Antenna device

Abstract

To provide an antenna device for a vehicle capable of reducing a height from a mounting surface to 50 mm or less.SOLUTION: There is provided an antenna device 1 containing an antenna element for receiving signals of an FM band and an AM band in an antenna accommodating body. The antenna element includes a first element for receiving a signal in the FM band and a second element for receiving a signal in the AM band around the first element. Both of the elements have capacity plates (conductive plates) of a predetermined area, which are arranged in the same or substantially the same plane. Each end of the first element includes a plurality of coils 14a and 14b whose one ends are connected to the capacity plate 12 of the first element, and these coils operate together with the capacity plate 12 as a series resonance circuit in the FM band.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a low-profile antenna device that can be mounted on, for example, a vehicle body. [Background technology]

[0002] As an FM band and AM band antenna device that can be attached to a vehicle body, for example, there are antenna devices disclosed in Patent Document 1 and Patent Document 2. The antenna device disclosed in Patent Document 1 has an antenna base and an antenna element composed of two types of helical parts in a shark fin-shaped antenna case. The two types of helical parts have a first helical part on the side closer to the antenna base and a second helical part on the side farther from the antenna base. The first helical part is composed of a line pattern or a plate-shaped conductive material. On the other hand, the second helical part has a larger surface area per unit length than the first helical part, and is composed of a line-shaped, solid pattern, a solid pattern and wire, or a plate-shaped conductive material bent into an approximately U-shape (a horizontally long spiral-shaped element).

[0003] In addition, in the antenna device disclosed in Patent Document 2, the antenna element is composed of a spiral antenna element and a plate-shaped element. The antenna element is wound around an imaginary axis extending from the antenna base toward the top of the vehicle antenna device. The plate-shaped element is a conductive plate, and is disposed on the open end side of the spiral antenna element so as to cover the top while being electrically connected, and to be positioned so as to intersect perpendicularly or obliquely with the imaginary axis. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2012-161075 A [Patent Document 2] JP 2013-106146 A Summary of the Invention [Problem to be solved by the invention]

[0005] The antenna device disclosed in Patent Document 1 is aimed at making the entire antenna element function as an antenna efficiently within a limited space. However, in such an antenna device, two types of helical parts are provided at a certain interval in the height direction. In particular, when the second helical part is made of a plate-shaped conductive material, the surface part is erected on the antenna base, resulting in a so-called vertical structure. Therefore, there is a limit to how low the height can be reduced, and the height can only be reduced to about 70 mm.

[0006] The antenna device disclosed in Patent Document 2 is able to ensure a nearly constant antenna gain over a wide band despite its low profile, due to the effect of the plate-like element attached to the tip of the antenna element. However, since this antenna device is composed of a single antenna element and a plate-like element, there is a limit to how high the antenna gain can be. In addition, it is said that the antenna height can only be reduced to about 50 mm to 70 mm.

[0007] In view of the above problems, the present invention has an object to provide an antenna device having a structure that can maintain antenna gain and other antenna performance equivalent to those of conventional antenna devices even when the device is made low-profile with a height of 50 mm or less. [Means for solving the problem]

[0008] The antenna device of the present invention comprises an antenna element that receives signals in a first frequency band and a second frequency band that is lower than the first frequency band, and is housed in an antenna housing that is approximately 15 mm high from a mounting surface. The antenna element includes a first element that receives signals in the first frequency band, and a second element that receives signals in the second frequency band around the first element, and the first element and the second element each have a conductor plate with a predetermined area, and these conductor plates are disposed in the same or approximately the same plane. The first element further includes a plurality of reactance elements each having one end connected to the conductor plate of the first element, and these reactance elements operate as a series resonant circuit together with the conductor plate in the first frequency band. Effect of the Invention

[0009] Since the conductor plate of the first element and the conductor plate of the second element are arranged on the same or approximately the same plane, there are no protruding parts, making it easy to reduce the height of the antenna element. In addition, since multiple reactance elements are connected to the conductor plate of the first element and operate as a series resonant circuit in the first frequency band, the VSWR is better than when one reactance element is used, and the radiation efficiency is improved. [Brief description of the drawings]

