Patent ID: 12191577

DESCRIPTION OF EMBODIMENTS

First Embodiment

In a first embodiment, the case in which the present invention is applied to a low-profile antenna device that is operable in the VHF band, such as the FM band (76 MHz to 90 MHz), and the MF band, such as the AM band (0.520 MHz to 1.710 MHz), is described. This antenna device is configured such that an antenna case, which is an example of an antenna housing, accommodates an antenna element and used in the state in which this antenna device is attached to, for example, a vehicle roof.

FIG.1is an exterior perspective view of an antenna device1according to the first embodiment.FIG.2Ais a top view of the antenna device1,FIG.2BtoFIG.2Eare side views thereof, andFIG.3is an exploded perspective view of the antenna device1. Referring to these drawings, the height of the antenna case of the antenna device1is 15 mm to 12 mm when measured from the mounting surface of the vehicle, with the attachment surface being set at a ground potential. The antenna case includes a cover portion10that is capable of transmitting radio waves and a resin base portion30. The shape of the cover portion10is a cylinder having an opening plane and a bottom surface, and the inner wall (the bottom portion) of the cover portion10is formed in a plane or substantially plane surface.

The antenna elements are accommodated in the antenna case. Each of the antenna elements includes a conductive plate of a predetermined area and a reactance element. One of the two conductive plates is used mainly for receiving the FM band and the opposing surface faces the surface at the ground potential, that is, the attachment surface of the vehicle. With this configuration, capacitance which is caused between the conductive plate and the attachment surface is loaded into the conductive plate (capacitance loading). For this reason, this conductive plate is hereinafter referred to as a “capacitance loading plate”. The other of the two conductive plates is used for receiving the AM band and positioned on the outer side of the capacitance loading plate. Thus, the conductive plate on the outer side is hereinafter referred to as an “outer side plate”. A capacitance loading plate12is a rectangular conductive plate of the area of 14850 mm2(=110 mm×135 mm). An outer side plate11is a U-shaped (in a quadrate shape without one edge) conductive plate of the area of 5700 mm2(=(15 mm×150 mm)+(10 mm×120 mm)+(15 mm×150 mm)). The outer side plate11and the capacitance loading plate12are fixed to the inner wall of the cover portion10. This means that the outer side plate11and the capacitance loading plate12of the antenna element are provided in an identical plane or a substantially identical plane. In the example inFIG.3, a plurality of holes are formed in the outer side plate11and the capacitance loading plate12. However, these holes do not substantially affect electrical characteristics because these are screw holes and guide holes for positioning.

The two reactance elements in the example of this embodiment are both linear conductors wound in a helical shape. In the first embodiment, as the reactance elements, linear conductors that are held by a first holder13aand a second holder13bare used. The holders13aand13bare made of resin and fixed to the inner wall of the cover portion10with the capacitance loading plate12interposed therebetween. This means that, while in an example the reactance element is downsized by winding a linear conductor around a dielectric body, in the first embodiment, an example in which the reactance element is formed by only a linear conductor is shown. Accordingly, for simplification of description, each of the two reactance elements is referred to as a “coil”.

A first coil14ais held by being wound around the surface of the first holder13a. A second coil14bis held by being wound around the surface of the second holder13b. One end of the first coil14ais coupled to a first end portion of the capacitance loading plate12and the other end is coupled to a feeding point. One end of the second coil14bis coupled to a second end portion, which is different from the first end portion, of the capacitance loading plate12and the other end is coupled to a ground conductor. Connection forms of these will be described in detail later.

The outer side plate11is capable of receiving radio waves at various frequencies. In the first embodiment, the outer side plate11is used for receiving AM band signals (AM signals); in other words, the outer side plate11per se forms an AM broadcast receiving element. AM signals received by the outer side plate11are led to a printed circuit board16described below via a feeding portion111that is at the end portion of the outer side plate11. In contrast, the capacitance loading plate12is capable of receiving FM band signals (FM signals) by being coupled to the first coil14aand the second coil14b; in other words, the capacitance loading plate12and the two coils14aand14bform an FM broadcast receiving element that resonates in the FM band. Received FM band signals are led to the printed circuit board16via the feeding point to which the first coil14ais coupled.

