Antenna device

An antenna device includes a first antenna conductor, a ground conductor, an artificial magnetic conductor sandwiched between the first antenna conductor and the ground conductor, and disposed separately from the first antenna conductor and the ground conductor, and a second antenna conductor disposed on a side opposite to the artificial magnetic conductor across the first antenna conductor and disposed furthest away from the ground conductor.

BACKGROUND

1. Technical Field

The present disclosure relates to an antenna device.

2. Description of the Related Art

Patent Literature (PTL) 1 discloses an antenna device including an artificial magnetic conductor (AMC) reflection plate that uses an AMC.

SUMMARY

It is an object of the present disclosure to provide an antenna device that easily adjusts an operation frequency applicable for wireless communication and maintains frequency characteristics of an operation frequency band.

The present disclosure is an antenna device including a first antenna conductor, a ground conductor, an artificial magnetic conductor sandwiched between the first antenna conductor and the ground conductor, and disposed separately from the first antenna conductor and the ground conductor, and a second antenna conductor disposed on a side opposite to the artificial magnetic conductor across the first antenna conductor and disposed furthest away from the ground conductor.

According to the present disclosure, an antenna device can easily adjust an operation frequency applicable for wireless communication and maintain frequency characteristics of an operation frequency band.

DETAILED DESCRIPTION

(Circumstance that Leads to the Present Disclosure)

In an antenna device of known art, e.g., PTL 1, an AMC reflection plate is disposed in an intermediate layer in the entire antenna device. Therefore, when the antenna device is manufactured and attached in an actual arrangement environment, it has been difficult to adjust an operation frequency (i.e., communication frequency) band applicable for wireless communication performed by the antenna device. For example, when the antenna device is attached in the actual arrangement environment (e.g., in a space where metal is provided), the operation frequency band corresponding to the antenna device can be shifted to a high frequency side. In the case of occurrence of such shift, in order to finely adjust the operation frequency band to match a desired frequency band (e.g., 2450 MHz in the case of Bluetooth (registered trademark)), an operation, e.g., adjustment of the length of a patch element of the AMC reflection plate, has been needed. In other words, an operation of remaking an antenna device is practically generated, causing a reduction in convenience of an operator.

Thus, in an exemplary embodiment below, a description is given of an example of an antenna device that easily adjusts an operation frequency applicable for wireless communication and maintains frequency characteristics of an operation frequency band. For example, as the operation frequency band of the antenna device, 2.45 GHz band of Bluetooth (registered trademark) is indicated. Note that the operation frequency band of the antenna device may not be a frequency band of Bluetooth (registered trademark), but may be a frequency band corresponding to wireless local area network (LAN), e.g., Wi-Fi (registered trademark).

The exemplary embodiment that specifically discloses the antenna device of the present disclosure is described in detail below with reference to the drawings properly. However, a detailed description more than necessary may be omitted. For example, a detailed description of a well-known matter or a redundant description regarding the substantially same configuration may be omitted. The reason for this is to avoid unnecessary redundancy of the following description and to help a person of ordinary skill in the art to achieve easy understanding. The accompanying drawings and the following description are provided in order for a person of ordinary skill in the art to get a sufficient understanding of the present disclosure, and therefore, this is not intended to impose a limitation on a subject matter that is recited in a claim.

The antenna device according to the exemplary embodiment below is used, for example, in an electronic device mounted in an aircraft. In the case of an economy class, for example, the antenna device is disposed in a housing of a seat monitor set on the rear surface of a seat of the aircraft. In the case of a first class, for example, the antenna device is disposed in a housing of a cabin monitor set on a wall surface of a cabin. Examples of intended purposes of the antenna device include not only the monitor, but also many IoT (Internet of Things) devices including a main phone and a secondary phone of a cordless telephone unit, an electronic shelf label (e.g., a card-type electronic device that is attached to a store shelf in a retail store and displays a selling price of a product), a smart speaker, an automotive device, a microwave oven, and a refrigerator.

The antenna device of the exemplary embodiment includes a dipole antenna that forms a parallel resonant circuit. The dipole antenna is formed such that a metal foil on a surface of a printed circuit board, which is a laminated board, is, for example, etched away. The laminated board is formed of a plurality of layers including a copper foil and glass epoxy.