[0010] [Figure 1] 1 is an external perspective view of an antenna device according to a first embodiment. [Diagram 2] (a) is a top view of the antenna element, and (b) to (e) are side views. [Diagram 3] FIG. 2 is an exploded perspective view of the antenna device according to the first embodiment. [Figure 4] FIG. 4 is a top view showing the positional relationship between the outer plate and the capacitive plate in the first embodiment. [Diagram 5] (a) is a schematic diagram showing the positional relationship between the capacitive plate and each coil, and (b) is a schematic diagram. [Figure 6] (a) is a schematic diagram showing the positional relationship between the reference plate and the reference coil of the reference antenna, and (b) is a schematic diagram. [Figure 7] FIG. 4 is a diagram showing radiation efficiency characteristics in the FM band in the first embodiment. [Figure 8] (a) is a diagram of FM band directional characteristics, and (b) is a diagram of AM band directional characteristics. [Figure 9] 10A is a top view of an antenna element according to a second embodiment, and FIG. 10B is a schematic diagram showing an example of the structure thereof. [Figure 10]FIG. 13A is a schematic diagram of the structure of an FM antenna in a third embodiment, and FIG. 13B is a schematic diagram of the structure of an FM antenna in a comparative example. [Figure 11] FIG. 11 is a diagram showing radiation efficiency characteristics in the FM band in the third embodiment. [Figure 12] 13A and 13B are schematic diagrams of an FM element according to a fourth embodiment. [Figure 13] FIG. 13 is a diagram showing radiation efficiency characteristics in the FM band in the fourth embodiment. [Figure 14] 13A is a top view of an antenna element in a fifth embodiment, and FIG. 13B is a schematic diagram of the antenna element. [Figure 15] FIG. 13A is a schematic diagram of an FM element in a sixth embodiment, and FIG. [Figure 16] FIG. 13 is a diagram showing radiation efficiency characteristics in the FM band in the sixth embodiment. [Figure 17] FIG. 13A is a schematic diagram of an FM element in the seventh embodiment, and FIG. [Figure 18] FIG. 13 is a diagram showing radiation efficiency characteristics in the FM band in the seventh embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] [First embodiment] In the first embodiment, an example will be described in which the present invention is applied to a low-profile antenna device that can be used in the VHF band, for example, the FM band (76 MHz to 90 MHz) and the MF band, for example, the AM band (0.520 MHz to 1.710 MHz). This antenna device is configured by housing an antenna element in an antenna case, which is an example of an antenna housing, and is used by being attached to, for example, the roof of a vehicle.

[0012] Fig. 1 is an external perspective view of the antenna device 1 according to the first embodiment. Fig. 2(a) is a top view of the antenna device 1, (b) to (e) are side views, and Fig. 3 is an exploded perspective view of the antenna device 1. Referring to these figures, the height of the antenna case of the antenna device 1 is 15 mm to 12 mm from the mounting surface of the vehicle that is at ground potential. The antenna case is configured to include a radio wave permeable cover part 10 and a resin base part 30. The cover part 10 is a cylindrical shape with a bottom having an opening surface, and its inner wall (bottom) is formed to be flat or approximately flat.

[0013] The antenna element housed in the antenna case includes two conductor plates, each with a specified area, and two reactance elements. One of the conductor plates is used primarily for receiving FM band signals, and the opposing surface faces the surface at ground potential, i.e., the mounting surface of the vehicle. As a result, the electrostatic capacitance generated between the plate and the mounting surface is loaded onto the plate itself (capacitive loading). Therefore, hereinafter, this conductor plate will be referred to as the "capacitive plate." The other conductor plate is used for receiving AM band signals, and is disposed outside the capacitive plate. Therefore, hereinafter, the outer conductor plate will be referred to as the "outer plate." Capacitive plate 12 has a dimension of 14,850 [mm 2 The outer plate 11 is a rectangular conductive plate having an area of ​​5700 mm 2 ] (= (15 [mm] × 150 [mm]) + (10 [mm] × 120 [mm]) + (15 [mm] × 150 [mm])). Outer plate 11 and capacitive plate 12 are fixed to the inner wall of cover part 10. In other words, outer plate 11 and capacitive plate 12 of the antenna element are disposed in the same or approximately the same plane. In the example of FIG. 3, outer plate 11 and capacitive plate 12 each have a plurality of holes, but these are screw holes or guide holes for positioning and have almost no effect on the electrical characteristics.

[0014] In this embodiment, both of the two reactance elements are linear conductors wound in a helical shape. In the first embodiment, linear conductors held by resin holders 13a and 13b fixed to the inner wall of the cover portion 10 with the capacitance plate 12 sandwiched therebetween are used as the reactance elements. That is, as an example of a reactance element, a linear conductor may be wound around a dielectric to achieve miniaturization, but the first embodiment shows an example in which the reactance element is composed of only a linear conductor. Therefore, for convenience of explanation, the two reactance elements are each referred to as a "coil".

[0015] The first coil 14a is wound around and held on the surface of the first holder 13a. The second coil 14b is wound around and held on the surface of the second holder 13b. The first coil 14a has one end connected to a first end of the capacitive plate 12, and the other end connected to a power supply point. The second coil 14b has one end connected to a second end of the capacitive plate 12 different from the first end, and the other end connected to a ground conductor. These connection forms will be described in detail later.

[0016] The outer plate 11 can receive radio waves of various frequencies. In the first embodiment, the outer plate 11 is used to receive signals in the AM band (AM signals). That is, the outer plate 11 itself constitutes an AM element. The AM signals received by the outer plate 11 are guided to a printed circuit board (Printed Circuit Board) 16 (described later) through a power supply section 111 at the end of the outer plate 11. On the other hand, the capacitive plate 12 is connected to the first coil 14a and the second coil 14b, thereby enabling it to receive signals in the FM band (FM signals). That is, the capacitive plate 12 and the two coils 14a and 14b constitute an FM element that resonates in the FM band. The received FM band signals are guided to the printed circuit board 16 through a power supply point to which the first coil 14a is connected.