The printed circuit board16is positioned below the first holder13aand the second holder13b. The printed circuit board16contains an electronic circuit. The electronic circuit includes, for example, a first input terminal to which AM signals received by the outer side plate11are input and a second input terminal in communication with the feeding point of the first coil14a. The electronic circuit also includes an AM amplifying circuit that amplifies AM signals input from the first input terminal and an FM amplifying circuit that amplifies FM signals input from the second input terminal. The electronic circuit also includes output terminals that output AM signals amplified by the AM amplifying circuit and FM signals amplified by the FM amplifying circuit. A synthesis circuit that synthesizes AM signals and FM signals may be provided at a stage before the output terminal. A filter, a tuning circuit, or the like may be provided at a stage before the AM amplifying circuit.

In the printed circuit board16, a GND pattern in communication with ground terminals of the amplifying circuits or the like is formed. A pair of GND terminals15aand15b, which are made of metal, are fixed to the GND pattern. The GND terminals15aand15bare members used for conducting with a conductive base19made of metal. A cable holder17is fixed to the back surface of the printed circuit board16. The cable holder17holds signal cables electrically coupled to a first output terminal and a second output terminal.

The resin base portion30is formed by a frame301protruding upwardly from a portion slightly inside the outer periphery of the resin base portion30and a bottom portion302surrounded by the frame301, the frame301and the bottom portion302being integrally formed. The frame301is formed in a size substantially identical to the size of the opening plane of the cover portion10. The frame301has an outer side surface in which a groove is formed to extend along the entire periphery of the frame301. An O-ring20formed of an elastic member is fitted into the groove. The depth of the groove is shorter than the diameter of the O-ring20. As a result, when the cover portion10is engaged in the resin base portion30, the O-ring20seals the space inside the bottom portion302in a watertight manner.

In the bottom portion302of the resin base portion30, a depression303, which accommodates and fixes the printed circuit board16, and a hole portion304, which enables a prelock member18and the conductive base19to project downwardly, are formed. The prelock member18is a member for temporary fixing the antenna device1when the antenna device1is attached to, for example, a vehicle roof. A fixing base305to which the prelock member18and the conductive base19are screwed is joined to the bottom portion302. The conductive base19is used for firmly fixing the antenna device1to, for example, a vehicle roof, and when attached, the conductive base19causes the GND pattern of the printed circuit board16to be set at the ground potential by using the GND terminals15aand15b.

A positional relationship between the outer side plate11and the capacitance loading plate12is illustrated inFIG.4. The plurality of holes illustrated inFIG.3are omitted inFIG.4. Referring toFIG.4, the outer side plate11surrounds about ¾ of the outer periphery of the capacitance loading plate12. Further, the outer side plate11and the capacitance loading plate12are positioned with a predetermined spacing therebetween not to position one end portion and the other facing end portion in an overlapping manner. As described above, the outer side plate11and the capacitance loading plate12are positioned in an identical plane or a substantially identical plane and there is thus no projection. This simplifies the exterior of the cover portion10and contributes to low-profile design of the antenna device1. Since the facing end portions of the outer side plate11and the capacitance loading plate12are spaced apart from each other and not positioned in an overlapping manner, no interference occurs.

One feature of the antenna device1according to the first embodiment is the configuration of the antenna element, in particular, the configuration of the FM broadcast receiving element. These configurations are described in detail below.FIG.5Ais a schematic diagram illustrating a positional relationship between the capacitance loading plate12included in the antenna device1, and the first coil14aand the second coil14b, andFIG.5Bis a simple diagram thereof. The shape of the printed circuit board16is similar to the shape of the capacitance loading plate12while the size of the printed circuit board16is slightly larger than the size of the capacitance loading plate12, however, the size difference matters little. A ground conductor GND illustrated inFIG.5Bis a portion of a vehicle roof in communication with the GND terminals15aand15b, and the conductive base19that are illustrated inFIG.3. The first holder13a, the second holder13b, and other members are omitted for the sake of convenience.

The size of the capacitance loading plate12is as described above and the height of the capacitance loading plate12from the ground conductor GND is approximately 10 mm. The major and minor diameters (the long diameter and the short diameter) of the first coil14aand the major and minor diameters (the long diameter and the short diameter) of the second coil14bare about ½ of the dimensions (105 mm×70 mm) of the capacitance loading plate12. The first coil14aand the second coil14bare both wound at a predetermined winding pitch and have the same inductance value. The first coil14aand the second coil14bare spaced about 5 mm apart from each other and provided not to be positioned in an overlapping manner.

The major and minor diameters (the long diameter and the short diameter), the shape, and the size of each of the first coil14aand the second coil14bare not limited to the examples described above and may be optionally modified depending on, for example, the installation space. The same holds for the spaced distance between the first coil14aand the second coil14b.

