Patent ID: 12261379

DETAILED DESCRIPTIONS

To begin with, examples of relevant techniques will be described. According to a comparative example, an antenna device includes a ground plate that is a conductive plate having a ground potential, separately from a radiating element.

In the antenna device, when a size of the ground plate is insufficient for wavelength of a radio wave which is transmitted and/or received by the antenna device, a leakage current that is a current leaking from the ground plate to the cable may increase, thereby decreasing a gain and/or destabilizing directivity.

In the comparative example, the leakage current to the cable is reduced by filtering high-frequency current using a filter element that is a circuit element that functions as a low-pass filter.

Since the configuration of the comparative example requires the filter element, a total cost may increase by an amount of cost for the filter element.

In contrast to the comparative example, according to the present disclosure, a leakage current to a cable can be reduced while reducing an increase in cost.

According to an aspect of the present disclosure, an antenna device, for example, includes a ground plate and an antenna element. The ground plate is a conductor member with a flat rectangular shape. The antenna element is a conductor member having a feed point electrically connected to a feeder line. A length of the ground plate in a predetermined direction is shorter than a target wavelength that is a wavelength of a radio wave to be transmitted or received. The ground plate is connected to a grounding cable at a connection position on the ground plate. The connection position is shifted from an edge of the ground plate by an odd multiple of ¼ of the target wavelength.

The developers of the present disclosure have conducted experiments on an operation of an antenna device with a ground plate having a small area by simulation and other means, and found that positions on the ground plate shifted from an edge of the ground plate by odd multiples of ¼ of the target wavelength behave as nodes in a potential distribution. A node in the potential distribution is a position where an electric potential is minimum. The above configuration has been created based on these findings. According to the configuration in which the grounding cable is connected to the ground plate at the connection position shifted by the odd multiple of ¼ of the target wavelength from the edge of the ground plate, a potential difference is unlikely to occur between the grounding cable and the ground plate. Therefore, leakage current can be reduced. Further, according to the above configuration, a filter element for reducing the leakage current to the cable is not required. That is, the leakage current to the cable can be reduced while an increase in cost is reduced.

Hereinafter, an embodiment of the present disclosure will be described below with reference to the drawings. In the following embodiment, members having the same function will be assigned the same reference numeral, and the descriptions thereof will be omitted. When only a part of a configuration is described, the other parts of the configuration may employ a preceding configuration described in the embodiment.

FIG.1is an exterior perspective view illustrating an example of a schematic structure of an antenna device1according to the present embodiment.FIG.2is a cross sectional view of the antenna device1along the line II-II illustrated inFIG.1. The antenna device1is used while being installed in a moving body such as a vehicle.

The antenna device1is configured to transmit and receive radio waves at a predetermined target frequency. Of course, as another mode, the antenna device1may be used only for transmission or only for reception. Since transmission and reception of radio waves are reversible, a configuration capable of transmitting radio waves at a predetermined frequency is also a configuration capable of receiving radio waves at the predetermined frequency.

Herein, the target frequency is, for example, 2.45 GHz. Of course, the target frequency may be set appropriately. In another embodiment, the target frequency may be, for example, 300 MHz, 760 MHz, 850 MHz, 900 MHz, 1.17 GHz, 1.28 GHz, 1.55 GHz, or 5.9 GHz. The antenna device1is capable of transmitting and receiving not only a radio wave having the target frequency but also a radio wave having a frequency within a predetermined range that has been determined with reference to the target frequency. For example, the antenna device1is configured to be capable of transmitting and receiving radio waves having frequencies belonging to a 2.4 GHz band, which is a band from 2400 MHz to 2500 MHz.

That is, the antenna device1is capable of transmitting and receiving radio waves in a frequency band used in short-range wireless communication such as Bluetooth (registered trademark) Low Energy, Wi-Fi (registered trademark), and ZigBee (registered trademark). In other words, the antenna device1is configured to be capable of transmitting and receiving radio waves in the frequency band (so-called ISM band) specified by the International Telecommunication Union and allocated for general use in the industrial, scientific, and medical fields.

“λ” hereinafter represents a target wavelength. The target wavelength is a wavelength of a radio wave having the target frequency. For example, “λ/2” and “0.5λ” indicate a half of the target wavelength, and “λ/4” and “0.25λ” indicate one quarter of the target wavelength. The wavelength (i.e., λ) of the 2.4 GHz radio wave in vacuum and air is 125 mm.

The antenna device1is connected via a cable to a communication ECU (i.e., Electronic Control Unit) installed in the vehicle, and signals received by the antenna device1are sequentially output to the communication ECU. Also, the antenna device1converts an electric signal input from the communication ECU into a radio wave, and radiates the radio wave. The communication ECU uses signals received by the antenna device1, and also supplies the antenna device1with high-frequency power corresponding to transmission signals.

