Abstract:
An antenna apparatus is mounted on a vehicle body provided with a spoiler made of an insulating resin. A feeder element is disposed in the spoiler such that the vehicle body serves as ground without providing a ground element in the spoiler.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to automobile antenna apparatus used on a vehicle body provided with a spoiler. 
   In recent years, from the point of view of design, a spoiler having a wing-shaped cross section is often placed on a vehicle body in order to increase the tire contact pressure during high speed running to thereby permit stable driving.  FIG. 5  is a view showing the state where a spoiler  12  is placed at a roof end of a vehicle body  10  of a station wagon type. Likewise in a minivan type or a truck type, a spoiler  12  is often placed at a roof end. On the other hand,  FIG. 6  is a view showing the state where a spoiler  12  is placed on a trunk of a vehicle body  10  of a sedan type. Likewise in a coupe type, a spoiler  12  is often placed on a trunk. 
   These spoilers  12  are mainly molded from insulating resin in order to save the weight or for any other purpose. Of them, there are some spoilers  12  having antenna apparatus disposed therein.  FIG. 7  is a view showing an example of the structure of related-art antenna apparatus disposed in a spoiler  12 . As shown in  FIG. 7 , the related-art antenna apparatus is constituted by a feeder element  14  formed into an L-shaped (L-type feeder element) in a spoiler  12 , and a ground element  16  formed into an L-shaped likewise. The feeder element  14  and the ground element  16  are disposed substantially symmetrically. A signal line is electrically connected to the feeder element  14  through a coaxial cable or the like, while the ground element  16  is electrically connected to a vehicle body  10  serving as the ground. When the antenna apparatus is placed in the spoiler  12 , the antenna apparatus does not project from the vehicle body  10  so as to be excellent in terms of design. 
   In the related-art antenna apparatus shown in  FIG. 7 , the pattern of the directivity in a horizontal plane with respect to vertically polarized signals is necked and nowhere close to a circle as shown in  FIG. 8 . The antenna apparatus can be hardly regarded as nondirectional. Thus, there is a problem that the sensitivity to AM broadcasting signals and FM broadcasting signals transmitted as vertically polarized signals varies widely in accordance with the direction of the antenna apparatus, that is, the direction of the vehicle body  10 . 
   In the related-art antenna apparatus, both the feeder element  14  and the ground element  16  are long in horizontal size but short in vertical size inevitably because they are disposed in the spoiler  12 . It can be therefore considered that a signal current flowing vertically through the feeder element  14  and the ground element  16  is limited so that nondirectivity cannot be obtained in a horizontal plane with respect to vertically polarized signals. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the invention to provide automobile antenna apparatus disposed in a spoiler while nondirectivity can be obtained in a horizontal plane with respect to vertically polarized signals. 
   In order to achieve the above object, according to the invention, there is provided an antenna apparatus mounted on a vehicle body provided with a spoiler comprised of an insulating resin, comprising a feeder element, disposed in the spoiler such that the vehicle body serves as ground without providing a ground element in the spoiler. 
   In such a configuration, the antenna apparatus does not exposed from the vehicle body so as to be excellent in terms of design. In addition, since the vehicle body serves as the ground, most of a signal current can flow a long distance substantially vertically through the vehicle body so that substantial nondirectivity can be obtained in a horizontal plane with respect to vertically polarized signals. Thus, it is possible to prevent such a problem that the receiving sensitivity varies in accordance with the direction of the vehicle. 
   Preferably, the spoiler is disposed on a rear end portion of a roof of the vehicle body. 
   In such a configuration, the degree of capacitive coupling between the feeder element and the vehicle body is so small that high receiving sensitivity can be obtained in a required band. In addition, when the vehicle body is of a station wagon type, a minivan type or a truck type, a rear pillar is substantially vertical. Thus, most of a signal current can flow a long distance vertically through the vehicle body including the rear pillar so that the nondirectivity in a horizontal plane with respect to vertically polarized signals is more improved. 
   Alternatively, the spoiler may be disposed on a trunk of the vehicle body, such that a height of the spoiler from the trunk is at least 150 mm. 
   In such a configuration, the degree of capacitive coupling between the feeder element and the vehicle body can be reduced so that high receiving sensitivity can be obtained in a required band. 
   Preferably, the feeding element comprises a T-type element operable to receive both of an AM broadcasting signal and an FM broadcasting signal. 
   Alternatively, the feeding element may comprise at least one of an F-type element operable to receive an FM broadcasting signal and an L-type element operable to receive an AM broadcasting signal. 
   In such a configuration, high receiving sensitivity can be obtained for at least one of the AM and FM broadcasting signals. 