One end141aof the first coil14ais coupled to the first end portion of the capacitance loading plate12while another end142ais coupled to a feeding point50via a wire pattern of the printed circuit board16. One end141bof the second coil14bis coupled to the second end portion (an end portion opposite to the first end portion), which is different from the first end portion described above, of the capacitance loading plate12, while another end142bis coupled to the ground conductor GND via the GND pattern of the printed circuit board16. With this configuration, the first coil14aand the second coil14boperate, in conjunction with the capacitance loading plate12, as a series resonance circuit in the FM band. This means that the electrical length from the other end142aof the first coil14avia the capacitance loading plate12to the other end142bof the second coil14bis equal to a resonant length in the FM band (an electrical length of ½ of a wave length λ of a frequency used in the FM band, the same holds for the following description). FM signals can be obtained from the feeding point50.

The present inventors made a reference antenna for the purpose of comparing electrical characteristics. The reference antenna includes: 1) a reference plate of which the material and the area are identical to those of the capacitance loading plate12included in the antenna device1; and 2) a reference coil of which the wire material and the wire diameter are identical to those of the first coil14aand the second coil14b, and the diameter defines the area identical to the area defined by the total of the diameters of the first coil14aand the second coil14b.FIG.6(a)is a schematic diagram illustrating a positional relationship between a reference plate (a conductive plate corresponding to the capacitance loading plate12)61R and a reference coil (a reactance element)64R that are included in a reference antenna1R, andFIG.6Bis a simple diagram thereof. The reference plate61R is illustrated transparently inFIG.6Afor the sake of convenience. One end641R of the reference coil64R is coupled to an end portion of the reference plate61R while another end642R is coupled to the feeding point50. The material and the size ratio of the printed circuit board66R, the connection state of the reference coil64R and the feeding point50, the distance between the ground conductor GND and the base end of the reference coil64R, the distance between the ground conductor GND and the distal end of the reference coil64R, and the distance between the distal end of the reference coil64R and the lower surface of the reference plate61R are identical to those of the antenna device1.

Concerning the reference antenna1R, when the current that flows through the reference coil64R is I1, the antenna impedance is Z1, and the radiated power (used synonymously with receiving power; the same holds for the following description) is P1, P1is expressed as Z1×I12. The value of the antenna impedance is, for example, an impedance value on a real axis when a Smith chart is used. As the antenna impedance approaches the feeding impedance (50Ω in this embodiment), the radiation efficiency (used synonymously with reception efficiency; the same holds for the following description) increases, and as a result, the electric power increases. According to a simulation experiment of the present inventors, the antenna impedance of the reference antenna1R was 0.06Ω.

Contrary to this, concerning the FM antenna included in the antenna device1of the first embodiment, when a radiated power P2is identical to the radiated power P1of the reference antenna1R and the current that flows through the first coil14aand the second coil14bis I2, the current I2is ½ of the current I1. Thus, an antenna impedance Z2is four times as much as Z1. This means that, with respect to the reference antenna1R having one coil, the antenna impedance increases in proportion to the square of the number of coils. The present inventors confirmed that the antenna impedance of the FM antenna of the first embodiment increased to 0.23Ω, which is four times as much as the antenna impedance of the reference antenna1R.

FIG.7is a diagram of radiation efficiency characteristic in the FM band. In the drawing, a solid line indicates the radiation efficiency in the FM band of the antenna device1according to the first embodiment and a dashed line indicates the radiation efficiency of the reference antenna1R. In the FM band in Japan that is indicated between thick lines, the average radiation efficiency of the reference antenna1R was −25.2 dB while the average radiation efficiency of the FM antenna according to the first embodiment was −19.6 dB. As described above, by increasing the number of coils coupled to the capacitance loading plate12, the antenna impedance is increased, and as a result, the reception gain and the radiation efficiency in the FM band are greatly improved. Although it is omitted in the diagram, the average radiation efficiency in the AM band was −70.0 dB.

FIG.8Ais a directional characteristic diagram of the antenna device1according to the first embodiment in a horizontal plane with vertical polarization in the FM band andFIG.8Bis a directional characteristic diagram in a horizontal plane with vertical polarization in the AM band. As seen from these characteristic diagrams, the antenna device1of the first embodiment is omnidirectional in the horizontal plane with vertical polarization in the FM band and also in the horizontal plane with vertical polarization in the AM band.