FIG. 1is an external perspective view of antenna device101according to the exemplary embodiment. Antenna device101includes printed circuit board1having an elongated plate shape. Front surface1aof printed circuit board1is a secondary element surface on which secondary element15is centrally disposed. Back surface1bof printed circuit board1is a ground conductor surface on which ground conductor8(seeFIG. 2) is formed entirely. Here, a direction perpendicular to the surface of printed circuit board1is an x direction. A direction parallel to and extending longitudinally along the surface of printed circuit board1is a y-direction. A direction parallel to and extending transversely along the surface of printed circuit board1is a z direction.

FIG. 2is a vertical cross-sectional view taken along line2-2ofFIG. 1. Printed circuit board1is a laminated board on which dielectric substrate12on which ground conductor8is formed, dielectric substrate11on which artificial magnetic conductor (i.e., AMC7) is formed, dielectric substrate10on which antenna conductors2,3(an example of a first antenna conductor) and parasitic conductor6(seeFIG. 3) are formed, and dielectric substrate14on which secondary element15(an example of the second antenna conductor) is formed are stacked in order. Dielectric substrates10,11,12,14(an example of the substrate) are formed, for example, of glass epoxy. AMC7is an artificial magnetic conductor having perfect magnetic conductor (PMC) characteristics, and is formed of a predetermined metal (e.g., copper) pattern. AMC7is stacked for a reduction in thickness and an increase in gain of antenna device101. Note that, here, dielectric substrate11on which AMC7is formed is separated from dielectric substrate12on which ground conductor8is formed. However, the AMC may be formed on the surface (surface in the x-direction) of a common dielectric substrate, and the ground conductor may be formed on the back surface (surface in a −x direction).

FIG. 3is a plan view illustrating each layer constituting antenna device101. Antenna device101includes a ground (GND) layer including ground conductor8, an AMC layer including AMC7, an antenna layer including antenna conductors2,3and parasitic conductor6, and a secondary element layer including secondary element15.

The antenna layer includes antenna conductor2, which is a strip conductor as an example of the feed antenna, antenna conductor3, which is a strip conductor as an example of the parasitic antenna, and parasitic conductor6disposed on sides of antenna conductors2,3. Antenna conductors2,3have, as an example, a width dimension of 1 mm. Antenna conductor2is an example of the feed-side antenna conductor. Antenna conductor3is an example of the ground-side antenna conductor.

Here, the longitudinal direction of antenna device101and antenna conductors2,3is a y-axis direction (seeFIG. 1). The width direction of antenna device101and antenna conductors2,3is a z-axis direction (seeFIG. 1). The thickness direction of antenna device101is an x-axis direction perpendicular to an xy plane (seeFIG. 1).

In printed circuit board1, via conductors4,5are formed in substantially opposite positions immediately below respective feedpoints Q1, Q2. Note that printed circuit board1of antenna device101may be mounted, for example, on a printed circuit board of an electronic device.

Parasitic conductor6is electrically separated from antenna conductors2,3. Antenna conductors2,3are connected respectively to via conductors4,5of printed circuit board1. Via conductor4constitutes a feed wire between feedpoint Q1of antenna conductor2and a wireless communication circuit (not illustrated). The wireless communication circuit is mounted, for example, on back surface1bof printed circuit board1. Via conductor5constitutes a ground wire between feedpoint Q2of antenna conductor3and the aforementioned wireless communication circuit.

Antenna conductors2,3are formed on the surface of dielectric substrate10to constitute a dipole antenna such that the longitudinal direction extends on a straight line in the y direction and in the −y direction and ends of antenna conductors2,3adjacent to respective feedpoints Q1, Q2are separated from each other at a predetermined distance.

Parasitic conductor6is disposed adjacently to antenna conductors2,3with a predetermined distance. The predetermined distance is, for example, within a quarter of received radio wave wavelength. Parasitic conductor6is disposed on one side surface side of antenna conductors2,3so as to be in parallel to a direction that antenna conductors2,3are disposed (i.e., in the y direction and the −y direction). As parasitic conductor6is electrostatically coupled to AMC7similar to antenna conductors2,3, parasitic conductor6can increase electrostatic capacitance between antenna conductors2,3and AMC7and shift a radio frequency handled by antenna device101to a low frequency side. Note that a size, a shape, a number, and the like of parasitic conductor6are not particularly limited. As long as parasitic conductor6is present on the same side of antenna conductors2,3and electrostatically coupled to AMC7, parasitic conductor6may not be disposed on the same surface as antenna conductors2,3, but may be disposed on the same surface as AMC7.

