Antenna device capable of being commonly used at a plurality of frequencies and electronic equipment having the same

[00002] The present invention relates to an antenna device and an electronic equipment having the antenna device, in particular to an antenna device contained in an electronic equipment which can be commonly used at a plurality of frequencies capable of being utilized in a radio communication.

[00003] In addition to Local Area Network(LAN)s which are widely used in desk-top type computers, wireless LANs, for example, the Bluetooth system, that can be used also in portable type computers have been spread in a computer network in recent years. As a specification of an antenna device for a radio communication used in such a portable type computer, the following items are required.

[00004] Namely, a multi-frequency operation, for example, one frequency band of 2.4 GHz and another frequency band of 5.2 GHz. Herein, in order that the portable type computer may be responsive to both the frequency bands, it has been conventionally required that the portable type computer has two kinds of antenna devices. However, it is difficult to obtain spaces for mounting the two kinds of antenna device, respectively, since the portable type computer is designed to be as small as possible in size and weight. Accordingly, it is required that even a single antenna device can be responsive to both the frequency bands so that the spaces for mounting the antenna device may be as small as possible in a portable type computer.

[00005] Further, since the portable type computer is designed to be as small as possible in size and weight, as mentioned above, it is preferable that the antenna device can be contained in the portable type computer. Accordingly, it is, of course, required that the antenna device is small in size. In addition, it is further required that the antenna device is not easily influenced electrically from an adjacent housing body, or the like.

[00006] For example, a wire antenna, such as a dipole antenna, a monopole antenna, and the like resonates at a frequency of integer times (one, two, three . . . ) as large as a predetermined frequency, in a case that the wire antenna has an antenna-length of approximately ninety-five percentages as long as a wave-length of as half as the predetermined frequency. However, between the two frequencies usable in the wireless LAN (hereunder called first and second frequencies), second frequency is not integer times (one, two, three . . . ) as large as first frequency, as mentioned before. As a result, concerning a conventional dipole antenna, or the like, a single antenna device cannot be responsive to both the frequency bands.

[00007] Accordingly, an example of a conventional antenna device is disclosed in unexamined Japanese patent publication Hei2-57003, namely, 57003/1990. In order to be responsive to both the frequency bands mentioned above, the conventional antenna device disclosed therein has two dipole antennas resonating at first, second frequency, respectively. The two dipole antennas are located in parallel in the same feed line and supplied with electric power transversely. However, in the conventional antenna device having two dipole antennas, not only a structure of the antenna device inevitably becomes large in size but also a constitution of an impedance matching section becomes complicated. Further, the conventional antenna device having the two dipole antennas is disadvantageous in actual use, since loss is increased in the feeder thereof, and the like.

[00008] On the other hand, an input impedance of a conventional dipole antenna, and the like becomes low almost down to short-circuit impedance near a metal conductor, particularly when an interval between the conventional dipole antenna and the metal conductor is not longer than a wave-length of one-tenth of the predetermined frequency. In addition, each of the first and the second resonant frequencies comes to a frequency characteristic having a narrow band. As a result, when the dipole antenna, and the like is contained in a computer, it becomes difficult to obtain impedance matching between an antenna element and a feeding system thereof. Further, it also becomes difficult to generally use the dipole antenna, and the like by way of a coaxial cable, and the like.

[00009] Accordingly, as an antenna device capable of being commonly used at the first and the second frequencies, a proposal is made about an antenna device in which a parasitic element resonating at the second frequency is additionally located in a dipole antenna resonating at the first frequency. For example, not only in unexamined Japanese utility model publication Sho62-191207, namely, 191207/1987 but also in unexamined Japanese patent publication Sho63-171004, namely, 171004/1988, disclosure is, respectively, made about an antenna device that a parasitic element consisting of a feed-less element is additionally located near a dipole antenna resonating at the first frequency, so that a resonant characteristic of the second frequency can be obtained in the antenna device.

