Wideband antenna mountable in vehicle cabin

Abstract: An antenna (10) includes a radiation conductor unit (12) constructed with a plurality of radiation conductors (12a,12b) having different lengths which extend in parallel to each other from an electricity-supplying conductor (13) and a grounded conductor unit (14) which opposes said plurality of radiation conductor in an approximately parallel manner with a predetermined distance therebetween. The radiation conductor unit and the grounded conductor unit are contained in an insulating casing (11a,11b) constructed by a pair of cases which is fixed to each other. The casing is provided with a plurality of projections (17a-17e) for positioning the radiation conductor unit and the grounded conductor unit.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to antennas, and more particularly relates to an on-board antenna used for receiving terrestrial television broadcast signals, etc. 2. Description of the Related Art 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. 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. 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. Prior art documents are EP 1 077 505 A and AU 55 898 A, the first one being pursuant to Article 54(3) and (4) EPC. SUMMARY OF THE INVENTION 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. 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. 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. The radiation conductor unit may include two radiation conductors which are arranged in parallel to each other with a slit therebetween. 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. 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. 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. 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. An embodiment of the present invention, will now be described, with reference to the accompanying diagrammatic drawings, in which: Fig. 1 is an exploded perspective view of an antenna according to an embodiment of the present invention; 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; Figs. 3A and 3B are explanatory drawings showing a manner in which a radiation conductor unit of an antenna not according to an embodiment of the present invention is installed; Fig. 4 is an exploded perspective view of an antenna not according to an embodiment of the present invention; Fig. 5 is a perspective view of a conventional on-board antenna; and Figs. 6A and 6B are side views of a vehicle showing manners in which the conventional on-board antenna is mounted. Fig. 1 is an exploded perspective view of an antenna 10 according to an embodiment of the present invention. The antenna 10 includes a casing which is constructed by fixing a first case 11a and a second case 11b together, 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. The first and the second cases 11a and 11b are constructed of an insulating and heat-resistant material such as ABS plastic. The first case 11a has the shape of an open container, and the second case 11b has the shape of an inverted open container. The first case 11a functions as a main case, and the second case 11b functions as a cover. Five fixing projections 17a to 17e and 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 19a as shown in Fig. 2B. In addition, a semicircular tube 11c is formed at the upper edge of an end surface of the first case 11a, and a semicircular tube 11d is formed at a lower edge of an end surface of the second case 11b. 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 12a and 12b have different lengths and are arranged in parallel to each other. The electricity-supplying conductor 13 is connected to each of the radiation conductors 12a and 12b at one longitudinal end thereof. The radiation conductors 12a and 12b and 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 12a and the second radiation conductor 12b, and the first radiation conductor 12a extends along the slit 20 in a form of a plate. The second radiation conductor 12b also 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 12b is bent in the shape of a bracket. The bottom plate portion of this bracket forms an attachment tab 12c having an insertion hole 16b. The electricity-supplying conductor 13 is provided with a receiving portion 13a, which is electrically connected to an inner conductor 15a of the coaxial cable 15, and an attachment tab 13b having an insertion hole 16a at the lower side thereof. The receiving portion 13a and the attachment tab 13b are integrally formed in the shape of a step. The attachment tab 13b and the above-described attachment tab 12c are formed in the same plane, and are fixed to the inwardly facing bottom surface of the first case 11a by inserting the fixing projections 17a and 17b through the insertion hole 16a and 16b, respectively. The grounded conductor unit 14 opposes the first and the second radiation conductors 12a and 12b in an approximately parallel manner with a predetermined distance therebetween. The grounded conductor unit 14 includes a receiving portion 14a and a holding portion 14b at 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 16c and 16d at one end, and an insertion hole 16e at the other end. The receiving portion 14a is provided for electrically connecting an outer conductor 15b of the coaxial cable 15 thereto, and is formed in the shape of a bracket so that the outer conductor 15b can be inserted therein. The holding portion 14b is provided for supporting an insulator 15c of the coaxial cable 15, and is formed