Via conductors4,5are formed such that a conductor is charged into an open hole, which is a through-hole or a via hole, formed in the direction of the thickness through front surface1aand back surface1bof printed circuit board1. Antenna conductor2, which functions as a feed antenna, is connected via via conductor4to a power feed terminal of the wireless communication circuit (see the above) on back surface1bof printed circuit board1. Moreover, antenna conductor3, which functions as a parasitic antenna, is connected via via conductor5to AMC7and ground conductor8of printed circuit board1, and a ground terminal of the wireless communication circuit (see the above).

Via conductor4is a feed wire having, for example, a cylindrical shape and feeding electric power for driving antenna conductor2as an antenna. Via conductor4electrically connects antenna conductor2formed on front surface1aof printed circuit board1to the power feed terminal of the wireless communication circuit (see the above). Via conductor4is formed to be substantially coaxial with via conductor insulation holes17,18formed on AMC7and ground conductor8, respectively, so as not to be electrically connected to AMC7and ground conductor8. Via conductor4has a diameter smaller than the diameters of via conductor insulation holes17,18(seeFIG. 2).

Meanwhile, via conductor5electrically connects antenna conductor3to the ground terminal of the wireless communication circuit (see the above). Via conductor5is electrically connected to ground conductor8and AMC7. The surface of the AMC layer, which corresponds to antenna conductor2, and the surface of the ground (GND) layer are not connected (i.e., non-conductive), and the surface of the antenna layer and the surface of the AMC layer, which correspond to antenna conductor3, and the surface of the GND layer are connected (i.e., conductive). However, via conductor5may not be electrically connected to AMC7, and the surface of the AMC layer, which correspond to antenna conductor3, and the surface of the GND layer may not be connected.

As illustrated inFIG. 3, slit71is formed to extend through AMC7in a central portion in the y-axis direction to a vicinity of ends in the width direction. Slit71is a portion of the AMC layer where the artificial magnetic conductor is not formed. Slit71can separate AMC7in accordance with the positions of antenna conductors2,3to increase electrostatic coupling between antenna conductor2and a right half portion of AMC7(i.e., the −y direction illustrated inFIG. 3) and electrostatic coupling between antenna conductor3and a left half portion of AMC7(i.e., the y direction illustrated inFIG. 3). Note that slit71may be formed to reach both ends of AMC7in the width direction to completely separate AMC7into two.

Ground conductor8is an earth region connected to the ground terminal of the wireless communication circuit (see the above). Ground conductor8includes via conductor insulation hole18formed to cause via conductor4to extend through and to be electrically insulated from ground conductor8and a hole formed to cause via conductor5to extend through and to be electrically insulated from ground conductor8.

In antenna device101, the plane shape of AMC7is, as compared with the plane shape of ground conductor8, slightly smaller (substantially the same) in the length direction and the width direction. Moreover, AMC7and ground conductor8are formed to face each other and to be overlapped at a predetermined interval in the thickness direction. Specifically, ground conductor8has a plane shape having the same dimension as the surface of dielectric substrate12(as one example, width of 6 mm). AMC7is formed to have a width of 5 mm to leave a margin (clearance) of 0.5 mm at ends in up-and-down direction (z direction and −z direction) with respect to dielectric substrate11having a width of 6 mm. Accordingly, the length of AMC7in the longitudinal direction is formed to be substantially the same as the length of ground conductor8in the longitudinal direction. Thus, one of AMC7and ground conductor8does not protrude over the other, making a contribution to reducing the size of printed circuit board1, eventually resulting in a reduction in size of antenna device101.

Secondary element15is provided to improve the antenna performance of antenna device101. Secondary element15is disposed at the center of the surface of dielectric substrate14and is formed of a copper foil to have an elongated plate shape. The dimension of secondary element15is, as an example, a length of 10 mm and a width of 1 mm. Secondary element15is stacked and exposed on the surface of antenna device101. Therefore, the dimension can be adjusted after manufacture of antenna device101. Secondary element15includes feed-side terminal15pof via conductor4that is inserted into hole21through which via conductor4extends and conductively connected to secondary element15, and ground-side terminal15qof via conductor5that is inserted into hole22through which via conductor5extends and conductively connected to secondary element15.