[00010] However, the resonant characteristic of the second frequency is obtained in the antenna device by additionally locating the parasitic element, limitation is caused to occur for a position and a size of the parasitic element in the antenna device. Further, the antenna device becomes large in size by a size of the parasitic element. In view of a radiation characteristic of the dipole antenna, it is necessary for the antenna device to be separated from the adjacent metal conductor, and the like by a distance of a quarter wave-length of the first frequency approximately, and integer times as large as the first frequency in addition thereto. As a result, a space of not smaller than the wave-length of one-fourth of the first frequency is required for mounting the antenna device in the computer.

[00011] Under the circumstances, as an antenna device capable of being contained in a computer by readily obtaining impedance matching between an antenna element and a feeding system thereof, a proposal is made about an antenna device, such as a loop antenna, a folded dipole antenna, and the like, each of which is a wire antenna that an input impedance is increased by folding an antenna element.

[00012] However, in the above-mentioned antenna device that is a wire antenna, such as a loop antenna, a folded dipole antenna, and the like, a resonant frequency of the antenna device depends on an antenna length thereof. It is therefore difficult to adjust the second frequency after the first frequency has been adjusted.

[00013] Therefore, a feature of the present invention is to provide an antenna device which is capable of being commonly used at a multi-frequency operation and being contained in an electronic equipment.

[00014] Another feature of the present invention is to provide an electronic equipment having an antenna device of the type described.

[00015] Other features of the present invention will become clear as the description proceeds.

[00016] According to an aspect of the present invention, there is provided an antenna device for use in an electronic equipment, comprising: a first conductor having an electrically half wave-length of a first resonant frequency; a feed point to which an end of said first conductor is connected; a plate-shaped second conductor on which said feed point is located and on which another end of said first conductor is grounded; and an impedance element which is loaded on said first conductor and which varies at least one of said first resonant frequency and a second resonant frequency.

[00017] The impedance element may vary said first resonant frequency.

[00018] The impedance element may vary said second resonant frequency.

[00019] The impedance element may vary both said first resonant frequency and said second resonant frequency.

[00020] The first conductor may be formed to be semi-rectangular.

[00021] The first conductor may be line-shaped.

[00022] The first conductor may be belt-shaped.

[00023] The first conductor may have a primary portion elongating from said plate-shaped second conductor and a secondary portion other than said primary portion, wherein said primary portion may be formed to have a length between 0.05 and 0.10, both inclusive, of a wave-length of a first resonant frequency.

[00024] Preferably, the length of said primary portion may be between 0.07 and 0.08, both inclusive, of said wave-length of said first resonant frequency.

[00025] The impedance element may be located on said secondary portion with being offset from a center of said secondary portion towards a side of a portion on which said first conductor is grounded.

[00026] The impedance element may be a lumped capacitance or inductance element.

[00027] The lumped capacitance or inductance element may be formed to be rectangular.

[00028] The first conductor may be formed on a dielectric block having a hexahedron shape.

[00029] The first conductor may be formed on a dielectric substrate.

[00030] The first conductor may be formed on said plate-shaped second conductor as a complement pair structure.

[00031] According to another aspect of the present invention, there is also provided an electronic equipment including said antenna device, said electronic equipment transmitting information to the outside thereof and receiving information from the outside thereof by a radio communication using said antenna device.

[00068] Now, referring to FIGS. 1 through 9, description will proceed to antenna devices according to first and second embodiments of the present invention. FIG. 1 is a perspective view for schematically showing an antenna device according to the first embodiment of the present invention.

[00069] As illustrated in FIG. 1, the antenna device 10 comprises an antenna element (first conductor) 11, a feed point 12, a conductive plate (second conductor) 13, and an impedance element 14. The antenna element 11 is formed to have a shape of a semi-rectangular line. One end of the antenna element 11 is connected to the feed point 12 while another end of the antenna element 11 is grounded on the conductive plate 13 through a ground portion 11a. The feed point 12 is located on the conductive plate 13 through an insulating layer (not shown). The impedance element 14 is loaded halfway on the antenna element 11.