A case in which the antenna device1and the communication ECU are connected by AV wires will be described as an example. Each AV wire is a low-voltage wire for automobiles, which is realized by sheathing a soft copper stranded wire with an insulating material such as polyvinyl chloride, for example. “A” in the term “AV wire” indicates low voltage automotive wires, and the “V” indicates vinyl. The AV wires connected to the antenna device1include a grounding cable that is an AV wire for providing a ground potential, and a signal cable that is an AV wire through which data signals are transmitted. A thin low-voltage wire for automobiles (AVSS cable) or a compressed conductor ultra-thin vinyl chloride insulated low-voltage wire for automobiles (CIVUS cable) can be used as a connection cable between the antenna device1and the communication ECU. “SS” in the term “AVSS” indicates an ultra-thin type. “C”, “I”, “V” and “US” in the term “CIVUS” indicate a compressed conductor type, ISO standards, vinyl, and an ultra-thin wall type, respectively. The cable connecting the antenna device1and the communication ECU may be any other communication cable such as a coaxial cable and a feeder line. An impedance matching circuit or the like may be provided at a joint portion between the antenna device1and the cable.

Hereinafter, a specific structure of the antenna device1will be described. As shown inFIG.1, the antenna device1includes a ground plate10, a support plate20, an opposing conductive plate30, and a short-circuit portion40. For convenience, each part will be described below while the antenna device1is assumed to be arranged such that a surface of the ground plate10on which the opposing conductive plate30is provided faces upward. That is, a direction from the ground plate10toward the opposing conductive plate30corresponds to an upward direction for the antenna device1. A direction from the opposing conductive plate30toward the ground plate10corresponds to a downward direction for the antenna device1.

The ground plate10is a conductive member having a plate shape and made of conductor such as copper. The ground plate10is provided along a lower surface of the support plate20. The plate shape here also includes a thin film shape such as a metal foil. That is, the ground plate10may be a pattern that is formed on a surface of a resin plate such as a printed wiring board by electroplating or the like. The ground plate10may also be provided as a conductor layer arranged inside a multilayer substrate having conductor layers and insulating layers. The ground plate10is electrically connected to a grounding cable51and provides a ground potential (in other words, ground) in the antenna device1. The ground plate10corresponds to a conductive plate that is directly or indirectly connected to the grounding cable51. The grounding cable51can also be called a grounding wire. The grounding cable51may be an outer conductor of a coaxial cable. A position of a cable connection point11at which the grounding cable51connects to the ground plate10, will be described later.

The ground plate10is formed in a rectangular shape. An electrical length of a short side of the ground plate10is set to 0.4λ, for example. Further, an electrical length of a long side of the ground plate10is set to 1.2λ. In this disclosure, an electrical length is an effective length in consideration of a fringing electric field, a wavelength shortening effect caused by a dielectric substance, and the like. This shape of the ground plate10corresponds to a rectangular shape in which a length of the ground plate10in a widthwise direction is set to be shorter than the target wavelength and a length of the ground plate10in a lengthwise direction is set to be twice the length in the widthwise direction or more. The length of the short side of the ground plate10may be 0.6λ or 0.8λ, for example. The short side of the ground plate10is longer than λ/4 at least. The length of the ground plate10in the lengthwise direction is at least longer than the length of the ground plate10in the widthwise direction, and may be 1.0λ or 1.5λ, for example. A length ratio of the short side to the long side of the ground plate10can be set to approximately 1:2, 1:3, 1:4, 2:3, or 2:5, for example. When the support plate20is formed of a dielectric material having a relative permittivity of 4.3, the target wavelength on the surface of the ground plate10is theoretically about 60 mm due to the wavelength shortening effect caused by the dielectric material of the support plate20. Therefore, the electrical length of 1.2λ corresponds to 72 mm.

The X-axis shown in the various drawings such asFIG.1represents the lengthwise direction of the ground plate10, the Y-axis represents the widthwise direction of the ground plate10, and the Z-axis represents a vertical direction. The Y-axis direction corresponds to a predetermined direction. A three-dimensional coordinate system including the X-axis, the Y-axis, and the Z-axis is a concept for describing the configuration of the antenna device1. As another aspect, when the ground plate10has a square shape, the direction along any one side can be the X-axis.

The ground plate10is at least larger than the opposing conductive plate30. The dimensions of the ground plate10can be changed as appropriate. The electrical length of one side of the ground plate10may be set to a value smaller than 1.0λ, for example, ⅓ of the target wavelength. Further, a planar shape that is a shape of the ground plate10viewed from above may be appropriately changed. Here, as an example, the planar shape of the ground plate10is a rectangular shape, but alternatively, as another aspect, the planar shape of the ground plate10may be a square shape. The planar shape of the ground plate10may be another polygonal shape. For example, the ground plate10may have a square shape in which an electrical length of one side is set to 1.0λ. The rectangular shape includes rectangle and square.