A use state of antenna device101having the aforementioned configuration is indicated.

Antenna device101is, as an example, incorporated into a metal frame attached to the front surface of the interior of the housing of the cabin monitor.FIG. 4is a partially enlarged cross-sectional view illustrating metal frame200into which antenna device101is incorporated. Metal frame200is formed of a metal material, e.g., steel. Metal frame200provides support to reinforce protective glass, which is a part of a liquid crystal display that is fit inside. At an upper portion of metal frame200, pocket210having a rectangular hollow shape is formed. Antenna device101is fixed to a bottom surface of pocket210with an adhesive, a screw, or the like. When antenna device101is fixed to the bottom surface of pocket210, the distance between antenna device101and the bottom surface and the back surface of metal frame200facing antenna device101is kept constant. When the distance between the metal that becomes closer in the space where antenna device101is incorporated and antenna device101becomes constant, the antenna performance of antenna device101that transmits and receives an electromagnetic wave of a high frequency band, e.g., a microwave, becomes stable. Moreover, cover220having an L angled shape is fit to a peripheral portion of pocket210of metal frame200to cover pocket210. The material of cover220is nonmetal, e.g., resin. Note that here is indicated the case where the two surfaces of the pocket into which the antenna device is incorporated, the bottom surface and the back surface, are a metallic frame, but only one surface, i.e., the bottom surface, may be a metallic frame, or the three surfaces, the bottom surface, the back surface, and the upper surface, may be a metallic frame.

Moreover, the antenna device may be bonded to a back side of the protective glass with a double-sided tape. When the antenna device is bonded to the protective glass with a double-sided tape, under the absence of the secondary element layer, the distance between the surface of the antenna layer and the surface of the protective glass varies with the thickness of the double-sided tape. Therefore, when the thickness of the double-sided tape is not constant due to the material or the like, the distance between the antenna conductor disposed on the surface of the antenna layer and the metal frame present behind the antenna device is not stable, which affects the antenna performance. Meanwhile, in the exemplary embodiment, because the secondary element layer is provided on the front surface of the antenna layer, the distance between the surface of the antenna layer and the surface of the protective glass varies with the thickness of the double-sided tape and the thickness of the secondary element layer. Because the thickness of the secondary element layer is constant, even when the thickness of the double-sided tape is not constant due to the material or the like, variations in distance between the surface of the antenna layer and the surface of the protective glass are mitigated as a whole. Thus, variations in distance with respect to the metal frame present behind antenna device101are suppressed, thereby suppressing an adverse effect on the antenna performance.

FIG. 5is a view illustrating cabin monitor250set in cabin150of an aircraft, and passenger hm. Passenger hm is assumed to watch cabin monitor250in a state of leaning against seat300in cabin150. One part of the upper portion of metal frame200of cabin monitor250is covered with cover220. Antenna device101is incorporated into metal frame200covered with cover220. Passenger hm wears headphone310that can receive a radio wave for short range communication (e.g., radio waves of 2.4 GHz band). Headphone310receives, for example, a radio wave of 2.4 GHz band that is transmitted by antenna device101in the direction of passenger hm (x direction), and, on the basis of an audio signal included in the received signal, outputs an audio synchronized with a video shown on cabin monitor250.

Next, characteristics of radio frequency of antenna device101of the exemplary embodiment are described.

FIG. 6is a graph illustrating a change in gain with respect to a frequency in an X-Y plane with respect to antenna device101with secondary element15of the exemplary embodiment and an antenna device without a secondary element of a comparative example. The horizontal axis of the graph indicates a frequency of 2.40 GHz to 2.48 GHz band. The vertical axis of the graph indicates mean effective gain (MRG).

In the case of antenna device101with secondary element15, as indicated by graph g21, the gain in the X-Y plane is high, indicating a value around 3.5 dBi to 4 dBi in the frequency bandwidth of 2.40 GHz to 2.48 GHz. Meanwhile, in the case of the antenna device without the secondary element, as indicated by graph g22, the gain is lower than the gain of antenna device101, indicating a value around 1.5 dBi to 2.5 dBi in the frequency bandwidth of 2.40 GHz to 2.48 GHz. Thus, the antenna device including the secondary element increases the gain in the X-Y plane of the antenna device.

