Wideband antenna mountable in vehicle cabin

[0002] 1. Field of the Invention

[0003] The present invention relates to antennas, and more particularly relates to an on-board antenna used for receiving terrestrial television broadcast signals, etc.

[0004] 2. Description of the Related Art

[0005] A conventional on-board antenna 50 for receiving terrestrial television broadcast signals is shown in FIG. 5. This conventional antenna 50 includes a rod-shaped radiation conductor 51 which is adjusted so as to resonate at a desired frequency. The angle between the radiation conductor 51 and a pedestal 52 is freely adjusted by inclining a supporting portion 53 relative to the pedestal 52. As shown in FIGS. 6A and 6B, this antenna 50 is attached on a rear window 61 or on a roof 62 of a vehicle 60.

[0006] Generally, to solve the problem of fading, which particularly occurs when signals are received by a moving antenna, a diversity receiving system is adopted in vehicles. In this system, a plurality of the antennas shown in FIG. 5 are used, and one of the antennas which exhibits the highest receiving level is selected.

[0007] With respect to the conventional antennas as described above, the operational bandwidth of a single antenna is not sufficiently wide. Thus, when a wide bandwidth must be covered, as in a case of receiving television broadcast signals, multiple antennas having different operational bandwidths are prepared. In addition, external circuits such as tuning circuits and amplifying circuits are attached. Accordingly, there has been a problem in that a considerably high total cost is incurred to obtain a wide operational bandwidth. In addition, since a plurality of antennas, each of which is relatively large, is used, the antennas are necessarily attached to the exterior of the vehicle. Thus, there are risks in that the antennas will be damaged or stolen. In addition, there is a problem in that the appearance of the vehicle is degraded.

[0008] In consideration of the above-described situation of the conventional technique, an object of the present invention is to provide an inexpensive and compact wideband antenna which is mountable in a vehicle cabin, which is fabricated by a simple process, and which causes small variation in characteristics.

[0009] To this end, an antenna of the present invention comprises a radiation conductor unit including an electricity-supplying conductor and a plurality of radiation conductors having different lengths which extend in parallel to each other from the electricity-supplying conductor; a grounded conductor unit which opposes the radiation conductors in an approximately parallel manner with a predetermined distance therebetween; an insulating casing which contains the radiation conductor unit and the grounded conductor unit; and a plurality of projections in an inwardly facing surface of the insulating casing for positioning the radiation conductor unit and the grounded conductor unit.

[0010] According to the antenna which is constructed as described above, multiple resonances occur between the radiation conductors having different lengths and the grounded conductor unit. Accordingly, overall frequency characteristics are improved in a frequency band including multiple resonance frequencies, and the operational bandwidth is increased. In addition, since the radiation conductors arranged in parallel to each other individually serve as radiators, the size of the antenna is reduced compared to conventional dipole antennas, so that the installation in a vehicle cabin is realized. In addition, since the projections for positioning the radiation conductors and the grounded conductor unit are provided on the inwardly facing surface of the casing, the radiation conductors and the grounded conductor unit are easily mounted at predetermined positions in the casing. Accordingly, fabrication process is simplified and variation in characteristics is reduced.

[0011] The radiation conductor unit may include two radiation conductors which are arranged in parallel to each other with a slit therebetween. Preferably, in such a case, one or more of the projections are inserted through the slit and are engaged with each of the radiation conductors. Accordingly, the risk is reduced in which the radiation conductors will be excessively close to each other and the characteristics of the antenna will be degraded.

[0012] One or more of the projections preferably abut against the peripheral edges of the radiation conductors at a plurality of positions. Accordingly, displacement of the radiation conductors is restrained, so that the positioning accuracy is increased.

[0013] One or more of the projections are preferably provided with a thinned portion for restraining the displacement of the radiation conductors in the thickness direction by fitting the radiation conductors on the thinned portion. Accordingly, the distance between the radiation conductors and the grounded conductor unit may be maintained constant.

[0014] Preferably, the insulating casing is constructed by fixing a pair of cases to each other with screws, and one or more of said projections serve as screw-receiving portions. Accordingly, number of projections which exclusively serve for positioning the radiation conductors is reduced, and the positioning accuracy is increased.

[0015] FIG. 1 is an exploded perspective view of an antenna according to a first embodiment of the present invention;

[0016] FIGS. 2A and 2B are explanatory drawings showing a manner in which the displacement of the radiation conductor unit shown in FIG. 1 is restrained;

[0017] FIGS. 3A and 3B are explanatory drawings showing a manner in which a radiation conductor unit of an antenna according to a second embodiment of the present invention is installed;

[0018] FIG. 4 is an exploded perspective view of an antenna according to a third embodiment of the present invention;

[0019] FIG. 5 is a perspective view of a conventional on-board antenna; and

[0020] FIGS. 6A and 6B are side views of a vehicle showing manners in which the conventional on-board antenna is mounted.

