Abstract:
An antenna unit transmit and receives radio signals having two different polarizations. The antenna unit includes a slot integrated in a microstrip element and an aperture integrated in a conductive surface on the microstrip element. The aperture is arranged in the conductive surface parallel to its polarization direction. The slot is arranged in an underlying layer directly below the aperture. The conductive surface of the microstrip element is arranged to transmit or receive with a vertical polarization and a first horizontal beam width. The slot is arranged to transmit or receive with a horizontal polarization and a second horizontal beam width. The second beam width is substantially equal to the first beam width. The antenna unit is very compact and light and only causes low power losses. A number of antenna units can be used to design sector antennas or antenna arrays.

Description:
This application claims priority under 35 U.S.C. §§119 and/or 365 to SE 9700667-0 filed in Sweden on Feb. 25, 1997; the entire content of which is hereby incorporated by reference. 
     1. Technical Field 
     The present invention relates to an antenna device and an antenna apparatus for transmitting and receiving radio signals, in particular one that is located on a base station in a mobile communications system. 
     2. Background 
     An important part of the planning and dimensioning of a communications system for radio signals is the properties of the antennas. These properties affect, among other things, the cell planning (size, pattern, number). One of these properties is the radio coverage area of the antenna. 
     Originally, only so called omni antennas were used, having a coverage in all directions seen from the base station. If a larger coverage area was necessary, a new cell was introduced adjacent to the first one and a new base station was placed in the middle of it. 
     Later on it was discovered that it was advantageous from a system point of view to divide the coverage area into sectors, for example, three sectors in one full circle. Antennas intended for this coverage are called sector antennas. This becomes particularly advantageous if the base station is placed in the intersection point between the cells. Each of the sector antennas then covers one cell and the base station thus serves several cells at a time. 
     The coverage area of a sector antenna is determined by the antenna&#39;s beam width in the horizontal plane. 
     Another important property of the antennas is their polarization, or rather the polarization of the signals transmitted or received by the antenna. Originally only vertical polarization was used in the base station antennas. Nowadays, often two linear polarizations are used at the same time (polarization diversity), for example in the horizontal and the vertical planes, here referred to as 0 and 90 degrees, or in the tilted planes between them, +/−45 degrees. Usually the antenna must have the same coverage for both polarizations. 
     The sector antennas used today for two polarizations have a beam width of approximately 60-70 degrees. At present antennas with a wide lobes can only be made with one polarization direction. Now many operators want antennas for two polarizations having beam widths of 80-90 degrees to adapt the coverage area of the base station to existing systems and the surrounding terrain. 
     A sector antenna comprises a column with some type of antenna element receiving and/or transmitting in one or two polarizations within a limited coverage area. These antenna elements may be implemented, for example, as so called microstrip elements. A microstrip element has a radiating body in the form of a conducting surface, often called a patch, located in front of an earth plane. The space between them may be filled with a dielectric material or air. Air has the advantages of being light, inexpensive and causing no power loss. For the microstrip element to function efficiently the length of the patch must correspond to a resonant length in the polarization direction, usually about half a wavelength. 
     The beam width in a certain plane of an antenna is inversely proportional to the dimension of the antenna in the same plane. Base station antennas often have a vertical beam width of 5-15 degrees, which is dictated by the topography of the surroundings of the base station. This beam width may easily be adjusted by changing the number of elements in the vertical direction of the antenna. In the horizontal direction the antenna cannot be made narrower than one element. If, for example, the polarization of the antenna is horizontal, the width of the element is determined by the resonance condition mentioned above. 
     A known antenna apparatus with two different polarization directions comprises a number of microstrip elements whose radiating elements have a square shape. Each radiating element has two different feeders. One feeder transmits or receives a signal having a certain polarization different from the one transmitted or received by the other feeder. This implies that the microstrip elements must be resonant in two directions (one for each polarization direction) which implies that the width of the radiating elements must correspond to half a wavelength. This in turn means that it is very difficult to generate lobes that are wider than 60-70 degrees. One known way to widen the lobe is to fill the microstrip element with a dielectric substance having a dielectric constant greater than one. This reduces the wavelength and thus also the resonant dimension of the patch. This procedure, however, causes reduced performance because of inevitable power losses in the substance as well as a higher weight and cost. 
