Abstract:
The invention relates to a compact and robust broadband antenna, for the frequency range from about 800 MHz up to 9 GHz, in particular, for use on railways. The antenna includes a monopole antenna element, extending in a longitudinal direction essentially parallel to and at a distance above a planar electrically-conducting base plate, between two opposing ends of the antenna element. The antenna element is electrically connected to the base plate at the first end thereof and is insulated from the base plate at the second end thereof, creating an injection point by means of which the antenna element may be connected to a device operating at high frequency.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation of International Application Serial No. PCT/CH03/000275, having an international filing date of Apr. 28, 2003, which designated the United States, the entirety of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to the field of antenna technology. It relates in particular to a physically compact and mechanically robust monopole broadband vehicle antenna capable of operating in the frequency range between about 800 MHz and 9 GHz, and which has a sufficiently high current carrying capacity to be suitable for specific use in high current applications (on board trains, for example). 
   BACKGROUND OF THE INVENTION 
   The geometric dimensions of antennas are closely related to the wavelength of the waves which are intended to be emitted or received via the antennas. Frequently the space available for the antennas is restricted (in the case of portable radio appliances, for example). Consequently, the antennas should be designed to be as compact as possible, without restricting the bandwidth or the efficiency of the antennas. 
   A number of proposals have already been made in the past for folded monopoles to be used as compact antennas. These monopole antennas are arranged in the form of a hairpin above an electrically conductive base plane, with one end being conductively connected (grounded) to the base plane, while the other end is used as a feed point independently of the base plane. See, for example, U.S. Pat. Nos. 3,295,137 and 3,508,271. 
   The characteristic properties of unfilled and filled monopoles folded two or more times have also been investigated in an article by B. J. Lamberty, “A Class of Low Gain Broadband Antennas”, 1958 IRE Wescon Convention Report, pp. 251-259 (August 1958). 
   More recently, folded monopoles have been used to allow a radio link for portable computers (see, for example, U.S. Pat. No. 6,054,955) or to provide wire-free price labeling systems via LANs (see, for example, U.S. Pat. No. 5,668,560). 
   Vehicle antennas for use in the railroad field are subject to unique requirements. Local antennas such as these should be designed in individual cases for a frequency range from 870 MHz to 2170 MHz with a VSWR of &lt;2, thus being suitable for the GMS 900, GSM 1800 and UMTS ranges. They should also be compact and mechanically robust in order to allow a reliable radio link despite the vibration, bugs and other environmental influences. Furthermore, owing to possible contact with the overhead wire on electrified sections of track, local antennas must, in accordance with the relevant test instructions, withstand in particular voltages of 16.6 kV and currents of 40 kA, in which case a voltage of no more than 60 V may occur at the RF connection, in order that personnel in the locomotive are not endangered by the downward feed cable. Examples of local antennas such as these are the K70 20 21 type antennas for the frequency range from 410 to 470 MHz, and the 742 325 type for the frequency range from 870 to 2170 MHz from the Kathrein Company. A broadband vehicle antenna which is resistant to heavy currents is also described in DE-A1-199 24 349. 
   SUMMARY OF THE INVENTION 
   One object of the invention is to provide a broadband antenna arrangement which is not only physically compact but is also mechanically robust and, in particular, is suitable for use as a vehicle antenna in the railroad field. A further object is for the capability to use the antenna arrangement in a frequency range from about 800 MHz up to several GHz (e.g., 9 GHz). 
   In accordance with one embodiment of the present invention, an antenna element is provided which is in the form of a monopole and extends a distance above a planar, electrically conductive base plate in a longitudinal direction essentially parallel to the base plate between two opposite ends of the antenna element and is electrically conductively connected at the first end of the base plate, and has a feed point, which is isolated from the base plate, at the second end, via which feed point the antenna element can be connected to a radio-frequency device. 
   According to a first preferred embodiment, a high degree of mechanical robustness and a high current carrying capacity as well as simplified assembly are achieved, because of several combined features: (1) the antenna element is in the form of a solid metal plate which has a constant thickness transversely with respect to the longitudinal direction and is composed of aluminum or an aluminum alloy; (2) the antenna element has a foot at the first end; (3) the antenna element is attached to the base plate by the foot; and (4) the foot has at least one threaded hole or attachment hole, through which the foot is screwed to the base plate. 