FIG. 7is a graph illustrating a change in gain with respect to a frequency in the X-Z plane with respect to antenna device101with secondary element15of the exemplary embodiment and the antenna device without the secondary element of the comparative example. The horizontal axis of the graph indicates a frequency of 2.40 GHz to 2.48 GHz band. The vertical axis of the graph indicates mean effective gain (MRG).

In the case of antenna device101with secondary element15, as indicated by graph g23, the gain in the X-Z plane is high, indicating a value around 3.5 dBi to 5.5 dBi in the frequency bandwidth of 2.40 GHz to 2.48 GHz. Meanwhile, in the case of the antenna device without the secondary element, as indicated by graph g24, the gain is lower than the gain of antenna device101, indicating a value around 2.5 dBi to 3.5 dBi in the frequency bandwidth of 2.40 GHz to 2.48 GHz. Thus, the antenna device including the secondary element increases the gain in the X-Z plane of the antenna device.

FIG. 8is a view for explaining length L of secondary element15. As described above, secondary element15is disposed at the center of the surface of dielectric substrate14and is formed of a copper foil to have an elongated plate shape. Secondary element15is disposed on the surface of dielectric substrate14, which is the outermost of antenna device101. Therefore, the dimension of secondary element15can be adjusted by cutting or the like even after manufacture of antenna device101. Length L (distance in the y direction) and width W (distance in the z direction) of secondary element15, which are dimensions of secondary element15, are changed. Note that the thickness (distance in the x direction) of secondary element15may be changed.

FIG. 9is a graph illustrating a change in antenna characteristics of antenna device101in a case where length L of secondary element15is changed. The vertical axis indicates a voltage standing wave ratio (VSWR), and the horizontal axis indicates a frequency. The graph illustrated inFIG. 9indicates center frequencies of VSWR and bandwidths corresponding to three different lengths L. The VSWR represents the degree of impedance matching (that is to say, degree of reflection) by a rate of a traveling wave and a reflected wave in a standing wave. In particular, the VSWR is calculated using a rate of maximum amplitude and minimum amplitude of a voltage of a radio wave that is a standing wave. The closer the VSWR is to 1, the less the reflected wave and the impedance matching is achieved. Accordingly, the closer the VSWR is to 1, the higher the transmission efficiency of a radio wave. Moreover, in the exemplary embodiment, a frequency band with a VSWR of less than or equal to 3.0 is determined as a fractional bandwidth, and whether the frequency band is a wide band or a narrow band is determined by the fractional bandwidth. The fractional bandwidth is calculated when the bandwidth with a VSWR of less than or equal to 3.0 is divided by the center frequency.

FIG. 9indicates, as an example, the center frequency of the VSWR and the fractional bandwidth in a frequency band near 2.2 GHz. When length L of secondary element15is 5 mm, the center frequency of the VSWR is 2.32 GHz. When length L is 10 mm, the center frequency of the VSWR is 2.26 GHz. When length L is 15 mm, the center frequency of the VSWR is 2.18 GHz. Thus, center frequency of the VSWR shifts to low frequency with an increase in length of the secondary element15. In setting the communication frequency, by increasing length L of secondary element15, the communication frequency can be adjusted to shift to a low frequency side. Moreover, by reducing length L of secondary element15, the communication frequency can be adjusted to shift to a high frequency side.

Moreover, when the curve of the VSWR is assumed to be substantially symmetrical relative to the center frequency, the fractional bandwidth is a value obtained when the bandwidth from the center frequency to the high frequency-side frequency where the VSWR is 3.0 is doubled. When length L is 5 mm, the fractional bandwidth of the VSWR is 0.55 GHz×2. When length L is 10 mm, the fractional bandwidth of the VSWR is 0.9 GHz×2. When length L is 15 mm, the fractional bandwidth of the VSWR is 1.1 GHz×2. Thus, the longer the length of secondary element15, the larger the value of the fractional bandwidth of the VSWR. That is, a change to wide band is promoted. Accordingly, it is possible to make adjustment to increase the fractional bandwidth of the VSWR by increasing length L of secondary element15. Such shifting of the communication frequency to a low frequency side and a change to a wide band are presumable due to the fact that an increase in width of secondary element15increases the electrical length (path length) of AMC7, thereby causing parallel resonance to occur easily.