The support plate20is a plate-shaped member and causes the ground plate10and the opposing conductive plate30to be separated by a predetermined distance so as to face each other. The support plate20has a rectangular flat plate shape, and a size of the support plate20is substantially the same as a size of the ground plate10when viewed from above. The support plate20is provided as a dielectric material having a predetermined relative permittivity, such as glass epoxy resin. Here, as an example, the support plate20is provided as a glass epoxy resin having a relative permittivity of 4.3, so-called FR4 (Flame Retardant Type 4).

In the present embodiment, as an example, the thickness H1of the support plate20is 1.5 mm. The thickness H1of the support plate20corresponds to a distance between the ground plate10and the opposing conductive plate30. By adjusting the thickness H1of the support plate20, the distance between the opposing conductive plate30and the ground plate10can be adjusted. The specific value of the thickness H1of the support plate20may be appropriately determined by simulations or experiments. The thickness H1of the support plate20may be 2.0 mm or 3.0 mm, for example. The wavelength in the support plate20is about 60 mm due to the wavelength shortening effect of the dielectric material. Therefore, the value of 1.5 mm in thickness electrically corresponds to 1/40 of the target wavelength (that is, λ/40).

The support plate20fulfills at least the above-mentioned function, and the shape of the support plate20can be changed as appropriate. A configuration causing the opposing conductive plate30and the ground plate10to be arranged to face each other may be multiple columns. Further, in the present embodiment, a configuration in which a resin as a support plate20is filled between the ground plate10and the opposing conductive plate30is adopted, but the present embodiment may not be limited to this. The gap between the ground plate10and the opposing conductive plate30may be hollow or vacuum. The support plate20may have a honeycomb structure, for example. In addition, the structures exemplified above may be combined. When the antenna device1is provided as a printed wiring board, conductor layers included in the printed wiring board may be used as the ground plate10and the opposing conductive plate30, and a resin layer separating the conductor layers may be used as the support plate20.

The thickness H1of the support plate20also functions as a parameter for adjusting a length of the short-circuit portion40, as described later. In other words, the thickness H1of the support plate20functions as a parameter for adjusting an inductance provided by the short-circuit portion40. In addition, the thickness H1also functions as a parameter for adjusting a capacitance formed by the ground plate10and the opposing conductive plate30facing each other.

A transmitting/receiving circuit70may be arranged on an upper surface20aof the support plate20on which the opposing conductive plate30is arranged. The transmitting/receiving circuit70is a circuit module that performs at least one of modulation, demodulation, frequency conversion, amplification, digital-to-analog conversion, and detection. The transmitting/receiving circuit70is an electrical assembly of various parts such as an IC, an analog circuit element, and a connector. The transmitting/receiving circuit70is electrically connected to the opposing conductive plate30through a feeder line71. The feeder line71is a microstrip, for example. The transmitting/receiving circuit70is also connected to the ground plate10through vias, short-circuit pins, or the like. The transmitting/receiving circuit70is also electrically connected to an AV wire used as the signal cable. That is, the transmitting/receiving circuit70is connected to the communication ECU via the signal cable. A position of connection between the signal cable and the antenna device1can be arbitrarily set on the antenna device1.

The opposing conductive plate30is a conductive member having a plate shape and made of conductor such as copper. As described above, the plate shape here also includes a thin film shape such as copper foil. The opposing conductive plate30is arranged so as to face the ground plate10via the support plate20. Similar to the ground plate10, the opposing conductive plate30may also be a pattern formed on a surface of a resin plate such as a printed wiring board. In the present disclosure, “parallel” is not limited to a completely parallel state. For example, the expression “parallel” also includes a state inclined about 30 degrees. That is, the expression “parallel” includes a substantially parallel state. The expression “vertical” in the present disclosure is not limited to a completely vertical state, and includes a state inclined at an angle of from several degrees to about 30 degrees.

By arranging the opposing conductive plate30and the ground plate10so as to face each other, a capacitance is generated according to an area of the opposing conductive plate30and the distance between the opposing conductive plate30and the ground plate10. The opposing conductive plate30has a size so as to generate a capacitance that resonates in parallel with the inductance of the short-circuit portion40at the target frequency. The area of the opposing conductive plate30is at least appropriately designed so as to provide a desired capacitance. The desired capacitance is a capacitance that operates at the target frequency in cooperation with the inductance of short-circuit portion40. When f is the target frequency, L is the inductance of the short-circuit portion40, and C is the capacitance formed between the opposing conductive plate30and the ground plate10, a relational expression of f=1/{2π√(LC)} is established. A person skilled in this art can determine an appropriate area of the opposing conductive plate30based on the relational expression.

For example, the opposing conductive plate30is formed in a square shape having a side of an electrical length corresponding to 12 mm. Since the wavelength on the surface of the opposing conductive plate30is about 60 mm due to the wavelength shortening effect of the support plate20, 12 mm electrically corresponds to 0.2λ. Of course, the length of one side of the opposing conductive plate30may be changed as appropriate, and may be 14 mm, 15 mm, 20 mm or 25 mm, for example. The planar shape of the opposing conductive plate30may be a circle, a regular octagon or a regular hexagon, for example. Further, the opposing conductive plate30may have a rectangular shape or a long ellipse shape.

