Abstract:
The present invention relates to the field of arrangements in reflector antennas, and in particular to that part of this field that concerns reflector antennas that include subreflectors. The invention is concerned chiefly with an improved subreflector ( 5   b ) which when used in a reflector antenna ( 1 ) enables the antenna to obtain a radiation diagram with high suppression of side lobes in both the H-plane and the E-plane. The reflective structure of the subreflector ( 5   b ) includes at least two mutually different geometries ( 15, 17 ) that have been configured specifically to obtain radiation diagrams with good suppression of side lobes in both the E-plane and the H-plane.

Description:
FIELD OF INVENTION  
         [0001]    The present invention relates to the field of arrangements relating to reflector antennas, and in particular to that part of this field concerned with reflector antennas that include subreflectors.  
         BACKGROUND OF THE INVENTION  
         [0002]    Many technical applications include some form of antenna function for sending or receiving radio signals. Examples of such applications are radio apparatus, TV apparatus, mobile telephony systems, radio communication links and radar systems.  
           [0003]    The requirement placed on the directional effect of an antenna varies in accordance with the application concerned. A radio apparatus shall be capable of receiving signals from different radio stations, regardless of where the apparatus is located, and the antenna should therefore be equally as receptive in all directions in a horizontal plane. On the other hand, a TV receiver shall only be receptive to signals arriving from the nearest TV mast or from a TV satellite. Thus, the antenna of a TV receiver should be positioned so that it is particularly receptive to signals arriving from a certain direction, and signals that arrive from other directions shall be suppressed to the greatest possible extent. This also applies, for instance, to antennas for radio links. Radar apparatus shall normally both transmit and receive in a certain direction, which shall also be capable of being changed so that the radar can receive omnidirectional information relating to the surroundings. It is also desirable in respect of radar apparatus that the antenna will function to suppress signals from directions other than the direction in which the radar currently transmits and receives at that moment in time.  
           [0004]    A common type of directional antenna is the so-called reflector antenna. A reflector antenna will normally include a main reflector and a feed. The feed is placed in front of the main reflector and is adapted to transmit or receive electromagnetic radiation reflected onto the main reflector. A common type of feed includes a waveguide or corresponding device, and a subreflector. In the transmission of electromagnetic radiation, the waveguide is excited to deliver electromagnetic radiation of a predetermined kind. The radiation emitted from the waveguide is first reflected against the subreflector and then against the main reflector. Electromagnetic signals can also be received by the reflector antenna. In the case of reception, the beam path will, of course, travel in the reverse direction to that travelled in the case of transmission. The dimensions of the main reflector are conveniently much larger than the wavelength of the signals used in the application in question. The main reflector is formed to combine signals that are transmitted (or are incoming) in a certain direction, in a manner that is suitable in the context concerned. The directional sensitivity, or receptiveness, of the reflector antenna can be changed by realigning the antenna mechanically.  
           [0005]    Technical specifications have been compiled in order to characterise the quality of the directional properties of reflector antennas. For instance, ETSI (European Telecommunications Standard Institute) have produced a specification—ETS 300 833—that specifies radio link antenna requirements. The specification states requirements concerning the radiation diagram of the reflector antenna in a horizontal plane. For a number of frequency ranges, the specification states, inter alia, the requirement regarding side lobe levels (both with regard to co-polarisation and cross-polarisation). Several numbered classes are specified for each frequency range and the greater the number, the stricter the requirements placed on the suppression of side lobes.  
           [0006]    In order to better utilise the radio frequency spectrum, it is usual to adapt radio link antennas for the use of either horizontal polarisation (horizontal E-field) or vertical polarisation (vertical E-field). It is, of course, beneficial if one and the same reflector antenna can be used for both horizontal polarisation and vertical polarisation, for instance by rotating the feed and the subreflector. In order to make this possible, it is therefore necessary for the reflector antenna to be adapted to enable the quality requirements placed on the radiation diagram (for instance, in accordance with the above-mentioned ETSI specification) to be achieved both in an E-plane in respect of horizontal polarisation and in an H-plane in respect of vertical polarisation.  