FIG. 10is a view illustrating surfaces of secondary element layers on which secondary elements15having different width W are disposed.FIG. 10illustrates surfaces of secondary element layers having width W of 0.6 mm, 1.0 mm, 1.5 mm, and 2.0 mm. Note that, as another example, a surface of a secondary element layer in a case where two via conductors are not conductively connected to a secondary element is indicated. Note that the two via conductors and the secondary element of the aforementioned another example correspond to via conductors4,5and secondary element15of the exemplary embodiment, respectively.

FIG. 11is a graph illustrating frequency characteristics of VSWR corresponding to width W of secondary element15. When width W of secondary element15is 0.6 mm, as indicated by graph g11, the center frequency of the VSWR is 2.22 GHz and the fractional bandwidth is about 0.26 GHz. When width W of secondary element15is 1.0 mm, as indicated by graph g12, the center frequency of the VSWR is 2.18 GHz and the fractional bandwidth is about 0.26 GHz. When width W of secondary element15is 1.5 mm, as indicated by graph g13, the center frequency of the VSWR is 2.16 GHz and the fractional bandwidth is about 0.26 GHz. When width W of secondary element15is 2.0 mm, as indicated by graph g14, the center frequency of the VSWR is 2.11 GHz.

Thus, the center frequency of antenna device101shifts to a low frequency side with an increase in width W of the secondary element15. This is presumable due to the fact that an increase in width of secondary element15increases the electrical length (path length) of AMC7, thereby causing parallel resonance to occur easily. However, no large change can be seen regarding the fractional bandwidth. Accordingly, in setting the operation frequency, by increasing width W of secondary element15, it is possible to make adjustment to shift the operation frequency to a low frequency side. Moreover, by reducing width W of secondary element15, it is possible to make adjustment to shift the operation frequency to a high frequency side.

Note that when two via conductors4,5are not conductively connected to secondary element15, as indicated by graph g15, the center frequency of the VSWR is as high as 2.38 GHz and the fractional bandwidth is as narrow as 0.16 GHz. In other words, for example, when two via conductors4,5are connected (conductive) to secondary element15, but, by making adjustment to cut the connection (conduction) between via conductors4,5and secondary element15, the operation frequency (center frequency) of the antenna device can be shifted to a high frequency side. Accordingly, in the case of the antenna device in which two via conductors4,5are not conductively connected to secondary element15, it is difficult to shift the communication frequency to a low frequency side and make a change to a wide band. Moreover, regarding two via conductors4,5, in either of the cases where via conductor4is conductively connected and via conductor5is not conductively connected and where via conductor4is not conductively connected and via conductor5is conductively connected, shifting of the center frequency of the VSWR to a low frequency side or increasing the fractional bandwidth were not confirmed. Accordingly, in the present disclosure, it is preferable that two via conductors4,5be conductively connected to secondary element15.

FIG. 12is a directivity characteristic view illustrating a radio wave radiation pattern in an X-Y plane.FIG. 12illustrates radiation pattern p2in the X-Y plane in the case where antenna device101is disposed in a free space. Radiation pattern p2has a peak of gain when the radiation direction of a radio wave is the x direction (0 degree direction). Moreover, the gain on the front side of antenna device101(270 degrees-0 degree-90 degrees) is larger than the gain on the back side (90 degrees-180 degrees-270 degrees). Moreover, in radiation pattern p2, a slight fluctuation in gain is not generated with the radiation direction of a radio wave.

Meanwhile,FIG. 12indicates radiation pattern p1in the X-Y plane obtained when antenna device101is incorporated into pocket210of metal frame200of cabin monitor250. Radiation pattern p1has the peak of gain when the radiation direction of a radio wave is the x direction (0 degree direction), i.e., on the user side watching cabin monitor250. Moreover, the gain on the front side of antenna device101(270 degrees-0 degree-90 degrees) is larger than the gain on the back side (90 degrees-180 degrees-270 degrees). Moreover, in radiation pattern p1, the gain slightly fluctuates with the radiation direction of a radio wave. This is presumable due to the fact that, because antenna device101is incorporated into pocket210of metal frame200of cabin monitor250, the gain is influenced by inner components of cabin monitor250including metal frame200.