   
     BRIEF DESCRIPTION OF THE INVENTION 
     The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein: 
       FIG. 1  is a view showing an automobile antenna apparatus according to a first embodiment of the invention; 
       FIG. 2  is a diagram showing the directivity of the automobile antenna apparatus of  FIG. 1 , in a horizontal plane with respect to vertically polarized signals; 
       FIG. 3  is a view showing an automobile antenna apparatus according to a second embodiment of the invention; 
       FIG. 4  is an explanatory view of a height of a spoiler from a trunk of a sedan type vehicle body; 
       FIG. 5  is a view showing the state in which a spoiler is disposed at a roof end of a station wagon type vehicle body; 
       FIG. 6  is a view showing the state in which a spoiler is disposed on a trunk of a sedan type vehicle body; 
       FIG. 7  is a view showing a related-art antenna apparatus disposed in a spoiler; and 
       FIG. 8  is a diagram showing the directivity of the antenna apparatus of  FIG. 7 , in a horizontal plane with respect to vertically polarized signals. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Preferred embodiments of the invention will be described below in detail with respect to the accompanying drawings. 
     FIG. 1  shows an automobile antenna apparatus according to a first embodiment of the invention. In this embodiment, a T-type feeder element  18  is disposed in a spoiler  12  placed at a roof end of a vehicle body  10  of a station wagon type shown in  FIG. 5 . The T-type feeder element  18  is electrically connected to a receiver-side via a coaxial cable  20  and, if necessary, through a not-shown amplifier or a not-shown matching circuit. Unlike in the configuration shown in  FIG. 7 , a ground element  16  is not disposed in the spoiler  12 . 
   In the automobile antenna apparatus configured thus, the vehicle body  10  serves as the ground so that a signal current flows into the vehicle body  10  in accordance with a signal received by the feeder element  18 . A substantially vertical pillar is provided in a rear end portion of the station-wagon type vehicle body  10 . Accordingly, most of the signal current can flow a long distance vertically through the vehicle body  10  including the rear pillar. As a result, substantial nondirectivity can be obtained in a horizontal plane with respect to vertically polarized signals as shown in  FIG. 2 . In addition, in the T-type feeder element  18 , sufficient receiving sensitivity can be obtained for both AM broadcasting signals and FM broadcasting signals. 
     FIG. 3  shows an automobile antenna apparatus according a second embodiment of the invention. In this embodiment, two feeder elements, namely, an F-type feeder element  22  and an L-type feeder element  24  are disposed in a spoiler  12  substantially symmetrically. The F-type and L-type feeder elements  22  and  24  are properly electrically connected via coaxial cables  20 , respectively. 
   From each of the F-type and L-type feeder elements  22  and  24 , most of a signal current flows a long distance through the vehicle body  10  including the rear pillar and serving as the ground. Accordingly, substantial nondirectivity could be obtained in a horizontal plane with respect to vertically polarized signals in the same manner as in the automobile antenna apparatus in  FIG. 1 . However, as for the F-type feeder element  22 , good receiving sensitivity to FM broadcasting signals can be indeed obtained, but receiving sensitivity to AM broadcasting signals is not good. On the other hand, as for the L-type feeder element  24 , good receiving sensitivity to AM broadcasting signals can be indeed obtained, but receiving sensitivity to FM broadcasting signals is not good. Therefore, in order to receive both AM broadcasting signals and FM broadcasting signals, both the F-type and L-type feeder elements  22  and  24  are required. Not to say, when only AM broadcasting signals or FM broadcasting signals are to be received, a suitable one of the feeder elements may be disposed in the spoiler  12 . 
   A similar result can be obtained in automobile antenna apparatus when the spoiler  12  is disposed at the roof end of a vehicle body  10  of a minivan type or a truck type. 
   However, a satisfactory result can not be obtained in automobile antenna apparatus when the spoiler  12  is disposed on a trunk of a vehicle body  10  of a sedan type or a coupe type shown in  FIG. 6 . Particularly, the receiving sensitivity was not satisfactory. The difference in structure between a roof end of a station wagon type and a trunk of a sedan type is whether a rear window is close to the spoiler  12  or not. It is considered that the degree of capacitive coupling occurring between the feeder element  18 ,  22 ,  24  disposed in the spoiler  12  and the vehicle body  10  may not be so large when the rear window was close to the spoiler  12 , but the degree of capacitive coupling occurring between the feeder element  18 ,  22 ,  24  disposed in the spoiler  12  and the vehicle body  10  may be large when the rear window was not close to the spoiler  12 . 
   As a result of an experiment that a height h (see  FIG. 4 ) of the spoiler  12  from the trunk of the vehicle body  10  is varied, it is confirmed that practically sufficient receiving sensitivity can be obtained when the height h is at least 150 mm. Further, in a case where the spoiler  12  is disposed at the roof end of the sedan type vehicle body  10 , receiving sensitivity similar to that in the case where the spoiler  12  is disposed on a vehicle  10  of a station wagon type can be obtained. In view of the above, it is confirmed that the directivity and the receiving sensitivity are not so affected by the difference in type of vehicle, but the degree of capacitive coupling between the vehicle body  10  and the feeder element  18 ,  22 ,  24  has a large influence on the receiving sensitivity in a required band. 
   The dimensions of the feeder element  18 ,  22  may be set suitably to be able to resonate with FM broadcasting signals. A matching circuit or an amplifier circuit may be provided suitably in accordance with necessity when the receiving sensitivity is measured. 
   Although the present invention has been shown and described with reference to specific preferred embodiments, various changes and modifications will be apparent to those skilled in the art from the teachings herein. Such changes and modifications as are obvious are deemed to come within the spirit, scope and contemplation of the invention as defined in the appended claims.