[00070] FIG. 2 is a perspective view for schematically showing an antenna device according to the second embodiment of the present invention. The antenna device 20 according to the second embodiment illustrated in FIG. 2 has a structure basically similar to that of the antenna device 10 illustrated in FIG. 1 except that an antenna element (first conductor) 21 is formed to have a shape of a semi-rectangular belt in the antenna device 20. Similar portions are designated by like reference numerals and explanations thereof are omitted accordingly. With the structure, the antenna device 20 also brings meritorious operations and effects similar to those of the antenna device 10 illustrated in FIG. 1. Besides, description is hereunder made as regards operations and effects of only the antenna device 10 illustrated in FIG. 1, for convenience of explanation.

[00071] Referring to FIGS. 3A and 3B with reference to FIG. 1 continued, description proceeds to the operations and the effects of the antenna device 10. As illustrated in FIG. 1, the antenna element 11 of the antenna device 10 includes not only portions elongating perpendicularly from the conductive plate 13, namely, a feeding side perpendicular portion 11b and a grounding side perpendicular portion 11c but also a portion other than the perpendicular portions 11b and 11c, namely, a parallel portion 11d. The antenna element 11 is formed on a condition that an added value of length (h) of the feeding side perpendicular portion 11b, length (h) of the grounding side perpendicular portion 11c, and length (b) of the parallel portion 11d is electrically a half length (0.5 λ₁) of a wave-length of the first resonant frequency (f1). The antenna element 11 having such a structure can be considered as depicted in FIGS. 3A and 3B, when the impedance element 14 is ignored.

[00072] Namely, as illustrated in FIG. 3A, the antenna element 11 located on the conductive plate 13 can be picked up its electrical image shown by a broken line at a symmetrical position with respect to the conductive plate 13. Further, as illustrated in FIG. 3B, it can be considered that a loop antenna 1 is composed of both the antenna element 11 and the electrical image thereof to have the perimeter of a wave-length (1λ) of the first resonant frequency (f1).

[00073] In other words, the antenna element 11 located on the conductive plate 13 becomes equal to a half of the loop antenna 1 formed by setting a conductive plain plate on a central plane perpendicular to a loop plane of the loop antenna 1 including the feed point 2. In this case, a voltage (V) of the feed point 2 is equivalently divided into a half voltage (V/2) on portions above and under the conductive plain plate, respectively. In addition, the portions above and under the conductive plain plate each having the half voltage (V/2) are connected in series to each other. At this time, an input impedance and a radiation resistance of the antenna element 11 located on the conductive plate 13, that is, a half of the loop antenna 1, as mentioned before, become one half of those of the original loop antenna 1. On the other hand, a radiation characteristic of the antenna element 11 becomes similar to that of the original loop antenna 1. The antenna device 10 can therefore be constituted so that not only an impedance matching between the antenna element 11 and the feed point 12 may be readily obtained but also the size of the antenna device 10 may be made compact without changing the radiation characteristic.

[00074] Herein, FIG. 4 is a graph for showing a relation between a normalized by wave length (h) of the feeding side perpendicular portion 11b (the grounding side perpendicular portion 11c) and an input impedance (input resistance) (Rin[Ω]) in the first and the second frequencies (f1[GHz]), (f2[GHz]), respectively, of the antenna element 11 located on the conductive plate 13. As will be clearly understood from the graph depicted in FIG. 4, an impedance matching can be readily obtained at a characteristic impedance (Z0) of 50 [Ω], when the length (h) of the feeding side perpendicular portion 11b (the grounding side perpendicular portion 11c) is between 0.05 and 0.10, both inclusive, preferably 0.07 and 0.08, both inclusive.

[00075] The impedance element (Z1=R1+jX1) 14 is, for example, a capacitance, an inductance, or the like used in an electronic circuit or a lumped capacitance or inductance element composed of an element having certain size and configuration. In this embodiment, a pure reactance element (X1) of no loss (R1=0) is used as the impedance element 14. The reactance element (X1) may be either capacitive (X1<0) and inductive (X1>0). The resonant frequency of the antenna device 10 can be made higher by loading the reactance element (X1) capacitive (X1<0). On the contrary, the resonant frequency of the antenna device 10 can be made lower by loading the reactance element (X1) inductive (X1>0). Accordingly, the antenna device 10 can obtain resonant characteristic at a desirable frequency with the impedance element 14 being optimized.