The opposing conductive plate30has a feed point31. The feed point31is a portion where the feeder line71and the opposing conductive plate30are electrically connected. In this configuration, the feed point31can be arranged at any position on the opposing conductive plate30. The feed point31is at least located at a position where an impedance matching with the feeder line71can be obtained. In other words, the feed point31is at least provided at a position where a return loss becomes a predetermined allowable level. The feed point31may be arranged at an arbitrary, for example, in a central region or an edge of the opposing conductive plate30. Here, as an example, the feed point31is positioned on a straight line passing through a center of the opposing conductive plate30and parallel to the X-axis.

As a method of feeding power to the opposing conductive plate30, various methods such as a direct connection power supply method and an electromagnetic coupling method can be adopted. The direct connection power supply method refers to a method in which the feeder line71and the opposing conductive plate30are directly connected. The electromagnetic coupling method refers to a power supply method using electromagnetic coupling between a microstrip line or the like for power supply and the opposing conductive plate30.

The short-circuit portion40is a conductive member that electrically connects the ground plate10and the opposing conductive plate30. The short-circuit portion40may be provided as a short-pin that is a conductive pin. The inductance of the short-circuit portion40can be adjusted by adjusting a diameter and a length of the short-pin of the short-circuit portion40.

The short-circuit portion40is at least a linear member having one end electrically connected to the ground plate10and the other end electrically connected to the opposing conductive plate30. When the antenna device1is provided as a printed wiring board as a base material, a via provided in the printed wiring board can be used as the short-circuit portion40.

The short-circuit portion40is, for example, located at a conductive-plate-center. Here, the conductive-plate-center is a center of the opposing conductive plate30. The conductive-plate-center corresponds to a center of gravity of the opposing conductive plate30. Since the opposing conductive plate30has the square shape in the present embodiment, the conductive-plate-center corresponds to an intersection of two diagonal lines of the opposing conductive plate30. When the ground plate10and the opposing conductive plate30are arranged to be concentric, the center of the opposing conductive plate30and a center of the ground plate10overlap in top view.

A position where the short-circuit portion40is located may not exactly coincide with the conductive-plate-center. The short-circuit portion40may be deviated by about several millimeters from the conductive-plate-center. The short-circuit portion40may be formed within a central region of the opposing conductive plate30. The central region of the opposing conductive plate30is a region inside a line connecting points that internally divide line segments from the center to edges in a ratio of 1:5. From another point of view, the central region corresponds to a region of a figure that has a similar shape of and about ⅙ the size of the opposing conductive plate30and is Concentrically Overlapped with the Opposing Conductive Plate30.

<Position of Opposing Conductive Plate30Relative to Ground Plate>

As shown inFIG.3, the opposing conductive plate30is disposed to face the ground plate10in such a manner that one set of opposite sides of the opposing conductive plate30is parallel to the X-axis and another set of opposite sides is parallel to the Y-axis. For example, the opposing conductive plate30is arranged at a position where its center is shifted from the center of the ground plate10by a predetermined offset amount ΔX in a negative direction of the X-axis. The offset amount ΔX can be, for example, 0.125λ, 0.25λ or 0.5λ, for example. The opposing conductive plate30may be aligned with an edge of the ground plate10that faces in the negative direction of the X-axis. The offset amount ΔX can be appropriately changed within a range in which the opposing conductive plate30does not protrude outward of the ground plate10when viewed from above. The opposing conductive plate30is arranged so that at least the entire region (i.e., entire surface) of the opposing conductive plate30faces the ground plate10. The offset amount ΔX corresponds to an amount of deviation between the center of the ground plate10and the center of the opposing conductive plate30.

InFIG.3, the support plate20, the transmitting/receiving circuit70, etc. are transparent in order to clarify the positional relationship between the ground plate10and the opposing conductive plate30. That is, illustrations of the support plate20, the transmitting/receiving circuit70, etc. are omitted inFIG.3. The alternate long and short dash line Lx1shown inFIG.3represents a straight line passing through the center of the ground plate10and parallel to the X-axis, and the alternate long and short dash line Ly1represents a straight line passing through the center of the ground plate10and parallel to the Y-axis. The alternate long and two short dash line Ly2represents a straight line that passes through the center of the opposing conductive plate30and is parallel to the Y-axis. From another point of view, the line Lx1corresponds to the axis of symmetry for the ground plate10and the opposing conductive plate30. The line Ly1corresponds to the axis of symmetry for the ground plate10. The line Ly2corresponds to the axis of symmetry for the opposing conductive plate30. The alternate long and short dash line Lx1also passes through the center of the opposing conductive plate30. That is, the alternate long and short dash line Lx1corresponds a straight line parallel to the X-axis and passing through the center of the ground plate10and the center of the opposing conductive plate30. The intersection of the line Lx1and the line Ly1corresponds to the center of the ground plate, and the intersection of the line Lx1and the line Ly2corresponds to the conductive-plate-center.

