Abstract:
An inventive antenna comprises two antenna elements forming a planar slotline antenna. The antenna furthermore comprises absorber elements surrounding the antenna elements on two layers. The absorber elements are shaped to partially cover the antenna elements and partially not cover the antenna elements. Moreover, they are shaped to dampen at least a dipole-mode of the antenna elements and not to dampen a slotline-mode of the antenna elements.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to an improved slotline antenna 
       BACKGROUND OF THE INVENTION 
       [0002]    For creating broadband antennas with low production costs, conventionally Vivaldi-antennas are used. Vivaldi-antennas consist of a tapered slotline antenna on a circuit board. Regular Vivaldi-antennas though create an electrically short antenna with a dipole-mode of radiation at signal frequencies where the antenna width W is approximately shorter than half of the signal wave length (W&lt;λ/ 2 ). At those frequencies, unwanted radio frequency currents flow on the outer shield of the feeding coaxial cable. Thus, the cable provides unbalanced feeding and becomes an antenna part, too. This deforms the radiation pattern towards a dipole-like radiation pattern. For example, the European Patent EP 1 425 818 B1 shows such a Vivaldi-antenna. Furthermore the electrically short antenna provides typically poor reflection coefficient at the feeding port. 
         [0003]    Accordingly, the object of the invention is to create a broadband antenna with a highly directive radiation pattern and low reflection coefficient. 
       SUMMARY OF THE INVENTION 
       [0004]    The object is solved by the features of the independent claims. The dependent claims contain further developments. 
         [0005]    In a first aspect of the inventive antenna, the antenna comprises two antenna elements forming a planar slotline antenna. The antenna furthermore comprises absorber elements surrounding the antenna elements on two layers. The absorber elements are shaped to partially cover the antenna elements and partially not cover the antenna elements. Moreover, they are shaped to dampen at least a dipole-mode of the antenna elements and not to dampen a slotline-mode of the antenna elements. By use of the setup, a high directivity and a high bandwidth of the antenna can be achieved. 
         [0006]    According to a second aspect of the present invention, the absorber elements are advantageously of identical shape on both layers or at least of nearly identical shape. This leads to a highly symmetrical radiation pattern of the antenna. 
         [0007]    In a third advantageous aspect of the present invention, the antenna elements are tapered. The setup allows for an especially high bandwidth of the antenna, low reflection coefficient and a desired directivity. 
         [0008]    In a fourth advantageous aspect of the present invention, the antenna elements form a Vivaldi-antenna. A high directivity and bandwidth can therefore be reached. 
         [0009]    In a fifth advantageous aspect of the invention, the antenna elements are formed by a metallization layer on a circuit board. The absorber elements are shaped to cover outer sections of the antenna elements and not to cover inner sections of the antenna elements. Since for low frequencies inciting the undesired dipole-mode, the current flows in the outer sections of the antenna elements, at least the dipole-mode can be dampened by surrounding these outer sections of the antenna elements by the absorber elements. For high frequencies, at which the desired slotline-mode is emitted, the currents flow mainly in the inner sections of the antenna elements and therefore should not be dampened. Therefore, absorber elements are not arranged there. 
         [0010]    In a sixth advantageous aspect of the invention, the antenna elements are formed by a metallization layer on a circuit board. The absorber elements are shaped to cover outer sections of the antenna elements. The circuit board and the absorber elements both extend beyond these outer sections of the antenna elements. This is done, since the electrical field created by the currents flowing in the outer sections of the antenna elements at low frequencies extends beyond the area covered by the antenna elements. By placing absorber elements supported by the circuit board in these areas beyond the outer sections of the antenna elements, this electrical field is dampened. 
         [0011]    According to a seventh advantageous aspect, the antenna elements are also formed by a metallization layer on a circuit board. The absorber elements are located on the metallization layer side of the circuit board and on a non-metallization layer side of the circuit board. By dampening the electrical field not only on the metallization layer side of the circuit board but also on a non-metallization layer side of the circuit board, the directivity and the bandwidth of the antenna can be further increased. 
         [0012]    According to an eighth advantageous aspect of the present invention, the antenna elements are also formed by a metallization layer on a circuit board. The metallization layer is largely covered by a protective coating. The protective coating though is interrupted in a feed line connection area. This feed line connection area is located at the narrowest point between the two antenna elements. By covering the majority of the surface area, the antenna is ideally protected. By leaving the feed line connection area uncovered, radio frequency influences are minimized in the especially sensitive area with lowest distance between the two antenna elements. 