Second Embodiment

Next, a second embodiment of the present invention is described. In the second embodiment, with regard to the outer side plate11and the capacitance loading plate12included in the antenna device1of the first embodiment, the materials and the thicknesses are not changed while the shapes and the layouts are changed. The structures of other members including the first coil14aand the second coil14bare similar to those of the first embodiment and thus denoted by the same names and the same reference characters, and redundant descriptions thereof are omitted.

FIG.9Ais a top view of an antenna element included in an antenna device2according to the second embodiment andFIG.9Bis a schematic diagram illustrating a structure of the antenna element. InFIG.9B, the capacitance loading plate is illustrated transparently. The antenna device2of the second embodiment includes a rectangular capacitance loading plate22and a rectangular-annular outer side plate21that surrounds the entire periphery of the capacitance loading plate22in an identical plane or a substantially identical plane. The outer side plate21and the capacitance loading plate22are spaced about 5 mm apart from each other so that the facing end portions are not positioned in an overlapping manner. The area of the capacitance loading plate22is 14400 mm2(=120 mm×120 mm). The area of the outer side plate21is 5600 mm2(=(10 mm×150 mm)+(10 mm×130 mm)+(10 mm×150 mm)+(10 mm×130 mm)). The distance between the ground conductor GND and the outer side plate21and the distance between the ground conductor GND and the capacitance loading plate22are identical to those of the antenna device1of the first embodiment. A resin base portion230is slightly larger in size than the outer side plate21.

AM signals received by the outer side plate21are led to an electronic circuit of a printed circuit board26on a resin base portion230via a feeding portion211at an end portion of the outer side plate21. That is, similarly to the first embodiment, the outer side plate21operates as an AM broadcast receiving element.

One end141aof the first coil14ais coupled to a first end portion of the capacitance loading plate22while another end142ais coupled to a feeding point50via a wire pattern of the printed circuit board26. One end141bof the second coil14bis coupled to a second end portion (an end portion opposite to the first end portion), which is different from the first end portion described above, of the capacitance loading plate22, while another end142bis coupled to the ground conductor GND via a GND pattern of the printed circuit board26. With this configuration, similarly to the first embodiment, the first coil14aand the second coil14boperate, in conjunction with the capacitance loading plate22, as a series resonance circuit in the FM band. FM signals can be obtained from the feeding point50.

According to the observation of the present inventors, the average radiation efficiency and the directivity in a horizontal plane with vertical polarization in the FM band in Japan were similar to those of the antenna device1of the first embodiment. The antenna impedance was also nearly unchanged from that of the first embodiment. This means that the radiation efficiency or the like in the FM band are similar to those of the antenna device1of the first embodiment. Also in the AM band, the directivity in a horizontal plane with vertical polarization was not changed from that of the antenna device1of the first embodiment and the radiation efficiency was substantially equal to that of the antenna device1of the first embodiment.

As described above, although the antenna device2of the second embodiment employs the configuration in which the entire periphery of the rectangular capacitance loading plate22is surrounded by the rectangular-annular outer side plate21in an identical plane or a substantially identical plane, the radiation efficiency in the AM band substantially equal to the radiation efficiency of the antenna device1of the first embodiment can be achieved. Furthermore, as long as the shape and the size (the area) of the outer side plate11are determined, the capacitance loading plate22can be accordingly formed simply by, for example, punching, and this results in the simplification of manufacturing processing.

The outer side plate21may be formed such that the height of part or all of the outer edge decreases toward the outer periphery without changing the area. In this case, since the height of a portion of the outer side plate21is relatively low, the radiation efficiency in the AM band becomes slightly low, however, this does not substantially affect in actual use. This configuration has an advantage that, for example, the cover portion10included in the antenna device1of the first embodiment can be downsized.

Third Embodiment

Next, a third embodiment of the present invention is described. Concerning an antenna device3of the third embodiment, in an FM broadcast receiving element including a conductive plate of a predetermined area and two reactance elements, a first coil34aand a second coil34badjacent to each other are wound in directions opposite to each other. The structures of the outer side plate11, the capacitance loading plate12, and other members are similar to those of the first embodiment and thus denoted by the same names and the same reference characters, and redundant descriptions thereof are omitted.

FIG.10Ais a diagram schematically illustrating a structure of an FM antenna of the antenna device3of the third embodiment andFIG.10Bis a diagram schematically illustrating a structure of an FM antenna of an antenna device9of a comparative example. For the sake of convenience, the capacitance loading plate12is illustrated transparently.