[0017] The printed circuit board 16 is disposed below the first holders 13a and 13b. An electronic circuit is mounted on the printed circuit board 16. The electronic circuit includes, for example, a first input terminal for inputting an AM signal received by the outer plate 11, and a second input terminal that is electrically connected to the power supply point of the first coil 14a. The electronic circuit also includes an AM amplifier circuit for amplifying the AM signal input from the first input terminal, and an FM amplifier circuit for amplifying the FM signal input from the second input terminal. The electronic circuit further includes output terminals for outputting the AM signal amplified by the AM amplifier circuit and the FM signal amplified by the FM amplifier circuit. A synthesis circuit for synthesizing the AM signal and the FM signal may be provided in front of the output terminal. A filter, a tuning circuit, or the like may be provided in front of the AM amplifier circuit. A GND pattern that is electrically connected to the ground terminals of the amplifier circuits and the like is also formed on the printed circuit board 16. A pair of metallic GND terminals 15a, 15b are fixed to the GND pattern. The GND terminals 15a, 15b are components that are electrically connected to a metallic conductive base 19. A cable holder 17 that holds a signal cable electrically connected to the first output terminal and the second output terminal is fixed to the back surface of the printed circuit board 16.

[0018] The resin base portion 30 is integrally formed with a frame 301 that protrudes upward from a position slightly inside the outer periphery, and a bottom portion 302 that is surrounded by this frame 301. The frame 301 is formed to be approximately the same size as the opening surface of the cover portion 10. The frame 301 has an outer surface, and a groove is formed on the outer surface around the entire periphery. An O-ring 20 made of an elastic member is fitted into this groove. The depth of the groove is shallower than the outer diameter of the O-ring 20. Therefore, when the cover portion 10 is fitted onto the resin base portion 30, the O-ring 20 watertightly seals the space inside the bottom portion 302.

[0019] A recess 303 for accommodating and fixing the printed circuit board 16 and a hole 304 for allowing the pre-lock 18 and the conductive base 19 to protrude downward are formed in the bottom 302 of the resin base portion 30. The pre-lock 18 is a part for temporarily fixing the antenna device 1 when it is attached to a vehicle roof or the like. A fixing base 305 for fixing the pre-lock 18 and the conductive base 19 with screws is fixed to the bottom 302. The conductive base 19 firmly fixes the antenna device 1 to a vehicle roof or the like, and also sets the GND pattern of the printed circuit board 16 to the ground potential through the GND terminals 15a and 15b when it is attached.

[0020] The positional relationship between the outer plate 11 and the capacitive plate 12 is shown in Figure 4. The multiple holes shown in Figure 3 have been omitted. Referring to Figure 4, the outer plate 11 surrounds approximately 3 / 4 of the outer periphery of the capacitive plate 12, and is arranged with a predetermined gap so that their opposing ends do not overlap. As described above, these are arranged in the same or approximately the same plane, so there are no protruding parts, which simplifies the appearance of the cover part 10 and contributes to reducing the height of the antenna device 1. Since the opposing ends of the outer plate 11 and the capacitive plate 12 are spaced apart and do not overlap, no interference occurs.

[0021] The antenna device 1 of the first embodiment has a feature in the configuration of the antenna element, especially the FM element, which will be described in detail below. Fig. 5(a) is a schematic diagram showing the positional relationship between the capacitive plate 12 and the first and second coils 14a and 14b of the antenna device 1, and Fig. 5(b) is a schematic diagram thereof. The printed circuit board 16 is similar in shape to the capacitive plate 12 and slightly larger in size, but the difference in size is not a big problem. The ground conductor GND shown in Fig. 5(b) is a part of the vehicle roof that is electrically connected to the GND terminals 15a, 15b and the conductive base 19 shown in Fig. 3. The first holder 13a, the second holder 13b, and other parts are omitted for convenience.

[0022] The size of the capacitive plate 12 is as described above, and the height from the ground conductor GND is about 10 [mm]. The first coil 14a and the second coil 14b each have a major axis and a minor axis that are approximately half the size of the capacitive plate 12 (105 [mm] × 70 [mm]), are wound at a predetermined winding pitch, and have the same inductance value. They are spaced apart from each other by about 5 [mm] and are arranged without overlapping. The major axis and minor axis or the shape and size of the first coil 14a and the second coil 14b are not limited to the above example and may be changed as desired depending on the installation space, etc. The same applies to the separation distance between the first coil 14a and the second coil 14b.

[0023] One end 141a of the first coil 14a is connected to a first end of the capacitive plate 12, and the other end 142a is connected to a power feed point 50 via a wiring pattern of the printed circuit board 16. One end 141b of the second coil 14b is connected to a second end (opposite end of the first end) of the capacitive plate 12 different from the first end, and the other end 142b is connected to a ground conductor GND via a GND pattern of the printed circuit board 16. This allows the first coil 14a and the second coil 14b to operate as a series resonant circuit together with the capacitive plate 12 in the FM band. That is, the electrical length from the other end 142a of the first coil 14a through the capacitive plate 12 to the other end 142b of the second coil 14b is the resonant length in the FM band (electrical length of 1 / 2 the wavelength λ of the frequency used in the FM band, the same below). An FM signal can be taken out from the power feed point 50.