   The design and assembly are particularly simple and functionally reliable if, according to a second preferred embodiment, a coaxial plug connector which is at a right angle to the base plate, and in particular is in the form of a connecting socket, is arranged on the lower face of the base plate in the area of the feed point, with its outer conductor being electrically conductively connected to the base plate and its inner conductor being electrically conductively connected through an opening in the base plate to the feed point of the antenna element. It is also preferred that the connection between the feed point and the inner conductor of the coaxial plug connector is designed to be detachable, in which case an electrically conductive connecting pin which projects downwards at a right angle to the base plate is arranged at the feed point of the antenna element and fits in a socket which is fitted to the inner conductor of the coaxial plug connector. 
   The connecting pin may in this case be integrally formed on the antenna element. It is then also machined out during the mechanical machining of the antenna element. However, it may also be in the form of a separate push-in pin, and may be pushed into a corresponding opening in the antenna element. This has the advantage that the connecting pin may be composed of a different material, which is optimized for making contact, and may be produced more easily. 
   In order to protect the antenna element against damaging environmental influences, it is advantageous for the antenna element to be covered on the outside by a removable shroud, which is detachably connected, in particular screwed, to the base plate. 
   According to another embodiment of the invention, the antenna element has a bar-like base element which runs in the longitudinal direction and to which additional resonant structures are fitted, distributed non-uniformly transversely with respect to the longitudinal direction. The resonant structures preferably comprise elements which are integrally formed on the base element and are oriented at right angles to the base plate. 
   The resonant structures may in this case be rectangular and/or angled, and may have inclined and/or rounded corners. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained in more detail in the following text using exemplary embodiments and in conjunction with the drawing, in which: 
       FIG. 1(   a ) is a longitudinal cross-sectional view and  FIG. 1(   b ) is a front view of a first embodiment of an antenna according to the present invention, and  FIG. 1(   c ) is a detailed view of the plug-in connection; 
       FIG. 2(   a ) is a longitudinal cross-sectional view,  FIG. 2(   b ) is a front view and  FIG. 2(   c ) is a view from underneath of the antenna element shown in  FIGS. 1(   a )- 1 ( c ); 
       FIG. 3  is a view from above of the base plate of the antenna arrangement shown in  FIGS. 1(   a )- 1 ( c ); 
       FIG. 4  is an enlarged side view of the connecting pin at the feed point of the antenna arrangement shown in  FIGS. 1(   a )- 1 ( c ); 
       FIG. 5  is a longitudinal cross-sectional view of a second embodiment of the antenna of the present invention; 
       FIG. 6  is a side view of the second embodiment of the antenna element of the antenna arrangement shown in  FIG. 5 ; 
       FIG. 7  is a plan view from underneath of the base plate of the antenna arrangement shown in  FIG. 5 ; 
       FIG. 8  is a perspective illustration of an antenna arrangement according to a third embodiment of the present invention, with angled and inclined resonant elements; and 
       FIG. 9  is a graph showing the relationship between the return loss and the frequency for the antenna arrangement shown in  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1(   a )- 1 ( c ) show different views of a first embodiment of an antenna arrangement according to the present invention.  FIG. 1(   a ) shows a longitudinal cross section of the antenna arrangement  10 , which is formed on an electrically conductive base plate  18  and is covered by a shroud  11  in the form of a cup. The shroud  11  is preferably composed of a fiber-reinforced plastic and is screwed to the base plate  18  by means of four screws  20 , which are screwed into the corresponding threaded holes  34  ( FIG. 3)  in the base plate. In the first embodiment, the shroud  11  has a flange-like edge with a circumferential groove, in which a sealing ring  76  is inserted to improve the sealing of the internal area from the external area. 