In the antenna device3of the third embodiment, the second coil34bis wound in a direction opposite to a direction in which a second coil94bincluded in the antenna device9of the comparative example is wound. The material, the length (the winding pitch), and the diameter of the linear conductor are identical to those of the first coil34a. One end341aof the first coil34ais coupled to the first end portion of the capacitance loading plate12, while another end342ais coupled to a feeding point, which is not illustrated in the drawing, via a wire pattern of a printed circuit board36. One end341bof the second coil34bis coupled to the second end portion (an end portion opposite to the first end portion) of the capacitance loading plate12different from the first end portion described above, while another end342bis coupled to the ground conductor GND via a GND pattern of the printed circuit board36. In this configuration, a current ia flowing through the first coil34aand a current ib flowing through the second coil34bflow in the same direction at a portion where the first coil34aand the second coil34bare adjacent to each other. Contrary to this, in the antenna device9of the comparative example, a current i1flowing through a first coil94aand a current i2flowing through the second coil94bflow in directions opposite to each other at a portion where the first coil94aand the second coil94bare adjacent to each other, and thus, the current i1and the current i2cancel each other out.

FIG.11is a diagram of radiation efficiency characteristic in the FM band. A solid line represents the antenna device3of the third embodiment and a dashed line represents the antenna device9of the comparative example. As seen from the characteristic diagram, in the case of the antenna device9of the comparative example, since the first coil94aand the second coil94bare both wound in the same direction, the current i1and the current i2cancel each other out. The inductance value thus decreases, and as a result, the frequency characteristic is moved to a higher range side in comparison to the antenna device3of the third embodiment. Contrary to this, in the case of the antenna device3of the third embodiment, since the currents flowing through the coils adjacent to each other do not cancel each other out, the decrease in inductance value is suppressed. This means that, the coil length for resonance at a desired frequency is reduced, and as a result, in comparison to the antenna device9of the comparative example, the conductor loss decreases and the radiation efficiency increases.

Fourth Embodiment

Next, a fourth embodiment of the present invention is described. The first embodiment is described on the basis that the winding pitches (the coil lengths) of the two coils are identical (5:5) to each other. When the electrical length from the other end142aof the first coil14avia the capacitance loading plate12to the other end142bof the second coil14bis a resonant length (½ of a wave length λ of a frequency in use) of the FM band, the winding pitches of the two coils14aand14bare not necessarily identical to each other. In the fourth embodiment, the case in which the winding pitch of a coil is different from the winding pitch of another coil in the antenna device1of the first embodiment is described. The structures of the outer side plate11, the capacitance loading plate12, and other members are similar to those of the first embodiment and thus denoted by the same names and the same reference characters, and redundant descriptions thereof are omitted.

FIG.12Bincludes simple diagrams of FM broadcast receiving elements of an antenna device4of the fourth embodiment.FIG.12Aillustrates the antenna device4in which the winding pitches of a first coil44aand a second coil44bare 6:4 andFIG.12Billustrates an antenna device5in which the winding pitches of a first coil54aand a second coil54bare 4:6.

FIG.13is a diagram of radiation efficiency characteristic in the FM band. A solid line represents a characteristic of the antenna device4, a long-dashed line represents a characteristic of the antenna device1of the first embodiment in which the winding pitches are 5:5, and a short-dashed line represents a characteristic of the antenna device5. The average radiation efficiencies in the FM band in Japan, which is indicated between thick lines, were −19.1 dB for the antenna device4, −19.6 dB for the antenna device1, and −20.2 dB for the antenna device5. This indicates that a coil close to the feeding point50(the first coil in this embodiment) is configured to have a higher level of inductance (to be specific, for example, by increasing the number of turns). This increases the average radiation efficiency in the FM band.

Fifth Embodiment

Next, a fifth embodiment of the present invention is described. While the second embodiment describes the example of the antenna device2including the rectangular capacitance loading plate22and the rectangular-annular outer side plate21surrounding the entire periphery of the capacitance loading plate22in an identical plane or a substantially identical plane, these conductive plates can be of any shape when the area of the outer side plate21and the area of the capacitance loading plate22are identical to each other. In the fifth embodiment, the case in which the capacitance loading plate is formed in a circular plate-like shape and the outer side plate positioned along the entire circumference of the capacitance loading plate is formed in an annular shape is described. The structures of other members are similar to those of the first embodiment and thus denoted by the same names and the same reference characters, and redundant descriptions thereof are omitted.