         [0013]    According to a ninth advantageous aspect, the antenna elements are also formed by a metallization layer on a circuit board. The antenna comprises a connector for connecting an external line. Moreover, it comprises a feed line connected to the connector and to the antenna elements for feeding a signal from the external line to the antenna elements. The feed line in this case may advantageously form an impedance transformer for transforming the impedance of a coaxial feed line to the antenna impedance. For example, a narrowing feed line can be used. This allows for an optimal power transfer if antenna impedance differs from line impedance. 
         [0014]    According to a tenth advantageous aspect of the present invention, the antenna elements are also formed by a metallization layer on a circuit board. The antenna elements comprise radiating edges. The shape of the circuit board follows the shape of the radiating edges of the antenna elements in general and extends beyond the radiating edges slightly. The antenna elements are therefore ideally supported by the circuit board. Detrimental effects by unnecessary circuit board area are prevented which reduces dispersion in antenna. By cutting the circuit board in the slot between the antenna elements the ratio of the signal power propagating in the surrounding air to the signal power propagating in the circuit board is increased. Then the transmission line medium becomes more uniform and dispersion effect is reduced. This prevents directivity drop especially at the higher bound of frequency range. 
         [0015]    According to an eleventh advantageous aspect of the invention, an antenna system comprises two previously described antennas. The two antennas are mounted perpendicularly, intersecting along a symmetry axis between their antenna elements. The two planar antenna elements form a three-dimensional antenna system. When using only one of the previously described antennas, a linear polarization of the emitted signal is achieved. By use of a here described antenna system, it is possible to achieve arbitrarily polarized electromagnetic emission or two orthogonal linear polarizations. Alternatively by adding a 90° phase shift between the signals of the two antennas, it is possible to reach a circular-polarized emitted signal. 
         [0016]    According to a twelfth advantageous aspect, the antenna system comprises a base plate mounted perpendicularly to both antennas on a non-radiating side of the antennas and absorbers mounted on the base plate, advantageously extending in the direction towards a radiating side of the antenna. Undesired cross-effects between the two antennas and reflection from the base plate can therefore be prevented. 
         [0017]    In an advantageous thirteenth aspect, an antenna comprises two antenna elements forming a planar slotline antenna. The antenna elements are formed by a metallization layer on a circuit board. The antenna elements in this case comprise radiating edges. The shape of the circuit board follows the shape of the radiating edges and extends beyond the radiating edges slightly. As described earlier, a high directivity can be achieved by reduction of signal dispersion. 
         [0018]    In a fourteenth advantageous aspect of the present invention, a measurement chamber for measuring emissions of a wireless device comprises a container sealed against electromagnetic radiation and a previously described antenna or antenna system. The device under test is placed within the test chamber and can easily be measured. The test chamber can have a low size, since the inventive antenna and antenna system only require a small space. 
         [0019]    The motivation for introducing the absorbers is to advantageously dampen all other modes except the slotline mode. At the frequencies where an antenna width W or an antenna length L are longer as λ/2, some parasitic electromagnetic modes might be excited along the metallic edges of the antenna. The absorbers shall dampen these modes as well. As a result broad bandwidth, high directivity and low reflection coefficient is maintained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    These and other features of the subject invention will be better understood in connection with the Detailed Description in conjunction with the Drawings of, Exemplary embodiments of the invention are now further explained with respect to the drawings, in which 
           [0021]      FIG. 1  shows a first embodiment of the inventive antenna in a front- and back-view with hidden absorbers; 
           [0022]      FIG. 2  shows a second view of the first embodiment of the inventive antenna in a front- and back-view; 
           [0023]      FIG. 3  shows a VSWR achievable by use of an inventive antenna; 
           [0024]      FIG. 4  shows an absolute gain achievable by use of an inventive antenna without a cut; 
           [0025]      FIG. 5  shows an achievable XPR by use of an inventive antenna; 
           [0026]      FIG. 6  shows a second embodiment of the inventive antenna in a front- and back-view with hidden absorbers; 
           [0027]      FIG. 7  shows an absolute gain achievable by use of an inventive antenna with a cut; 
           [0028]      FIG. 8  shows a third embodiment of the inventive antenna; 
           [0029]      FIG. 9  shows a fourth embodiment of the inventive antenna in an antenna system in a front- and back-view and 
           [0030]      FIG. 10  shows an embodiment of the inventive measurement chamber. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    First, we demonstrate the general construction and function of a first embodiment of the inventive antenna along  FIG. 1-2 . Afterwards we show the achievable measurement results using an embodiment of the inventive antenna along  FIG. 3-FIG .  5 . By use of  FIG. 6-9  additional embodiments of the inventive antenna are described in detail. Finally, along  FIG. 10 , an embodiment of the inventive measuring chamber is described. Similar entities and reference numbers in different figures have been partially omitted. 
