reference numeral  2  and separated by dotted lines. Vivaldi antennas arrays may be formed from any number of Vivaldi antenna elements. 
         [0004]      FIG. 3  is a schematic illustration of a further example of a Vivaldi antenna array. This example array is a dual-polarised Vivaldi antenna array comprising six interlocking Vivaldi antenna arrays, each array according to the example array of  FIG. 2 . The six arrays of the dual-polarised Vivaldi antenna array are arranged such that three arrays are vertically polarised (these arrays are indicated in  FIG. 3  by the reference numerals  301 ), and three arrays are horizontally polarised (these arrays are indicated in  FIG. 3  by the reference numerals  302 ). The arrays are arranged such that the Vivaldi antenna elements  2  of each horizontally polarised array  301  are separated by a vertically polarised array  302 , and vice versa. 
         [0005]    Dual polarised Vivaldi antenna arrays, such as that illustrated in  FIG. 3 , are conventionally assembled by making use of slots extending half the length of the antenna elements. The slots are conventionally square sided, and as they are cut into the board forming the antenna array, no metallisation is applied to the edges. After the boards are assembled, the corners of the resulting square cells are soldered along the complete depth of the board. This is difficult to perform, and this difficulty is exacerbated by the consideration that good electrical contact is required in all of these areas for the antenna array to perform well. Furthermore, the physical sizes involved are small, e.g. the width of the antenna element may be e.g. 5 mm, in which case the resulting square cell size is e.g. 5 mm×5 mm, which therefore makes accessing with a soldering iron or other equipment along the full depth of the board e.g. 50 mm difficult. 
       SUMMARY OF THE INVENTION 
       [0006]    In a first aspect, the present invention provides a planar Vivaldi antenna array comprising a plurality of slots at an end of the antenna array, the slots extending through the whole thickness of the planar structure of the antenna array, and a plurality of grooves extending from the slots, wherein the grooves do not extend through the whole thickness of the planar structure of the antenna array, and the cross-sectional shape of the grooves is complementary to the cross-sectional shape of the slots. 
         [0007]    Surfaces of the planar Vivaldi antenna array that result from forming of the grooves and/or slots may be electrically conductive surfaces. 
         [0008]    The grooves may be substantially v-shaped. 
         [0009]    The slots may be substantially bow-tie shaped. 
         [0010]    A shape of a slots may be a shape that is formed by performing the following: forming first a V-shaped groove in a top surface of the planar Vivaldi antenna array, forming second a V-shaped groove in a bottom surface of the planar antenna array, the second V-shaped groove being opposite the first V-shaped groove, and extending the first V-shaped grove and/or the second V-shaped groove through the planar Vivaldi antenna array towards the opposite groove such that the whole thickness of the planar structure of the antenna array is extended through. 
         [0011]    Conductive epoxy may be provided on the grooves and/or the slots. 
         [0012]    In a further aspect, the present invention provides a Vivaldi antenna array comprising at least two of the planar Vivaldi antenna arrays of any of the above aspects fixed together by means of the grooves of one and the slots of the other. 
         [0013]    Conductive epoxy in the slots and/or grooves conductively may bond the two planar Vivaldi antenna arrays at those places. 
         [0014]    In a further aspect, the present invention provides a method of forming a Vivaldi antenna array, the method comprising: providing at least two planar Vivaldi antenna arrays according to any of the above aspects, and sliding the grooves of one planar Vivaldi antenna arrays along the slots of a different planar Vivaldi antenna array. 
         [0015]    The grooves and/or the slots may first be provided with conductive epoxy. 