FIG.14Ais a top view of an antenna element of an antenna device6according to the fifth embodiment andFIG.14Bis a diagram schematically illustrating a structure of the antenna element.

The antenna device6includes a capacitance loading plate62in a circular plate-like shape and an outer side plate61in an annular shape positioned along the outer circumference of the capacitance loading plate62. The capacitance loading plate62and the outer side plate61surrounding the capacitance loading plate62are spaced about 5 mm apart from each other not to be positioned in an overlapping manner. The area of the capacitance loading plate62is 14527 mm2(=a diameter of 68 mm). The area of the outer side plate61is 5426 mm2(=a diameter of 84 mm and a width of 11 mm). InFIG.14B, the capacitance loading plate62and the outer side plate61are illustrated transparently. The outer circumference of a first coil64aand the outer circumference of a second coil64bare each formed in a semicircle and the total area defined by the outer circumferences is similar to that of the capacitance loading plate62.

A printed circuit board66corresponding to the printed circuit board16described in the first embodiment is formed in a shape and a size that are similar to those of the outer side plate61, however, any shape and any size can be used. A resin base portion630below the printed circuit board66is formed in a size larger than the size of the antenna element and the size of the printed circuit board66so as to accommodate the antenna element and the printed circuit board66. A member corresponding to the cover portion10of the first embodiment is formed in a cylinder having a bottom surface, which is omitted in the drawing.

AM signals received by the outer side plate61are led to an electronic circuit of the printed circuit board66via a feeding portion611at an end portion of the outer side plate61. One end641aof the first coil64ais coupled to a first end portion of the capacitance loading plate62, while another end642ais coupled to a feeding point, which is not illustrated in the drawing, via a wire pattern of the printed circuit board66. One end641bof the second coil64bis coupled to a second end portion (an end portion opposite to the first end portion), which is different from the first end portion described above, of the capacitance loading plate62while another end642bis coupled to the ground conductor GND via a GND pattern of the printed circuit board66. With this configuration, similarly to the first embodiment, the first coil64aand the second coil64boperate, in conjunction with the capacitance loading plate62, as a series resonance circuit in the FM band. FM signals are output from the feeding point. The distance between the ground conductor GND and the outer side plate61and the distance between the ground conductor GND and the capacitance loading plate62are identical to those of the antenna device1of the first embodiment.

The average radiation efficiency in the FM band of the antenna device6of such a configuration was −19.5 dB and this achieves the radiation efficiency similar to that of the antenna device1of the first embodiment. The average radiation efficiency in the AM band was −70.0 dB and this achieves the radiation efficiency similar to that of the antenna device1of the first embodiment. Concerning the directivity, both in the AM and FM bands, the antenna device6is omnidirectional in a horizontal plane with vertical polarization.

In the antenna device6of the fifth embodiment, the first coil64aand the second coil64bmay be wound in directions opposite to each other and the ratio of winding pitches may be changed. Moreover, the capacitance loading plate62may be formed in a substantially circular plate-like shape or a substantially oval shape. In this case, the outer side plate61, the first coil64a, and the second coil64bare also formed in shapes that match the shape of the capacitance loading plate62.

Sixth Embodiment

A sixth embodiment of the present invention is described. The sixth embodiment is a modified exemplary embodiment of the first embodiment and the structures of members are similar to those of the first embodiment and thus denoted by the same names and the same reference characters, and redundant descriptions thereof are omitted.

FIG.15Ais a diagram schematically illustrating a structure of an FM broadcast receiving element of an antenna device7of the sixth embodiment andFIG.15Bis a simple diagram. The FM broadcast receiving element of the antenna device7of the sixth embodiment includes a first FM broadcast receiving element and a second FM broadcast receiving element, each of which resonates in the FM band.

The first FM broadcast receiving element includes a first capacitance loading plate721positioned in such a manner that capacitance is caused between the first capacitance loading plate721and the ground conductor GND and the capacitance is loaded into the first FM broadcast receiving element. The first FM broadcast receiving element also includes a first coil74aand a second coil74beach coupled to the first capacitance loading plate721at one end thereof.

The second FM broadcast receiving element includes a second capacitance loading plate722positioned in such a manner that capacitance is caused between the second capacitance loading plate722and the ground conductor GND and the capacitance is loaded into the second FM broadcast receiving element. The second FM broadcast receiving element also includes a third coil74cand a fourth coil74deach coupled to the second capacitance loading plate722at one end thereof. In the example inFIG.15A, for ease of description, the first capacitance loading plate721and the second capacitance loading plate722are illustrated transparently. The area of the first capacitance loading plate721and the area of the second capacitance loading plate722are each 7350 mm2(=105 mm×70 mm) and the two areas together are similar to the area of the capacitance loading plate12included in the antenna device1of the first embodiment. The height from the ground conductor GND is approximately 10 mm.