       First Embodiment 
       [0032]      FIG. 1  shows a first embodiment of the inventive antenna  1 . In  FIG. 1 , for reasons of clarity and comprehensibility, not all components of the antenna have been depicted. In  FIG. 2 , a view of the antenna showing all components is depicted. On the left side of the  FIG. 1 , a front-view of the antenna  1  is shown. On the right side, a back-view of the antenna  1  is shown. 
         [0033]    The antenna  1  comprises a circuit board  10  and two antenna elements  12 ,  13  formed in a metallization layer  11  on the front side of the circuit board  10 . The antenna elements  12 ,  13  are not connected electrically. The antenna element  13  is directly connected to a connector  17 , while the antenna element  12  is connected to the connector  17  through a wire  19  and a feed line  18 . The connector  17  is for example a coaxial connector. The antenna element  13  in this case is connected to the shielding of the coaxial connector, while the antenna element  12  is connected to the center line of the coaxial connector  17 . 
         [0034]    The antenna elements  12 ,  13  are arranged symmetrically on the front-side of the circuit board  10 . The circuit board  10  extends outwardly from the symmetrical axis beyond the extent of the antenna elements  12 ,  13 . Moreover, the antenna elements  12 ,  13  comprise recesses  14 ,  15  at their outer edges regarding the symmetry axis. 
         [0035]    In  FIG. 2 , the antenna  1  from  FIG. 1  is shown including all relevant components. Identical elements have been partially omitted in the description of  FIG. 2 . Absorber elements  20 ,  21 ,  22  and  23  are mounted on two layers surrounding the antenna elements  12 ,  13 . The absorber elements  20 ,  21 ,  22  and  23  are mounted on the front-side and the back-side of the circuit board  10 . The absorber elements  20 - 23  are advantageously formed from a foam material having a dielectric constant ∈ r  between 10 and 100. 
         [0036]    The distance d 1  between the absorber elements  20 ,  21  and  22 ,  23  advantageously is between 20 mm and 100 mm, most advantageously about 60 mm. Moreover d 1  is in the range of 30% to 70% of the entire width of the antenna. Most advantageously, d 1  is 50% of the width of the entire antenna. 
         [0037]    The entire width of the antenna W is between 50 mm and 200 mm, preferably between 80 mm and 140 mm, most advantageously about 120 mm. 
         [0038]    The absorber elements  20 - 23  are mostly symmetrical regarding the circuit board  10  and regarding a symmetry axis of the antenna elements  12 ,  13 . 
         [0039]    The absorber elements  20 - 23  are arranged in an outer section  35  of the circuit board  10  above and below the antenna elements. The outer section  35  is outer in regard to the central symmetry axis of the antenna elements  12 ,  13 . The outer absorber element areas  110  of the absorber elements  20 - 23  extend further outwards than the antenna elements  12 ,  13  regarding the central symmetry axis. 
         [0040]    An inner section  34  regarding the central symmetry axis of the antenna elements  12 ,  13  is not covered by the absorber elements  20 - 23 . Moreover, the absorber elements  20 - 23  form recesses  33  regarding an emitting edge of the antenna elements  12 ,  13 . Also, the absorber elements  20 - 23  form recesses  24 ,  25 ,  28 ,  29  in the outer sections  35 . These recesses  24 ,  25 ,  28 ,  29  can advantageously be used for mounting the antenna. Also, the absorber elements  20 - 23  form recesses  26 ,  27 ,  30 ,  31  at a non-emitting side of the antenna  1 . These recesses  26 ,  27 ,  30 ,  31  can also be used for mounting the antenna  1 . 
         [0041]    The metallization layer  11  shown in  FIG. 1  is largely covered by a protective coating. The protective coating is therefore placed on the circuit board  10  directly where no antenna elements  12 ,  13  are formed and on the antenna elements  12 ,  13  where they are formed. The protective coating is advantageously placed on the top and bottom of the circuit board. Near a feed line connection area  39 , a recess  32  within the protective coating is formed. This is done, so that the protective coating does not influence the antenna radio frequency behavior in the especially sensitive section of the antenna, where the antenna elements  12 ,  13  have minimal distance. The recess  32  within the protective coating extends until the distance between the antenna elements  12 ,  13  towards the emitting side of the antenna reaches d 2 . Advantageously, d 2  is between 2 mm and 8 mm, most advantageously 5 mm. 
