Abstract:
A turnstile antenna has two dipoles which have a galvanic contact at the crossing point. The dipoles are arranged in a geometrically asymmetrical manner with respect to the crossing point. The turnstile antenna can be arranged either in a free space or over a metal plate.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     An antenna structure of a circular-polarized antenna for a vehicle is described. 
     The document WO 01/76007 A1 discloses an antenna structure of a circular-polarized antenna for a vehicle or for a portable communication or navigation appliance. The known antenna structure is envisaged for the use of particular frequencies and has a conductive baseplate and a first and a second conductive element that cross one another. The crossing elements are spaced apart from one another at a crossing point, so that any electrical contact is avoided and no substantial capacitive coupling with respect to one another occurs at the crossing point either. Each element half has a length that corresponds approximately to one quarter wavelength for the envisaged frequency, each element having at least one end section that is arranged generally at right angles to the baseplate and has at least one further section that is provided parallel to the planar conductive baseplate. 
     The elements and the baseplate generally define a volume, wherein each end of each element is electrically coupled to the baseplate and wherein the elements are coupled to one another via a 90° phase shifter. Hence, this antenna structure has two feed points. Furthermore, the crossing point of the two crossing elements is arranged such that a geometric symmetry is obtained for the crossing point of the crossing elements. In order to be able to use a geometrically symmetrical crossed dipole of this kind to receive circular-polarized satellite signals, the envisaged 90° phase shifter is absolutely necessary. The provision and connection of the phase shifter to the two ends of the crossing antenna elements is also costly. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an antenna structure of a circular-polarized antenna for a vehicle that can be integrated into existing antenna modules in a space-saving and inexpensive manner. 
     This object is achieved by means of the subject matter of the independent claims, and advantageous developments of the invention can be found in the dependent claims. 
     The invention proposes an antenna structure of a circular-polarized antenna that can be arranged on an electrically conductive surface that has a first λ/2 dipole, a second λ/2 dipole and a crossing point for the first and second λ/2 dipoles to form a crossed dipole. The crossing dipoles are electrically conductively connected at the crossing point. In relation to the crossing point, they have a geometric asymmetry in at least one longitudinal direction of the dipoles or in both longitudinal directions such that the lengths of the dipole limbs from the crossing point to the ends are different, with the asymmetry setting a desired phase shift. In this case, the antenna structure has only one feed point, which is positioned at one end of one of the dipoles, this determining the direction of rotation of the circular polarization. 
     The antenna structure according to the invention therefore provides two asymmetrically crossing dipoles, particularly above an electrically conductive surface. This antenna structure is fed from a single feed point at one dipole end and, by virtue of the asymmetry, a phase shift is achieved within the desired frequency range when the second dipole is excited. Furthermore, these dipoles can be punched from a metal plate, which gives a great deal of latitude in the configuration or inclination of the polar diagram. These dipoles can alternatively be realized on printed circuit boards, such as FR 4  material. Preferably, the first λ/2 dipole is bent in a U shape and the second λ/2 dipole is likewise bent in a U shape. In this preferred embodiment of the invention, the antenna structure therefore has a first λ/2 dipole bent in a U shape and a second λ/2 dipole bent in a U shape. The first λ/2 dipole and the second λ/2 dipole form a crossing point for a crossed dipole, wherein the crossing λ/2 dipoles bent in a U shape are electrically conductively connected at the crossing point and are arranged geometrically asymmetrically in relation to the crossing point. 
     In comparison with the antenna structure known from the document above, such an antenna structure of an asymmetric crossed dipole has the advantage that a phase shifter is not required, since the configuration of the asymmetry can be used to set a circular polarization for the crossed dipole according to the invention. The use of the asymmetry and of the direct electrical contact between the two dipoles means that only a single feed pin is required. Furthermore, the shapes of polar diagrams for this asymmetric crossed dipole can be broadly matched to the requirements of electrically conductive surfaces such as vehicle roofs, this being possible only to a very restricted degree with the patch antennas used as standard for satellite surfaces, for example. The additional elements that are otherwise customary for patch antennas, such as a base or a shielding chamber beneath the patch antenna in order to achieve an inclination for the polar diagram in relation to a curved electrically conductive surface, are therefore not necessary given the asymmetric configuration of the crossed dipole. 
     The free configuration of the dipoles makes it possible to achieve complete compensation for severely curved roofs or compensation for windowpane inclinations. This configuration of the crossed dipole provides for the individual dipoles not to be arranged centrally above one another perpendicularly but rather to be provided with an offset. This crossed dipole can also be fed just from a single end of one of the λ/2 dipoles, and there is no requirement for a 90° phase-shifting additional connection to the feed ends of the crossing dipoles. The position of this single feed point and also the asymmetric offset and the configuration of the dipoles allow circular polarization to be produced for the reception of satellite services, as shown by the appended figures. 