[00076] Herein, referring to FIGS. 5 and 6, description is made as regards loading position of the reactance element (X1) of the capacitance or inductance in the antenna element 11 of the antenna device 10. Further, description is also made as regards variation of an input impedance (Zin=Rin+jXin) at the first and the second frequencies (f1) and (f2), when the loading position of the reactance element (X1) of the capacitance or inductance is moved in the antenna element 11 of the antenna device 10. As illustrated in FIG. 5, each the length (h) of the feeding side perpendicular portion 11b and the grounding side perpendicular portion 11c is 0.125 λ₁ while the length (b) of the parallel portion 11d is 0.25 Ω1 in the antenna element 11.

[00077] Further, the loading position of the reactance element (X1) is moved as illustrated in FIG. 5. Namely, first, the reactance element (X1) is loaded at a position (A)[b/4] near the feed point 12, as illustrated in FIG. 5. Second, the reactance element (X1) is loaded at a central position (B)[b/2] of the parallel portion 11d. Third, the reactance element (X1) is loaded at a position (C)[3b/4] near the ground portion 11a. Subsequently, variation of the input impedance (Zin=Rin+jXin) at the positions (A), (B), and (C) illustrated in FIG. 5 are depicted in FIGS. 6A, 6B, and 6C, respectively.

[00078] In the position (A)[b/4] near the feed point 12, variation of an input impedance (Zin) at the first frequency (f1) becomes small as shown by an actual line illustrated in FIG. 6A, when the reactance element (X1) is capacitive (X1<0). On the other hand, variation of an input impedance (Rin) at the second frequency (f2) becomes large as shown by an alternate long and short dash line illustrated in FIG. 6A.

[00079] Further, variation of an input impedance (Zin) at the first frequency (f1) becomes large as shown by a dotted line illustrated in FIG. 6A, when the reactance element (X1) is inductive (X1>0). On the other hand, variation of an input impedance (Rin) at the second frequency (f2) becomes small as shown by an alternate long and two short dash line illustrated in FIG. 6A. Accordingly, variation of an input impedance (Xin) becomes gentle as shown by an alternate long and two short dash line illustrated in FIG. 6A.

[00080] At the center of the parallel portion 11d, variation of an input impedance (Zin) at the first frequency (f1) substantially keeps a certain value as shown by an actual line illustrated in FIG. 6B, when the reactance element (X1) is capacitive (X1<0). On the other hand, variation of an input impedance (Rin) at the second frequency (f2) becomes small as shown by an alternate long and short dash line illustrated in FIG. 6B. Accordingly, variation of an input impedance (Xin) becomes large as shown by an alternate long and short dash line illustrated in FIG. 6B.

[00081] Further, when the reactance element (X1) is inductive (X1>0), variation of an input impedance (Zin) at the first frequency (f1) substantially keeps a certain value as shown by a dotted line illustrated in FIG. 6B. On the other hand, variation of an input impedance (Zin) at the second frequency (f2) becomes large as shown by an alternate long and two short dash line illustrated in FIG. 6B.

[00082] In the position (C)[3b/4] near the ground portion 11a, variation of an input impedance (Rin) at the first frequency (f1) becomes gentle as shown by an actual line illustrated in FIG. 6C, when the reactance element (X1) is capacitive (X1<0). On the other hand, variation of an input impedance (Rin) at the second frequency (f2) becomes small as shown by an alternate long and short dash line illustrated in FIG. 6C. Accordingly, variation of an input impedance (Xin) becomes large as shown by an alternate long and short dash line illustrated in FIG. 6C.