[0024] The present inventors created a reference antenna for comparing electrical characteristics. The reference antenna has a reference plate made of the same material and with the same area as the capacitive plate 12 of the antenna device 1, and one reference coil made of the same wire material and with the same wire diameter as the first coils 14a and 14b, and with an outer diameter that is the same as the area defined by the sum of the outer diameters of the first coils 14a and 14b. FIG. 6(a) is a schematic diagram showing the positional relationship between a reference plate (a conductor plate corresponding to the capacitive plate 12) 61R and a reference coil (reactance element) 64R of the reference antenna 1R, and FIG. 6(b) is a schematic diagram. In FIG. 6(a), the reference plate 61R is shown in a transparent manner for convenience. One end 641R of the reference coil 64R is connected to the end of the reference plate 61R, and the other end 642R is connected to the power supply point 50. The material and size ratio of the printed circuit board 66R, the connection state between the reference coil 64R and the power supply point 50, the distance between the ground conductor GND and the base end and tip end of the reference coil 64R, and the distance between the tip end of the reference coil 64R and the underside of the reference plate 61R are all the same as those of the antenna device 1.

[0025] In this reference antenna 1R, if the current flowing through the reference coil 64R is I1, the antenna impedance is Z1, and the radiated power (synonymous with received power, the same applies below) is P1, then P1 is Z1×I1 2 The antenna impedance is, for example, an impedance value on the real axis when using a Smith chart, and the closer it is to the power supply impedance (50[Ω] in this embodiment), the higher the radiation efficiency (synonymous with reception efficiency, the same applies below) and the larger the power. According to a simulation experiment by the present inventors, the antenna impedance of the reference antenna 1R was 0.06Ω. In contrast, in the FM antenna of the antenna device 1 of the first embodiment, if the radiation power P2 is the same as the radiation power P1 of the reference antenna 1R and the current flowing through the first coil 14a and the second coil 14b is I2, each current I2 is 1 / 2 of the current I1. Therefore, the antenna impedance Z2 is four times Z1. In other words, for the reference antenna 1R with one coil, the antenna impedance increases in proportion to the square of the number of coils. The inventors have confirmed that the antenna impedance of the FM antenna of the first embodiment increases to 0.23Ω, which is four times the antenna impedance of the reference antenna 1R.

[0026] FIG. 7 is a radiation efficiency characteristic diagram in the FM band. In the figure, the solid line indicates the radiation efficiency in the FM band of the antenna device 1 according to the first embodiment, and the dashed line indicates the radiation efficiency of the reference antenna 1R. In the Japanese FM band shown by the thick line, the radiation efficiency was -25.2 [dB] on average for the reference antenna 1R, whereas the FM antenna according to the first embodiment had an average of -19.6 [dB]. In this way, by increasing the number of coils connected to the capacitance plate 12 and increasing the antenna impedance, the reception gain and radiation efficiency in the FM band were significantly improved. Although not shown in the figure, the average radiation efficiency in the AM band was -70.0 [dB].

[0027] 8(a) is a diagram showing the directional characteristics of the antenna device 1 of the first embodiment in the horizontal plane of vertical polarization in the FM band, and (b) is a diagram showing the directional characteristics of the antenna device 1 of the first embodiment in the horizontal plane of vertical polarization in the AM band. As is clear from these characteristic diagrams, the antenna device 1 of the first embodiment is omnidirectional in the horizontal plane of vertical polarization in the FM band, and is also omnidirectional in the horizontal plane of vertical polarization in the AM band.

[0028] [Second embodiment] Next, a second embodiment of the present invention will be described. In the second embodiment, the outer plate 11 and the capacitive plate 12 of the antenna device 1 of the first embodiment are modified in shape and layout without changing the material and thickness of these. The structures of other parts including the first coils 14a and 14b are the same as those of the first embodiment, so the same names and symbols are used and descriptions of overlapping parts are omitted. FIG. 9(a) is a top view of an antenna element included in an antenna device 2 according to a second embodiment, and FIG. 9(b) is a schematic diagram showing the structure of this antenna element. In FIG. 9(b), the capacitive plate is shown in a transparent manner. The antenna device 2 of the second embodiment comprises a rectangular capacitive plate 22 and a rectangular annular outer plate 21 that surrounds the entire periphery of the capacitive plate 22 on the same plane or approximately the same plane. The outer plate 21 and the capacitive plate 22 are arranged with a gap of about 5 mm between them so that their opposing ends do not overlap. The area of ​​the capacitive plate 22 is 14,400 mm 2 ] (= 120 [mm] × 120 [mm]). The area of ​​the outer plate 21 is 5600 [mm 2 ] (=(10 [mm] × 150 [mm]) + (10 [mm] × 130 [mm]) + (10 [mm] × 150 [mm]) + (10 [mm] × 130 [mm])). The distances between the ground conductor GND and the outer plate 21 and the capacitive plate 22 are the same as those in the antenna device 1 of the first embodiment. The resin base portion 230 is slightly larger in size than the outer plate 21.