           [0007]    WO, A1, 87/07771 teaches a reflector antenna comprising a feed—a so-called hat feed—that includes a subreflector. It would appear that the subreflector is constructed, inter alia, to achieve with the reflector antenna a radiation diagram which in an H-plane coincides with a radiation diagram in an E-plane to the greatest possible extent. The subreflector is rotationally symmetrical about a centre axis and includes a centrally positioned conical spreader which is intended to be placed in front of the aperture of a waveguide in the feed. That part of the reflective structure of the subreflector located outside the spreader is essentially planar, although it includes circular corrugations (grooves) of a constant depth correspond approximately to one-quarter wavelength. The construction of the subreflector also enables the hat feed to be made very compact.  
           [0008]    Reflector antennas that include hat feeds function very efficiently in general. However, reflector antennas that include a hat feed do not function satisfactorily in some cases. For instance, when measuring at high frequencies on a 0.3 m reflector antenna that included a hat feed, it was found that the reflector antenna was unable to meet the requirements of ETSI class 3 (30-47 GHz) in the H-plane. The radiation exceeded specified levels in the region nearest the main lobe and for angles around 60° in relation to the main lobe (so-called spillover lobes, in other words direct radiation from the feed that failed to impinge on the main reflector). The reflector antenna equipped with the hat feed essentially met the requirements of ETSI class 3 in respect of the E-plane.  
           [0009]    One drawback with the hat feed is thus that it is unable to achieve a radiation diagram with high suppression of side lobes in both the H-plane and the E-plane under all circumstances.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention addresses chiefly the problem of obtaining an improved subreflector which, when used in a reflector antenna, enables the reflector antenna to obtain a radiation diagram with high suppression of side lobes in both the H-plane and the E-plane.  
           [0011]    In brief, the problem addressed above is solved by providing the subreflector with an improved reflective structure.  
           [0012]    Accordingly, one object of the invention is to provide a subreflector that is an improvement with respect to achieving radiation diagrams of predetermined quality in different planes, wherein the invention also includes a feed that includes one such subreflector and also a reflector antenna that includes such a subreflector.  
           [0013]    More specifically, the above addressed problem is solved as follows: The reflective structure of the subreflector includes at least two different geometries which have been designed specifically to obtain radiation diagrams that have effective suppression of side lobes in both the E-plane and the H-plane.  
           [0014]    An essential advantage afforded by the invention is that it enables the procurement of reflector antennas that can be used for both horizontal polarisation and vertical polarisation in applications where high quality is required of the reflector antenna radiation diagram in a horizontal plane (or in a vertical plane).  
       
    
    
       [0015]    The invention will now be described in more detail with reference to preferred exemplifying embodiments thereof and also with reference to the accompanying drawings.  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a sectioned view of a reflector antenna.  
         [0017]    [0017]FIG. 2 is a plan view of a subreflector of the reflector antenna.  
         [0018]    [0018]FIG. 3 is a first sectioned view of the subreflector.  
         [0019]    [0019]FIG. 4 is a second sectioned view of the subreflector.  
         [0020]    [0020]FIG. 5 is a sectioned view of a feed element belonging to the reflector antenna.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]    [0021]FIG. 1 is a cross-sectional view of one embodiment of the invention that includes a reflector antenna  1 . The reflector antenna  1  includes a dish-shaped main reflector  3  (generally parabolic) and a feed element  5  mounted on a vertex plate  7  located centrally in the main reflector  3 . The feed element  5  is disposed along a centre axis  8  of the main reflector  3  and includes a waveguide  5   a,  a subreflector  5   b  and a holder  5   c  that functions to secure the subreflector  5   b  in a predetermined position relative to the waveguide  5   a.  In the illustrated embodiment, the waveguide  5   a  has a circular cross-section, although it may, alternatively, have some other cross-sectional shape, such as a rectangular shape. The waveguide  5   a  is adapted for excitation to a predetermined propagation mode, for instance TE11. A tubular attenuator  9  has a first edge that lies against the edge of the main reflector  3 . The material composition of the attenuator  9  and its dimensions are adapted so that the attenuator  9  will suppress spillover lobes. A radome  11  is disposed at the other edge of the attenuator  9 , this edge being opposite to said first edge.  