         [0042]    In  FIG. 3  the performance of an example of an inventive antenna is shown. The VSWR (Voltage Standing Wave Ratio) of different antenna types is depicted over frequency. The curve  40  and the curve  41  show exemplary regular antennas. The curve  42  shows an exemplary embodiment of the inventive antenna. It can clearly be seen that the performance of the inventive antenna is most advantageous. 
         [0043]    Moreover, in  FIG. 4  the absolute gain of different antenna types over frequency is shown. The curve  50  shows the performance of an inventive antenna, while the curves  51 - 54  show the performance of regular broadband antennas. It can clearly be seen that the inventive antenna is very advantageous. 
         [0044]    Moreover, in  FIG. 5 , the cross-polarization of different antenna types is depicted. Curve  60  shows the cross-polarization XPR of an exemplary embodiment of the inventive antenna while the curve  61  and  62  show the cross-polarization of regular broadband antennas. Also here it can be seen that the inventive antenna is most advantageous. 
       Second Embodiment 
       [0045]    In  FIG. 6 , a second embodiment of the inventive antenna is shown. In this embodiment, the antenna  2  does not necessarily comprise absorber elements. The circuit board  70  of the antenna  2  here furthermore comprises a recess  72  at the emitting side of the antenna  2 . The shape of the circuit board  70  follows the shape of emitting edges  71  of the antenna elements. The circuit board  72  though extends beyond the shape of the antenna elements into the emitting direction of the antenna slightly. A current flowing in the antenna elements at the emitting edge of the antenna elements results in an electromagnetic field along the emitting edge of the antenna elements being present in the surround air and in the circuit board dielectric. These two media have different electrical permittivity creating dispersion effect. The cut  72  reduces the dispersion and increase radiation directivity. 
         [0046]    Moreover, in  FIG. 7 , the absolute gain 111 of an embodiment as shown in  FIG. 6  is depicted. Gain does not drop above 12 GHz like it was shown in  FIG. 4  for an antenna without the cut  72 . 
         [0047]    Alternatively, the features of the embodiments shown in  FIG. 1  and  FIG. 2  and  FIG. 6  can be combined. Then the absorber elements  20 - 23  of  FIG. 2  are arranged on the circuit board  72  of  FIG. 6 . In this combined embodiment a high directivity and a high bandwidth can be achieved. 
       Third Embodiment 
       [0048]    In  FIG. 8 , a third embodiment of an inventive antenna  83  is shown. The antenna  83  is part of an antenna system  3  which is comprised by the antenna  83 , a base plate  80 , on which the antenna  83  is mounted perpendicularly, an absorber base  81  mounted on the base plate  80  and a plurality of absorbers mounted on the absorber base  81 . The absorbers  82  extend from a non-emitting side of the antenna towards the emitting side of the antenna  83  and are mounted parallel to the antenna. The absorbers advantageously are shorter than the antenna  83 . The antenna  83  is an antenna according to one of the previously shown embodiments of the inventive antenna. 
       Fourth Embodiment 
       [0049]    In  FIG. 9 , an embodiment of the inventive antenna system  4  is shown. Two antennas  93  and  94  are arranged perpendicularly. They intersect at a central symmetry axis defined by the antenna elements. The antennas  93 ,  94  are mounted on a base plate  90 , on which also an absorber base  91  and absorbers  92  are mounted. On the left side of  FIG. 9 , the antennas  93  and  94  and the absorber base  91  and the absorbers  92  are depicted. For reasons of clarity, on the right side of  FIG. 9 , the antennas  93 ,  94  and the base plate  90  are shown on their own. 
       Fifth Embodiment 
       [0050]    In  FIG. 10 , finally an embodiment of the inventive measurement chamber  5  is depicted. The measurement chamber  5  comprises a container  101 , which is sealed against electromagnetic radiation and at least an antenna  100  or an antenna system according to one of the previous embodiments. The antenna  100  or the antenna system is mounted on an inner surface of the container  101 . The device under test  102  is placed within the container  101 . The inner surface of the container  101  is completely covered with absorbers. For reasons of clarity, only a part of these absorbers are depicted here. 
         [0051]    The invention is not limited to the examples depicted here. The invention discussed above can be applied not only to sending antennas but also to receiving antennas. Also a use outside of measurement chambers, for example in base stations, etc. is possible. The characteristics of the exemplary embodiments can be used in any combination. 
         [0052]    Having now described a few embodiments of the invention, and some modifications and variations thereto, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by the way of example only. Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention as limited only by the appended claims and equivalents thereto.