JP2012054915A - Antenna structure and diversity antenna structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize miniaturization and space saving while sufficient directivity is given to respective antennas in the same specified directions in an antenna structure where a plurality of antennas are erected on a plate and a diversity antenna structure of a space diversity system.SOLUTION: The diversity antenna structure 10 includes a first antenna 11 and a second antenna 12, which are separately arranged in forward/backward directions of a vehicle 100. An element length L1 of the first antenna 11 is shorter than an element length L2 of the second antenna 12. Thus, the first antenna 11 behaves as a waveguide from a viewpoint of the second antenna 12 and the second antenna 12 comes to have sufficient directivity in front of the vehicle. The second antenna 12 behaves as a reflector from a viewpoint of the first antenna 11 and the first antenna 11 comes to have sufficient directivity in the front of the vehicle. Therefore, even if the antennas 11 and 12 are brought close, they can sufficiently receive radio waves from the front.

Description

  The present invention relates to an antenna structure and a diversity antenna structure in which a plurality of antennas are erected on a ground plane.

  As antennas mounted on vehicles for vehicle-to-vehicle communication and road-to-vehicle communication, multiple antennas are installed on the ground plane in order to receive radio waves of a desired frequency even in a multipath environment. A diversity antenna structure is often used.

  In addition, the antenna structure having a configuration in which a plurality of antennas are erected on the ground plane is not limited to the diversity antenna structure, and for example, radio waves of a plurality of different types of communication systems (a plurality of types of communication systems having different operating frequencies) can be used. Sometimes they are used to send and receive.

And since such an antenna structure and a diversity antenna structure are mounted in a vehicle, good reception performance is requested | required and size reduction is also requested | required.
With regard to reception performance, for example, in a system that communicates with another vehicle in front of the host vehicle, such as a collision prevention system, particularly good directivity in the front direction (front of the host vehicle) is required.

  With regard to miniaturization, for example, in the case of a diversity antenna structure, it is usually desirable that the wavelength of the communication frequency be λ / 2 or more between the two antennas constituting the diversity antenna structure. However, for example, in a diversity antenna structure for receiving a radio wave having a frequency of 720 MHz, which is scheduled to be used in a vehicle-to-vehicle communication system, separating antennas from each other by λ / 2 or more requires separating them by about 20 cm or more. Therefore, the antenna mounted on the vehicle is not practical because it is large. Therefore, it is desirable to realize miniaturization by making the antenna interval as close as possible to λ / 2.

  One of the simplest methods for reducing the size of a space diversity antenna is to make a plurality of (for example, two) antennas constituting a diversity antenna structure close to each other.

  However, when the two antennas are brought closer to each other, the closer the distance is, the stronger the coupling with the other antenna, and the other antenna functions as a reflector / radiator. It will have directivity in different directions. This will be described more specifically with reference to FIG.

  FIG. 16 shows the horizontal plane directivity of each antenna when the distance between the antennas is changed with respect to the diversity antenna structure composed of two monopole antennas (first antenna and second antenna) vertically installed on the ground plane. Is shown. The upper part of FIG. 16 shows the directivity when the distance between the two antennas is λ / 2, and the lower part of the figure shows the directivity when the distance between the two antennas is λ / 8.

  As shown in the upper part of FIG. 16, when the distance between the antennas is relatively far from λ / 2, the coupling between the antennas is also weak, and the directivity (omnidirectionality) in the case where each exists alone is almost the same. The pattern is similar. For this reason, both the first antenna and the second antenna can satisfactorily receive radio waves from a desired direction (for example, forward; left direction in the figure).

  On the other hand, when the distance between the antennas is relatively close to λ / 8, the coupling between the antennas becomes strong and the other antenna functions as a reflector. As such, the directivity of each antenna is destroyed. In other words, when they exist alone, each of them is omnidirectional, but has a biased directivity in which the gain is large only in a specific direction and the gain is small in other directions.