   As can be seen from  FIGS. 1(   a ) and  3 , the essentially rectangular base plate  18  has a raised mounting platform  32  in the central area, adjacent to which are two attachment flanges  31 ,  31 ′ on the transverse faces with two attachment holes  33  provided at each corner. The base plate  18  and thus the entire antenna arrangement  10  can be tightly fastened to a planar outer surface of a vehicle (a locomotive) by means of the four attachment holes  33 . Electrical access to the antenna arrangement  10  is gained via an opening in the outer surface of the vehicle, through which a coaxial antenna socket  17 , which is arranged on the lower face of the base plate  18 , projects into the interior of the vehicle, where it can be connected to a coaxial plug on an antenna cable. The connecting socket  17  is sealed on the outside by a sealing ring  16  which surrounds it and is inserted into an annular groove  38  on the lower face of the base plate  18 . When not connected, the coaxial connecting socket  17  can be protected by a protective cap  19 . 
   Three different views of the antenna element  14  which is the central component of the antenna arrangement  10  are shown in  FIGS. 2(   a )- 2 ( c ). The integral antenna element  14 , which acts as a monopole, is composed of a semi-hard aluminum alloy. It may be machined from a corresponding metal plate. However, it may also be cast in this shape. It is also feasible for a mount body to be formed from a plastic, which is then provided with an electrically conductive surface coating. The antenna element  14  has a foot  22  with a thickness of about 5 mm in the form of a rectangular plate with rounded corners and external dimensions of about 30 mm×38 mm ( FIG. 2(   c )). The antenna element  14  is inserted with the foot  22  into a corresponding milled area  35  in the mounting platform  32  ( FIG. 3) , and is screwed to the base plate  18  by means of four threaded holes  30  on the foot  22  and through-holes  36  in the base plate  18 . The upper face of the foot  22  is in this case adjacent, without any step, to the upper face of the mounting platform  32 , so that, for antenna purposes, the foot  22  is part of the base plate  18 . A bar-like base element  27 , which extends parallel to the foot plane and at a distance from it, is arranged above the foot  22  (underneath the foot as illustrated in  FIGS. 2(   a )- 2 ( c ) which are rotated 180°) of the antenna element  14 . The base element  27  has a square cross section with a side length of about 8 mm. It can withstand a current surge of 40 kA for a duration of 100 ms without any problems. One end of the base element  27  is connected by a vertical section  26  to the foot  22 , and is thus grounded. 
   An attachment  29  which points towards the foot  22  and in which a blind hole  24  is incorporated from the foot  22  is arranged at its other end. A rectangular resonant structure  28  with a square cross section (8 mm side length) and the height of 2 mm is arranged on the upper face of the base element  27 . The blind hole  24  is accessible from the outside through a concentric through-hole  23  with a larger diameter in the foot  22 . A connecting pin  13 , which is in the form of a push-in pin and is composed of brass ( FIG. 4 ), is pushed with its foot  39  into the blind hole  24 , so that it projects into the through-hole  23  in the foot  22  and contacts the coaxial connecting socket  17  (feed point  12  of  FIG. 1(   a )) through a corresponding through-hole  37  in the base plate  18  ( FIG. 3) . This is done by means of the inner conductor  74  of the coaxial connecting socket  17 , at whose upper end a longitudinally slotted socket  21  is arranged for holding the connecting pin  13  ( FIG. 1(   c )). The connecting socket  17  is attached by means of four screws, which are screwed into the threaded holes  30  in the foot  22  of the antenna element  14  through the through-holes  36  in the base plate  18 . The outer conductor of the coaxial connecting socket  17  is thus conductively connected e.g., grounded to the base plate  18 . This configuration results in a functionally reliable and compact connection, which is particularly simple to assemble, between the antenna element  14  and the coaxial connecting socket  17 . As can be seen in  FIG. 2 , the blind hole  24  is connected to the external area through a horizontal connecting channel  25 . The connecting channel  25  ensures that, when the connecting pin  13  is pushed in, the expanding air can escape without causing any damage to the rest of the blind hole  24 , when the antenna element  14  is heated severely, such as, for example, in the event of possible contact with the overhead line, where the short-circuit current is in the kilo ampere range. 
   With the abovementioned dimensions and the comparatively small resonator structure  28  on the upper face of the base element  27 , the antenna arrangement  10  is designed for a comparatively narrow frequency range from about 5.15-5.875 GHz, as is required for some WLL (wireless local loop) and WLAN (wireless local area network) applications. 