[0021] Embodiments of the present invention will be described below in conjunction with the accompanying drawings. FIG. 1 is an exploded perspective view of an antenna 10 according to a first embodiment of the present invention. The antenna 10 includes a casing which is constructed by fixing a first case 11aand a second case 11btogether, a radiation conductor unit 12, and a grounded conductor unit 14. The radiation conductor unit 12 and the grounded conductor unit 14 are installed in the casing, and are supplied with electricity via a coaxial cable 15 which is led out from the casing.

[0022] The first and the second cases 11aand 11bare constructed of an insulating and heat-resistant material such as ABS plastic. The first case 11ahas the shape of an open container, and the second case 11bhas the shape of an inverted open container. In the first embodiment, the first case 11afunctions as a main case, and the second case 11bfunctions as a cover. Five fixing projections 17ato 17eand a pair of struts 18 are formed on the inwardly facing bottom surface of the first case 11a.The struts 18 are provided with restraining projections 19 on the upper side thereof, and each of the restraining projections 19 has a thinned portion 19aas shown in FIG. 2B. In addition, a semicircular tube 11cis formed at the upper edge of an end surface of the first case 11a,and a semicircular tube 11dis formed at a lower edge of an end surface of the second case 11b.

[0023] The radiation conductor unit 12 includes a first radiation conductor 12a,a second radiation conductor 12b,and an electricity-supplying conductor 13. The radiation conductors 12aand 12bhave different lengths and are arranged in parallel to each other. The electricity-supplying conductor 13 is connected to each of the radiation conductors 12aand 12bat one longitudinal end thereof. The radiation conductors 12aand 12band the electricity-supplying conductor 13 are integrally formed by bending a plate constructed of a highly conductive metal such as Cu, Al, etc. A slit 20 is formed between the first radiation conductor 12aand the second radiation conductor 12b,and the first radiation conductor 12aextends along the slit 20 in a form of a plate. The second radiation conductor 12balso extends along the slit 20 in a form of a plate, but is longer than the first radiation conductor 12a.The leading end of the second radiation conductor 12bis bent in the shape of a bracket. The bottom plate portion of this bracket forms an attachment tab 12chaving an insertion hole 16b.The electricity-supplying conductor 13 is provided with a receiving portion 13a,which is electrically connected to an inner conductor 15aof the coaxial cable 15, and an attachment tab 13bhaving an insertion hole 16aat the lower side thereof. The receiving portion 13aand the attachment tab 13bare integrally formed in the shape of a step. The attachment tab 13band the above-described attachment tab 12care formed in the same plane, and are fixed to the inwardly facing bottom surface of the first case 11aby inserting the fixing projections 17aand 17bthrough the insertion hole 16aand 16b,respectively.

[0024] The grounded conductor unit 14 opposes the first and the second radiation conductors 12aand 12bin an approximately parallel manner with a predetermined distance therebetween. The grounded conductor unit 14 includes a receiving portion 14aand a holding portion 14bat one end, which are integrally formed by bending a plate constructed of a highly conductive material such as Cu, Al, etc. The grounded conductor unit 14 is provided with three insertion holes: insertion holes 16cand 16dat one end, and an insertion hole 16eat the other end. The receiving portion 14ais provided for electrically connecting an outer conductor 15bof the coaxial cable 15 thereto, and is formed in the shape of a bracket so that the outer conductor 15bcan be inserted therein. The holding portion 14bis provided for supporting an insulator 15cof the coaxial cable 15, and is formed in the shape of a bracket so that the insulator 15ccan be inserted therein. The fixing projections 17c,17d,and 17eare inserted through the insertion holes 16c,16d,and 16e,respectively, to fix the grounded conductor unit 14 to the inwardly facing bottom surface of the first case 11a.

[0025] The coaxial cable 15 is constructed by forming the insulator 15cand the outer conductor 15 around the inner conductor 15adisposed in the center. The inner conductor 15ais connected to the receiving portion 13aof the electricity-supplying conductor 13 by soldering, and the outer conductor 15bis clamped by the receiving portion 14aof the grounded conductor unit 14. Accordingly, the electricity-supplying conductor 13 and the grounded conductor unit 14 are supplied with electricity through the inner conductor 15aand outer conductor 15b.In addition, the insulator 15cof the coaxial cable 15 is clamped by the holding portion 14bof the grounded conductor unit 14, and the exterior of the coaxial cable 15 is sandwiched by the semicircular tubes 11cand 11dof the first and the second cases 11aand 11b.

