Patent Publication Number: US-8994598-B2

Title: Circularly polarized wave reception antenna

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a National Phase Patent Application and claims the priority of International Application Number PCT/JP2008/069395, filed on Oct. 21, 2008, which claims priority of Japanese Patent Application Number 2007-290036, filed on Nov. 7, 2007. 
     TECHNICAL FIELD 
     The present invention relates to a circularly polarized wave reception antenna. The present invention particularly relates to an improvement of the gain of a loop antenna used attached to a dielectric body portion of an automobile or other vehicle and receiving circularly polarized waves. 
     BACKGROUND ART 
     In the past, automobiles and other vehicles have been equipped with antennas enabling the reception of radio waves even during movement. Generally, the radio waves received by a vehicle have for long years principally been the medium waves (MW) for AM radio and the very high frequency (VHF) or ultrahigh frequency (UHF) waves for FM radio or television. 
     However, in recent years, the types of antennas mounted at vehicles have been increasing. For example, antennas for global positioning systems (GPS) or antennas for receiving radio waves for digital terrestrial broadcasts have been increasingly becoming mainstream. Antennas receiving radio waves for digital terrestrial TV broadcasts hereinafter will be referred to as “DTV antennas”. 
     Circularly polarized waves have been used for the GPS radio waves or terrestrial digital TV broadcast radio waves received by such antennas mounted on vehicles. Further, for conventional circularly polarized wave antennas, patch antennas have usually been used. However, such a patch antenna is contained inside an antenna case. The case is tall and therefore the appearance was bad. Therefore, recently, film antennas used attached to the windows of the vehicles have been used (for example, see Japanese Patent Publication (A) No. 2005-102183). 
     However, the film antennas disclosed in Japanese Patent Publication (A) No. 2005-102183 etc. were not sufficient in reception performance. 
     DISCLOSURE OF THE INVENTION 
     Therefore, the present invention has as its object to provide a circularly polarized wave reception antenna able to be increased in gain, able to be improved in reception performance, and able to provide sufficient performance even as a film antenna. 
     A circularly polarized wave reception antenna of the present invention for achieving this object comprises a loop antenna provided with two feed terminals, a parasitic element positioned near the loop antenna and comprised from a conductor independent of the antenna conductor of the loop antenna, and a conductor positioned so as to surround the vicinity of the loop antenna and parasitic element. This conductor can be made a looping line conductor. 
     According to the antenna of the present invention, there is provided an antenna with a simple structure and good reception performance able to send and/or receive circularly polarized waves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a plan view showing the structure of a circularly polarized wave reception antenna of a first embodiment of the present invention. 
         FIG. 1B  is a perspective view showing an example of installation of the antenna shown in  FIG. 1A  at the front window of an automobile. 
         FIG. 2  is a plan view showing the structure of a circularly polarized wave reception antenna of a second embodiment of the present invention. 
         FIG. 3A  is a view showing a modification of a circularly polarized wave reception antenna of the first embodiment shown in  FIG. 1A  wherein a looping line conductor with an exterior rectangular shape has a lateral direction length X much longer than a longitudinal direction length Y. 
         FIG. 3B  is a view showing a modification of a circularly polarized wave reception antenna of the first embodiment shown in  FIG. 1A  wherein a looping line conductor with an exterior rectangular shape has a lateral direction length X slightly longer than a longitudinal direction length Y. 
         FIG. 3C  is a view showing a modification of a circularly polarized wave reception antenna of the first embodiment shown in  FIG. 1A  wherein a looping line conductor with an exterior rectangular shape has a lateral direction length X nearly equal to a longitudinal direction length Y. 
         FIG. 3D  is a view showing a modification of a circularly polarized wave reception antenna of the first embodiment shown in  FIG. 1A  wherein a looping line conductor with an exterior rectangular shape has a lateral direction length X slightly shorter than a longitudinal direction length Y. 
         FIG. 3E  is a view showing a modification of a circularly polarized wave reception antenna of the first embodiment shown in  FIG. 1A  wherein a looping line conductor with an exterior rectangular shape has a lateral direction length X much shorter than a longitudinal direction length Y. 
         FIG. 4A  is a perspective view showing the appearance of a connector and coaxial cable connected to a feed terminal of a loop antenna. 
         FIG. 4B  is a disassembled perspective view of the connector shown in  FIG. 4A . 
         FIG. 5A  is a view of an example of the circuit board shown in  FIG. 4B  seen from the bottom surface. 