  Specifically, for the first antenna on the front side, the gain on the front side is maintained, but the gain on the rear side is reduced, and radio waves from the rear side cannot be received well. In contrast to this, with respect to the second antenna on the rear side, the gain on the rear side is maintained, but the gain on the front side is reduced, and radio waves from the front side cannot be satisfactorily received.

  For this reason, only the first antenna on the front side can receive radio waves from the front, and the performance of the spatial diversity antenna used for receiving radio waves from the front is greatly deteriorated. . In other words, the presence of the second antenna on the rear side is meaningless for the purpose of favorably receiving radio waves from the front.

  On the other hand, as a configuration example of a diversity antenna structure in which the directivity in the front (traveling direction) is improved while narrowing the distance between the two antennas, a plurality of distributors are arranged in the feeder circuit, and distribution is performed in each distributor. A technique is known in which directivity is controlled by providing signals having different phases to the two antennas so that the signals have different phases, and the directivity diversity system is used (see, for example, Patent Document 1). .)

Japanese Patent Laid-Open No. 2003-249809

  However, in the technique described in Patent Document 1, the two antennas are fed with a phase difference from each other, thereby providing directivity in a specific direction (forward) as a whole while bringing the two antennas close to each other. However, it is realized as a combined gain of two antennas, and each of the two antennas cannot function effectively independently.

  In addition, since the technique described in Patent Document 1 is a structure for realizing a directional diversity method by power feeding switching, a so-called antenna selection method (selective combining) that selects and receives one of the antennas with good reception sensitivity. (Method) is not applicable.

  Even in an antenna structure configured such that two antennas are used in a plurality of types of communication systems having different operating frequencies, each antenna is good in a specific direction (for example, in front of the vehicle) while being close to each other. There are cases where it is required to have directivity, and in such a case, among the two antennas, radio waves from the specific direction cannot be satisfactorily received for the antenna on the side opposite to the specific direction. The same problem as in the case of the diversity antenna structure described above arises.

  The present invention has been made in view of the above problems, and in an antenna structure in which a plurality of antennas are erected on a ground plane and a diversity antenna structure of a spatial diversity system, each of the plurality of antennas has the same specific direction. The objective is to achieve downsizing and space saving while maintaining good directivity.

  In order to solve the above-mentioned problem, the invention according to claim 1 is configured so that radio waves can be transmitted or received by each of at least two antennas of the first antenna and the second antenna. Each of the antenna structures is erected on the ground plane in a direction parallel to the plate surface, and the first resonance length λ1 is a wavelength (resonance length) corresponding to the resonance frequency of the first antenna. It is shorter than the second resonance length λ2, which is the wavelength (resonance length) corresponding to the resonance frequency of the second antenna.

  In the antenna structure configured in this way, the first resonance length λ1 is shorter than the second resonance length λ2, so that when viewed from the second antenna, the first antenna behaves as a director. Thus, the second antenna has good directivity in the direction toward the first antenna (hereinafter also referred to as “first direction”), and the gain in the first direction is improved. On the other hand, when viewed from the first antenna, the second antenna behaves as a reflector, whereby the first antenna has good directivity in the direction opposite to the second antenna side (first direction).

  That is, in the conventional diversity antenna structure in which each element length is equal, the directivity of the second antenna cannot be directed in the first direction because the first antenna behaves as a reflector with respect to the second antenna. However, in the diversity antenna structure of the present invention, both the first antenna and the second antenna have good directivity in the first direction.

  Therefore, according to the antenna structure of the first aspect, it is possible to achieve miniaturization and space saving while giving each antenna good directivity in the same specific direction (first direction). It becomes.

  In this specification, “transmission or reception” and “transmission / reception” mean a concept including both transmission only, reception only, and both transmission and reception, that is, meaning of at least one of transmission and reception. It is.

  Here, various specific configurations for making the first resonance length λ1 shorter than the second resonance length λ2 are conceivable. For example, the first resonance length λ1 can be realized by making a difference in element length as described in claim 2. Alternatively, for example, it may be realized by providing a matching circuit in at least the second antenna as described in claim 3.