     By relocating the feed point for a dipole to another, for example opposite, end of the dipole, it is possible to change from left-circulating to right-circulating polarization, and vice versa. Furthermore, the configuration allows height compensation for the antenna structure in relation to a curved electrically conductive surface such as an automobile roof, the U-shaped configuration of the dipoles facilitating this task. 
     As already mentioned above, it is also possible for the antenna characteristic or the polar diagram of the antenna to be reconfigured by reconfiguring the individual dipole arms by means of intrinsic antenna geometry changes. In addition, it is possible to dispense with the phase shifter, reducing costs for hardware and assembly. In addition, inexpensive manufacture by means of punched antenna structures is possible. 
     In this case, a preferred embodiment of the invention can have provision for a punched and bent sheet metal material to be provided to the crossing λ/2 dipoles that are bent in a U shape, the sheet metal material preferably having a copper alloy that may be protected from corrosion by a gold coating, for example. 
     In this case, the crossing λ/2 dipoles may be arranged on a printed circuit board material. In addition, the electrically conductive surface is preferably formed by a vehicle roof. 
     In a further embodiment of the invention, the circular-polarized antenna has an SDARS (Satellite Digital Audio Radio System) antenna for satellite communication frequencies f SDARS  between 2.320 GHz≦f SDARS ≦2.345 GHz. These high satellite communication frequencies mean that a relatively small design is also obtained for the crossed dipoles, with a relatively low height in order to space the crossed dipole apart from an antenna circuit board or from a curved electrically conductive surface of a vehicle roof. 
     Secondly, the SDARS antenna may be arranged on an electrically conductive top of a shielding chamber for the antenna circuit board, wherein the shielding chamber can enclose an antenna matching circuit, a tuner and/or an amplifier. 
     In addition, such an antenna structure is used for a GPS (Global Positioning System) antenna for satellite navigation frequencies f GPS  between 1.574 GHz≦f GPS ≦1.577 GHz. In this case too, the antenna structure of the GPS antenna may be arranged on an electrically conductive top of a shielding chamber of an antenna circuit board, wherein the shielding chamber encloses an antenna matching circuit, a tuner and/or an amplifier. 
     In a further embodiment of the invention, the antenna structure is connected to a matching network and a coaxial feed line or to a receiver or transceiver by means of a limb of one of the crossing λ/2 dipoles. In this case, it is possible to dispense completely with the coupling of a second end of a limb of a crossing λ/2 dipole via a phase shifter, which has the effect of saving space and cost. 
     In addition, in a further embodiment of the invention, the height h and hence the limb length of the four limbs of the λ/2 dipoles bent in a U shape matches the curvature of the electrically conductive surface such as a vehicle roof such that a horizontal crossing plane for the crossing λ/2 dipoles is obtained. 
     The overall length of each λ/2 dipole is retained despite the different bends and the different limb heights, the effective wavelength for the respective frequency range being provided for λ. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The invention will now be explained in more detail with reference to the appended figures. 
         FIG. 1  shows a basic illustration of the antenna structure according to the invention 
         FIG. 2  shows a schematic perspective view of an antenna structure of an asymmetric crossed dipole according to an embodiment of the invention; 
         FIG. 3  shows a schematic perspective view of a crossed dipole as shown in  FIG. 1  on a shielding chamber; 
         FIG. 4  shows a schematic perspective view of the asymmetric crossed dipole shown in  FIG. 1  on a curved vehicle roof; 
         FIG. 5  shows a schematic perspective view of an antenna structure of an symmetric crossed dipole according to a further embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a basic illustration of the antenna structure  1  according to the invention. The basic illustration of the antenna structure  1  according to the invention comprises two asymmetrically crossing dipoles  5  and  6  that are electrically connected at their crossing point  7  and can be arranged or are arranged above an electrically conductive surface. This antenna structure  1  is fed from a single feed point  17  at one dipole end  24  and, by virtue of the asymmetry, a phase shift is achieved within the desired frequency range when the second dipole  6  is excited. Furthermore, these dipoles  5  and  6  can be punched from a metal plate, which gives a great deal of latitude in the configuration or inclination of the polar diagram. These dipoles  5  and  6  can alternatively be realized on printed circuit boards such as FR 4  material. 