         FIG. 5B  is a block circuit diagram showing the internal structure of an amplifier mounted on the circuit board shown in  FIG. 5A . 
         FIG. 5C  is a view of another example of a circuit board shown in  FIG. 4B  seen from the bottom surface. 
         FIG. 6A  is a plan view showing the structure of a modification of the antenna of the first embodiment of the present invention. 
         FIG. 6B  is a plan view showing the general structure of a DTV reception antenna. 
         FIG. 6C  is a plan view showing a different structure of a DTV reception antenna. 
         FIG. 6D  is a perspective view of the front windshield of an automobile to which antennas etc. shown from  FIG. 6A  to  FIG. 6C  are attached and the surroundings of the same seen from the vehicle interior. 
         FIG. 7  is a circuit diagram showing the connection of the antenna shown in  FIG. 6D  to a navigation system mounted in a vehicle. 
         FIG. 8A  is a plan view showing the structure of a circularly polarized wave reception antenna of a third embodiment of the present invention. 
         FIG. 8B  is a plan view showing the structure of a modification of the antenna of the third embodiment of the present invention. 
         FIG. 9A  is a perspective view showing an example of use attaching the antenna of the first embodiment of the present invention on the back mirror of an automobile. 
         FIG. 9B  is a perspective view showing an example of use burying the antenna of the first embodiment of the present invention in the back mirror of an automobile. 
         FIG. 10A  is a perspective view showing an example of use incorporating the antenna of the present invention inside the rear spoiler of an automobile. 
         FIG. 10B  is a lateral view showing an example of use incorporating the antenna of the present invention in the rear spoiler of an automobile. 
         FIG. 11  is a directivity diagram comparing the gain when setting the antenna of the present invention near the top end of the front windshield of an automobile to when using a conventional antenna. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Below, drawings will be used to explain preferred embodiments of the present invention. The same component parts will be explained assigned the same reference notations. Note that, in general, an antenna both sends and receives radio waves. However, in the embodiments below, to facilitate understanding, only the case where the antenna receives radio waves will be explained. The explanation for the case where the antenna sends radio waves will be omitted. Needless to say, the transmission of radio waves from the antenna is included in the present invention however. 
       FIG. 1A  shows the structure of a GPS antenna  13  of a first embodiment of the present invention. The GPS antenna  13  of this embodiment is a loop antenna comprised of a sheet-like transparent film  14  on which a rectangular antenna conductor  15  and a parasitic element  16  not electrically connected to the antenna conductor  15  are formed. The antenna  13  can receive a circularly polarized wave from a GPS satellite and can send a circularly polarized wave. On the other hand, there are feed terminals  17 ,  18  at the two ends of the antenna conductor  15 . Later explained connectors are connected to these feed terminals  17  and  18 . The antenna conductor  15 , parasitic element  16 , and feed terminals  17  and  18  are formed by conductive ink or copper foil or another conductor on the sheet-like transparent film  14 . 
     The GPS antenna  13  of this embodiment has a rectangular looping line conductor  19  around the antenna conductor  15 , parasitic element  16 , and feed terminals  17 ,  18 . The looping line conductor  19  is also formed by conductive ink or copper foil or another conductor on the sheet-like transparent film  14 . The dimensions when arranging this GPS antenna  13  on a glass-like dielectric body are as follows for example. The length Z of one side of the rectangular antenna conductor  15  is 30 mm or so, the length of the distant part P of the parasitic element  16  is 40 mm or so, and the length of the parallel part Q is 20 mm or so. 
     Further, the length X of the looping line conductor  19  in the lateral direction can be made 90 mm or so, and the length Y of the looping line conductor  19  in the longitudinal direction can be made 90 mm or so. The total length of the looping line conductor  19  in this case is 180 mm or so. The aspect ratio can be changed according to the size of the loop antenna inside. Further, the optimum length of the looping line conductor  19  and the size of the GPS antenna  13  are determined by the dielectric constant of the dielectric body that the GPS antenna  13  is attached to. 
     Further, if setting the GPS antenna  13  on plastic foam, it is sufficient if the length Z of one side of the loop of the GPS antenna  13  is 50 mm or so, the length of the distant part P of the parasitic element  16  is 60 mm or so, and the length of the parallel part Q is 30 mm or so. 