     U.S. Pat. No. 5,223,848 describes an antenna comprising microstrip elements having a pair of rectangular radiating elements. Each radiating element is fed to transmit and receive with both a vertical and a horizontal polarization simultaneously. The radiating elements may be conducting surfaces or other radiating elements. Both radiating elements in the pair transmit and receive on two frequencies with different polarization directions. 
     SUMMARY 
     The present invention attacks a problem that arises when a sector antenna implemented using plane conductor technology is to be able to generate efficiently very wide antenna lobes (more than 70 degrees) simultaneously, with two different polarization directions, while at the same time being compact, light and inexpensive. 
     More specifically, the problem arises when the antenna elements of the antenna must be resonant in two directions to be able to transmit and receive with two polarization directions. This limits the possibility to design a compact, light and inexpensive antenna generating small losses. 
     A similar problem arises when a narrow sector antenna is to generate two antenna lobes of the same width, and having two different polarization directions, in the horizontal plane. 
     The purpose of the present invention is thus to achieve a compact, light and inexpensive antenna with small losses having two antenna lobes of substantially the same width, greater than a certain width, and having two different polarization directions. 
     More specifically the present invention is intended to achieve an antenna in which the width of the antenna lobes in the horizontal plane is greater than 70 degrees. 
     According to the invention two different types of antenna element are used in one common unit, in which the type and geometrical shape of the antenna elements enable a unit that is as compact and light as possible. Each type of antenna element is arranged to transmit or receive with one particular polarization. 
     More specifically, the invention relates to an antenna unit having a narrow antenna element of a first type, for example, a microstrip element, in combination with a narrow and light antenna element of a second type, for example, a slot in an earth plane. The first type of antenna element is only designed for a first polarization direction, while the second type of antenna element is only designed for a second polarization direction, different from the first polarization direction. These antenna elements may be arranged to occupy a very small surface. This means that the antenna may be built for antenna lobes greater than a certain angle, for example 70 degrees, without the antenna becoming heavy and/or expensive. 
     The invention also relates to an antenna apparatus comprising a certain number of said antenna units. These antenna units may, for example, be arranged in a column forming a sector antenna. The sector antenna, too, may be built for antenna lobes greater than a certain angle, for example 70 degrees, without the antenna becoming heavy and/or expensive. 
     One advantage of the present invention is that the antenna can have a very wide lobe (70-90 degrees) in the horizontal plane for two different polarization directions. When both antenna lobes have substantially the same width, considerable advantages are achieved from a system point of view. Among other things, polarization diversity may be utilized in the whole coverage area of the antenna. 
     Further advantages is that it becomes very easy to make a compact, light and inexpensive antenna. This is particularly true for sector antennas. 
     The invention also enables the construction of two dimensional antenna arrays having a distance of less than half a wavelength between the antenna columns (rows of antenna elements). This enables the generation of one or more antenna lobes with great output angles without so called grid lobes being generated. 
     The antennas mentioned above can also generate one or two circular polarizations in a large angular area, trough a combination of the individual radio signals to the respective antenna elements, in ways known in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the appended drawings. 
     FIG. 1 is an explanatory sketch of antenna lobes from a sector antenna seen from above. 
     FIG. 2 is a cross-sectional view of a first microstrip element. 
     FIG. 3 is a cross-sectional view of a second microstrip element. 
     FIG. 4 is a cross-sectional view of a slot in an earth plane with a supply conductor of a plane conductor type. 
     FIG. 5 is a front view of a slot in an earth plane. 
     FIG. 6 is a front view of microstrip elements which can transmit and/or receive with two different polarization directions. 
     FIG. 7 is a cross-sectional view of the antenna shown in FIG.  6 . 
     FIG. 8 is a front view of a second prior art antenna. 
     FIG. 9 is a front view of a first embodiment of an inventive antenna unit. 
     FIG. 10 is a cross-section of the antenna unit shown in FIG.  9 . 