[00083] Further, when the reactance element (X1) is inductive (X1>0), variation of an input impedance (Rin) at the first frequency (f1) substantially keeps a certain value as shown by an actual line illustrated in FIG. 6C. Accordingly, variation of an input impedance (Xin) becomes gentle as shown by a dotted line illustrated in FIG. 6C. On the other hand, variation of an input impedance (Zin) at the second frequency (f2) becomes large as shown by an alternate long and two short dash line illustrated in FIG. 6C.

[00084] In the interim, it is required not only that variation of an input impedance (Rin) is small but also that variation of an input impedance (Xin) is large in order that the resonant frequency may be adjustable. Accordingly, in order that the second resonant frequency (f2) may be adjustable, it is necessary that the reactance element (X1<0) is loaded between the central position (B)[b/2] of the parallel portion 11d and the position (C)[3b/4] near the ground portion 11a.

[00085] Besides, in a case that the impedance element 14 is a lumped capacitance or inductance element, the impedance element 14 is basically located on the antenna element 11 with the impedance element 14 being perpendicular to a principal surface of the conductive plate 13, as illustrated in FIG. 1. Alternatively, the impedance element 14 of a lumped capacitance or inductance element may be located on the antenna element 11 with the impedance element 14 being parallel to a principal surface of the conductive plate 13, as illustrated in FIG. 7. Further, the impedance element 14 of a lumped capacitance or inductance element may be located on the antenna element 11 with the impedance element 14 being inclined to a principal surface of the conductive plate 13 at a predetermined angle α, that is, between 0° and 90° (0[°]<α [°]<90[°]), as illustrated in FIG. 8.

[00086] Herein, description is made about operational effects, in a case that the impedance element 14 of a lumped capacitance or inductance element is located in parallel to the principal surface of the conductive plate 13, as illustrated in FIG. 7, in a case that the impedance element 14 is located in perpendicular to the principal surface of the conductive plate 13, as illustrated in FIG. 1, respectively. FIGS. 9A through 9D show a relation between the frequency (f[GHz]) and the return loss (RL[dB]), when a distance (S1), namely, a distance from the feed point 12 to a loading position of the impedance element 14 is varied.

[00087] As will be understood from FIGS. 9A through 9D, even though the impedance element 14 of a lumped capacitance or inductance element is located in parallel or perpendicular to the principal surface of the conductive plate 13, the second resonant frequency (f2) can be adjustable by loading the reactance element (X1) capacitive (X1<0) between the central position (B)[b/2] and the position (C)[3b/4] near the ground portion 11a.

[00088] Next, referring to FIGS. 10 through 12, description will proceed to antenna devices according to third, fourth, and fifth embodiments of the present invention.

[00089] FIG. 10 is a perspective view for schematically showing an antenna device according to the third embodiment of the present invention.

[00090] As illustrated in FIG. 10, the antenna device 30 comprises an antenna element (first conductor) 31, a feed point 32, a conductive plate (second conductor) 33, an impedance element 34, and a dielectric block 35. The antenna element 31 is formed to have a shape of a semi-rectangular line, similarly to the antenna element 11 of the first embodiment. The antenna element 31 also includes a ground portion 31a, a feeding side perpendicular portion 31b, a grounding side perpendicular portion 31c, and a parallel portion 31d. The feed point 32 is located an end of the conductive plate 33. The impedance element 34 is similar to the impedance element 14 of the first embodiment, namely, a capacitance, an inductance, or the like used in an electronic circuit or a lumped capacitance or inductance element composed of an element having certain size and configuration. The dielectric block 35 is formed to have a hexahedron shape. In this embodiment, the antenna element 31 is formed on an upper surface and side surfaces opposite to each other of the dielectric block 35. Namely, the parallel portion 31d of the antenna element 31 is formed on an upper surface of the dielectric block 35 while the feeding side perpendicular portion 31b and the grounding side perpendicular portion 31c are formed on side surfaces of the dielectric block 35. The impedance element 34 is formed on the upper surface of the dielectric block 35 to be located halfway on the antenna element 31. With the structure, the dielectric block 35 is mounted on the conductive plate 33. Further, the feeding side perpendicular portion 31b is connected to the feed point 32 by way of a feed line formed on the conductive plate 33, for example a microstrip line 33a. On the other hand, the grounding side perpendicular portion 31c is grounded on the conductive plate 33 through the ground portion 31a.