     If arranging the rectangular looping line conductor  19  around the antenna conductor  15 , parasitic element  16 , and feed terminals  17 ,  18 , making the total length (2X+2Y) of the looping line conductor  19  about three times (about 2.7 to 3.3 times) the total length (4Z) of the antenna conductor  15  will increase the gain of the GPS antenna  13 . Further, the ratio (X:Y) of the length X of the lateral direction of the looping line conductor  19  to the length Y of the longitudinal direction is optimally 1:1, but there will be improved gain also with a range of 1:2 to 2:1. 
     The GPS antenna  13  with the above such structure can be set near the top end of the front windshield  1  of the automobile  60  as shown in  FIG. 1B  for example. Depiction of the transparent film is omitted in this drawing. The GPS antenna  13  is connected to a feed circuit comprising a connector  20  and coaxial cable  22 . The coaxial cable  22  is positioned along an A pillar  3  of the automobile  60  and is connected to a digital TV tuner not shown in the drawing.  8  is a car navigation system installed in an instrument panel  9  of the automobile. This receives an image signal from the tuner as input. 
     As explained above, by setting a GPS antenna  13  comprised of an antenna conductor  15 , parasitic element  16 , and feed terminals  17 ,  18  surrounded by a rectangular and looping line conductor  19  near the top end of the front windshield  1  of the automobile  60 , as shown in  FIG. 11 , there will be an effect of an increase in gain of approximately 2 dB in comparison to a case with no looping line conductor  19 . 
       FIG. 2  shows the structure of a GPS antenna  13  of a second embodiment of the present invention. The GPS antenna  13  of this embodiment also uses a loop antenna comprised of a sheet-like transparent film  14  on which a rectangular antenna conductor  15  and a parasitic element  16  not electrically connected to the antenna conductor  15  are formed. There are feed terminals  17 ,  18  on the two ends of the antenna conductor  15 . Connectors are connected to these feed terminals  17 ,  18 . This is the same as in the first embodiment. 
     In the first embodiment, the antenna conductor  15 , parasitic element  16 , and feed terminals  17 ,  18  were surrounded by the rectangular looping line conductor  19 . On the other hand, in the second embodiment, the antenna conductor  15 , parasitic element  16 , and feed terminals  17 ,  18  are surrounded by a vertically long elliptical looping line conductor  19 . Here as well, making the total length of the looping line conductor  19  three times or so the total length ( 4 Z) of the antenna conductor  15  will increase the gain of the GPS antenna  13 . Further, in this case, the ratio (X:Y) of the length X of the minor axis of the elliptical line conductor  19  to the length Y of major axis is optimally 1:1, but there is an effect of raising the gain even in a range of 1:2 to 2:1. 
     Note that, the antenna  13  of the first embodiment, as shown in  FIG. 3C , preferably has a ratio (X:Y) of the length X of the lateral direction of the looping line conductor  19  to the length Y of the longitudinal direction of 1:1 or so. However, even if changing the X:Y ratio, without changing the sum of the side X and side Y, by making the length of the side X longer and conversely making the length of the side Y shorter so as to obtain the antenna  13  in the state shown in  FIG. 3B , the gain is greater than in an antenna  13  of a state without a looping line conductor  19 . Similarly, even if changing the ratio of X:Y, without changing the sum of the side X and side Y, by making the length of the side X even longer and making the length of the side Y even shorter so as to obtain the antenna  13  (X:Y=2:1) in the state shown in  FIG. 3A , the gain is greater than in an antenna  13  of a state without a looping line conductor  19 . Further, even if changing the X:Y ratio, without changing the sum of the side X and side Y, by making the length of the side X shorter and making the length of the side Y longer so as to obtain the antenna  13  in the state shown in  FIG. 3D  or  FIG. 3E , the gain is greater than in an antenna  13  of a state without a looping line conductor  19 . Further, if making the X:Y ratio 1:2 like the antenna  13  in the state shown in  FIG. 3E , the gain will be no different from the antenna  13  shown in  FIG. 3A . 
       FIGS. 4A and 4B  show the appearance of the connector  20  shown in  FIG. 1B  and the connector  20  in a disassembled state. As shown in  FIG. 4A , the connector  20  comprises a combination of an inner case  21  and outer case  25 . The surface of the inner case  21  (the surface the antenna  10  is mounted to) has two openings  21 A,  21 B. Connection terminals  31 ,  32  having springiness protrude from these openings  21 A,  21 B. The connector  20  is fixed on top of each of the feed terminals  17 ,  18  with two-sided adhesive tape or other adhesive stuck on the surface of the inner case  21 . 