     FIG. 11 is a front view of a first embodiment of a sector antenna comprising the first embodiment of the inventive antenna unit. 
     FIG. 12 is a front view of a second embodiment of the inventive antenna unit. 
     FIG. 13 is a cross-sectional view of the antenna unit shown in FIG.  12 . 
     FIG. 14 is a front view of a second embodiment of the sector antenna comprising the second embodiment of the inventive antenna unit. 
     FIG. 15 is a front view of a third embodiment of the sector antenna comprising the first embodiment of the inventive antenna unit. 
     FIG. 16 is a front view of a fourth embodiment of the sector antenna comprising the second embodiment of the inventive antenna unit. 
     FIG. 17 is a front view of an embodiment of an antenna array comprising the second embodiment of the inventive antenna. 
     FIG. 18 shows three examples of slots that may be used in all the embodiments listed above. 
     FIG. 19 is a front view of an example of a gridded patch. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a top view of antenna lobes from an antenna  30  transmitting or receiving in a particular direction. Such an antenna  30  is called a sector antenna. The main part of the radiation from a sector antenna is found in a particular limited area  31  referred to as the front lobe of the antenna. So called side lobes  32   a-b  and back lobes  33  also arise. The beam width  34  of the antenna is the part of the front lobe  31  in which the field strength F of the antenna exceeds F max /2 in which F max  is the maximum field strength in the front lobe  31 . 
     Microstrip elements  40 , see FIGS. 2-3, and slots in earth planes  60 , see FIGS. 4-5, are examples of different types of antenna elements. 
     FIG. 2 is a cross-section of a first microstrip element  40 . The microstrip element  40  comprises an electrically insulating volume  41  having a certain dielectric constant ∈, an earth plane  42  consisting of an electrically conductive substance, for example, copper, below the insulating volume  41  and a limited surface (patch)  43  of an electrically conductive substance, for example, a square copper surface arranged above the insulating volume  41 . The conductive surface  43  is an example of a radiating element that can transmit or receive signals from air. In the following, the conductive surface  43  on the microstrip element  40  will be referred to as a surface element  43 . The dimensions of the surface elements  43  are determined, among other things, by the polarization and wavelength of the signal concerned. A sector antenna comprises a column having a well defined number of microstrip elements  40  arranged in a common antenna structure. 
     The surface element  43  on the microstrip element  40  can, if necessary, be arranged on a disc  44  of an electrically insulating material. The surface element  43  may then be arranged above, as in FIG. 2, or below the disc  44 . 
     The surface element may also be arranged on one or more support units  51   a-b  between the surface element  43  and the earth plane  42 , see FIG. 3, which shows another embodiment of a microstrip element  40 . 
     FIG. 4 is a cross-sectional view of an antenna element  60  having a slot  61  in an earth plane  62  and a feeder  63  of a plane conductor type for the supply to and from the slot  61 . The feeder  63  to the slot  61  in the earth plane  62  is arranged below the slot  61 . An electrically insulating volume  64  is arranged between the feeder  63  and the earth plane  62 . Signals to and from the slot  61  are transmitted to/from the feeder  63  by electromagnetic transmission through the volume  64  (the slot  61  is excited). 
     FIG. 5 is a cross-sectional view of the antenna element  60  comprising the slot  61  in the earth plane  62 . The slot  61  in the earth plane  62  is another example of a radiating element which, like the surface element  43  mentioned, can transmit or receive signals from air. 
     As mentioned above a prior art antenna uses microstrip elements having square radiating elements of the surface element type, which can transmit and/or receive with two different polarization directions from each surface element. FIG. 6 is a view of such an antenna  80  comprising three surface elements  81   a-c . The surface elements  81   a-c  are resonant in two directions (horizontally and vertically) in order to generate the 0/90 degrees polarization mentioned above. Each surface element  81   a-c  has a feeder  82   a-c  for the horizontal polarization and a feeder  83   a-c  for the vertical polarization. 
     FIG. 7 (cf. FIG. 2) is a cross-sectional view of the antenna  80  with the surface element  81   a  and an underlying earth plane  91 . Between them, a dielectric volume  92  is arranged. If the dielectric volume  92  is air the beam width  34  of the front lobe  31 , see FIG. 1, will be between 60 and 70 degrees in the two polarization directions. 