<Principle of Operation of Antenna Device>

Here, the operation of the antenna device1will be described. In the antenna device1, the opposing conductive plate30is short-circuited to the ground plate10by the short-circuit portion40provided in the center region of the opposing conductive plate30, and the area of the opposing conductive plate30is set to cause an electrostatic capacitance that resonates in parallel with the inductance of the short-circuit portion40at the target frequency.

Therefore, when a high-frequency signal is input from the transmitting/receiving circuit70, an LC parallel resonance occurs due to an energy exchange between the inductance and the capacitance, and a vertical electric field perpendicular to the ground plate10and the opposing conductive plate30is generated between the ground plate10and the opposing conductive plate30. This vertical electric field propagates from the short-circuit portion40toward the edge of the opposing conductive plate30. Then, at the edge of the opposing conductive plate30, the vertical electric field becomes a ground-plate vertically-polarized wave that is a linearly polarized wave with a polarization plane perpendicular to the ground plate10, and propagates through space. That is, a structure including the short-circuit portion40and the opposing conductive plate30functions as a radiating element, in other words, as an antenna element2. The ground-plate vertically-polarized wave here is a radio wave in which the vibration direction of the electric field is perpendicular to the ground plate10and the opposing conductive plate30.

The antenna device1has directivity in an antenna horizontal direction at the target frequency. When the antenna device1is installed in the vehicle with the ground plate10being horizontal, the antenna device1functions as an antenna having a main beam in the horizontal direction. The antenna horizontal direction here is a direction from the center of the opposing conductive plate30toward the edge thereof. According to another viewpoint, the antenna horizontal direction is perpendicular to a perpendicular line of the ground plate10that passes through the center of the opposing conductive plate30. The antenna horizontal direction corresponds to a transverse direction (i.e., lateral direction) of the antenna device1.

The operation for transmitting (i.e. radiating) radio waves and the operation for receiving radio waves are mutually reversible in the antenna device1. That is, the antenna device1is capable of receiving the ground-plate vertically-polarized wave coming in the antenna horizontal direction.

<Position of Cable Connection Point on Ground Plate>

In the present disclosure, the cable connection point11, which is the point of connection between the grounding cable51and the ground plate10, is located at a position shifted by a distance a equal to λ/4 from an edge (i.e., right edge in the drawing) of the ground plate10facing in a positive direction of the X-axis. Specifically, the cable connection point11is arranged at a position shifted inward from an antenna far edge12by λ/4 on the line Lx1which is passing through the center of the ground plate10and parallel to the X-axis. The antenna far edge12is farther one of the opposite edges of the ground plate10in the lengthwise direction from the opposing conductive plate30of the antenna element2. Hereinafter, the position on the ground plate10that is away by λ/4 from the antenna far edge12is also referred to as a λ/4 point.

The cable connection point11may be arranged at a position shifted from the edge of the ground plate10facing in the positive direction of the X-axis by a distance three or five times as large as λ/4. The cable connection point11is at least arranged at a position shifted by the distance a equal to λ/4×N (N is an odd number) from the edge of the ground plate10. The cable connection point11is at least arranged at the position that is an odd multiple of λ/4 away from the antenna far edge12. Thus, the position of the cable connection point11in the Y direction is not limited to on the line Lx1. The cable connection point11may be arranged at a position shifted in a positive or negative direction of the Y-axis from the position of the cable connection point11shown inFIG.3.

The grounding cable51may extend from the cable connection point11parallel to the Y-axis, or located at least λ/20 away from the ground plate10. According to this configuration, the grounding cable51can be prevented from being electrically or electromagnetically coupled to the ground plate10at locations other than the cable connection point11.

<Operations and Effects>

As a result of simulations, it has been confirmed that a current flowing through the ground plate10due to the LC parallel resonance mainly flows from the short-circuit portion40toward the edges of the ground plate10. The current that flows into the ground plate10from the opposing conductive plate30through the short-circuit portion40flows from the short-circuit portion40toward both sides of the ground plate10in the lengthwise direction. That is, the current flowing through the ground plate10flows from the short-circuit portion40toward the antenna far edge12.

Here, since the current can be zero at the antenna far edge12, as shown inFIG.4, a potential at the antenna far edge12can be maximum, and the potential can be minimum at the position shifted by λ/4×N from the antenna far edge12. The potential of the ground plate10at a point where the potential is minimum does not change even when a conductor approaches to the point. Therefore, also a current at the point where the potential is minimum does not change even when a conductor approaches the point. Consequently, according to the configuration in which the cable connection point11is arranged at the position that is an odd multiple of λ/4 away from the antenna far edge12, it is possible to reduce a leakage current from the ground plate10to the grounding cable51.