     The connection terminals  31 ,  32 , as shown in  FIG. 4B , are mounted on one surface of the circuit board (dielectric board)  30  built in the inner case  21  and outer case  25 . The circuit board  30  is connected to a coaxial cable  22 . The other surface of the circuit board  30  is equipped with an integrated circuit  40  to be mentioned later. Generally, the connection terminal  31  is the hot side (signal transmission side) terminal, and the connection terminal  32  is the ground side terminal. 
       FIG. 5A  shows the general structure of the circuit board  30  inside the connector  20  shown in  FIG. 4B  excluding the inner case  21  and outer case  25 . Connection terminals  31 ,  32  are mounted on the bottom surface side of the circuit board  30  and are led to the top surface side of the circuit board  30  by the through holes  33 ,  34 . In this example, the through hole  33  is connected to the input terminal of the integrated circuit  40  mounted on the top surface of the circuit board  30 , and the through hole  34  is connected to the ground line (outside conductor)  22 B of the coaxial cable  22 . The integrated circuit  40  amplifies and otherwise processes the signals received by the antennas and outputs the processed signals to a center conductor (inner side conductor)  22 A of the coaxial cable  22 . 
       FIG. 5B  shows the internal structure of the integrated circuit  40  shown in  FIG. 5A . The integrated circuit  40  has inside it a filter  41  connected to the antenna  10 , an amplifier  42  amplifying a signal output from the filter  41 , and a filter  43  determining the signal band output from the amplifier  42 . This filter  43  is connected to the center conductor  22 A of the coaxial cable  22  through a capacitor  44  which blocks direct current. This coaxial cable  22  is a cable also supplying power. The power voltage (direct current) is supplied to the amplifier  42  through the coil  45  blocking the alternate current component. 
       FIG. 5C  shows the structure of a circuit board  30  different from the connector  20  shown in  FIG. 5A  excluding the inner case  21  and outer case  25 . In the circuit board  30  of the connector  20  shown in  FIG. 5A , the connection terminal  31  is the hot side (signal transmission side) terminal which is connected to the input terminal of the integrated circuit  40  through the through hole  33 , and the connection terminal  32  is the ground side terminal which is connected to the ground line  22 B of the coaxial cable  22  through the through hole  34 . On the other hand, in the circuit board  30  of the connector  20  shown in  FIG. 5C , the connection terminal  31  is the ground side terminal and is connected to the ground line  22 B of the coaxial cable  22  through the through hole  34 , and the connection terminal  32  is the hot side terminal and is connected to the input terminal of the integrated circuit  40  through the through hole  33 . In this way, the connection terminal  31  may also be made the ground side terminal and the connection terminal  32  the hot side terminal. 
     In the first embodiment, it was found by experiments that the rectangular looping line conductor  19  surrounding the antenna conductor  15 , parasitic element  16 , and feed terminals  17 ,  18  is effective even if the conductor is not continuous across the entire circumference. Further, it was found that the rectangular looping line conductor  19  surrounding the feed terminals  17 ,  18  of the GPS antenna  13  had a total length close to the loop length of the loop antenna that the DTV antenna is comprised from. Thus, the inventors proposed cutting out a portion of the rectangular looping line conductor  19 , forming the feed terminals  11 ,  12  at the cut-out ends shown in  FIG. 6A , and making the rectangular looping line conductor  19  a DTV antenna  10 A. 
     In this case, an integrated antenna  10 A,  13  in which the GPS antenna  13  and DTV antenna  10 A are combined as shown in  FIG. 6A  is positioned at the top left corner of the front windshield  1  of the automobile  60 . In addition, the DTV antenna  10 D shown in  FIG. 6B , the DTV antenna (with feed terminals  11 ,  12  offset to one side)  10 B shown in  FIG. 10C , and the DTV antenna  10 C which is a mirror image of the DTV antenna  10 D shown in  FIG. 10C  can be arranged in a line from the integrated antenna  10 A,  13  on the top end of the front windshield  1  of the automobile  60  as shown in  FIG. 6D  to form an antenna system. Note that, in the antenna system shown in  FIG. 6D , the feed terminals of the antennas are connected to connectors so depiction of the feed circuits comprised of the connectors and coaxial cables is omitted. 
       FIG. 7  is a circuit diagram showing the connection of the antenna system comprised of the antennas  10 A,  13 ,  10 B,  10 C,  10 D of  FIG. 6D  to the navigation system  8  mounted in a vehicle. In this embodiment, there is a built-in TV tuner  5  in the navigation system  8 , however, the TV tuner  5  may also be separate from the navigation system  8 . 