         [0016]    The conductive epoxy may be applied in the form of one or more drops, and wherein the epoxy is then spread along the grooves/slots when the planar Vivaldi antenna arrays are slid together. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  a schematic illustration of a top surface and a side surface of a typical conventional Vivaldi antenna element; 
           [0018]      FIG. 2  is a schematic illustration of an example of a Vivaldi antenna array comprising four Vivaldi antenna elements; 
           [0019]      FIG. 3  is a schematic illustration of a further example of a Vivaldi antenna array; 
           [0020]      FIG. 4  is a schematic illustration of a Vivaldi antenna array according to a first embodiment of the present invention; 
           [0021]      FIG. 5  is a schematic illustration of a cross-section of the Vivaldi antenna array of  FIG. 4 ; 
           [0022]      FIG. 6  is a schematic illustration of a further cross-section of the Vivaldi antenna array of  FIG. 4 ; 
           [0023]      FIG. 7  is a schematic illustration showing how portions of two Vivaldi antenna arrays interlock; and 
           [0024]      FIG. 8  is a schematic illustration of a dual polarised Vivaldi antenna array according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    In the following description, terminology such as “vertical”, “horizontal” etc. is adopted to describe elements of the invention. It will be appreciated by the skilled person that such terminology is not limiting and is used merely to refer to the position of one element relative to other elements. 
         [0026]      FIG. 4  is a schematic illustration of a Vivaldi antenna array  50  according to a first embodiment of the present invention. In particular,  FIG. 4  shows features of a top surface  53  of the Vivaldi antenna array  50 . Features of further surfaces of the Vivaldi antenna array  50  are described in more detail later below with reference to  FIGS. 5 and 6 . In this embodiment, the Vivaldi antenna array  50  comprises four Vivaldi antenna elements  2  arranged in a row  502 . In the row  502 , the four Vivaldi antenna elements  2  are positioned side-by-side such that the open ends  12  of the Vivaldi antenna elements  2  are level and form a first edge  51  of the Vivaldi antenna array  50 , along the direction of the dotted line X-X shown in  FIG. 4 . Furthermore, the opposite ends of the four Vivaldi antenna elements  2  are level and form a second edge  52  of the Vivaldi antenna array  50 , along the direction of the dotted line Y-Y shown in  FIG. 4 . Thus the first edge  51  and the second edge  52  form two opposite outside edges of the Vivaldi antenna array  50 . The first edge  51  and the second edge  52  are parallel. 
         [0027]    Each Vivaldi antenna element  2  is provided with a connector  16  at the second edge  52 . The connector provides a connection to the conductor layer of the top surface  53 , and a separate connection to a conductor layer on the opposite side of the board, i.e. on a bottom surface  54  (described further below with reference to  FIGS. 5 and 6 ). In this embodiment the conductor layer on the opposite surface provides further Vivaldi antenna elements, but this need not be the case, and in other embodiments a planar ground plane may be provided on the bottom surface instead. 
         [0028]    In this embodiment, the length of each Vivaldi antenna element  2  is approximately 50 mm (i.e. the distance from the first edge  51  to the second edge  52 ). However, in other embodiments, other lengths may be provided. 
         [0029]    In this embodiment, each Vivaldi antenna element  2  is approximately 15 mm wide, allowing a top frequency of operation of approximately 10 GHz. In other embodiments other widths may be used, allowing other top frequencies. For example, the Vivaldi antenna elements may be 5 mm wide, allowing a top frequency of operation of approximately 30 GHz. 
         [0030]    The Vivaldi antenna elements  2  are interspaced by V-shaped grooves  60  extending from the first edge  51  to a central axis  500  of the Vivaldi antenna array  50 , as described in greater detail later below with reference to  FIG. 5 . 
         [0031]    The Vivaldi antenna elements  2  are further interspaced by slots  62  extending from the second edge  52  to the central axis  500  of the Vivaldi antenna array  50 , as described in greater detail later below with reference to  FIG. 6 . 
         [0032]      FIG. 5  is a schematic illustration of a cross-section of the Vivaldi antenna array  50  of  FIG. 4 , at a position corresponding to the dotted line X-X. 