     The size of the antenna  80  may be reduced by selecting a dielectric volume  92  having a dielectric constant ∈ r  greater than, for example,  2 , thus achieving a wide front lobe  31 . This, however, increases the loss in the antenna  80  and makes it heavier and more expensive. 
     FIG. 8 shows an antenna  100  having microstrip elements according to the above mentioned U.S. Pat. No. 5,223,848. A first  101  and a second  102  rectangular surface element have two feeders  103 - 106  each, for two different polarization directions per surface element  101 - 102 . Each surface element  101 - 102  transmits and receives with two different frequencies fl and f 2 . A first frequency fl is used for the horizontal polarization in the first surface element  101  and for the vertical polarization in the second surface element  102 , whereas the other frequency f 2  is used for the vertical polarization in the first surface element  101  and for the horizontal polarization in the second surface element  102 . These surface elements  101 - 102  may be replaced by another type of radiating element with two feeders. 
     In the embodiments described below the antennas are designed with a layer type structure. The antennas are described as if horizontally oriented and having an upper, a lower and an intermediate layer. Of course the antennas may be arranged with another orientation, for example, standing, in which case the upper layer corresponds to a front layer, the lower layer corresponds to a back layer and something being located under the antenna corresponds to something being located behind it. 
     FIG. 9 is a front view of a first embodiment  110  of an antenna unit according to the present invention, for transmitting and receiving with a polarization of 0/90 degrees. The antenna unit  110  is here shown in a rectangular design. The antenna unit  110  comprises a combination of a microstrip element  111  having a rectangular surface element  112  in the upper layer and a rectangular slot  113  in an earth plane  114  in the intermediate layer (the earth plane is not shown in FIG.  9 ). 
     The surface element  112  has a well defined length l e1  and width w e1 . The slot  113  also has a well defined length l s1  and width w s1 . These lengths l e1  and l s1  are dependent on the wavelength with which the antenna unit is to transmit and receive. The width w el  determines the beam width of the element in the horizontal plane. The width w s1  substantially determines the bandwidth of the slot. The surface element  112  is arranged on the antenna unit  110  so that, for example, its lower edge  115  levels with an upper edge  116  of the slot  113 . 
     FIG. 10 is a cross-sectional view of the antenna unit  110 . The antenna unit  110  comprises a first disc  121  of an electrically insulating material, in the upper layer of which the surface element  112  is arranged. In the lower layer a second disc  123  of an electrically insulating material is arranged having a feeder  124  to the slot  113 . In the intermediate layer an earth plane  114  is arranged. The slot  113  is arranged in the earth plane  114  so that it is not covered by a thought projection of the surface element  112  onto the earth plane  114 . A first dielectric volume  122 , for example air, is arranged between the first disc  121  of an electrically insulating material and the earth plane  114 . A second dielectric volume  125 , for example air, is arranged between the earth plane  114  and the second disc  123  of an electrically insulating material. If the dielectric volumes  122  and  125  consist of air, of course, side walls are arranged in a suitable way to support the discs  121  and  123 , and the earth plane  114 . 
     The earth plane  114  may, for example, consist of an electrically conductive material comprising said slot  113  or a disc of an electrically conductive material on which an electrically conductive surface with the slot  113  is arranged. 
     FIG. 11 is a front view of a first embodiment of a sector antenna  130  comprising the first embodiment of the inventive antenna unit, to transmit and receive with a polarization of 0/90 degrees. The antenna  130  is here shown in a rectangular embodiment. The antenna  130  comprises four antenna units  110   a-d  (not marked out in FIG. 11) each similar to the ones shown in FIGS.  9  and  10 , and arranged one after the other, the antenna units  110   a-d  being integrated with each other in a common structure. 
     The rectangular surface elements  112   a-d , see FIG. 11, of the respective antenna unit  110   a-d , are arranged in a column, short sides facing each other, with a certain, for example constant, first centre distance a c1  between the centres of the surface elements. They are also arranged so that their longitudinal axes are parallel with the longitudinal axis of the antenna. The centre distance a c1  corresponds to a wavelength in the medium in which the wave is propagating when passing through feeders and microstrip elements. 