   In contrast, the antenna arrangement  40  of a second embodiment as illustrated in  FIGS. 5-7  can be used for a considerably wider frequency range (e.g., 870 to 2170 MHz). In this case, the antenna arrangement  40 , as shown in  FIG. 5 , has an electrically conductive base plate  48  (see also  FIG. 7 ) composed of a semi-hard aluminum alloy with a plate thickness of 15 mm, an antenna element  44  which acts as a monopole and is mounted on the base plate  48 , a coaxial connecting socket  47  whose inner conductor  75  is connected at the feed point  42  to a connecting pin  43  on the antenna element  44 , and a protective shroud  41 . The lower edge of the shroud  41  is seated in an (elliptical) annular groove  58  (see  FIG. 7 ) in the base plate  48  and is attached to the base plate  48  by means of screws  50 , which are screwed into the shroud  41  from underneath through attachment holes  59  in the base plate  48 . The coaxial connecting socket  47  is in this case pushed from above through a through-hole  63  in the base plate making electrical contact with an attachment nut  45  which is screwed onto an external thread on the connecting socket  47 . A sealing ring  46  is once again provided around the connecting socket  47  and is inserted into a corresponding annular groove  62  on the lower face of the base plate  48 . The connecting socket  47  can in turn be protected by a removable protective cap  49 . Outside the shroud  41 , attachment holes  61  ( FIG. 7 ) are provided in the base plate  48 , for attaching the antenna arrangement  40  to the outside of a vehicle. 
   The antenna element  44  ( FIG. 6 ) comprises a plate composed of semi-hard aluminum alloy with a plate thickness of 10 mm (the plate thickness extends at right angles to the plane of the drawing in  FIG. 6 ). The antenna element  44  has a base element  54 , which is like a bar, extends parallel to and at a distance from the base plate  48 , and has a height h 1  of 15 mm and a length of (a 1 +a 2 +a 3 +a 4 +a 5 )=140 mm. An attachment with the connecting pin  43  projecting downwards is provided at one end of the base element  54  (on the right in  FIG. 6 ), towards the base plate  48 . The connecting pin  43  is machined from solid in the example. However, it can just as well be in the form of a push-in pin once again. At the other end (on the left in  FIG. 6 ), a foot  52  with an attachment hole  53  is integrally formed, projecting downwards, on the base element  54 . The antenna element  44  can be mounted detachably on the base plate  48  by means of the foot  52  and a fastening screw  51 . A corresponding threaded hole  60  is provided in the base plate  48  for installing the fastening screw ( FIG. 7 ). 
   A plurality of resonant structures ( 55 - 57 ) are integrally formed on the base element  54  of the antenna element  44 . The first resonant structure  55 , for example, corresponds to a step with a height h 1  of 15 mm after a distance a 1  of 30 mm. The second resonant structure  56 , for example, is a vertical bar with a height h 3  of about 50 mm and a width a 3  of 30 mm. The distance a 2  from the step is, for example, about 30 mm. The third resonant structure  57 , for example, is a short vertical bar with a width a 5  of 10 mm and a height h 4  of 9 mm. The dimensions of the resonant structures are one specific embodiment of the present invention provided by way of example. The location and configuration of the resonant structures ( 55 - 57 ) may be varied in different embodiments of the present invention, as appropriate. Further, additional resonant structures may be provided between the two ends of the base element  54 , or beyond the screw connection  51 . 
   The resonant structures may also be folded and their ends may be inclined or rounded. One example of an antenna element that has been modified in this way is illustrated in  FIG. 8 . The antenna element  66  of the antenna arrangement shown in  FIG. 8  comprises a bar-like base element  70  which extends parallel to and at a distance from the base plate  65 , and which is attached (e.g., by screwing) to the base plate  65  at one end by means of a foot  69 . A feed point  67  is located at the other end, at which the antenna element  66  is accessible via a coaxial connecting socket. A plurality of resonant structures  71 ,  72  and  73  are integrally formed on the base element  70 . The resonant structures  71  and  72  are angled (folded); their corners are partially inclined and partially rounded. The corners of the resonant structure  73 , which is in the form of a bar, are also rounded. 
   The antenna element  66  shown in  FIG. 8  has dimensions similar to those of the antenna element  44 , shown in  FIG. 6 , providing coverage of the following frequency bands: 
                                                   GSM 900   870-960   MHz           GSM 1800   1710-1880   MHz           PCS 1900   1850-1990   MHz           1800/UMTS   1710-2170   MHz           WLL/WLAN   2.4-2.7   GHz;               3.4-3.7   GHz;               5.15-5.875   GHz                        
The resonant structure  71  broadcasts and receives the low frequencies and the frequencies around 1 GHz. The resonant structure  72  likewise broadcasts and receives low frequencies, but resonates (transmits) mainly below 1 GHz. In this case, the lower face of the base element  70  does not run completely parallel to the base plate  65 , but is slightly stepped.
 