[0029] The AM signal received by the outer plate 21 is guided through the power supply section 211 at its end to the electronic circuit of the printed circuit board 26 on the resin base 230. In other words, the outer plate 21 operates as an AM element, similar to the first embodiment. One end 141a of the first coil 14a is connected to a first end of the capacitive plate 22, and the other end 142a is connected to a power feed point 50 via a wiring pattern of the printed circuit board 26. One end 141b of the second coil 14b is connected to a second end (an end opposite to the first end) of the capacitive plate 22 which is different from the first end, and the other end 142b is connected to a ground conductor GND via a GND pattern of the printed circuit board 26. As a result, the first coil 14a and the second coil 14b operate as a series resonant circuit together with the capacitive plate 22 in the FM band, similarly to the first embodiment. An FM signal can be extracted from the power feed point 50.

[0030] According to actual measurements by the inventors, the average radiation efficiency and the directivity in the vertically polarized horizontal plane in the Japanese FM band were almost the same as those of the antenna device 1 of the first embodiment. The antenna impedance was also almost the same as that of the first embodiment. In other words, the radiation efficiency and the like in the FM band were almost the same as those of the antenna device 1 of the first embodiment. In the AM band, the directivity in the vertically polarized horizontal plane was the same as that of the antenna device 1 of the first embodiment, and the radiation efficiency was also equivalent to that of the antenna device 1 of the first embodiment.

[0031] In this way, in the antenna device 2 of the second embodiment, the entire periphery of the rectangular capacitive plate 22 is surrounded by the rectangular annular outer plate 21 on the same plane or approximately the same plane, and the AM band radiation efficiency can be equivalent to that of the antenna device 1 of the first embodiment. Also, once the shape and size (area) of the outer plate 11 are determined, the capacitive plate 22 can be simply formed by punching or the like, which has the effect of simplifying the manufacturing process.

[0032] Alternatively, the outer plate 21 may be formed so that a part or all of its outer edge becomes lower toward the outer periphery without changing the area of ​​the outer plate 21. In this case, since a part of the height of the outer plate 21 becomes lower, the radiation efficiency in the AM band becomes slightly lower, but this is not a big problem in practical use. Such a configuration has an advantage that, for example, the cover part 10 of the antenna device 1 of the first embodiment can be made smaller.

[0033] [Third embodiment] Next, a third embodiment of the present invention will be described. In the antenna device 3 of the third embodiment, the winding directions of the adjacent first coil 34a and second coil 34b are reversed in an FM element including a conductor plate of a predetermined area and two reactance elements. The structures of the outer plate 11, the capacitive plate 12, and other components are the same as those of the first embodiment, so the same names and symbols are used, and descriptions of overlapping parts are omitted.

[0034] 10(a) is a diagram showing a schematic structure of an FM antenna in an antenna device 3 of the third embodiment, and (b) is a diagram showing a schematic structure of an FM antenna in an antenna device 9 of a comparative example. For convenience, a capacitive plate 12 is shown in a see-through manner. In the antenna device 3 of the third embodiment, the second coil 34b is wound in the opposite direction to the second coil 94b of the antenna device 9 of the comparative example. The material, length (winding pitch), and outer diameter of the linear conductor are the same as those of the first coil 34a. One end 341a of the first coil 34a is connected to a first end of the capacitance plate 12, and the other end 342a is connected to a power supply point (not shown) via a wiring pattern of the printed circuit board 36. One end 341b of the second coil 34b is connected to a second end (opposite end of the first end) of the capacitance plate 12 different from the first end, and the other end 342b is connected to the ground conductor GND via the GND pattern of the printed circuit board 36. With this configuration, the current ia flowing through the first coil 34a and the current ib flowing through the second coil 34b are in the same direction at adjacent portions. In contrast to this, in the antenna device 9 of the comparative example, the current i1 flowing through the first coil 94a and the current i2 flowing through the second coil 94b flow in opposite directions in adjacent portions and are cancelled out.

[0035] FIG. 11 is a radiation efficiency characteristic diagram in the FM band. The solid line is that of the antenna device 3 of the third embodiment, and the dashed line is that of the antenna device 9 of the comparative example. As is clear from this characteristic diagram, in the case of the antenna device 9 of the comparative example, the first coil 94a and the second coil 94b are wound in the same direction, so that the current i1 and the second current i2 are offset. Therefore, it can be seen that the inductance value is reduced, and the frequency characteristic is shifted to the high frequency side compared to the antenna device 3 of the third embodiment. On the other hand, in the case of the antenna device 3 of the third embodiment, the current flowing through the adjacent coils is not canceled, so that the reduction in the inductance value can be suppressed. This means that the coil length when resonating at a desired frequency is shortened, and as a result, compared to the antenna device 9 of the comparative example, the conductor loss is reduced and the radiation efficiency is increased.