in the shape of a bracket so that the insulator 15c can be inserted therein. The fixing projections 17c, 17d, and 17e are 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. The coaxial cable 15 is constructed by forming the insulator 15c and the outer conductor 15 around the inner conductor 15a disposed in the center. The inner conductor 15a is connected to the receiving portion 13a of the electricity-supplying conductor 13 by soldering, and the outer conductor 15b is clamped by the receiving portion 14a of 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 15a and outer conductor 15b. In addition, the insulator 15c of the coaxial cable 15 is clamped by the holding portion 14b of the grounded conductor unit 14, and the exterior of the coaxial cable 15 is sandwiched by the semicircular tubes 11c and lid of the first and the second cases 11a and 11b. Next, the fabrication process of the antenna 10 having the above-described construction will be explained below. First, the fixing projections 17a to 17e are respectively inserted through the insertion holes 16a and 16b formed in the radiation conductor unit 12 and the insertion holes 16c to 16e formed in the grounded conductor unit 14. Then, the attachment tabs 12c and 13b and the grounded conductor unit 14 are fixed to the inwardly facing bottom surface of the first case 11a by deforming the ends of the fixing projections 17a to 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 12a and 12b are supported by the strut 18, and the restraining projections 19 are inserted through the slit 20. Accordingly, the thinned portions 19a of the restraining projections 19 are engaged with the edge portions of the radiation conductors 12a and 12b. Thus, the radiation conductor unit 12 and the grounded conductor unit 14 are positioned and fixed by the fixing projections 17a to 17e. In addition, the radiation conductors 12a and 12b are positioned by the struts 18 and the restraining projections 19. Accordingly, the width of the slit 20 and the distance between the radiation conductors 12a and 12b and 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 15b and the insulator 15c are inserted into the receiving portion 14a and the holding portion 14b, respectively. The inner conductor 15a at 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 15c is clamped and fixed by the holding portion 14b, and the outer conductor 15b is clamped and fixed by the receiving portion 14a. Thus, the outer conductor 15b is electrically and mechanically connected to the grounded conductor unit 14. The inner conductor 15a is soldered on and electrically connected to the receiving portion 13a. The outer conductor 15b may also be soldered on the receiving portion 14a to ensure reliability. In addition, the outer conductor 15b and the insulator 15c may also be fixed by means other than clamping, for example, by press fitting. Lastly, the opening at the upper side of the first case 11a is covered by the second case 11b in a manner such that the coaxial cable 15 is led out through the semicircular tubes 11c and lid. The first and the second cases 11a and 11b are 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 11a and 11b is completed. In the above-described antenna 10, multiple resonances occur between the first and the second radiation conductors 12a and 12b having 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 12a and 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. As described above, the receiving portions 13a of the radiation conductor unit 12 and the receiving portion 14a of 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 11c and lid of the first and the second cases 11a and 11b. Accordingly, the operation of connecting the coaxial cable 15 is easily performed while the second case 11b is removed. In addition, the fixing projections 17a to 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 17a to 17e position and fix the radiation conductor unit 12 and grounded conductor unit 14. The struts 18 determine the vertical position of the radiation conductors 12a and 12b, and the restraining projections 19 restrain the displacement of the radiation conductors 12a and 12b by using the slit 20. Thus, the radiation conductor unit 12 and the grounded conductor unit 14 are easily installed inside the first case 11a at 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. When the attachment tabs 12c and 13b are formed not in a horizontal manner but in an inclined manner toward the lower side, the insertion holes 16a and 16b and the fixing projections 17a and 17b are 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 11a by pressing the attachment tabs 12c and 12b against the inwardly facing bottom surface of the first case 11a. At this time, the edge portions of the radiation conductors 12a and 12b are fitted into the thinned portions 19a of the restraining projections 19, while the attachment tabs 12c and 12b are deformed. Accordingly, the radiation conductors 12a and 12b are pressed upward against the top portion of the restraining projections 19 by an opposing force generated by the attachment tabs 12c and 13b. Thus, the radiation conductors 12a and 12b and the receiving portion 13a are positioned and supported at predetermined positions. Figs. 3A and 3B show a part of an antenna not acrcording to an embodiment of the present invention. According to this a second case 11b, which functions as a cover, is provided with a plurality of projections for positioning the radiation conductors 12a and 12b of 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 12a and 12b, and one of two screw-receiving portions 32, which are used for fixing the second case 11b to 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 12a and 12b, and is designed so as to abut against the end surface of the first radiation conductor 12a and on the side surface of the second radiation conductor 12b. 