         [0022]    [0022]FIG. 2 is a plan view that illustrates the subreflector  5   b  in more detail, from its reflective side. In the illustrated case, the subreflector  5   b  has a circular periphery. The reflective structure of the subreflector  5   b,  however, is not rotationally symmetrical. Instead, the subreflector  5   b  includes different reflective geometries that have specially adapted properties. For instance, the subreflector  5   b  includes a first reflective geometry  15  in mutually opposing first and second sectors  19  and  21 . The subreflector  5   b  also includes a second reflective geometry  17  in mutually opposing third and fourth sectors  23  and  25  that are perpendicular in relation to the first and the second sectors  19  and  21 .  
         [0023]    [0023]FIG. 3 is a sectional view of the subreflector  5   b,  taken on the line A-A in FIG. 2. The section A-A is taken through a first plane that includes a centre axis  27  of the subreflector  5   b  and that divides said first and second sectors  19  and  20  centrally in two. The first plane constitutes an H-plane when using the subreflector, in other words the first plane is parallel with the magnetic field strength (H) of an electromagnetic field  50  reflected by the subreflector  5   b.  The section A-A illustrates the design of the first geometry  15 . The first geometry  15  includes nearest the centre axis  27  a conical spreader  29  which the first geometry  15  shares with the second geometry  17 . A first corrugation (groove)  31  is located outside the spreader  29 . This first corrugation has a circular configuration whose centre point lies on the centre axis  27  of the subreflector  5   b.  The first corrugation  31  thus passes through all four sectors  19 ,  21 ,  23  and  25 , and is thus common to the first and the second geometries  15  and  17  respectively. In addition to contributing towards the reflective properties of the subreflector  5   b,  the first corrugation  31  has the function of providing means for anchoring the subreflector  5   b  to the holder  5   c,  as will be made apparent hereinafter. Disposed outside the first corrugation  31  is a circular second corrugation  33  whose centre point lies on the centre axis  27  of the subreflector  5   b.  The second corrugation  33  passes through all four sectors  19 ,  21 ,  23  and  25  and is thus common to the first and the second geometries  15  and  17 . The first geometry  15  includes outwardly of the second corrugation  33  a first sloping reflector surface  35  that is disposed in the first sector. The first sloping reflector surface  35  is not perpendicular in relation to the centre axis  27 , but defines an acute angle α (see FIG. 3) relative to said centre axis  27 . In the illustrated case, the first sloping surface  35  has a conical shape. The first geometry  15  also includes a second sloping reflector surface  37  disposed in the second sector  21 . The second sloping reflector surface  37  is opposite the first sloping reflector surface  35 , although it is formed in a corresponding manner to the first sloping reflector surface  35  in other respects. The sloping reflector surfaces  35  and  37  vary linearly in the sectional view A-A of FIG. 3, although the sloping reflector surfaces  35  and  37  may, alternatively, be slightly curved in the section A-A and therewith be more dish-shaped than conical.  
         [0024]    [0024]FIG. 3 is a sectional view of the subreflector  5   b  taken on the line B-B in FIG. 2. The section B-B is taken through another plane which includes the centre axis  27  of the subreflector  5   b  and which divides centrally in two the third and the fourth sectors  23  and  25 . When using the subreflector  5   b,  the second plane constitutes an E-plane, in other words the second plane is parallel with the electric field strength of the electromagnetic field  50  reflected by the subreflector  5   b.  The section B-B illustrates the configuration of the second geometry  17 . The second geometry  17  in the third sector  23  includes outside the spreader  29  and the first and the second corrugations  31  and  33  a circular third and fourth corrugation  39  and  41 . The second geometry  17  in the fourth sector  25  also includes a fifth and a sixth corrugation  55  and  47 , these corrugations being circular and opposite the third and the fourth corrugations  39  and  41 .  