   The graph in  FIG. 9  shows the calculated return loss of the antenna arrangement  64  shown in  FIG. 8 , plotted against the frequency. As can be seen, the return loss is less than 10 dB between approximately 800 MHz and 9 GHz. 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               LIST OF REFERENCE SYMBOLS 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               10, 40, 64 
               Antenna arrangement 
             
             
                 
               11, 41 
               Shroud 
             
             
                 
               12, 42, 67 
               Feed point 
             
             
                 
               13, 43 
               Connecting pin 
             
             
                 
               14, 44, 66 
               Antenna element (monopole) 
             
             
                 
               15, 68 
               Screw 
             
             
                 
               16, 46, 76 
               Sealing ring 
             
             
                 
               17, 47 
               Connecting socket (coaxial) 
             
             
                 
               18, 48, 65 
               Base plate 
             
             
                 
               19, 49 
               Protective cap 
             
             
                 
               20, 50 
               Screw 
             
             
                 
               21 
               Socket 
             
             
                 
               22, 52, 69 
               Foot 
             
             
                 
               23 
               Through-hole 
             
             
                 
               24 
               Blind hole 
             
             
                 
               25 
               Connecting channel 
             
             
                 
               26 
               Section (vertical) 
             
             
                 
               27, 54, 70 
               Base element (like a bar) 
             
             
                 
               28, 55, . . . , 
               Resonant structure 
             
             
                 
               57 
             
             
                 
               29 
               Attachment 
             
             
                 
               30 
               Threaded hole 
             
             
                 
               31, 31′ 
               Attachment flange 
             
             
                 
               32 
               Mounting platform 
             
             
                 
               33 
               Attachment hole 
             
             
                 
               34 
               Threaded hole 
             
             
                 
               35 
               Milled area 
             
             
                 
               36, 37 
               Through-hole 
             
             
                 
               38, 62 
               Annular groove (sealing ring) 
             
             
                 
               39 
               Foot 
             
             
                 
               45 
               Attachment nut 
             
             
                 
               51 
               Screw connection 
             
             
                 
               53 
               Attachment hole 
             
             
                 
               58 
               Annular groove (shroud) 
             
             
                 
               59 
               Attachment hole 
             
             
                 
               60 
               Threaded hole 
             
             
                 
               61 
               Attachment hole 
             
             
                 
               63 
               Through-hole 
             
             
                 
               71, 72, 73 
               Resonant structure 
             
             
                 
               74, 75 
               Inner conductor (connecting socket) 
             
             
                 
               a1, . . . , a4 
               Distance 
             
             
                 
               h1, . . . , h4 
               Height