         [0033]    The Vivaldi antenna array  50  comprises a first conductive layer  56 , a substrate  57 , and a second conductive layer  58 . The first conductive layer  56  is disposed on a first side of the substrate  57  corresponding to the top surface  53  of the Vivaldi antenna array  50 . The second conductive layer  58  is disposed on a second side of the substrate  57  corresponding to a bottom surface  54  of the Vivaldi antenna array  50 . In this embodiment the first conductive layer  56  and the second conductive layer  58  are copper. In this embodiment, the substrate  6  is alumina. 
         [0034]    The Vivaldi antenna elements  2  that are arranged to form the first edge— 51  are interspaced by V-shaped grooves  60  in both the top surface  53  and the bottom surface  54 . In this embodiment, the V-shaped grooves  60  separating a pair of Vivaldi antenna elements  2  are such that they are aligned, are substantially the same size, and such that material is present between the V-shaped grooves  60  in the top and bottom surfaces  53 ,  54 , i.e. the V-shaped grooves  60  in the top and bottom surfaces do not join/overlap. In other words, the grooves  60  do not extend through the whole thickness of the substrate  57  and the conductive layers  56 ,  58  on the substrate  57 , i.e. the grooves  60  do not extend through the whole thickness of the planar structure of the antenna  50 . 
         [0035]      FIG. 6  is a schematic illustration of a cross-section of the Vivaldi antenna array  50  of  FIG. 4 , at a position corresponding to the dotted line Y-Y. 
         [0036]    Each slot  62  is formed by a V-shaped groove in the top surface  53  and a V-shaped groove in the bottom surface  54  that join/overlap at the apexes of the respective V-shaped grooves to form a slot  62  in the Vivaldi antenna array  50  that passes from the bottom surface  54  to the top surface  53 . Accordingly, each slot  62  has an “hourglass”, or “bow tie”, shaped cross-section. In other words, the slots  62  do extend through the whole thickness of the substrate  57  and the conductive layers  56 ,  58  on the substrate  57 , i.e. the slots  62  do extend through the whole thickness of the planar structure of the antenna  50 . The function of the slots  62  will be described in greater detail later below with reference to  FIGS. 7 and 8 . 
         [0037]      FIG. 7  is a schematic illustration showing how portions of two of the above described Vivaldi antenna arrays  50 , hereinafter referred to as the “first array  50 A” and the “second array  50 B”, interlock. In this embodiment, the first array  50 A and the second array  50 B have substantially the same dimensions. 
         [0038]    The first array  50 A is positioned orthogonally to the second array  50 B such that the slots  62  of the first array  50 A align with the slots  62  of the second array  50 B. One such slot alignment is shown schematically in  FIG. 7 . 
         [0039]    When in an interlocked position, the first array  50 A and the second array  50 B are positioned such that a surface, hereinafter referred to as the “first joining surface  503 ”, of the first array  50 A is in contact with a surface, hereinafter referred to as the “second joining surface  505 ”, of the second array  50 B. In effect, the first array  50 A is in the position it would occupy if it were moved along the dotted arrow indicated by the reference numeral  510  in  FIG. 7 , whilst remaining orthogonal to the second array  50 B. 
         [0040]    The above described interlocked position of the first array  50 A and the second array  50 B provides that the V-shaped grooves  60  of the first array  50 A are engaged with the slot  62  of the second array  50 B. In other words, the surface of the V-shaped grooves  62  of the first array  50 A are in contact with the V-shaped surfaces of the second array  50 B formed by the hourglass-shaped slot  62  in the second array  50 B. 
         [0041]    Also, the V-shaped grooves  60  of the second array  50 B are engaged with the slot  62  of the first array  50 A. In other words, the surface of the V-shaped grooves  62  of the second array  50 B are in contact with the V-shaped surfaces of the first array  50 A formed by the hourglass-shaped slot  62  in the first array  50 A. 
         [0042]    The above described interlocked position of the first array  50 A and the second array  50 B provides that the first edge  51  of the first array  50 A is substantially level with the second edge  52  of the second array  50 B. Also, the first edge  51  of the second array  50 B is substantially level with the second edge  52  of the first array  50 A. 