[0036] [Fourth embodiment] A fourth embodiment of the present invention will be described. In the first embodiment, the winding pitch (coil length) of the two coils is the same (5:5). If the electrical length from the other end 142a of the first coil 14a through the capacitive plate 12 to the other end 142b of the second coil 14b is the resonance length of the FM band (1 / 2 the wavelength λ of the frequency used), the winding pitch of the two coils 14a, 14b does not necessarily have to be the same. In the fourth embodiment, an example in which the winding pitch is different from the winding pitch of the other coil in the antenna device 1 of the first embodiment will be described. The structures of the outer plate 11, the capacitive plate 12, and other parts are the same as those of the first embodiment, so the same names and symbols are used and the description of the overlapping parts is omitted.

[0037] Fig. 12 is a schematic diagram of an FM element in an antenna device 4 according to a fourth embodiment. Fig. 12(a) shows an antenna device 4 in which the winding pitch of the first coil 44a and the second coil 44b is 6:4, and Fig. 12(b) shows an antenna device 5 in which the winding pitch of the first coil 54a and the second coil 54b is 4:6. FIG. 13 is a radiation efficiency characteristic diagram in the FM band. The solid line indicates the characteristics of the antenna device 4, the long dashed line indicates the characteristics of the antenna device 1 of the first embodiment with a winding pitch of 5:5, and the short dashed line indicates the characteristics of the antenna device 5. The average radiation efficiency in the Japanese FM band indicated by the thick line is -19.1 [dB] for the antenna device 4, -19.6 [dB] for the antenna device 1, and -20.2 [dB] for the antenna device 5. In other words, the inductance of the coil (the first coil in this embodiment) closer to the power supply point 50 is set to be larger (specifically, for example, the number of turns is increased). This makes it possible to increase the average radiation efficiency in the FM band.

[0038] [Fifth embodiment] A fifth embodiment of the present invention will be described. In the second embodiment, an example of an antenna device 2 having a rectangular capacitive plate 22 and a rectangular annular outer plate 21 that surrounds the entire periphery of the capacitive plate 22 on the same plane or approximately the same plane has been described, but the shapes of these conductive plates are arbitrary as long as they have the same area as the outer plate 21 and the capacitive plate 22. In the fifth embodiment, an example will be described in which the capacitive plate is disk-shaped and the outer plate arranged along the entire periphery is annular. The structures of the other parts are the same as those of the first embodiment, so the same names and symbols are used and descriptions of overlapping parts will be omitted.

[0039] FIG. 14(a) is a top view of an antenna element of an antenna device 6 according to the fifth embodiment, and FIG. 14(b) is a diagram showing a schematic structure of this antenna element. The antenna device 6 has a disk-shaped capacitive plate 62 and an annular outer plate 61 arranged along the outer periphery of the disk-shaped capacitive plate 62. The capacitive plate 62 and the outer plate 61 around it are spaced apart by about 5 mm so as not to overlap. The area of ​​the capacitive plate 62 is 14527 mm 2 ] (= outer diameter 68 [mm]). The area of ​​the outer plate 61 is 5426 [mm 2 14(b), the capacitive plate 62 and the outer plate 61 are shown in a transparent manner. The first coil 64a and the second coil 64b each have a semicircular outer diameter, and the sum of the areas defined by the outer diameters is approximately the same as that of the capacitive plate 62.

[0040] The printed circuit board 66, which is equivalent to the printed circuit board 16 described in the first embodiment, is molded in approximately the same shape and size as the outer plate 61, but the shape and size are arbitrary. In addition, the resin base portion 630 below it is molded in a size larger than the antenna element and the printed circuit board 66 in order to accommodate them. Although not shown in the figure, the part corresponding to the cover portion 10 in the first embodiment also has a bottomed cylindrical shape.

[0041] The AM signal received by the outer plate 61 is guided to the electronic circuit of the printed circuit board 66 through the power supply unit 611 at the end. One end 641a of the first coil 64a is connected to the first end of the capacitive plate 62, and the other end 642a is connected to a power supply point (not shown) via the wiring pattern of the printed circuit board 66. One end 641b of the second coil 64b is connected to a second end (opposite end of the first end) of the capacitive plate 62 different from the first end, and the other end 642b is connected to the ground conductor GND via the GND pattern of the printed circuit board 66. As a result, the first coil 64a and the second coil 64b operate as a series resonant circuit in the FM band together with the capacitive plate 62, as in the first embodiment. An FM signal is output from the power supply point. The distances between the ground conductor GND and the outer plate 61 and the capacitive plate 62 are the same as those of the antenna device 1 of the first embodiment.

[0042] The average radiation efficiency of the antenna device 6 having such a structure in the FM band is -19.5 [dB], which is equivalent to the antenna device 1 of the first embodiment. The average radiation efficiency in the AM band is -70.0 [dB], which is equivalent to the antenna device 1 of the first embodiment. The directivity is omnidirectional in the vertical polarization horizontal plane in both the AM band and the FM band.

[0043] In the antenna device 6 of the fifth embodiment, the winding directions of the first coil 64a and the second coil 64b may be opposite to each other, and the ratio of the winding pitches may be changed. The capacitive plate 62 may be substantially disk-shaped or substantially elliptical. In this case, the outer plate 61, the first coil 64a, and the second coil 64b are also shaped to match the shape of the capacitive plate 62.