[00091] FIG. 11 is a perspective view for schematically showing an antenna device according to the fourth embodiment of the present invention. The antenna device 40 according to the fourth embodiment illustrated in FIG. 11 has a structure basically similar to that of the antenna device 30 illustrated in FIG. 10 except that, in spite of the above-mentioned dielectric block 35, the antenna device 40 has a rectangular dielectric substrate 45 having both ends folded substantially perpendicular thereto and that an antenna element (first conductor) 41 and an impedance element 44 are formed on an upper surface and side surfaces opposite to each other of the rectangular dielectric substrate 45. With the structure, the rectangular dielectric substrate 45 is mounted on the conductive plate 43. Further, the feeding side perpendicular portion 41b is connected to the feed point 42 by way of a feed line formed on the conductive plate 43, for example a microstrip line 43a. On the other hand, the grounding side perpendicular portion 41c is grounded on the conductive plate 43 through the ground portion 41a.

[00092] FIG. 12 is a view for schematically showing an antenna device according to the fifth embodiment of the present invention. The antenna device 50 according to the fifth embodiment illustrated in FIG. 12 has, what is called, a complement pair structure. Namely, the antenna device 50 comprises a conductive plate 53, an antenna element 51 which is composed of a slit having a shape of a semi-rectangular line formed in the conductive plate 53, and the impedance element 54 which is composed of a cut portion formed in the conductive plate 53, as illustrated in FIG. 12. Besides, the antenna device 50 further comprises a feed point 52. The antenna element 51 of a slit includes three slit portions which function as a feeding side perpendicular portion 51b, a grounding side perpendicular portion 51c, and a parallel portion 51d, respectively. Further, the feed point 52 is connected to both edges of the slit portion functioning as the feeding side perpendicular portion 51b.

[00093] Referring to FIGS. 13 through 20, description will proceed to variations of a configuration of the impedance element 14 (24, 34, 44, and 54) of a lumped capacitance or inductance element.

[00094] As illustrated in FIG. 1, the impedance element 14 basically has a rectangular configuration having length(x)×width(y). Alternatively, variations of the configuration illustrated in FIGS. 13 through 20 can be applied to the impedance element 14 (24, 34, 44, and 54) of a lumped capacitance or inductance element.

[00095] Namely, as illustrated in FIG. 13, an impedance element 141 has a rectangular configuration which has length(x)×width(y) and which is offset from the antenna element 11 by a predetermined length (x/2).

[00096] Next, an impedance element 142 illustrated in FIG. 14 has a trapezoidal configuration which has a taper of an angle (□). Further, an impedance element 143 illustrated in FIG. 15 has a circular configuration which has a radius of (r). Moreover, an impedance element 144 illustrated in FIG. 16 has a configuration which has a bump including width (w1) and width (w2) as well as length (l1) and length (l2). Furthermore, an impedance element 145 illustrated in FIG. 17 is composed of an antenna element 11 itself having a bump including width (w3) and width (w4).

[00097] On the other hand, an impedance element 146 illustrated in FIG. 18 is composed of an antenna element 11 itself cut halfway thereon and having a gap (g) between both the cut portions thereof. Further, an impedance element 147 illustrated in FIG. 19A (a plan view) and FIG. 19B (a side view) is composed of an antenna element 11 itself cut halfway thereon and both the cut portions thereof are partially overlapped on each other. The impedance element 147 can therefore be realized, for example, by a substrate of a stacked-layer structure. Moreover, an impedance element 148 illustrated in FIG. 20 has a box-like three-dimensional configuration. The impedance element 148 can be realized, for example, by folding both ends of a rectangular substrate so that the both ends may be perpendicular to the rectangular substrate.