         [0043]      FIG. 8  is a schematic illustration of a dual polarised Vivaldi antenna array  70  according to an embodiment of the present invention. The dual-polarised Vivaldi Antenna array  70  is formed from the interlocking of six Vivaldi antenna arrays  50  of the type described above with reference to  FIGS. 4-7 . The three horizontally positioned Vivaldi antenna arrays in  FIG. 8  are oriented the same as the first array  50 A in  FIG. 7 , and are thus indicated in  FIG. 8  by the reference numeral  50 A. Also, the three vertically positioned Vivaldi antenna arrays in  FIG. 8  are oriented the same as the second array  50 B in  FIG. 7 , and are thus indicated in  FIG. 8  by the reference numeral  50 B. 
         [0044]    As shown in  FIG. 8 , the Vivaldi antenna arrays  50 A,  50 B interlock such that each slot  62  of each first array  50 A is engaged with the V-shaped grooves  60  of a different second array  50 B. In other words, the V-shaped surfaces of each of the first arrays  50 A, formed by the hourglass-shaped slots  62  in the first arrays  50 A, are in contact with a surface of a V-shaped groove  62  of a different second array  50 B. Also, the Vivaldi antenna arrays  50 A,  50 B interlock such that each slot  62  of each second array  50 B is engaged with the V-shaped grooves  60  of a different first array  50 A. In other words, the V-shaped surfaces of each of the second arrays  50 B, formed by the hourglass-shaped slots  62  in the second arrays  50 B, are in contact with a surface of a V-shaped groove  62  of a different first array  50 A. 
         [0045]    The above described interlocked position of the first arrays  50 A and the second arrays  50 B provides that the first edges  51  of the first arrays  50 A are substantially level with the second edges  52  of the second arrays  50 B. 
         [0046]    Also, the first edges  51  of the second arrays  50 B are substantially level with the second edges  52  of the first arrays  50 A (this is not shown in  FIG. 8 ). 
         [0047]    Thus, the dual-polarised Vivaldi antenna array  70  comprising Vivaldi antenna arrays  50 A,  50 B with V-shaped grooves  60  and slots  62  is provided. 
         [0048]    The V-shaped grooves  60  and slots  62  tend to advantageously provide that the dual-polarised Vivaldi antenna array  70  is more structurally stable than conventional dual-polarised Vivaldi antenna arrays. 
         [0049]    A further advantage is that the dual-polarised Vivaldi antenna array  70  tends to be easier to construct than conventional dual-polarised Vivaldi antenna arrays. For example, the dual-polarised Vivaldi antenna array  70  tends to avoid the need for soldering from awkward positions. 
         [0050]    A further advantage is that the V-shaped grooves  60  and slots  62  tend to advantageously allow for better contact between the conductive layers of the interlocking antenna arrays. This advantageously tends to reduce problems caused by uncontrolled ground planes. Also, this tends to provide a low reflection coefficient. 
         [0051]    A further advantage is that due to the grooving and slots as described above, an increased bonding area is provided. 
         [0052]    It will be appreciated that the above advantages will tend to be achieved irrespective of the particular conductive bonding arrangement, e.g. soldering, conductive epoxy, etc. that is used to electrically connect the arrays at the interface between the slots and grooves. However, yet further advantages tend to be achieved by the present embodiment due to the particular conductive bonding arrangement employed, details of which will now be described. 