         [0025]    The corrugations  33 ,  39 ,  41 ,  45  and  47  all pass in one and the same plane, which is perpendicular to the first and the second plane. The invention is not restricted to corrugations  33 ,  39 ,  41 ,  45  and  47  that pass in the same plane, and said corrugations may, alternatively, be disposed to pass in different planes, for instance so that the reflective structure of the subreflector  5   b  will be slightly conical.  
         [0026]    As will be apparent from the section A-A, the second geometry has a configuration that corresponds to the reflective structure of the aforesaid hat feed subreflector. As before mentioned, the hat feed provides a radiation diagram in the E-plane that satisfies ETSI class 3, but, on the other hand, a radiation diagram in the H-plane that does not meet with the requirements of ETSI class 3. Accordingly, the subreflector  5   b  utilises in the second geometry  17  those advantages possessed by the hat feed subreflector in obtaining a radiation diagram that fulfils a predetermined quality in the second plane (E-plane). The first geometry  15  of the subreflector  5   b,  however, is designed to compensate for the deficiencies of the hat feed subreflector. Thus, the first geometry is designed especially to obtain with the reflector antenna  1  a radiation diagram that fulfils a predetermined quality also in the first plane (H-plane). In this regard, it is chiefly the sloping reflector surfaces  35  and  37  which enable the first geometry to compensate for the deficiencies of the second geometry with respect to the radiation diagram in the first plane (H-plane).  
         [0027]    When making measurements and calculations on a reflector antenna that included a subreflector as in the case illustrated in FIGS.  2  to  4 , it was found that the reflector antenna fulfilled ETSI class 3 in the H-plane and also in the E-plane in respect of the frequencies 37.0, 38.25, 39.5 GHz. Reflection and antenna gain are also roughly the same as that obtained with more conventional subreflectors.  
         [0028]    It will be understood that the first and the second geometries  15  and  17  are not restricted to precisely the configuration shown in FIGS.  2  to  4 , and that the first and the second geometries may, alternatively, be configured in some other way so as to obtain a radiation diagram of predetermined quality in the first and the second planes respectively.  
         [0029]    [0029]FIG. 5 is a sectional view which illustrates the feed  5  in more detail. The section shown in FIG. 5 is taken through a plane that corresponds to the first plane through which the section A-A in FIG. 3 is taken. In this example, the waveguide  5   a  is circular-cylindrical and includes a first and a second end  56  and  57 , which are both open. The holder  5   c  includes a first tubular end  55   a  whose outer diameter corresponds substantially to an inner diameter of the waveguide  5   a.  The first tubular end  55   a  of the holder  5   c  is inserted into the waveguide  5   a  at the first end  56  of said waveguide. The holder  5   c  also includes a stop shoulder  55   c  that lies against the first end  56  of the waveguide  5   a.  The stop shoulder  55   c  and the first tubular end  55   a  enable the holder  5   c  to be readily placed in a predetermined position in relation to the waveguide  5   a.  The holder  5   c  also includes a second tubular end  55   b  which is adapted to fit in the first corrugation  31  of the subreflector  5   b.  The holder  5   c  also includes a centrally disposed recessed part  55   d  that has a conical shape corresponding to the shape of the spreader  29 , said holder  5   c  being formed so that the spreader will lie against the walls of the recess  55   d.  The holder  5   c  is constructed so that the subreflector  5   b  will be located at a predetermined distance from the first end of the waveguide  5   a  and so that the centre axis  27  of the subreflector will coincide with a centre axis  58  of the waveguide  5   a.  In the illustrated case, the holder  5   c  is assumed to be made of PTFE (polytetrafluoroethylene), although it may alternatively consist of some other material found appropriate to this end by the person skilled in this art, for instance polystyrene. The design of the subreflector  5   b  enables the feed  5  to be made relatively short and compact.  
         [0030]    The invention can be applied in all antenna applications found appropriate by the person skilled in this art. The invention, however, is particularly suitable for use in radio links where different polarisation directions are used to reduce interference.