Thus, even when antenna device101is incorporated into pocket210of metal frame200, the antenna performance of antenna device101is not largely reduced. Rather, the gain on the front side (300 degrees-30 degrees) including the peak gain of radiation pattern p1of antenna device101incorporated into metal frame200is larger than the gain of radiation pattern p2of antenna device101disposed in the free space. Accordingly, antenna device101can efficiently emit a radio wave to the front side of cabin monitor250(x direction) in the X-Y plane.

FIG. 13is a directivity characteristic view illustrating a radio wave radiation pattern in an X-Z plane.FIG. 13illustrates radiation pattern p4in the X-Z plane in the case where antenna device101is disposed in the free space. Radiation pattern p4has a substantially uniform gain in the X-Z plane.

Meanwhile,FIG. 13illustrates radiation pattern p3in the X-Z plane obtained when antenna device101is incorporated into pocket210of metal frame200. Radiation pattern p3has a substantially uniform gain on the front side (300 degrees-90 degrees) of antenna device101in the radiation direction of a radio wave in the X-Z plane. Moreover, radiation pattern p3has a null between 240 degrees and 270 degrees of the radiation direction of a radio wave, and the gain is significantly reduced. This is presumable due to the fact that, because antenna device101is incorporated into metal frame200of cabin monitor250, the gain is influenced by inner components of cabin monitor250including metal frame200.

Thus, when antenna device101is incorporated into pocket210of metal frame200, the antenna performance is not largely reduced on the front side of antenna device101in the X-Z plane. Rather, the gain on the front side (330 degrees-90 degrees) of radiation pattern p3of antenna device101incorporated into metal frame200is larger than the gain of radiation pattern p4of antenna device101disposed in the free space. Accordingly, antenna device101can efficiently emit a radio wave to the front side of the cabin monitor (x direction) in the X-Z plane.

FIG. 14is a graph illustrating a change in peak gain with respect to a frequency of a radio wave in the X-Y plane. The vertical axis indicates peak gain (dBi), and the horizontal axis indicates a frequency band of 2.40 GHz to 2.48 GHz.FIG. 14illustrates peak gain g2in the X-Y plane obtained when antenna device101is disposed in the free space. In 2.40 GHz to 2.48 GHz, peak gain g2indicates a small value close to 0.5 dBi. Moreover,FIG. 14illustrates peak gain g1in the X-Y plane obtained when antenna device101is incorporated into pocket210of metal frame200. In 2.40 GHz to 2.48 GHz, peak gain g1indicates a large value in a range of 4.0 dBi to 3.0 dBi, indicating the tendency that the gain increases at a lower frequency.

As described above, according to a comparison between peak gain g1and peak gain g2, as compared with the case where antenna device101is disposed in the free space, when antenna device101is incorporated into pocket210of metal frame200of cabin monitor250, it is possible to strengthen a radio wave emitted from the front surface of antenna device101in the X-Y plane.

FIG. 15is a graph illustrating a change in peak gain with respect to a frequency of a radio wave in the X-Z plane. The vertical axis indicates peak gain (dBi), and the horizontal axis indicates a frequency band of 2.40 GHz to 2.48 GHz.FIG. 15illustrates peak gain g4in the X-Z plane obtained when antenna device101is disposed in the free space. In 2.40 GHz to 2.48 GHz, peak gain g4indicates a small value close to 1.0 dBi. Moreover,FIG. 15illustrates peak gain g3in the X-Z plane obtained when antenna device101is incorporated into pocket210of metal frame200. In 2.40 GHz to 2.48 GHz, peak gain g3indicates a large value in a range of 4.0 dBi to 5.0 dBi, indicating the characteristic that the gain is the largest near 2.4 GHz.

As described above, according to a comparison between peak gain g3and peak gain g4, as compared with the case where antenna device101is disposed in the free space, when antenna device101is incorporated into pocket210of metal frame200, it is possible to strengthen a radio wave emitted from the front surface of antenna device101in the X-Z plane.

As described above, antenna device101of the exemplary embodiment includes antenna conductors2,3, ground conductor8, AMC7sandwiched between antenna conductors2,3and ground conductor8so as to be disposed separately from antenna conductors2,3and ground conductor8, and secondary element15disposed on a side opposite to AMC7across antenna conductors2,3so as to be disposed furthest away from ground conductor8.