     The slots  113   a-d  in the earth plane  114  of each respective antenna unit  110   a-d  are also arranged in a column, short sides facing each other, with a certain, for example, constant second centre distance a c2  between the centres of the slots  113   a-d . The slots are arranged so that their longitudinal axes are parallel with the longitudinal axis of the antenna. It is feasible to let the centre distance a c2  be equal to the centre distance a c1 . 
     The column comprising the surface elements  112   a-d  and the column comprising the slots  113   a-d  are parallel displaced relative to each other and in the longitudinal direction of the sectors antenna. The columns are arranged with a certain distance a k  between them. The distance a k  is selected so that the function of the slots  113   a-d  is not disturbed by the surface elements  112   a-d.    
     The surface elements  112   a-d  are fed through a central feeding cable  131  and serially connected, from  112   c  to  112   d  and from  112   c  to  112   a , respectively, by means of three feeders  132   a-c  for the feeding to and from the surface elements  112   a-d . This implies that the surface elements  112   a-d  can transmit or receive with a vertical polarization with a first horizontal beam width  34 . 
     FIG. 11 also shows how the feeders  124   a-d  for the supply to and from the slots  113   a-d  are connected in parallel with the respective slot  113   a-d . The feeders  124   a-d  are arranged to excite the slots  113   a-d  so that they can transmit or receive with a horizontal polarization with a second horizontal beam width  34 . The second beam width is substantially equal to the first beam width. 
     The supply and the feeders to/from the slots  113   a-d  and the surface elements  112   a-d  can be arranged in more ways than what has been shown and described in connection with FIG.  11 . 
     The feeders  132   a  and  132   c  to the surface elements  112   a  and  112   d  can, for example, be connected directly to the central supply conductor  131  by parallel feeding. The supply to/from the surface elements  112   a-d  can also be arranged by means of a probe supply or an aperture supply instead of the central supply conductor  131 . 
     An apparatus for fixing the parts of the antenna  130  relative to each other may comprise, for example, a bar around the antenna  130 , suitable side walls or a support unit on either side of the antenna  130 . Another example is an enclosing housing, for example, a radome. Having an apparatus for fixing the parts is particularly useful when the dielectric volumes  122  and  125  consist of air. 
     An example of dimensions for a sector antenna  130  according to the first embodiment and with a wavelength of 16 cm is given in the following: 
     Length of surface elements l e1 =7.5 cm 
     Width of surface elements w e1 =4 cm 
     Length of slots l s1 =8 cm 
     Width of slots w s1 =0.5 cm 
     Distance a k =1 cm 
     Height of the first dielectric volume h d1 =1 cm 
     Height of the second dielectric volume h d2 =0.2 cm. 
     The dimensions listed above are estimated. 
     FIG. 12 is a front view of a second embodiment  140  of the inventive antenna unit for transmitting and receiving with a polarization of 0/90 degrees. The antenna unit  140  is here shown in a rectangular design. The embodiment is based on the first embodiment in connection with FIG. 9, the antenna unit  140  comprising a slot  151 , see FIG. 13, integrated in a microstrip element  143 , see FIG. 12, and an aperture  141  integrated in a surface element  142  on the microstrip element  143 . The surface element  142  with the integrated opening  141  will in the following be referred to as a radiating unit  144 . The aperture  141  is arranged in the surface element  142  parallel to its polarization direction in order not to intercede any current paths. This implies that the risk of a signal coupling between the two orthogonal polarization directions of the antenna unit  140  will be negligible. The surface element  142  has a well defined length l e2  and width w e2 . The length  1   e2  is dependent on the wavelength with which the antenna unit  140  is to transmit and receive. The width w e2  determines the beam width of the surface element in the horizontal plane. 
