Abstract:
A planar antenna device comprises a ground plane, a planar antenna element having a principal plane mounted above the ground plane, and a cavity, having an opening partially exposing the antenna element, placed on the ground plane in order to cover the entire antenna element contactlessly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-292298, filed Sep. 26, 2000, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an on-vehicle planar antenna device for receiving satellite broadcasting. 
     Conventionally, there has been no technique but decreasing an antenna element size when a planar antenna device is used for obtaining high electromagnetic field radiation characteristics within the range of a wide elevation angle. 
     FIG. 1 illustrates a structure of a general air patch antenna device. FIG. 1 shows a ground plane  11 , an antenna element  12  mounted on the ground plane  11  separated by a spacer  13 , and a feed point  14  to the antenna element  12 . 
     A microstrip antenna device stationed in the air (εr=1) has a high relative antenna device gain. On the other hand, however, the half-power angle generally becomes approximately 60° to 80° depending on antenna device shapes. Consequently, a gain remarkably decreases toward a low elevation angle. 
     To decrease the antenna element size for widening such a narrow elevation angle range, a dielectric must be used. 
     FIG. 2 illustrates an example structure of a dielectric patch antenna device using the dielectric. FIG. 2 shows a ground plane  21 , a dielectric plate  22  mounted on the ground plane  21 , an antenna element  23  provided on the dielectric plate  22 , and a feed point  24  to the antenna element  23 . 
     The size of the antenna element  23  is decreased by using the dielectric plate  22 . It becomes possible to obtain high electromagnetic field radiation characteristics within a wide elevation angle range. 
     However, the antenna element size is decreased for the dielectric patch antenna device in FIG.  2 . Compared to the air patch antenna device in FIG. 1, the antenna device gain greatly decreases. In addition, a loss due to the dielectric plate  22  further decreases the antenna device gain. As a result, the dielectric patch antenna device in FIG. 2 does not provide so high a radiation level toward a low elevation angle. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a planar antenna device which satisfies both of electromagnetic field radiation characteristics over a wide elevation angle range including a low elevation angle direction and a high antenna device gain. 
     A planar antenna device according to the present invention comprises: a ground plane; a planar antenna element having a principal plane mounted above the ground plane; and a cavity, having an opening partially exposing the antenna element, placed on the ground plane in order to cover the entire antenna element contactlessly. 
     Preferred manners for the above-mentioned planar antenna device are as follows. 
     (1) A feed point for supplying power supply to the antenna element is further provided. 
     (2) An area of the opening is smaller than a size of the antenna element. 
     (3) The opening is placed substantially parallel to a principal plane of the antenna element. 
     (4) The antenna element is an air patch antenna element mounted above the ground plane separated by a spacer. 
     Another planar antenna device according to the present invention comprises a ground plane; a planar antenna element having a principal plane mounted above the ground plane; and a planar conductor placed substantially parallel to a principal plane of the antenna element and having an opening at substantially a center thereof. 
     According to the present invention, it is possible to provide excellent electromagnetic field radiation characteristics over a wide elevation angle range including a low elevation angle direction and a high antenna device gain only by adding a cavity to a conventional air patch antenna device without decreasing the antenna element size, thereby maintaining sufficiently high antenna device gain. 
     Further, the present invention eliminates the need to use a dielectric for obtaining a gain toward a low elevation angle. It is possible to maintain a high antenna device gain without decreasing an antenna device gain due to a dielectric loss. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a perspective view exemplifying a structure of a conventional air patch antenna device; 
     FIG. 2 is a perspective view exemplifying a structure of a conventional dielectric patch antenna device; 
     FIG. 3 is a perspective view illustrating a structure of an antenna device according to an embodiment of the present invention for receiving BS digital broadcasting; 
     FIG. 4 is a sectional view of an antenna device structure taken along the line  4 — 4  of FIG. 3; 
     FIG. 5 shows VSWR characteristics of an antenna device according to an embodiment of the present invention; 
     FIG. 6 shows return loss characteristics of an antenna device according to an embodiment of the present invention; 
     FIG. 7 is a Smith chart for an antenna device according to an embodiment of the present invention; 
     FIG. 8 shows gain characteristics of an antenna device according to an embodiment of the present invention in comparison with conventional antenna devices corresponding to azimuth angles at a horizontal plane; 
     FIGS. 9A through 9C show directivities of an antenna device according to an embodiment of the present invention and conventional antenna devices; and 
     FIG. 10 is a modification of an antenna device according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the planar antenna device according to the present invention will be described in further detail with reference to the accompanying drawings. 
     FIG. 3 is a perspective view illustrating a structure of a planar antenna device according to the present invention. FIG. 4 is a sectional view taken along the line  4 — 4  of FIG.  3 . 
     In FIG. 3, an antenna element  33  is mounted above a ground plane  31  via a spacer  32  so that the antenna element  33  is separated from the ground plane  31 . This antenna element  33  is excited by power from the feed point  34 . The ground plane  31  is made of a metal plate such as brass, aluminum, stainless steel, and the like. The spacer  32  is made of synthetic resin such as polyacetal, polycarbonate, ABS, and the like. The antenna element  33  is made of a metal plate such as brass, aluminum, and the like. 