In the first FM broadcast receiving element, one end741aof the first coil74ais coupled to a first end portion of the first capacitance loading plate721while another end742ais coupled to the feeding point50via a wire pattern of a printed circuit board76. Regarding the second coil74b, one end741bis coupled to a second end portion (an end portion opposite to the first end portion of the first capacitance loading plate721) of the first capacitance loading plate721different from the first end portion described above, while another end742bis coupled to the ground conductor GND via a GND pattern of the printed circuit board76.

In the second FM broadcast receiving element, concerning the third coil74c, one end741cis coupled to a first end portion of the second capacitance loading plate722while another end742cis coupled to the ground conductor GND via the GND pattern of the printed circuit board76. Regarding the fourth coil74d, one end741dis coupled to a second end portion (an end portion opposite to the first end portion of the second capacitance loading plate722) of the second capacitance loading plate722different from the first end portion described above, while another end742dis coupled to the ground conductor GND via the GND pattern of the printed circuit board76.

The area of each of the coils74ato74ddefined by each outer diameter is about ½ of the capacitance loading plate721or722(105 mm long side×30 mm short side) and each coil is wound by a predetermined winding pitch to be formed in a helical shape. The coils74ato74dare spaced about 5 to 10 mm apart from each other and provided not to be positioned in an overlapping manner.

The first FM broadcast receiving element and the second FM broadcast receiving element operate as a series resonance circuit in conjunction with the ground conductor GND. Specifically, the first FM broadcast receiving element and the second FM broadcast receiving element both resonate at a desired frequency (for example, 84 MHz) and the first FM broadcast receiving element and the second FM broadcast receiving element are designed to collectively resonate at the frequency as a series resonance circuit.

The number of coils in the sixth embodiment is twice as many as the number of coils in the first embodiment. This means that the current flowing through the first coil74a, the current flowing through the second coil74b, the current flowing through the third coil74c, and the current flowing through the fourth coil74dare each equal to ½ of the current flowing through the first coil14aand the second coil14bin the first embodiment. For this reason, in comparison to the case of the antenna device1of the first embodiment in which the antenna impedance was 0.23Ω, the antenna impedance of the antenna device7of the sixth embodiment was 0.86Ω which is about four times as much as the antenna impedance of the antenna device1.

FIG.16is a diagram of radiation efficiency characteristic in the FM band, in which a solid line represents the antenna device7of the sixth embodiment and a dashed line represents the antenna device1of the first embodiment. As seen fromFIG.16, the radiation efficiency characteristic of the antenna device7has a steep feature in comparison to the antenna device1and the bandwidth is narrower than that of the antenna device1, however, the radiation efficiency at a desired frequency (84 MHz) is higher than that of the antenna device1. In the FM band indicated between thick lines, the average radiation efficiency was −18.1 dB, which is improved in comparison to the antenna device1. Concerning the directivity, similarly to the antenna device1, the antenna device7is omnidirectional in a horizontal plane with vertical polarization in the FM band.

The sixth embodiment is the example of coupling two coils to each of the first capacitance loading plate721and the second capacitance loading plate722, however, three coils may be coupled to at least one of capacitance loading plates. In this case, it is desired that a coil in the middle is wound in a direction opposite to the direction in which the other coils are wound. In addition, the ratio of winding pitches of the plurality of coils may be changed.

Seventh Embodiment

A seventh embodiment of the present invention is described. The seventh embodiment is a modified exemplary embodiment of the first embodiment and the structures of members are similar to those of the first embodiment and thus denoted by the same names and the same reference characters, and redundant descriptions thereof are omitted.FIG.17Ais a diagram schematically illustrating a structure of an FM broadcast receiving element of an antenna device8of the seventh embodiment andFIG.17Bis a simple diagram thereof.