[00098] Referring to FIGS. 21 through 29, description will proceed to variations of a location of the antenna device 10 (20, 30, 40, and 50).

[00099] As illustrated in FIG. 1, in the basic location of the antenna device 10, the antenna element 11 stands on the principal surface of the conductive plate 13. Alternatively, variations of the location illustrated in FIGS. 21 through 29 can be applied to the antenna device 10 (20, 30, 40, and 50).

[00100] Namely, in the antenna device 10 illustrated in FIG. 21, the antenna element 11 is located on an end of the conductive plate 13 to be elongated horizontally to the principal surface of the conductive plate 13.

[00101] Next, in the antenna device 10 illustrated in FIG. 22, the antenna element 11 is located on the end of the conductive plate 13 to be inclined to the principal surface of the conductive plate 13 at a predetermined angle θ1, that is, between 0° and 180° (0[°]<θ1[°]<180[°]). Further, in the antenna device 10 illustrated in FIG. 23, the antenna element 11 is located on a corner of the conductive plate 13 to be elongated horizontally to the principal surface of the conductive plate 13 with the antenna element 11 being folded as depicted in FIG. 23. Moreover, in the antenna device 10 illustrated in FIG. 24, the antenna element 11 is located on the corner of the conductive plate 13 to be inclined to the principal surface of the conductive plate 13 at a predetermined angle θ2, that is, between 0° and 180° (0[°]<θ2[°]<180[°]). Furthermore, in the antenna device 10 illustrated in FIG. 25A (a perspective view) and FIG. 25B (a side view), the antenna element 11 is located on the conductive plate 13 to be once elongated therefrom horizontally and then folded perpendicularly to the principal surface of the conductive plate 13.

[00102] On the other hand, in the antenna device 10 illustrated in FIG. 26, the conductive plate 13 further comprises two conductive plates 13A and 13B which are perpendicular to each other. With the structure, the antenna element 11 is located obliquely between ends of the two conductive plates 13A and 13B, as illustrated in FIG. 26. Further, in the antenna device 10 illustrated in FIG. 27, the conductive plate 13 further comprises two conductive plates 13A and 13B which are perpendicular to each other, similarly to the antenna device 10 illustrated in FIG. 26. With the structure, the antenna element 11 is located between ends of the two conductive plates 13A and 13B with a center of the antenna element 11 being substantially folded vertically, as illustrated in FIG. 27. Moreover, a plurality of antenna devices 10 may be located in parallel on the principal surface of the conductive plate 13 at predetermined pitches, as illustrated in FIG. 28. Furthermore, a plurality of antenna devices 10 may be located in series on the principal surface of the conductive plate 13 at predetermined pitches, as illustrated in FIG. 29.

[00103] Referring to FIGS. 30 through 34, description will proceed to variations of a configuration of the antenna element 11 (21, 31, 41, and 51).

[00104] As illustrated in FIG. 1, in the basic configuration, the antenna element 11 is formed on a condition that an added value of length (h) of the feeding side perpendicular portion 11b, length (h) of the grounding side perpendicular portion 11c, and length (b) of the parallel portion 11d is electrically a half length (0.5 λ₁) of a wave-length of the first resonant frequency (f1). Alternatively, variations of the configuration illustrated in FIGS. 30 through 34 can be applied to the antenna element 11 (21, 31, 41, and 51).

[00105] Namely, the antenna element 111 illustrated in FIG. 30 is formed to include an inclined portion. Namely, the antenna element 111 comprises the feeding side perpendicular portion 111b having a length (h1), the grounding side perpendicular portion 111c having a length (h2,[h2<h1]), and the inclined portion 111d having a length (b1). With the structure, the antenna element 111 illustrated in FIG. 30 is formed on a condition that an added value of length (h1) of the feeding side perpendicular portion 111b, length (h2) of the grounding side perpendicular portion 111c, and length (b1) of the inclined portion 111d is electrically a half length (0.5 λ₁) of a wave-length of the first resonant frequency (f1). Next, the antenna element 112 illustrated in FIG. 31 is formed to be arc-shaped. Namely, the antenna element 112 comprises the feeding side arc portion 112b and the grounding side arc portion 112c. With the structure, the antenna element 112 illustrated in FIG. 31 is formed on a condition that an added value (1) of a length of the feeding side arc portion 112b and a length of the grounding side arc portion 112c is electrically a half length (0.5 λ₁) of a wave-length of the first resonant frequency (f1).