[0026] Next, the fabrication process of the antenna 10 having the above-described construction will be explained below. First, the fixing projections 17ato 17eare respectively inserted through the insertion holes 16aand 16bformed in the radiation conductor unit 12 and the insertion holes 16cto 16eformed in the grounded conductor unit 14. Then, the attachment tabs 12cand 13band the grounded conductor unit 14 are fixed to the inwardly facing bottom surface of the first case 11aby deforming the ends of the fixing projections 17ato 17e,by using an adhesive, or by other means. FIG. 2A is a plan view of a part of the radiation conductor unit 12, and FIG. 2B is a cross sectional view of FIG. 2A along line IIB—IIB. As show in FIGS. 2A and 2B, the radiation conductors 12aand 12bare supported by the strut 18, and the restraining projections 19 are inserted through the slit 20. Accordingly, the thinned portions 19aof the restraining projections 19 are engaged with the edge portions of the radiation conductors 12aand 12b.Thus, the radiation conductor unit 12 and the grounded conductor unit 14 are positioned and fixed by the fixing projections 17ato 17e.In addition, the radiation conductors 12aand 12bare positioned by the struts 18 and the restraining projections 19. Accordingly, the width of the slit 20 and the distance between the radiation conductors 12aand 12band the grounded conductor unit 14 is maintained constant. The coaxial cable 15 is then introduced from the upper side of the first case 11a.The outer conductor 15band the insulator 15care inserted into the receiving portion 14aand the holding portion 14b,respectively. The inner conductor 15aat the leading end is put on the receiving portion 13a,and the exterior of the coaxial cable 15 is fitted into the semicircular tube 11c.Then, the insulator 15cis clamped and fixed by the holding portion 14b,and the outer conductor 15bis clamped and fixed by the receiving portion 14a.Thus, the outer conductor 15bis electrically and mechanically connected to the grounded conductor unit 14. The inner conductor 15ais soldered on and electrically connected to the receiving portion 13a.The outer conductor 15bmay also be soldered on the receiving portion 14ato ensure reliability. In addition, the outer conductor 15band the insulator 15cmay also be fixed by means other than clamping, for example, by press fitting. Lastly, the opening at the upper side of the first case 11ais covered by the second case 11bin a manner such that the coaxial cable 15 is led out through the semicircular tubes 11cand 11d.The first and the second cases 11aand 11bare then fixed to each other by screws, snaps, an adhesive, or by other means. Accordingly, the fabrication of the antenna 10 containing the radiation conductor unit 12 and the grounded conductor unit 14 in the first and the second cases 11aand 11bis completed.

[0027] In the above-described antenna 10, multiple resonances occur between the first and the second radiation conductors 12aand 12bhaving different lengths and the grounded conductor unit 14. Accordingly, overall frequency characteristics are improved in a frequency band including multiple resonance frequencies, and the operational bandwidth is increased. In addition, since the first and the second radiation conductors 12aand 12b,which are arranged in parallel to each other, individually serve as radiators, the size of the antenna 10 is reduced, so that the installation in a vehicle cabin is realized.

[0028] As described above, the receiving portions 13aof the radiation conductor unit 12 and the receiving portion 14aof the grounded conductor unit 14 are disposed in the first case 11a,and are covered by the second case 11b.In addition, the coaxial cable 15 for supplying electricity is sandwiched by the semicircular tubes 11cand 11dof the first and the second cases 11aand 11b.Accordingly, the operation of connecting the coaxial cable 15 is easily performed while the second case 11bis removed. In addition, the fixing projections 17ato 17e,the struts 18, and the restraining projections 19 are utilized in the process of installing the radiation conductor unit 12 and the grounded conductor unit 14 into the first case 11a.The fixing projections 17ato 17eposition and fix the radiation conductor unit 12 and grounded conductor unit 14. The struts 18 determine the vertical position of the radiation conductors 12aand 12b,and the restraining projections 19 restrain the displacement of the radiation conductors 12aand 12bby using the slit 20. Thus, the radiation conductor unit 12 and the grounded conductor unit 14 are easily installed inside the first case 11aat predetermined positions. Accordingly, the antenna 10 is fabricated by a significantly simple process. In addition, the radiation conductor unit 12 and the grounded conductor unit 14 are positioned with high accuracy, so that variation in characteristics of the antenna is reduced.