An FM broadcast receiving element of the antenna device8of the seventh embodiment is formed such that three coils84a.84b, and84care provided in one direction on a single capacitance loading plate12, in an identical plane or a substantially identical plane and the second coil84bin the middle is wound in a direction opposite to the direction in which the other coils84aand84care wound. For the sake of convenience, the capacitance loading plate12is illustrated transparently. The total area defined by the diameters of the coils84a,84b, and84cis similar to the area of the capacitance loading plate12(15750 mm2(=105 mm×150 mm). This means that the size of each of the coils84a,84b, and84cis about ⅓ of the size of the capacitance loading plate12(=105 mm×40 mm). The coils84a,84b, and84care provided not to be positioned in an overlapping manner. The height from the ground conductor GND to the capacitance loading plate12is similar to that of the first embodiment. A printed circuit board86is slightly larger in size than the capacitance loading plate12and formed in a rectangular shape.

One end841aof the first coil84ais coupled to the capacitance loading plate12and another end842ais coupled to the feeding point50via a wire pattern of the printed circuit board86. One end841bof the second coil84band one end841cof the third coil84care coupled to the capacitance loading plate12and another end842bof the second coil84band another end842cof the third coil84care coupled to the ground conductor GND via a GND pattern of the printed circuit board86. The one end841bof the second coil84bis electrically coupled to the substantially center portion of the capacitance loading plate12. The electrical length from the other end842aof the first coil84ato the other end842cof the third coil84cis a resonant length in the FM band, and similarly to the antenna device1of the first embodiment, the first coil84a, the second coil84b, the third coil84c, and the capacitance loading plate12operate as a series resonance circuit in the FM band.

The antenna impedance of the antenna device8was 0.86Ω, which is increased in comparison to that of the antenna device1of the first embodiment.

FIG.18is a diagram of radiation efficiency characteristic in the FM band. A solid line represents the antenna device8and a dashed line represents the antenna device1of the first embodiment. As seen fromFIG.18, the representation of the radiation efficiency of the antenna device8becomes steep toward the desired frequency (84 MHz) and the radiation efficiency at the frequency is higher than that of the antenna device1of the first embodiment. In addition, the average gain of the radiation efficiency is improved. The average in the FM band in Japan indicated between thick lines was −18.0 dB, which is improved in comparison to the antenna device1. Accordingly, when the number of coils coupled to a single capacitance loading plate12, is increased, the radiation efficiency at a desired frequency in the FM band can be greatly increased.

Modified Exemplary Embodiments

While in the first to seventh embodiments the height from the ground conductor GND to the capacitance loading plate12or the like is approximately 10 mm, when the area of the capacitance loading plate12(the total area in the case of a plurality of capacitance loading plates) is substantially maintained, the radiation efficiency increases as the height from the ground conductor GND to the capacitance loading plate increases. For example, in the antenna device1of the first embodiment, the height from the ground conductor GND to the back surface of the capacitance loading plate12may be determined as 14.9 mm (the height to the outer wall of the cover portion10is approximately 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 are increased in comparison to the case of 10 mm (the average in the FM band is −19.6 dB and the average in the AM band is −69.9 dB).

Furthermore, the first embodiment indicates the example in which three sides of the capacitance loading plate12are surrounded by the outer side plate11along the outer periphery of the capacitance loading plate12and the second embodiment indicates the example in which the outer periphery of the capacitance loading plate22is entirely surrounded by the outer side plate21. However, the configuration may be such that the outer side plate has a side of a length identical to the length of a side of the capacitance loading plate and is positioned with a predetermined spacing to the side of the capacitance loading plate. In this case, when the area (the height) of the outer side plate is similar to the area (the height) of, for example, the outer side plate11included in the antenna device1of the first embodiment, the radiation efficiency is not greatly changed in the case in which the shape varies. This means that the arrangement of the outer side plate can be changed in any manner depending on the shape of the cover portion10, resulting in improved flexibility of design.

Moreover, in the above embodiments, the example of using the FM band as the VHF band is described, however, the embodiments can be applied to the cellular band (800 MHz to 900 MHz) in the same manner by only changing the size.

According to the present disclosure, a conductive plate of the first element and a conductive plate of the second element are provided in an identical plane or a substantially identical plane, and there is thus no projection. This facilitates low-profile design of the antenna element. Additionally, by coupling a plurality of reactance elements to the conductive plate of the first element, the reactance elements and the conductive plate operate as a series resonance circuit in the first frequency band, and as a result, in comparison to the case of using one reactance element, the voltage standing wave ratio (VSWR) is improved and the radiation efficiency is thus increased.

While the first embodiment describes the example in which the antenna housing is the antenna case including the cover portion10and the resin base portion30, a housing space formed at any portion of a vehicle body may be used as an antenna housing instead of the antenna case that individually exists.