[00106] Further, the antenna element 113 is formed to have a bump, as illustrated in FIG. 32. Namely, the antenna element 113 includes the feeding side perpendicular portion 113b having a length (h3), the grounding side bump portion 113c which includes an upper stage having a length (h4) and a lower stage having a length (h5), the parallel portion 113d which includes an upper stage having a length (b2) and a lower stage having a length (b3). With the structure, the antenna element 113 illustrated in FIG. 32 is formed on a condition that an added value of length (h3) of the feeding side perpendicular portion 113b, length (h4) of the upper stage of the grounding side bump portion 113c, length (h5) of the lower stage of the grounding side bump portion 113c, length (b2) of the upper stage of the parallel portion 113d, and length (b3) of the lower stage of the parallel portion 113d is electrically a half length (0.5 λ₁) of a wave-length of the first resonant frequency (f1). In addition, the impedance element 14 is loaded on the upper stage of the parallel portion 113d.

[00107] On the other hand, the antenna element 114 illustrated in FIG. 33 has a structure similar to that of the antenna element 113 illustrated in FIG. 32 except that the impedance element 14 is loaded on the lower stage of the parallel portion 114d. Further, the antenna element 115 illustrated in FIG. 34 has a structure similar to that of the antenna element 113 illustrated in FIG. 32 except that the impedance elements 14 are loaded on the upper and the lower stages of the parallel portion 115d, respectively.

[00108] FIGS. 35A and 35B show a relation between the frequency (f[GHz]) and the return loss (RL[dB]), when the length (h2) of the grounding side perpendicular portion 111c of the antenna element 111 illustrated in FIG. 30, and the length (b1) of the inclined portion hid of the antenna element 111 illustrated in FIG. 30 are varied, respectively. As will be understood from FIGS. 35A and 35B, an input impedance can be adjusted by changing the length (h2) of the grounding side perpendicular portion 111c and the length (b1) of the inclined portion 11d.

[00109] FIGS. 36A and 36B show a relation between the frequency (f[GHz]) and the return loss (RL[dB]), when the length (b2) of the upper stage, the length (b3) of the lower stage of the parallel portion 113d of the antenna element 113 illustrated in FIG. 32 are varied, respectively. As will be understood from FIGS. 36A and 36B, an input impedance can be adjusted by changing the length (b2) of the upper stage of the parallel portion 113d and the length (b3) of the lower stage thereof.

[00110] As described above, according to the present invention, the antenna element 11(first conductor) located on the conductive plate 13 (second conductor) can be picked up its electrical image at a symmetrical position with respect to the conductive plate 13 (second conductor). Further, it can be considered that a loop antenna 1 is composed of both the antenna element 11 (first conductor) and the electrical image thereof to have the perimeter of a wave-length (1λ) of the first resonant frequency (f1). The antenna element 11(first conductor) can be resonated at a desirable second resonant frequency (f2) by loading a predetermined impedance element halfway on the antenna element 11(first conductor). Accordingly, a compact antenna device 10 can therefore be constituted so that an impedance matching between the antenna element 11 (first conductor) and the feed point 12 may be readily obtained. In addition, the antenna device 10 can be commonly used with respect to a plurality of frequencies.

[00111] While this invention has thus far been described in conjunction with several embodiments thereof, it will now be readily possible for one skilled in the art to put this invention into effect in various other manners. For example, in the embodiments mentioned above, description was made about a case that the antenna device was incorporated in a computer. However, the present invention is not restricted to such a case. The present invention can be applied to an electronic equipment capable of communication, such as a portable telephone, PDA (Personal Digital Assistants), and the like.