       FIG. 2  shows a schematic perspective view of an antenna structure  1  of a circular-polarized antenna  3  having an asymmetric crossed dipole  8  according to an embodiment of the invention. In this first embodiment of the invention, the crossed dipole  8  consists of a first λ/2 dipole  5  bent in a U shape and a second λ/2 dipole  6  bent in a U shape. In this arrangement, the length of half of the effective wavelength of the first λ/2 dipole  5  extends from a base point A, which is in the form of a feed point, to a base point B. In addition, the effective λ/2 length of the second λ/2 dipole  6  extends from a base point C to a base point D. The two λ/2 dipoles  5  and  6  are electrically conductively connected at a crossing point  7  at which they encounter one another at right angles. 
     In this embodiment of the invention, this is achieved by virtue of the entire antenna structure  1  of this crossed dipole  8  being punched from a copper sheet material  9  and bent. This punching and bending can take place in a single production step. In this case, the two λ/2 dipoles  5  and  6  are angled in a U shape. While the base points B, C and D of the limbs  14 ,  15  and  16  of the λ/2 dipoles  5  and  6  angled in a U shape are fixed on an electrically conductive surface  4  in a capacitive or resistive manner, the base point A of the limb  13  is in the form of a feed point and connected to a coaxial feed line  17 . 
     The limb lengths of the limbs  13  to  16  simultaneously define the heights h 13 , h 14 , h 15  and h 16  of an almost horizontal crossing plane  18  above an electrically conductive surface  4 , the horizontal crossing plane  18  containing horizontal sections of the two λ/2 dipoles. The horizontal sections for the limbs  13 ,  14 ,  15  and  16  are of different length in relation to the crossing point  7 , so that an antenna structure  1  of a circular-polarized antenna  3  having an asymmetric crossed dipole  8  is obtained. 
     The asymmetry prescribes the circular polarization. The latter is determined by the difference in the sum of the length (starting point is the crossing point  8 ) of the limbs  14  and  18  in comparison with the length of the limbs  24  and  16  and also by the difference in the length of the limbs  21 ,  22  and  15  in comparison with the sum of the length of the limbs  5  and  13 . This structural asymmetry achieves right-circular polarization of the antenna structure  1  if the feed point is maintained at the base point A. If the feed point is moved to the base point B, on the other hand, left-circular polarization is obtained. The different limb lengths of limbs  13  to  16  with the heights h 13  to h 16  and also the right-angled angling of the dipole with the limbs  21  and  22  allow—in addition to the phase shift—the shape of the polar diagram to be customarized taking account of the influence of the electrically conductive surfaces or a chassis. 
       FIG. 3  shows a schematic perspective view of the asymmetric crossed dipole  8  in  FIG. 2  on a shielding chamber  12 . Components having the same functions as in  FIG. 1  are denoted by the same reference symbols in the subsequent figures and are not discussed separately. This shielding chamber  12  can contain—shielded from the radiating elements of the asymmetric crossed dipole  8 —a circuit board having matching circuits, a tuner or an amplifier. From the shielding chamber, an output jack  23  for holding a feed line  17 , as shown in  FIG. 2 , may be provided below an electrically conductive surface  11 . By way of example, the electrically conductive top  11  can match the curvature of a vehicle roof. Furthermore, the shielding chamber  12 , which has a rectangular top  11  in this case, may also have a round or oval top  11 . 
       FIG. 4  shows a schematic perspective view of the asymmetric crossed dipole  8  shown in  FIG. 1  on a curved vehicle roof  10 , the circular-polarized antenna  3  being arranged under a flat plastic cover  19  and the vehicle roof  10  of the vehicle  20  being used as an electrically conductive surface for the radiating λ/2 dipoles  5  and  6 . 
       FIG. 5  shows a schematic perspective view of an antenna structure  2  of an asymmetric crossed dipole  8  according to a second embodiment of the invention. In this embodiment of the invention, the asymmetry of the crossed dipole is extreme, since the limb  13  of the U-shaped λ/2 dipole  5  is arranged directly next to the crossing point  7  and is connected to the feed line  17  via the base point A. In contrast to the preceding first embodiment of the antenna structure, this antenna structure is provided for use in the region of the windshield. 
     LIST OF REFERENCE SYMBOLS 
     
         
           1  Antenna structure (1 st  embodiment) 
           2  Antenna structure (2 nd  embodiment) 
           3  Circular-polarized antenna 
           4  Curved surface 
           5  First dipole 
           6  Second dipole 
           7  Crossing point 
           8  Crossed dipole 
           9  Sheet metal material 
           10  Vehicle roof 
           11  Top 
           12  Shielding chamber 
           13  Limb 
           14  Limb 
           15  Limb 
           16  Limb 
           17  Feed line 
           18  Crossing plane 
           19  Plastic cover 
           20  Vehicle 
           21  Section 
           22  Lug 
           23  Output jack 
           24  Dipole end 
         A Base point 
         B Base point 
         C Base point 
         D Base point 
         h Height