         [0053]    In this embodiment the grooves  60  and the slots  62  are metallised. Prior to assembling the various arrays together using the grooves and slots (as shown in  FIG. 7 ), conductive epoxy is applied to the slots  62 . In other embodiments, conductive epoxy may be applied to the grooves  60  instead of or in addition to being applied to the slots  62 . When the various arrays are assembled together, the conductive epoxy provides structural bonding and electrical connection. This tends to provide good structural stability and good quality electrical connection between elements, in particular due to the aspect that the epoxy can readily be applied such as to provide good coverage along the whole length of the slots and grooves. Moreover, since the conductive epoxy is applied, access to the grooves and/or slots for applying the epoxy is straightforward. In a preferred implementation, the conductive epoxy is applied in the form of spaced apart drops along slots and/or groove, and then when the arrays are slid together as shown in  FIG. 7 , the interlocking of the slots with respective grooves spreads or smears the epoxy along the slot/groove arrangement, thereby providing coverage along their lengths whilst nevertheless having only had to apply the epoxy in the easier drop by drop manner. Also, by virtue of the implementations described in this paragraph, a weight saving is achieved since e.g. excess solder or conductive epoxy can be alleviated or avoided compared to conventional approaches. Also, the grooves and slots tend to provide a channelling effect, which tends to confine the epoxy thereby alleviating or reducing any run out or leaking of the epoxy onto the conducting surfaces of the antenna elements which in conventional arrangements can occur thereby changing the antenna properties. 
         [0054]    In the above embodiments, the grooves  60  (that do not extend through the whole thickness of the planar structure of the antenna array  50 ) are V-shaped and the slots  62  (that do extend through the whole thickness of the planar structure of the antenna array  50 ) are shaped accordingly to co-operate with the V-shaped grooves, i.e. are “bow-tie” shaped. However, these particular shapes are not essential, and in other embodiments other interlocking or co-operating shapes of grooves that do not extend through the whole thickness of the planar structure of the antenna array  50  and slots  62  that do extend through the whole thickness of the planar structure of the antenna array  50  may be used. For example, curved cross-sectional profiles may be used (e.g. “C-shaped”). Yet further for example, even if the grooves and slots are (or are based on) square profiles rather than V-shapes, some of the above described advantages would still be obtainable compared to conventional arrangements (which do not provide any grooves that do not extend through the whole thickness of the planar structure of the antenna array  50 ), even if the square shapes were less advantageous than the above described embodiments. 
         [0055]    In the above embodiments, the Vivaldi antenna arrays comprise four Vivaldi antenna elements  2 . However, in other embodiments, the Vivaldi antenna arrays may comprise any appropriate number of antenna elements. 
         [0056]    In the above embodiments, the dual-polarised Vivaldi antenna array  70  comprises six Vivaldi antenna arrays. However, in other embodiments, the dual-polarised Vivaldi antenna array comprises a different number of Vivaldi antenna arrays. 
         [0057]    In the above embodiments, the first arrays  50 A and the second arrays  50 B have substantially the same dimensions. However, in other embodiments some or all of the first arrays have different appropriate dimensions. Also, in other embodiments, some or all of the second arrays have different appropriate dimensions. Also, in other embodiments some or all of the first arrays have different appropriate dimensions to some or all of the second arrays. 
         [0058]    In the above embodiment, the first conductive layer  56  is copper. However, in other embodiments the first conductive layer is a different conductive material. 
         [0059]    In the above embodiment, the second conductive layer  58  is copper. However, in other embodiments the second conductive layer is a different conductive material. 
         [0060]    In the above embodiments, the substrate is alumina. However, in other embodiments, the substrate is a different appropriate material. 
         [0061]    In the above embodiments, horizontally polarised Vivaldi antenna arrays (i.e. horizontally aligned antenna elements for operating with horizontally polarised signals) are interlocked with vertically polarised Vivaldi antenna arrays (i.e. vertically aligned antenna elements for operating with vertically polarised signals). However, in other embodiments, one or more of the vertically polarised antenna arrays may be substituted by an non-antenna structure of substantially the same shape, i.e. a structure having the same shape of the Vivaldi antenna array but comprising no antenna elements, or inactive antenna elements. For example, in other embodiments the horizontally polarised Vivaldi antenna arrays are interlocked with vertically aligned inert structures having the same shape as the Vivaldi antenna array. This tends to advantageously provide improved structural stability and/or conductive layer contact in a Vivaldi antenna array polarised (i.e. orientated) in a single direction.