Antenna device

Abstract: An antenna device (111) for surface mounting has a dielectric base plate (112), a chassis (113) made of an electrically conductive material and attached to the dielectric base plate, a feed electrode (119) and grounding electrodes. The chassis (113) has a planar radiative part (116) facing the upper surface of the dielectric base plate and side walls (135) at its edges. The side walls are provided with slits (135c, 135d) which separate portions of the side wall into removable parts (135e, 135f). The resonance frequency of the antenna device (111) can be adjusted by cutting off and removing one or more of these removable parts (135e, 135f) of the side walls. These removable parts (135e, 135f) can be cut off and removed even after the chassis (113) has been attached to the base plate (112) or the antenna device (111) has been mounted to a circuit board.

Description: BACKGROUND OF THE INVENTION This invention relates to an antenna device which can be surface-mounted to a circuit board or the like and hence is useful, for example, in a mobile communication system. In Fig. 5, there is shown a surface-mountable antenna device 111' which has been proposed in EP-A-621 653. This antenna is adapted for use in a mobile communication system and characterized as having a dielectric base plate 112 and a radiative chassis 113' attached to this dielectric base plate 112 with a space 121 therebetween. The radiative chassis 113' is formed by machine-processing an electrically conductive material such as copper or a copper alloy and, as shown in Fig. 6, has a rectangularly shaped planar radiative part 116. Planar first and second attachment parts 117 and 118 are formed by being bent downward from the shorter edges of this rectangularly shaped radiative part 116. A feed electrode 119 and a grounding electrode 120 are formed integrally at the tip of the first attachment part 117. Slits 120a and 118a for injecting solder thereinto are provided at the tips of the first and second attachment parts 117 and 118, respectively. The slit 120a on the first attachment part 117 is formed in the part where the grounding electrode 120 is provided. Stopper pieces 131 - 134, serving as a space-securing means for securing a space between the inner surface of the radiative part 116 and the upper surface of the dielectric base plate 112, are provided on both sides of the first and second attachment parts 117 and 118 so as to come into contact with the upper surface of the dielectric base plate 112, forming the space 121. These stopper pieces 131 - 134 are formed by bending portions of the radiative part 116 at positions further towards the center (or further away from the shorter edges) of the radiative part 116 than where the radiative part 116 is bent to form the first and second attachment parts 117 and 118, such that they can come into contact with the upper surface of the dielectric base plate 112. There are also side walls 135a and 135b, formed by bending the longer side edges of the radiative part 116 downward. These side walls 135a and 135b serve to improve the mechanical strength of the radiative chassis 113'. As shown in Fig. 7 in more detail, the dielectric base plate 112 is made of a ceramic or synthetic resin material and has a rectangular box-like shape. Grounding electrodes 114a and 114b are formed on the side surfaces along its longer edges, and connector electrodes 115a and 115b are formed on the side surfaces along its shorter edges. As shown more clearly in Fig. 8, a planar capacitor electrode 136 is formed inside the dielectric base plate 112 at an intermediate height and parallel to its principal surfaces, electrically connected to the connector electrode 115a. A planar grounding electrode grounding pattern 137 is similarly formed inside the dielectric base plate 112 parallel to and below the capacitor electrode 136, electrically connected to the grounding electrodes 114a and 114b, such that a capacitor is formed with the capacitor electrode 136, the grounding electrode pattern 137 and the portion of the dielectric material of the base plate 112 lying therebetween. In this manner, the antenna device 111' can be made compact and its resonance frequency can be lowered. When the radiative chassis 113' and the dielectric base plate 112 are assembled together, the base plate 112 is inserted between the first and second attachment parts 117 and 118 of the chassis 113' until the stopper pieces 131 - 134 come into contact with the upper surface of the base plate 112. The antenna device 111' is completed by soldering the first attachment part 117 to the connector electrode 115b and the second attachment part 118 to the connector electrode 115a. The soldering can be accomplished dependably by injecting a solder paste into the slits 118a and 120a. Because the second attachment part 118 and the connector electrode 115a are thus electrically connected, a capacitor comprising the capacitor electrode 136 and the grounding electrode pattern 137 as described above comes to be connected between the radiative chassis 113' and the grounding electrodes 114a and 114b. The stopper pieces 131 - 134, in contact with the upper surface of the dielectric base plate 112, serve to keep the space 121 between the lower surface of the radiative part 116 and the upper surface of the dielectric base plate 112 so as to limit the radiative loss of electromagnetic waves and improve the gain of the antenna device 111'. As shown in the equivalent circuit diagram of Fig. 9, the antenna device 111' thus formed may be viewed as being composed of inductive components L 1 and L 2 and a capacitive component C 1 . The first inductive component L 1 