     FIG. 12 shows the aperture  141  having a well defined length  1   a  and width w a  held within the surface element  142 . The length l a  of the aperture can also be longer than the length  1   e2  of the surface element, in which case the surface element will be divided into two elongated portions  191   a-b , see FIG.  19 . The surface element may also comprise more than two elongated portions  191   a-c  with apertures  192   a-b  between the portions. Such a surface element is commonly referred to as a gridded patch, see the article “Dual Polarised Aperture Coupled Printed Antennas”, pp. 79-89, from “Proc. Of 16 th  ESA Workshop on Dual Polarisation Antennas” in Noordwijk, The Netherlands, Jun. 8 th  -9 th , 1993. 
     FIG. 13 is a cross-sectional view of the antenna unit  140 . The antenna unit  140  comprises the first disc  121  of an electrically insulating material in the upper layer on which the radiating unit  144  (not marked out in FIG. 13) as shown in FIG. 12 is arranged, the intermediate layer with the earth plane  114 , and the first dielectric volume  122 , for example air, between them. In the earth plane  114 , the slot  151  is arranged. The slot  151  is arranged directly below the aperture  141 . The second dielectric volume  125 , for example air, is arranged between the earth plane  114  and the second disc  123  of electrically insulating material in the lower layer of which a feeder  152  to the slot  151  is arranged. If the dielectric volumes  122  and  125  consist of air, of course, side walls are arranged in a suitable way to support the discs  121  and  123  and the earth plane  114 . 
     The earth plane  114  may also in this case consist of, for example, an electrically conductive material with said slot  151  or a disc of an electrically insulating material, on which an electrically conductive surface comprising the slot  151  is arranged. 
     The slot  151  has a predetermined  1   s2  and width w s2 , for example, coinciding with the well defined length  1   a  and width w a  of the aperture  141 . The well defined length  1   s2  is dependent on the wavelength with which the antenna unit  140  is to transmit and receive. The width w s2  substantially determines the bandwidth of the slot. 
     The antenna unit  140  can be used, with an addition of technology known in the art, to generate a circular polarization in a large angular area. 
     FIG. 14 is a front view of a second embodiment of a sector antenna  160  comprising the second embodiment of the inventive antenna unit, for transmitting and receiving with a polarization of 0/90 degrees. The antenna  160  is here shown having a rectangular design. The antenna  160  comprises four antenna units  140   a-d  (not marked out in FIG.  14 ), each similar to the ones shown in FIGS. 12 and 13 and arranged one after the other in a common structure. This means that the antenna  160  comprises four rectangular radiating units  144   a-d  in the upper layer and four slots  151   a-d  (not shown in FIG. 14) in the intermediate layer. 
     The rectangular radiating units  144   a-d  on the respective antenna unit  140   a-d  are arranged in a column, the short sides facing each other, with a certain, for example, constant centre distance d c3  between the centres of the radiating units  144   a-d . The radiating units  144   a-d  are also positioned in such a way that their longitudinal axes are parallel to the longitudinal axis of the antenna. The centre distance d c3  correspond to a wavelength in the medium in which the wave is propagating when passing through feeders and microstrip elements. 
     The surface elements  142   a-d  in the respective radiating unit  144   a-d  are supplied through a central supply conductor  161  and serially connected., from  142   c  to  142   d  and from  142   c  to  142   a , respectively, by means of three pairs of parallel feeders  162   a-c . Because of the serial feeder, the surface elements  142   a-d  can transmit or receive with a vertical polarization and a first horizontal beam width  34 . Because of the parallel connectors  162   a-c  the current distribution over the surface elements will be even. 
     FIG. 14 also shows how the feeders  152   a-d  for the supply to/from the slots  151   a-d  (not shown in FIG. 14) in the respective antenna unit  140   a-d  are serially connected. Each of the feeders  152   a-d  is arranged under the corresponding slot  151   a-d  to excite them in a predetermined way. The slots  151   a-d , in turn, radiate through the apertures  141   a-d  in the radiating units  144   a-d  so that they can transmit or receive with a horizontal polarization with a second horizontal beam width  34 . The second beam width is substantially equal to the first beam width. 
     The supply and the feeders to and from the slots  151   a-d  and the surface elements  142   a-d  can be arranged in more ways than what was shown and described in connection with FIG.  14 . The feeders  152   a-d  to the slots  151   a-d  can, for example, be arranged in the same way as the feeders  124   a-d  to the slots  113   a-d  in FIG.  11 . 