     In this embodiment, the antenna conductor  19  in the integrated antenna  10 A,  13  and the film antennas  10 B,  10 C, and  10 D are DTV antennas, and the antenna conductor  15  in the integrated antenna  10 A,  13  is a GPS antenna. The DTV signals received by these film antennas  10 A,  10 B,  10 C, and  10 D are guided to the TV tuner  5  with cables  22  through integrated circuits  40  that are built inside the connectors and perform amplification and the like. A demodulated image is displayed in the display  6  when the navigation system  8  is in the TV mode. Further, the GPS signals received by the GPS antenna  13  (antenna conductor  15 ) mounted in the film antenna  10 AM are guided through an integrated circuit  40  and cable  22  to the ECU  4  of the navigation system  8  where the current location of the automobile is detected and displayed on the display  6  of the navigation system  8  together with map information. 
       FIG. 8A  shows the structure of the antenna  53  of a third embodiment of the present invention. The GPS antenna  13  of the third embodiment also uses a loop antenna comprised of a sheet-like transparent film  14  on which a rectangular antenna conductor  15  and a parasitic element  16  not electrically connected to the antenna conductor  15  are formed. It can receive a circularly polarized wave from a GPS satellite and, further, send a circularly polarized wave. On the other hand, there are feed terminals  17 ,  18  at the two ends of the antenna conductor  15 . Later explained connectors are connected to these feed terminals  17 ,  18 . The antenna conductor  15 , parasitic element  16 , and feed terminals  17 ,  18  are formed by conductive ink or copper foil or another conductor on a sheet-like transparent film  14  in the same way as the first embodiment. 
     In the GPS antenna  53  of the third embodiment, a metal sheet  51  having an opening of the same dimensions as the rectangular looping line conductor  19  explained by the first embodiment is attached on the transparent film  14  around the antenna conductor  15 , parasitic element  16 , and feed terminals  17 ,  18 . In the third embodiment, so long as the dimensions of the opening of the metal sheet  51  are the same, the size of the metal sheet  51  is not particularly limited. For example, when the length Z of one side of the rectangular antenna conductor  15  of the GPS antenna  13  is 32 mm or so, the length of the lateral direction of the opening of the metal plate  51  may be 95 mm or so and the length of the longitudinal direction 95 mm or so. 
       FIG. 8B  shows a modification of the antenna  53  of the third embodiment of the present invention. The only difference between the antenna  53  of this modification and the antenna  53  of the third embodiment explained in  FIG. 8A  is that instead of the metal sheet  51 , a metal mesh  52  is attached to the sheet-like transparent film  14 . The performance of the antenna  53  of this modification is not much different from that of the antenna  53  of the third embodiment. 
       FIG. 9A  shows an example of usee where the antenna  13 ,  53  of the first or third embodiment of the present invention is attached to the back mirror (inner rearview mirror  35 ) of an automobile. Further,  FIG. 9B  shows an example of use where the antenna  13 ,  53  of the first or third embodiment of the present invention is buried in the back mirror  35  of the automobile. By mounting the antenna  13 ,  53  of the present invention at this position, it can efficiently receive radio waves arriving from the upper front of the automobile. 
       FIGS. 10A and 10B  show, as different examples of vehicle positions to mount the antenna  13 ,  53  of the present invention, examples where the antenna  13 ,  53  is built inside the rear spoiler  36  of a wagon type automobile  37 . The directivity of the antenna  13 ,  53  at this position can be changed by the mounting angle of the antenna  13 ,  53  built inside the rear spoiler  36 . As shown in  FIG. 10A , by having the antenna  13 ,  53  built into the rear spoiler  37  tilted to the back direction, the directivity of the antennas  13 ,  53  is to the upper rear of the automobile  37 . Further, as shown in  FIG. 10B , by having the antenna  13 ,  53  built into the rear spoiler  36  tilted to the front, the directivity of the antennas  13 ,  53  is to the upper front of the automobile  37 . 
     The antennas  13 ,  53  of the present invention can be mounted at positions other than these mounting positions, for example, a plastic rooftop etc. of the vehicle. The shape of the antenna conductor of the GPS antenna  13  that can be used in the antennas  13 ,  53  of the present invention and the numbers and arrangements of the parasitic elements  16  are not limited to these embodiments.