corresponds mainly to the inductance of the radiative part 116 of the chassis 113'. The second inductive component L 2 corresponds mainly to the inductance between the feed electrode 119 and the grounding electrode 120 of the chassis 113'. The capacitive component C 1 corresponds to the floating capacitance between the grounding electrodes 114a and 114b of the dielectric base plate 112 and the radiative part 116 of the chassis 113'. In terms of L 1 , L 2 and C 1 , the resonance frequency f 0 ' of the antenna device 111' is expressed as follows: f 0 ' = 1/{(2π){C 1 (L 1 + L 2 )} 1/2 }. Because the antenna device 111' is compact with its maximum length less than 1/6 of that of common prior art whip antennas, its band width is less than 1/3 of such prior art whip antennas. Accordingly, the resonance frequency f 0 ' of the antenna device 111' must be more accurately adjusted in order to obtain a desired frequency band width therefor. According to the antenna described above (proposed in EP-A-621 653), the resonance frequency f 0 ' of the antenna device 111' is adjusted by varying the distance between the feed electrode 119 and the grounding electrode 120 to thereby change the value of the second inductive component L 2 . Since the shape of the antenna device 111' itself must be modified if the distance between its feed electrode 119 and grounding electrode 120 are to be changed, the desired adjustment of resonance frequency may become impossible after the chassis 113' has been fastened to the dielectric base plate 112 or the antenna device 111' has been mounted to a circuit board. In other words, even where there are fluctuations in the resonance frequency among antenna devices due to their mass production, their resonance frequency may not be adjustable after they are mounted to a circuit board. SUMMARY OF THE INVENTION It is therefore an object of this invention, in view of the above, to provide an antenna device the resonance frequency of which can be adjusted even after its assembly has been completed by attaching a radiative chassis to its dielectric base plate or after it has been mounted to a circuit board. It is another object of this invention to provide such antenna devices the resonance frequencies of which can be adjusted, even after they have been mounted, if there are fluctuations among them due to their mass production. An antenna device embodying this invention, with which the above and other objects can be accomplished, may be characterized not only as comprising a dielectric base plate, a chassis made of an electrically conductive material and attached to the dielectric base plate with a space therebetween, a feed electrode and grounding electrodes, but also wherein the side walls are formed with a plurality of removable parts serving as means for adjusting the resonance frequency of the antenna device. Portions of the side walls of an antenna device thus structured can be cut off and removed easily even after the chassis has been attached to the base plate or the antenna device has been mounted to a circuit board, and the inductance of the electrically conductive chassis is thereby increased, thereby reducing the resonance frequency of the antenna device. Fine adjustments of the resonance frequency become possible by increasing the number of such removable parts. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and form a part of this specification, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention. In the drawings: Fig. 1 is a perspective view of an antenna device embodying this invention; Fig. 2 is a perspective view of the radiative chassis of the antenna device of Fig. 1; Fig. 3 is a perspective view of the antenna device of Fig. 1 with one of the removable side wall parts removed; Fig. 4 is an equivalent circuit diagram of the antenna device of Fig. 1; Fig. 5 is a perspective view of an antenna device proposed in EP-A-621 653; Fig. 6 is a perspective view of the radiative chassis of the antenna device of Fig. 5; Fig. 7 is a perspective view of the dielectric base plate of the antenna device of Fig. 5; Fig. 8 is a sectional side view of a portion of the dielectric base of Fig. 7; and Fig. 9 is an equivalent circuit diagram of the antenna device of Fig. 5. DETAILED DESCRIPTION OF THE INVENTION Fig. 1 shows an antenna device 111 embodying this invention. For convenience of description, those of its components which are substantially like components of the antenna device 111' already described above are indicated by the same reference numerals as in Figs. 5 - 9. As shown in Fig. 1, the antenna device 111 according to this embodiment of the invention is formed by attaching a radiative chassis 113 to a dielectric base plate 112 with a space 121 kept above the dielectric base plate 112. This radiative chassis 113 is different from the chassis 113' of the antenna device 111' shown in Figs. 5 and 6 in that the side walls 135a and 135b of the chassis 113' of the antenna device 111' are replaced by a plurality of removable side wall parts 135e and 135f (four each in the embodiment shown in Figs. 1 and 2) which are mutually separated by a plurality of vertical slits 135c and 135d (three, respectively, in the embodiment shown in Figs. 1 and 2) provided at equal intervals. The chassis 113 and the dielectric base plate 112 are assembled together, to form antenna device 111, as explained above for the assembly of the antenna device 111'. Fig. 3 shows the antenna device 111 after one of the removable side wall parts 135e has been cut off and removed. It is to be appreciated that since these removable side wall parts 135e and 135f are separated from and independent of the dielectric base plate 112 and