     An apparatus for fixing the parts of the antenna  160  man, for example, comprise a bar around the antenna  160 , suitable side walls or a support unit on either side of the antenna  160 . Another example is a surrounding housing, for example, a radome. Having a device for fixing the parts is particularly useful when the dielectric volumes  122  and  125  consist of air. 
     An example of the dimensions of a sector antenna  160  according to the second embodiment, having a wavelength of 16 cm, is given in the following: 
     Length of surface elements l e2 =7.5 cm 
     Width of surface elements w e1 =4 cm 
     Length of apertures l a =Length of slots l s2 =7 cm 
     Width of apertures w a =Width of slots w s2 =0.5 cm 
     Height of the first dielectric volume h d1 =1 cm 
     Height of the second dielectric volume h d2 =0.2 cm. 
     The dimensions listed above are estimated. 
     FIG. 15 is a front view of a third embodiment of a sector antenna  170  comprising the first embodiment of the inventive antenna unit as shown in FIGS. 9 and 10. The third embodiment is based on the first embodiment in connection with FIG.  11 . The sector antenna  170  comprises four antenna units  110   a-d  according to the first embodiment, arranged one after the other, the antenna units being integrated in a common structure. The antenna units  110   a-d  are described in more detail in connection with FIGS. 9 and 10. The antenna units  110   a-d  are tilted 45 degrees anticlockwise relative to the first embodiment (FIG. 11) of the sector antenna  130 . This implies that the antenna  170  can transmit and receive with a polarization of ±45 degrees. The beam widths of the two polarizations are substantially equal. Apart from this, the design of the antenna corresponds to that of the antenna  130 . 
     The antenna units  110   a-d  may also be tilted an arbitrary number of degrees clockwise or anticlockwise. 
     FIG. 16 shows a fourth embodiment of a sector antenna  180  comprising the second embodiment of the inventive antenna unit, as shown in FIGS. 12 and 13. The fourth embodiment is based on the second embodiment in connection with FIG.  14 . The sector antenna  180  comprises four antenna units  140   a-d  according to the second embodiment, arranged one after the other, the antenna units  140   a-d  being integrated in a common structure. The antenna units  140   a-d  are described in more detail in connection with FIGS. 12 and 13. The antenna units  140   a-d  are tilted 45 degrees anticlockwise relative to the second embodiment (FIG. 14) of the sector antenna  160 . This implies that the sector antenna  180  can transmit and receive with a polarization of ±45 degrees. The beam widths of the two polarizations are substantially equal. Apart from that, the design of the sector antenna  180  corresponds to that of the sector antenna  160 . 
     The antenna units  140   a-d  may also be tilted an arbitrary number of degrees clockwise or anticlockwise. 
     FIG. 17 is a front view of an embodiment of an antenna array  190  comprising the second embodiment of the inventive antenna unit as shown in FIGS. 12 and 13 for transmitting and receiving in two polarization directions. The embodiment is based on the second embodiment in connection with FIG.  14 . The antenna array  190  comprises four parallel columns, each having four antenna units  140   a  according to the second embodiment, in each column. The antenna units  140  are integrated in a common structure forming a two-dimensional antenna array  190 . Each column may be connected, in a way known in the art, and separately for each polarization, to lobe shaping networks for generating one or more fixed or adjustable lobes in the horizontal plane. A centre distance d c4  between the centre lines of the columns may be smaller than a distance corresponding to half a wavelength in air. This enables large output angles from the antenna  190  and prevents the generation of gridded lobes. 
     The centre distance d c4  may be selected, for example to 7 cm for an antenna array having a wavelength of 16 cm. 
     In the examples of the invention described above, the slots  113   a-d ,  151   a-d  and the apertures  141   a-d  are rectangular. They may also have other shapes. FIG. 18 shows three examples of different shapes of the slots  113   a-d  and  151   a-d . Their shapes are shown in FIG.  18 . 
     FIG. 19 was described in connection with FIG.  12 .