[0044] [Sixth embodiment] A sixth embodiment of the present invention will be described. The sixth embodiment is a modified example of the first embodiment, and since the structure of parts is almost the same as in the first embodiment, the same names and symbols are used and descriptions of overlapping parts are omitted. 15(a) is a schematic diagram showing the structure of an FM element of an antenna device 7 of the sixth embodiment, and (b) is a schematic diagram. The FM element of the antenna device 7 of the sixth embodiment includes a first FM element and a second FM element that each resonate in the FM band. The first FM element has a first capacitive plate 721 arranged to generate a capacitance between itself and the ground conductor GND and load the capacitance onto the first FM element, and a first coil 74a and a second coil 74b, each having one end connected to the first capacitive plate 721. The second FM element has a second capacitive plate 722 arranged to generate a capacitance between itself and the ground conductor GND and load the capacitance on the second FM element, and a third coil 74c and a fourth coil 74d each having one end connected to the second capacitive plate 722. In the example of FIG. 15(a), for convenience of explanation, the first capacitive plate 721 and the second capacitive plate 722 are shown in a transparent manner, but both are 7350 [mm 2 ] (=105 [mm] × 70 [mm]), and the two of them together have approximately the same area as the capacitive plate 12 of the antenna device 1 of the first embodiment. The height from the ground conductor GND is approximately 10 [mm].

[0045] In the first FM element, one end 741a of the first coil 74a is connected to a first end of the first capacitive plate 721, and the other end 742a is connected to the power supply point 50 via the wiring pattern of the printed circuit board 76. One end 741b of the second coil 74b is connected to a second end of the first capacitive plate 721 (an end opposite to the first end of the first capacitive plate 721) different from the first end, and the other end 742b is connected to the ground conductor GND via the GND pattern of the printed circuit board 76. In the second FM element, the third coil 74c has one end 741c connected to a first end of the second capacitive plate 722, and the other end 742c connected to the ground conductor GND via the GND pattern of the printed circuit board 76. The fourth coil 74d has one end 741d connected to a second end of the second capacitive plate 722 (an end opposite to the first end of the second capacitive plate 722) different from the first end, and the other end 742d connected to the ground conductor GND via the GND pattern of the printed circuit board 76. Each of the coils 74a to 74d has an area defined by its outer diameter that is approximately half the area (long side 105 mm × short side 30 mm) of the capacitance plates 721, 722, and is helically wound with a predetermined winding pitch. The coils 74a to 74d are arranged with a space of about 5 to 10 mm between each other, without overlapping.

[0046] The first FM element and the second FM element operate as a series resonant circuit via the ground conductor GND. In other words, the first FM element and the second FM element each resonate at a desired frequency (e.g., 84 MHz), and the first FM element and the second FM element as a whole are designed to resonate at that frequency as a series resonant circuit. The number of coils in the sixth embodiment is twice as many as that in the first embodiment. That is, the currents flowing through the first coil 74a, the second coil 74b, the third coil 74c, and the fourth coil 74d are equivalent to half the currents flowing through the first coil 14a and the second coil 14b in the first embodiment. Therefore, compared with the antenna impedance of the antenna device 1 of the first embodiment being 0.23Ω, the antenna impedance of the antenna device 7 of the sixth embodiment is 0.86Ω, which is nearly four times higher.

[0047] Fig. 16 is a diagram showing radiation efficiency characteristics in the FM band, where the solid line is that of the antenna device 7 of the sixth embodiment and the dashed line is that of the antenna device 1 of the first embodiment. As is clear from Fig. 16, the radiation efficiency characteristic of the antenna device 7 is steeper than that of the antenna device 1, and the bandwidth is narrower, but the radiation efficiency at the desired frequency (84 MHz) is greater than that of the antenna device 1. Even in the FM band shown by the thick line, the average radiation efficiency is -18.1 [dB], which is improved over the antenna device 1. The directivity was also omnidirectional in the vertically polarized horizontal plane in the FM band, similar to that of the antenna device 1.

[0048] In the sixth embodiment, two coils are connected to each of the first capacitive plate 721 and the second capacitive plate 722, but three coils may be connected to at least one of the capacitive plates. In this case, it is preferable to wind the middle coil in the opposite direction to the other coils. Also, the ratio of the winding pitches of the multiple coils may be changed.

[0049] [Seventh embodiment] A seventh embodiment of the present invention will be described. The seventh embodiment is a modified example of the first embodiment, and the structure of the parts is almost the same as that of the first embodiment, so the same names and symbols are used and the description of the overlapping parts is omitted. Fig. 17(a) is a diagram showing the structure of the FM element of the antenna device 8 of the seventh embodiment, and (b) is a schematic diagram. The FM element of the antenna device 8 of the seventh embodiment has three coils 84a, 84b, and 84c arranged in the same direction on the same plane or approximately the same plane on one capacitive plate 12, and the winding direction of the second coil 84b in the middle is opposite to that of the other coils 84a and 84c. For convenience, the capacitive plate 12 is shown in a see-through manner. The sum of the areas defined by the outer diameters of the coils 84a, 84b, and 84c is the area of ​​the capacitive plate 12 (15750 [mm 2 ] (=105 [mm] × 150 [mm]). That is, each of the coils 84a, 84b, and 84c is approximately 1 / 3 (=105 [mm] × 40 [mm]) the size of the capacitive plate 12, and is arranged without overlapping with each other. The height from the ground conductor GND to the capacitive plate 12 is the same as in the first embodiment. The printed circuit board 86 is rectangular and slightly larger than the capacitive plate 12.