     A box-like cavity  35  is placed on the ground plane  31  so as to cover the entire antenna element  33 . The cavity  35  is made of a metal plate such as brass, aluminum, and the like. 
     The cavity  35  is provided so that it does not touch the antenna element  33  with a predetermined distance. A square opening  35   a , which is smaller than a size of the antenna element  33 , is formed at a surface a cavity  35  which is opposite to the antenna element  33 . 
     The opening  35   a  of this cavity  35  is formed in order to provide high electromagnetic field radiation characteristics in a wide range of elevation angles, especially toward a low elevation angle without reducing the size of the antenna element  33 . It is possible to change electromagnetic field radiation characteristics especially toward a low elevation angle by adjusting the size of the opening  35   a  with reference to the antenna element  33  and a distance between the opening  35   a  and the antenna element  33 . 
     In the above-mentioned antenna device structure, various characteristics observed from experiments will be described as follows. 
     First, characteristics of the antenna device itself will be described with reference to FIGS. 6 through 7. 
     FIGS. 5 through 7 show an experimental voltage standing-wave ratio (VSWR), a return loss corresponding to the VSWR, and a Smith chart, respectively. Any of the characteristics FIGS. 5 through 7 indicates that an excellent performance is available at approximately 2.34 GHz with an input impedance of 50 Ω. 
     FIGS. 8 through 9C exemplify characteristics of the antenna device according to the embodiment of the present invention in comparison with the air patch antenna device in FIG.  1  and the dielectric patch antenna device in FIG.  2 . 
     FIG. 8 shows gain characteristics corresponding to azimuth angles at a horizontal plane. A characteristic a indicated by a thin line corresponds to the air patch antenna device in FIG. 1. A characteristic β indicated by a broken line corresponds to the dielectric patch antenna device in FIG. 2. A characteristic γ indicated by thick lines corresponds to the antenna device with the cavity  35  in FIGS. 3 and 4 according to this embodiment. 
     As shown in FIG. 8, the air patch antenna device showing the characteristic α provides a high gain at around azimuth angle 0°, but causes large gain changes corresponding to azimuth angles. The air patch antenna device in FIG. 8 is found to be inappropriate for, especially, an on-vehicle antenna device which always changes antenna device angles according to directions of radio waves received. 
     The dielectric patch antenna device showing the characteristic β decreases the antenna element size and causes a dielectric loss, decreasing the total gain for the entire antenna device. 
     By contrast, the antenna device according to this embodiment showing the characteristic γ causes a little change in gains according to azimuth angles and is found to be suited for an antenna device which always changes antenna device angles in accordance with directions of radio waves received. 
     FIGS. 9A through 9C show directivities of the antenna devices explained in FIG.  8 . 
     FIG. 9A exemplifies a directivity of the air patch antenna device. The directivity is valid only in a front direction and within a high elevation angle range. It is understood that the directive range is very narrow. 
     FIG. 9B exemplifies a directivity of the dielectric patch antenna device. Compared to the air patch antenna device in FIG. 9A, the dielectric patch antenna device in FIG. 9B increases a characteristic at the azimuth angle and toward a low elevation angle. However, it is understood that the directivity is unsatisfactory. 
     FIG. 9C exemplifies a directivity of the antenna device with the cavity  35  according to this embodiment. The antenna device in FIG. 9C provides the directivity in a very wide range not only at the azimuth angle on the horizontal plane, but also at elevation angles especially ranging from low to high elevation-angle directions. 
     As mentioned above, the antenna device structure with the cavity  35  according to this embodiment of the present invention can maintain high electromagnetic field radiation characteristics over a wide elevation angle range from a low elevation-angle direction. It is also possible to provide a sufficiently high total gain for the entire antenna device. 
     Compared to a quadrifilar helical antenna device, a cross di-pole antenna device, and the like having high efficiency and low elevation-angle radiation characteristics, the antenna device according to this embodiment of the present invention provides the following advantages. 
     (1) Simplifying a structure of the entire antenna device including a feed structure. 
     (2) Providing a mechanically solid structure having the rigid cavity for guarding the antenna element with no sharp projections. 
     (3) Easily manufacturing the antenna device. 
     (4) Easily thinning the entire antenna device structure. 
     The antenna device according to the present invention can be easily mass-produced and be suitably mounted on vehicles such as cars. 
     The above-mentioned embodiment provides an air patch antenna device with the cavity  35 . The present invention is not limited thereto. 
     For example, in the embodiment, an elevation radiation characteristic is improved by providing the cavity, but a rectangular conductor  36  having an opening (or may be a circular conductor, or a linear conductor like a wire etc.) as shown in FIG. 10 may be provided like the cavity  35 . That is, any conductor may be used to define an aperture of the antenna. With this configuration, the same advantage can be obtained as the above-mentioned embodiment. 
     The present invention is not limited to above-mentioned embodiment, and can be achieved in a scope of the invention. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.