Thus, in antenna device101, unlike AMC7disposed on the intermediate layer, secondary element15disposed furthest away from ground conductor8is disposed on the outermost. Therefore, it is possible to easily adjust the operation frequency applicable for wireless communication and efficiently maintain the frequency characteristics of the operation frequency band with secondary element15.

Moreover, antenna device101further includes via conductor5that is disposed separately from the center of dielectric substrate14having a substantially rectangular shape on which secondary element15is disposed and that conductively connects antenna conductor3, secondary element15, AMC7, and ground conductor8. Thus, secondary element15has a function of an antenna conductor, and secondary element15can be included as a part of antenna device101. Thus, as the performance of antenna device101, it is possible to shift the operation frequency to a low frequency side and increase the gain.

Moreover, length L of secondary element15in the longitudinal direction is variable. Therefore, the center frequency of the VSWR shifts to a low frequency with an increase in length of secondary element15. Accordingly, in setting the operation frequency, by increasing length L of secondary element15, it is possible to make adjustment to shift the operation frequency to a low frequency side. Moreover, by reducing length L of secondary element15, it is possible to make adjustment to shift the operation frequency to a high frequency side. Moreover, the longer length L of secondary element15, the larger the value of the fractional bandwidth of the VSWR. Therefore, by increasing length L of secondary element15, it is possible to make adjustment to increase the fractional bandwidth of the VSWR.

Moreover, the length of secondary element15in the width direction, i.e., width W, is variable. Thus, the center frequency of antenna device101shifts to a low frequency side with an increase in width W of secondary element15. Accordingly, in setting the operation frequency, by increasing width W of secondary element15, it is possible to make adjustment to shift the operation frequency to a low frequency side. Moreover, by reducing width W of secondary element15, it is possible to make adjustment to shift the operation frequency to a high frequency side.

Moreover, antenna device101further includes parasitic conductor6provided on dielectric substrate10on which antenna conductors2,3are disposed. As parasitic conductor6is electrostatically coupled to AMC7similar to antenna conductors2,3, parasitic conductor6can increase electrostatic capacitance between antenna conductors2,3and AMC7and shift a radio frequency handled by antenna device101to a low frequency side.

Moreover, ground conductor8and AMC7are disposed to face each other and substantially overlap on plan view. Thus, one of AMC7and ground conductor8does not protrude over the other, making a contribution to reducing the size of printed circuit board1, eventually resulting in a reduction in size of antenna device101.

Moreover, antenna device101is incorporated into pocket210of metal frame200of cabin monitor250(i.e., disposed in a vicinity of a space that at least partially includes metal). Antenna device101improves the antenna performance with secondary element15. Therefore, even when incorporated into metal frame200, antenna device101can match the operation frequency band to a desired frequency band and maintain the antenna performance.

Moreover, antenna device101is a dipole antenna including antenna conductor2and antenna conductor3. Via conductor5on the ground side conductively connects secondary element15, antenna conductor3, AMC7, and ground conductor8. Via conductor4on the feed side conductively connects secondary element15and antenna conductor2. Thus, antenna device101can achieve a dipole antenna that allows easy adjustment of communication frequency (i.e., operation frequency) applicable for wireless communication.

Moreover, AMC7includes the slit that separates electrostatic coupling between antenna conductor2formed on the upper layer and antenna conductor3formed on the upper layer. Thus, it is possible to increase electrostatic coupling between antenna conductor2and a right half portion of AMC7(i.e., the +y direction illustrated inFIG. 3) and electrostatic coupling between antenna conductor3and a left half portion of AMC7(i.e., the −y direction illustrated inFIG. 3).

Heretofore, the exemplary embodiment has been described with reference to the accompanying drawings. However, the present disclosure is not limited to the example. It is apparent that those skilled in the art may conceive of various change examples, modification examples, replacement examples, addition examples, deletion examples, and equivalent examples within the scope of the claims, which are understood to fall within the technical scope of the present disclosure. Moreover, the constituent elements of the aforementioned exemplary embodiment may be optionally combined without departing from the gist of the present disclosure.

For example, the aforementioned exemplary embodiment indicates the case where the antenna device transmits a radio wave of a high frequency band of 2.4 GHz. However, the antenna device may transmit a radio wave of another frequency, e.g., 1.9 GHz or 1 GHz.

The present disclosure is useful as an antenna device that easily adjusts an operation frequency applicable for wireless communication and maintains frequency characteristics of an operation frequency band.