[0029] When the attachment tabs 12cand 13bare formed not in a horizontal manner but in an inclined manner toward the lower side, the insertion holes 16aand 16band the fixing projections 17aand 17bare not necessary for positioning and fixing the radiation conductor unit 12. In such a case, the radiation conductor unit 12 is installed in the first case 11aby pressing the attachment tabs 12cand 12bagainst the inwardly facing bottom surface of the first case 11a.At this time, the edge portions of the radiation conductors 12aand 12bare fitted into the thinned portions 19aof the restraining projections 19, while the attachment tabs 12cand 12bare deformed. Accordingly, the radiation conductors 12aand 12bare pressed upward against the top portion of the restraining projections 19 by an opposing force generated by the attachment tabs 12cand 13b.Thus, the radiation conductors 12aand 12band the receiving portion 13aare positioned and supported at predetermined positions.

[0030] FIGS. 3A and 3B show a part of an antenna according to a second embodiment of the present invention. According to the second embodiment, a second case 11b,which functions as a cover, is provided with a plurality of projections for positioning the radiation conductors 12aand 12bof the radiation conductor unit 12. These projections include three restraining projections 31, which are disposed so as to abut against the peripheral edges of the radiation conductors 12aand 12b,and one of two screw-receiving portions 32, which are used for fixing the second case 11bto the first case 11a(not shown) by screws. As shown in FIG. 3B, one of the screw-receiving portions 32 is disposed at an end of the slit 20. This screw-receiving portion 32 serves to position the radiation conductors 12aand 12b,and is designed so as to abut against the end surface of the first radiation conductor 12aand on the side surface of the second radiation conductor 12b.

[0031] Since the restraining projections 31 and one of the screw-receiving portions 32 abut against a plurality of positions in the peripheral edges of the radiation conductors 12aand 12b,the displacement of the radiation conductors 12aand 12bis restrained and the positioning accuracy is increased. In addition, one of the screw-receiving portions 32, which are necessary for fixing the first and the second cases to each other, is also used for positioning the radiation conductors 12aand 12b.Thus, the number of projections which exclusively serve to position the radiation conductors 12aand 12bis reduced.

[0032] When the second case 11bis capable of positioning the radiation conductors 12aand 12bas described above, there is no need to provide the struts 18 and restraining projections 19 in the first case 11aas shown in FIGS. 1 and 2. The attachment tabs 12cand 12bof the radiation conductor unit 12 and the grounded conductor unit (not shown in FIGS. 3A and 3B), however, are fixed to the inwardly facing bottom surface of the first case in a similar manner as described in the first embodiment. Thus, the fixing projections 17ato 17eshown in FIG. 1 are still necessary.

[0033] FIG. 4 is an exploded perspective view of an antenna 40 according to a third embodiment. The antenna 40 differs from the antenna 10 of the first embodiment shown in FIG. 1 in the following point. That is, the antenna 40 includes three struts 41ato 41cwhich are provided with positioning projections 42ato 42c,respectively, on the upper surfaces thereof. The positioning projections 42ato 42care inserted into insertion holes 21ato 21c,respectively, which are formed in the radiation conductors 12aand 12bof the radiation conductor unit 12. With reference to FIG. 4, the radiation conductor unit 12 is positioned and fixed on the inwardly facing bottom surface of the first case 11aby inserting the fixing projections 17aand 17binto the attachment tabs 12cand 13b.At this time, the first radiation conductor 12ais supported by the strut 41ain a manner such that the positioning projection 42ais inserted through the insertion hole 21a.Similarly, the second radiation conductor 12bis supported by the struts 41band 41cin a manner such that the positioning projections 42band 42care inserted through the insertion holes 21band 21c,respectively. Thus, the radiation conductors 12aand 12bare positioned with high accuracy, so that variation in characteristics of the antenna is reduced.

[0034] The grounded conductor unit 14 of the third embodiment has a different shape compared to that in the first embodiment. As shown in FIG. 4, the grounded conductor unit 14 of the antenna 40 is designed so as to be positioned and fixed on the inwardly facing bottom surface of the first case 11ain a manner such that fixing projections 17fand 17gare inserted through insertion holes 16fand 16g.In addition, the grounded conductor unit 14 is positioned directly below the slit 20 between the radiation conductors 12aand 12b.Other parts of the antenna 40 shown in FIG. 4 have the same constructions as those described in the first embodiment. Thus, components corresponding to those shown in FIG. 1 are denoted by the same reference numerals, and redundant explanations are thus omitted.