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 12a and 12b, the displacement of the radiation conductors 12a and 12b is 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 12a and 12b. Thus, the number of projections which exclusively serve to position the radiation conductors 12a and 12b is reduced. When the second case 11b is capable of positioning the radiation conductors 12a and 12b as described above, there is no need to provide the struts 18 and restraining projections 19 in the first case 11a as shown in Figs. 1 and 2. The attachment tabs 12c and 12b of 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 embodiment of the invention. Thus, the fixing projections 17a to 17e shown in Fig. 1 are still necessary. Fig. 4 is an exploded perspective view of an antenna 40 not according to an embodiment of the invention. The antenna 40 differs from the antenna 10 of the embodiment shown in Fig. 1 in the following point. That is, the antenna 40 includes three struts 41a to 41c which are provided with positioning projections 42a to 42c, respectively, on the upper surfaces thereof. The positioning projections 42a to 42c are inserted into insertion holes 21a to 21c, respectively, which are formed in the radiation conductors 12a and 12b of 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 11a by inserting the fixing projections 17a and 17b into the attachment tabs 12c and 13b. At this time, the first radiation conductor 12a is supported by the strut 41a in a manner such that the positioning projection 42a is inserted through the insertion hole 21a. Similarly, the second radiation conductor 12b is supported by the struts 41b and 41c in a manner such that the positioning projections 42b and 42c are inserted through the insertion holes 21b and 21c, respectively. Thus, the radiation conductors 12a and 12b are positioned with high accuracy, so that variation in characteristics of the antenna is reduced. The grounded conductor unit 14 in Fig. 4 has a different shape compared to that in the embodiment of the present invention. 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 11a in a manner such that fixing projections 17f and 17g are inserted through insertion holes 16f and 16g. In addition, the grounded conductor unit 14 is positioned directly below the slit 20 between the radiation conductors 12a and 12b. Other parts of the antenna 40 shown in Fig. 4 have the same constructions as those described in the embodiment of the invention. Thus, components corresponding to those shown in Fig. 1 are denoted by the same reference numerals, and redundant explanations are thus omitted.

Claims: An antenna (10) comprising: at least two radiation conductors (12a, 12b) formed in a rectangular shape and having different lengths which are disposed parallel to each other with a slit (20) therebetween in the widthwise direction; an electricity-supplying conductor (13) connected to said at least two radiation conductors (12a, 12b) at one identical end in the longitudinal direction of each of said at least two radiation conductors (12a, 12b); a grounded conductor unit (14) formed in a rectangular shape, said grounded conductor unit (14) opposing said at least two radiation conductors (12a, 12b) in the thickness direction, said grounded conductor unit (14) being disposed parallel to said at least two radiation conductors (12a, 12b) in the longitudinal direction, characterized by the width of said grounded conductor unit (14) being set to be smaller than the width between the outer edges of said at least two radiation conductors (12a, 12b); and an insulating casing (11a, 11b) containing said at least two radiation conductors (12a, 12b), said electricity-supplying conductor (13), and said grounded conductor unit, wherein: a plurality of projections (17a - 17e, 19) being provided in an inwardly facing surface of said insulating casing for positioning said at least two radiation conductors (12a, 12b) and said grounded conductor unit (14); and one or more of the plurality of projections (19) being inserted into the slit and being engaged with each of said at least two radiation conductors (12a, 12b). An antenna according to Claim 1, wherein one or more of said projection 17a-17e abut against the peripheral edges of said plurality of radiation conductors (12a, 12b). An antenna according to Claim 1 or 2, wherein one or more of said projections (19) are provided with a thinned portion for restraining the displacement of said plurality of radiation conductors (12a, 12b) in the thickness direction by fitting said plurality of radiation conductors (12a, 12b) on said thinned portions. An antenna according to any preceding claim, wherein said insulating casing 11a, 11b is constructed by fixing a pair of cases to each other with screws, and wherein one or more of said projections (17a-17e, 19) serve as screw-receiving portions.

Inventors: IKEDA TOMOKI, TAKAHASHI TOSHIYUKI, IKEDA, TOMOKI, TAKAHASHI, TOSHIYUKI, Ikeda, Tomoki, c/o Alps Electric Co., Ltd., Takahashi, Toshiyuki, C/o Alps Electric Co., Ltd.
Applicants: ALPS ELECTRIC CO LTD, ALPS ELECTRIC CO., LTD.
Application year: 2001
Main classification: H01Q9
IPC codes: H01Q5, H01Q1, H01Q9