FIG.5andFIG.6show results of analyses on change in directivity with and without the grounding cable51when the cable connection point11is shifted from the antenna far edge12by λ/2 and by λ/4, respectively.FIG.5shows the directivity simulation result when the cable connection point11is located λ/2 away from the antenna far edge12, andFIG.6shows the directivity simulation result when the cable connection point11is located λ/4 away from the antenna far edge12. InFIGS.5and6, the dashed lines show simulation results of the directivity when the grounding cable51does not exist, and the solid lines show the directivity simulation results when the grounding cable51exists. Thus, the gap between the dashed line and the solid line in each ofFIGS.5and6indicates a degree of influence on the directivity by the grounding cable51. As is clear from a comparison ofFIGS.5and6, according to the configuration in which the cable connection point11is arranged at the λ/4 point, the change in directivity due to the grounding cable51can be reduced. The change in directivity is caused by the leakage current to the grounding cable51.FIGS.5and6indirectly show that the leakage current to the grounding cable51can be reduced by the configuration in which the cable connection point11is set to the λ/4 point.

As described above, according to the above configuration, even when the size of the ground plate10is insufficient for the target wavelength, an amount of the current leakage to the grounding cable51can be reduced. The above described manner of connecting the cable to the ground plate10is particularly effective in a configuration in which the length of the ground plate10in the widthwise direction is less than 0.75λ. In addition, the above described arrangement of the cable works particularly well in a configuration in which the ground plate10has the rectangle shape having the lengthwise direction and the antenna element2is adjacent to the edge of the ground plate10that faces in the lengthwise direction. This is because antinodes and nodes in the voltage distribution are likely to be formed. In the configuration in which the antenna element2is adjacent to the edge of the ground plate10that faces in the lengthwise direction, the current on the ground plate10flows toward the antenna far edge12that is the opposite edge from the antenna element2, thereby causing the antinodes and nodes.

Since the antenna element2including the opposing conductive plate30and the short-circuit portion40causes LC parallel resonance, the antenna device1can transmit and receive the ground-plate vertically-polarized wave in the antenna horizontal direction. The antenna element2may be a monopole antenna as another embodiment capable of transmitting/receiving the ground-plate vertically-polarized wave. However, the embodiment using the monopole antenna as the antenna element2requires a height of λ/4. On the other hand, the antenna device1described above can be realized with a height (i.e., thickness) of about λ/100. That is, according to the configuration of the above-described disclosure, the height of the antenna device1can be reduced.

In addition, arranging the cable connection point11at the position shifted by an odd multiple of λ/4 from the edge of the ground plate10can reduce the leakage current to the grounding cable51without providing a circuit element such as a low-pass filter. It is possible to achieve both reduction of manufacturing costs and stabilization of antenna characteristics.

<Use of Antenna Device>

The antenna device1described above, for example, as shown inFIG.7, is used while being attached to an outer surface of a B pillar91of a vehicle, at least in an orientation where the ground plate10faces the surface of the B pillar91, and the X-axis is along a longitudinal direction of the B pillar91(i.e., vehicle height direction). Alternatively, the antenna device1may be attached in the same orientation described above to a portion inside a door panel that overlaps with the B pillar51.

According to the above attachment state, a positive direction of the Z-axis, which is the upward direction of the antenna device1, roughly corresponds to a width direction of the vehicle, and the antenna horizontal direction is along (i.e., parallel to) a lateral surface of the vehicle. According to this attachment state, it is possible to form a communication area along the lateral surface of the vehicle.

The attachment position and attachment orientation of the antenna device1may not be limited to the above examples. The antenna device1may be attached to an arbitrary position on the outer surface of the vehicle, such as an outer surface of an A-pillar92or a C-pillar, a rocker portion (i.e., side sill)94, and an inside/vicinity of an outer door handle95. For example, the antenna device1may be housed inside the outer door handle95such that the X-axis is along a longitudinal direction of the handle and the Y-axis is along the vehicle height direction. Also, the antenna device1may be installed in a roof portion93.

Although the embodiment of the present disclosure has been described above, the present disclosure is not limited to the above-mentioned embodiment, and various supplements and modifications described below are also included in the technical scope of the present disclosure. Furthermore, in addition to the following, various changes can be made within the range that does not deviate from the scope. For example, various modifications to be described below can be executed in combination as appropriate within a scope that does not cause technical inconsistency.

<Regarding Antenna Element>

In the above-described embodiment, the antenna device1has the configuration including the opposing conductive plate30and the short-circuit portion40as the antenna element2. In other words, a configuration in which a zeroth-order resonant antenna is used as the antenna element2has been disclosed above, but the antenna element2is not limited to the zeroth-order resonant antenna. The antenna element2may be a monopole antenna or a patch antenna. The antenna element2may be an inverted F antenna or a loop antenna. Various antenna configurations can be adopted as the antenna element2of the antenna device1.