[0050] The first coil 84a has one end 841a connected to the capacitive plate 12 and the other end 842a connected to the power supply point 50 via the wiring pattern of the printed circuit board 86. The second coil 84b and the third coil 84c have one ends 841b and 841c connected to the capacitive plate 12 and the other ends 842b and 842c connected to the ground conductor GND via the GND pattern of the printed circuit board 86. One end 841b of the second coil 84b is electrically connected to the approximate center of the capacitive plate 12. The electrical length from the other end 842a of the first coil 84a to the other end 842c of the third coil 84c is the resonant length in the FM band, and operates as a series resonant circuit in the FM band, similar to the antenna device 1 of the first embodiment.

[0051] Moreover, the antenna impedance of this antenna device 8 is 0.86Ω, which is increased compared to the antenna device 1 of the first embodiment. FIG. 18 is a radiation efficiency characteristic diagram in the FM band. The solid line is that of the antenna device 8, and the dashed line is that of the antenna device 1 of the first embodiment. As is clear from FIG. 18, the radiation efficiency of the antenna device 8 becomes steeper as it approaches the desired frequency (84 MHz), and is higher than that of the antenna device 1 of the first embodiment at that frequency. The average gain of the radiation efficiency is also improved. The average in the FM band in Japan, shown by the thick line, is -18.0 [dB], which is improved compared to the antenna device 1. Therefore, when the number of coils connected to one capacitance plate 12 is increased, it is possible to significantly increase the radiation efficiency at the desired frequency in the FM band.

[0052] [Variations] In the first to seventh embodiments, the height of the capacitive plate 12 from the ground conductor GND is about 10 mm, but when the area of ​​the capacitive plate 12 (the total area when there are multiple capacitive plates) is substantially the same, the higher the height of the capacitive plate from the ground conductor GND, the higher the radiation efficiency. For example, in the antenna device 1 of the first embodiment, the height from the ground conductor GND to the rear surface of the capacitive plate 12 may be 14.9 mm (the height to the outer wall of the cover part 10 is about 15 mm or less). In this case, the average radiation efficiency in the FM band is −16.6 [dB], and the average radiation efficiency in the AM band is −67.5 [dB], which can be further increased compared to the case of 10 mm (average −19.6 [dB] in the FM band, average 69.9 [dB] in the AM band).

[0053] In the first embodiment, the outer plate 11 surrounds the outer periphery of the capacitive plate 12 on three sides, and in the second embodiment, the outer plate 21 surrounds the entire outer periphery of the capacitive plate 22. However, the outer plate may be disposed at a predetermined distance from one side of the capacitive plate, with the same length as that of the capacitive plate. In this case, if the area (height) of the outer plate is set to be approximately the same as that of the outer plate 1 of the antenna device 1 of the first embodiment, the radiation efficiency does not change much even if the shapes are different. In other words, the arrangement of the outer plate can be arbitrarily changed according to the shape of the cover portion 10, and the flexibility of the design can be increased.

[0054] Furthermore, in the above embodiments, the FM band has been described as an example of the VHF band, but the present invention can also be applied to the cellular band (800 [MHz] to 900 [MHz]) with only the size being different.

[0055] In the first embodiment, an example was described in which the antenna housing is an antenna case including a cover portion 10 and a resin base portion 30, but the antenna housing may also be a storage space formed in any part of the vehicle body rather than an independently existing antenna case.

Claims

1. a conductor plate that generates a capacitance between itself and a ground conductor; a plurality of reactance elements, each of which has one end connected to the conductor plate; At least a portion of the conductor plate is disposed so as to become lower toward the outer periphery. Antenna device.

2. the plurality of reactance elements include a first reactance element and a second reactance element, the other end of the first reactance element is electrically connected to a power supply point, and the other end of the second reactance element is electrically connected to the ground conductor.

2. The antenna device according to claim 1.

3. the conductive plate loads the capacitance on an antenna element including the conductive plate and the plurality of reactance elements; 3. An antenna device according to claim 1 or 2.

4. the conductor plate, the first reactance element, and the second reactance element operate as a series resonant circuit; 3. The antenna device according to claim 2.

5. an electrical length from the other end of the first reactance element to the other end of the second reactance element is a resonant length of an antenna element including the conductor plate and the plurality of reactance elements in a first frequency band; 5. The antenna device according to claim 2 or 4.

6. a holding unit that holds the plurality of reactance elements; a substrate having the ground conductor and the power supply point; The substrate is disposed below the holder.

6. An antenna device according to claim 2, 4 or 5.

7. the first reactance element and the second reactance element are arranged spaced apart from each other without overlapping with each other; 6. An antenna device according to claim 2, 4 or 5.

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