the upper surface of a circuit board (not shown), these removable side wall parts 135e and 135f can be easily cut off and removed even after the chassis 113 has been attached to the dielectric base plate 112 or the antenna device 111 has been mounted to a circuit board or the like. As shown in the equivalent circuit diagram of Fig. 4, the antenna device 111 thus formed may be viewed as being composed of inductive components L 11 and L 22 and a capacitive component C 11 . The first inductive component L 11 corresponds mainly to the inductance of the radiative part 116 of the chassis 113. The second inductive component L 22 corresponds mainly to the inductance between the feed electrode 119 and the grounding electrode 120 of the chassis 113. The capacitive component C 11 corresponds to the floating capacitance between the grounding electrodes 114a and 114b of the dielectric base plate 112 and the radiative part 116 of the chassis 113. In terms of L 11 , L 22 and C 11 , the resonance frequency f 0 of the antenna device 111 is expressed as follows: f 0 = 1/{(2π){C 11 (L 11 + L 22 )} 1/2 }. As each of the removable side wall parts 135e and 135f is cut off and removed, this has the effect of changing the total area of the side walls 135a and 135b of the chassis 113, or that of increasing the value of the inductive component L 11 of the radiative part 116. In other words, the resonance frequency of the antenna device 111 according to this invention can be made smaller. Table 1 shows the change in the measured resonance frequency f 0 as the removable side wall parts 135e and 135f were cut off and removed one by one. The dimensions of the principal surface of the antenna device 111 used for this measurement were 10mm x 6.3mm and its height was 4mm. Number of removable side wall parts that were cut off Resonance frequency (GHz) 0 1.891 1 1.881 2 1.877 3 1.868 4 1.864 5 1.853 6 1.848 7 1.837 8 1.832 This shows that the resonance frequency f 0 can be made smaller by cutting off and removing an increasing number of removable side wall parts 135e and 135f. In other words, the resonance frequency f 0 of the antenna device 111 is easily adjustable. Although this invention has been described above with reference to only one example, this example is not intended to limit the scope of the invention. Many modifications and variations are possible within the scope of the invention. For example, the total number of the removable side wall parts 135e and 135f may be further increased such that the resonance frequency f 0 can be more finely adjustable. Grooves may be formed where the removable side wall parts 135e and 135f are attached to the radiative part 116 so as to facilitate their removal. Although the slits 135c and 135d are shown in Figs. 1 - 3 to extend over the entire height of what corresponds to the side walls 135a and 135b of the chassis 113', these slits 135c and 135d may be cut from the bottom edges of the side walls to only a part of the way upward to the radiative part 116 such that, even after a removable side wall part 135e or 135f is cut off and removed, a portion of the downwardly bent portion remains attached to the radiative part 116 and the mechanical strength of the chassis 113 will not be adversely affected. Furthermore, different means for being partially removed may be substituted if such means also serve to adjust the resonance frequency of the antenna device 111 and to improve the mechanical strength of the radiative chassis 113. Such removable portions may be provided, for example, on the side walls 135a and 135b. In summary, an antenna device according to this invention is capable of having its resonance frequency reduced because the inductance due to the radiative part of its chassis becomes larger as the area of its side surfaces connected to this radiative part is varied. Its resonance frequency can be adjusted by increasing the number of removable side wall parts to be cut off and removed, and since such removable side wall parts are separate from and independent of the dielectric base plate or the circuit board to which the antenna device is mounted, such adjustments of resonance frequency can be effected even after the radiative chassis has been attached to the dielectric base plate or the antenna device has been mounted to a circuit board.

Claims: An antenna device (111) for surface mounting, comprising: a dielectric base plate (112) having an upper surface; a chassis (113) made of an electrically conductive material and attached to said dielectric base plate, said chassis having a planar radiative part (116) with a lower surface opposite said upper surface of said dielectric base plate and side walls (135) at edges of said radiative part (116); and a feed electrode (119) and grounding electrodes on an external surface; characterized in that said side walls (135) include adjusting means (135e, 135f) adapted to be removed for adjusting the resonance frequency of said antenna device. The antenna device of claim 1 wherein said adjusting means comprises a part of said side walls separated into a plurality of removable parts (135e, 135f) by slits (135c, 135d). The antenna device of claim 2 wherein said slits (135c, 135d) are mutually parallel and provided at equal intervals. The antenna device of claim 1 wherein said side walls are formed as edge portions (135) of said chassis (113) bent from said radiative part (116).

Inventors: KATO MITSUHIDE, TSURU TERUHISA, KATO, MITSUHIDE, TSURU, TERUHISA, Kato, Mitsuhide, c/o Murata Manufacturing Co., Ltd, TSURU, TERUHISA, C/O MURATA MANUFACTURING CO., LTD
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1995
Main classification: H01Q9
IPC codes: H01Q13, H01Q9