<Manner of Connecting Grounding Cable to Ground Plate>

The grounding cable51may be vertically connected to the ground plate10at the cable connection point11using a connector52as shown inFIG.8. This connection manner can reduce electrical or electromagnetic coupling between the grounding cable51and the ground plate10at locations other than the cable connection point11.

As another connection manner, as shown inFIG.9, in a case where the ground plate has a slit13extending from the antenna far edge12to the λ/4 point with a width W, the grounding cable51may extend along a center line of the slit13. In the case, the grounding cable51is connected to the ground plate10at an innermost end of slit13in an inward direction. Here, the inward direction is a direction in which the slit13extends from the antenna far edge12to the opposite edge. In the connection configuration shown inFIG.8, since the connector52is perpendicular to the ground plate10, an overall height of the antenna device1is increased by a height of the connector52. On the other hand, according to the connection configuration shown inFIG.9, since the connector52is parallel to the ground plate10, the height of the antenna device1can be reduced. Therefore, ease of installation of the antenna device1in a space having a small thickness, such as B-pillar91, can be improved. The width W is at least wide enough to prevent electromagnetic coupling between the grounding cable51, which extends through the center of the slit13, and the ground plate10. For example, the width W can be λ/10 or more. According to this configuration, a distance between the grounding cable51and the ground plate10along the Y-axis is approximately λ/20 or more, and electromagnetic coupling can be reduced.

An insulating layer similar to the support plate20may be provided under the ground plate10. That is, the ground plate10may be provided as an inner layer of the printed circuit board. When the ground plate10is provided as an inner layer of an printed multilayer board, it is difficult to attach the connector52to the innermost end of the slit13. Therefore, as shown inFIG.10, the antenna device1may have an extension line14that is a conductive line extending from the λ/4 point11ato the antenna far edge12through the center of the slit13. The grounding cable51may be connected to an outer end of the extension line14. The extension line14corresponds to, for example, a circuit trace. The extension line14may be a micro strip or a strapline. According to this configuration, while the connector52is placed at the antenna far edge12, a substantial connection point can be set at the λ/4 point. Since the extension line14is connected to the grounding cable51in series, this configuration corresponds to a configuration in which the grounding cable51is electrically connected to the ground plate10via the extension line14. The extension line14may be at least mounted on an insulating layer that is formed over or under the ground plate10as the support plate20. When the antenna device1is provided as a multilayer substrate including multiple conductor layers and insulating layers, the signal cable can be electrically connected at any point on a conductor layer different from the conductor layer functioning as the ground plate10.

<Supplement to Shape and Position of Opposing Conductive Plate Relative to Ground Plate>

The ground plate10has at least a substantially rectangular shape, and thus may have rounded corners. A part or whole of the edge of the ground plate10may have a meander shape. The rectangular shape also includes a rectangular shape having minute projections and recesses on its edge. The projections and recesses provided on the edge of the ground plate10and the slit formed at a position away from the edge of the ground plate10can be ignored in design of the external shape of the ground plate10, as long as they do not affect the operations of the antenna device1. Here, the minute projections and recesses have sizes of about several millimeters.

The opposing conductive plate30may have slits or rounded corners. For example, a notch as a degeneracy separation element may be provided at a pair of corner portions diagonally facing each other. A part or whole of the edge of the opposing conductive plate30may have a meander shape. Projections and recesses provided at the edge of the opposing conductive plate30that do not affect the operations of the antenna device1can be ignored.

The shape of the ground plate10and the arrangement of the opposing conductive plate30relative to the ground plate10may not be limited to the configuration disclosed as the embodiment. The arrangement of the opposing conductive plate30relative to the ground plate10may be modified variously as illustrated inFIGS.11to13. For example, as shown inFIG.11, the opposing conductive plate30may be arranged so that an end of the opposing conductive plate30facing in the negative direction of the X-axis is aligned with an end of the ground plate10facing in the negative direction of the X-axis. InFIGS.11to13, illustrations of the support plate20, the transmitting/receiving circuit70, etc. are omitted in order to clarify the positional relationship between the ground plate10and the opposing conductive plate30. The dimensions of each drawing are examples and can be changed as appropriate.

Lx2shown inFIG.12is a straight line passing through the center of the opposing conductive plate30and parallel to the X-axis. AX inFIG.12represents an amount of offset of the opposing conductive plate30relative to the ground plate10along the X-axis, and MY represents an amount of offset in the Y-axis direction. AX and MY may have the same value or different values. The configuration disclosed inFIG.12corresponds to a configuration in which the opposing conductive plate30is arranged so as to be displaced by a predetermined amount along the X-axis and a predetermined amount along the Y-axis from a position concentric with the ground plate10.

An edge of the ground plate10used as a reference in setting the cable connection point11is not limited to the edge in the longitudinal direction. As shown inFIG.13, the cable connection point11may be arranged at a position that is an odd multiple of λ/4 away from an edge that is relatively distant from the short-circuit portion40among the edges in the widthwise direction of the ground plate10.

<Supplement to Overall Configuration of Antenna Device>

The antenna device1may include a case60for accommodating the ground plate10, the opposing conductive plate30, and the support plate20on which the short-circuit portion40is formed, as shown inFIG.14.FIG.14is a schematic diagram showing an internal configuration of the case60. In order to ensure the visibility of the drawing, hatching indicating the material type is omitted in theFIG.14. The case60is formed by combining, for example, an upper case and a lower case that are vertically separable. The case60is made of, for example, a polycarbonate (PC) resin. Various resins, such as synthetic resin obtained by mixing acrylonitrile-butadiene-styrene copolymer (so-called ABS) with PC resin, and polypropylene (PP), can be adopted as the material of the case60. The case60includes a case bottom portion61, a side wall portion62, and a case top plate portion63. The case bottom portion61provides a bottom of the case60. The case bottom portion61is formed in a flat plate shape. In the case60, a circuit board including the ground plate10, the opposing conductive plate30, the transmitting/receiving circuit70and the like is arranged so that the ground plate10faces the case bottom portion61.

The side wall portion62provides a side surface of the case60, and extends upward from an edge portion of the case bottom portion61. A height of the side wall portion62is designed so that, for example, a distance between an inner surface of the case top plate portion63and the opposing conductive plate30is λ/25 or less. The case top plate portion63provides an upper surface portion of the case60. The case top plate portion63in this embodiment is formed in a flat plate shape. The shape of the case top plate portion63may be various other shapes such as a dome shape. An inner surface of the case top plate portion63faces the upper surface20aof the support plate20. The side wall portion62has a cable lead-out portion64that is a hole through which the grounding cable51and the like are lead out. According to the configuration in which the cable lead-out portion64is arranged at the side wall portion62, it is possible to improve the ease of installation of the antenna device1in the B-pillar91or the like.

When the case top plate portion63is disposed in a region close to the opposing conductive plate30as in the above configuration, a wave of the vertical electric field radiated by the LC resonance mode can be prevented from propagating around the edge of the opposing conductive plate30to its upper side. Thus, a radiation gain in the antenna horizontal direction can be increased. The “region close to the opposing conductive plate30” is, for example, a region stretching from the opposing conductive plate30by an electrical length of 1/25 or less of the target wavelength.

In addition, as shown inFIG.14, the case top plate portion63may have an upper rib631that is in contact with the edge of the opposing conductive plate30. The upper rib631is formed on the inner surface of the case top plate portion63and protrudes downward. The upper rib631is formed so as to be in contact with the edge of the opposing conductive plate30. The upper rib631fixes the position of the support plate20in the case60, obstructs the propagation of the ground-plate vertically-polarized wave from the edge of the opposing conductive plate30to its upper side, and increases the radiation gain in the antenna horizontal direction. A metal trace such as copper foil may be printed on a vertical surface, i.e., an outer surface, of the upper rib631that is continuously connected to the edge of the opposing conductive plate30.

In addition, the inside of the case60is filled with a sealing material65such as silicon The sealing material65may be a urethane resin such as polyurethane prepolymer. The sealing material65may be selected from among various other materials such as epoxy resin and silicone resin. According to the configuration in which the case60is filled with the sealing material65, the sealing material65located above the opposing conductive plate30obstructs the propagation of the ground-plate vertically-polarized wave from the edge of the opposing conductive plate30to its upper side, thereby exerting the effect of increasing the radiation gain in the antenna horizontal direction. At least the side wall portion and the case top plate portion of the case60may be made of resin or ceramic having a predetermined relative permittivity. Further, according to the configuration in which the sealing material65fills the case60, waterproofness, dustproofness, and vibration resistance can be improved.

Of course, the filling of the case60with the sealing material65is an optional element. The upper rib631may be also an optional element. The case top plate portion63, the upper rib631, and the sealing material65correspond to a radio wave shield body that obstructs the propagation of the wave of the vertical electric field radiated by the LC resonance mode from the edge of the opposing conductive plate30to its upper side. The configuration disclosed above corresponds to a configuration in which the radio wave shield body containing a conductor or a dielectric material is arranged on the upper side of the opposing conductive plate30.

Either of the case bottom portion61or the case top plate portion63included in the case60may be omitted. When either the case bottom portion61and the case top plate portion63is omitted, the sealing material65may be a resin that is in a solid state within a predetermined operating temperature range assumed as a temperature range of an environment in which the antenna device1is used. The operating temperature range can be, for example, −30° C. to 100° C. A configuration in which one of the case bottom portion61and the case top plate portion63is omitted corresponds to a case in which the top surface or the bottom surface of the case is an opening.

While the present disclosure has been described with reference to embodiments thereof, it is to be understood that the disclosure is not limited to the embodiments and constructions. To the contrary, the present disclosure is intended to cover various modification and equivalent arrangements. In addition, while the various elements are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the present disclosure.