  That is, the invention according to claim 2 is the antenna structure according to claim 1, wherein the first element length L1 which is the element length of the first antenna is the second element length of the second antenna. It is shorter than the element length L2.

  Strictly speaking, the element length here is the length in the direction of standing from the ground plane, that is, the length (physical length) in the main polarization direction of each antenna. For example, each antenna is perpendicular to the ground plane. If the main polarization direction is vertical, the element length of each antenna is the length (height) from the ground plane to the tip.

  In general, the longer the antenna element length, the lower the resonant frequency. Therefore, by making the first element length L1 shorter than the second element length L2, the first resonance length λ1 can be made shorter than the second resonance length λ2, and thus the invention according to claim 1 is embodied. it can.

  On the other hand, the invention according to claim 3 is the antenna structure according to claim 1 or 2, wherein at least the second antenna is configured by connecting a matching circuit to one end of the power feeding side of the element. Yes. The second resonance length λ2 is a wavelength corresponding to the resonance frequency of the entire second antenna including the element and the matching circuit.

  The resonance length of the antenna basically depends on the element length. However, if a circuit or element such as a matching circuit is connected to the feeding part and it is also one of the components of the antenna, It also depends on the matching circuit. That is, even if the element length is the same (even if the length of the entire antenna is the same), the resonance length varies depending on the characteristics of the matching circuit. For example, the higher the impedance of the matching circuit, the higher the resonance frequency of the entire antenna (element + matching circuit) Becomes lower and the resonance length becomes longer.

  For this reason, the first resonance length λ1 can be made shorter than the second resonance length λ2 by configuring the at least second antenna to have a matching circuit, and thus the invention according to claim 1 can be embodied. .

  The matching circuit may be provided only in the second antenna, or may be provided in both the first antenna and the second antenna. In other words, as long as the first resonance length λ1 is configured to be shorter than the second resonance length λ2 as a result, whether or not the matching circuit is provided in the first antenna can be determined as appropriate.

  Further, by configuring at least the second antenna to have a matching circuit, the first element length L1 and the second element length L2 can be the same length as described in claim 4. That is, even if L1 = L2 (the same physical length), since at least the second antenna has a matching circuit, the resonance lengths can be different from each other. The resonance length λ1 <the second resonance length λ2.

  The antenna according to any one of claims 1 to 4 may be configured to use each antenna for different types of communication as described in claim 5, for example. As described in Item 6, the same communication may be used for communication by a spatial diversity method.

  That is, in the invention according to claim 5, the first antenna and the second antenna are each configured to transmit and receive radio waves in different frequency bands, and the frequency of the radio waves transmitted and received by the first antenna. The band is higher than the frequency band of radio waves transmitted and received by the second antenna. In the invention according to claim 6, the first antenna and the second antenna are configured to transmit and receive radio waves in the same frequency band by a spatial diversity method. In any case, each antenna has good directivity in the same direction (first direction), and therefore each antenna can perform transmission and reception of radio waves in the first direction.

  Next, in order to solve the above-mentioned problem, the invention according to claim 7 is configured so that radio waves can be received by a spatial diversity system by at least two antennas of the first antenna and the second antenna. A diversity antenna structure in which two antennas are provided on a ground plane so as to be spaced apart from each other in a direction parallel to the plate surface. The first element length L1 that is the element length of the first antenna is the second It is shorter than the second element length L2, which is the element length of the antenna.

  In the diversity antenna structure configured in this way, since the first element length L1 is shorter than the second element length L2, from the viewpoint of the second antenna, the first antenna behaves as a waveguide. As a result, the second antenna has good directivity in the first direction, and the gain in the first direction is improved. On the other hand, when viewed from the first antenna, the second antenna behaves as a reflector, whereby the first antenna has good directivity in the direction opposite to the second antenna side (first direction).

  That is, in the conventional diversity antenna structure in which each element length is equal, the directivity of the second antenna cannot be directed in the first direction because the first antenna behaves as a reflector with respect to the second antenna. However, in the diversity antenna structure of the present invention, both the first antenna and the second antenna have good directivity in the first direction.

  Therefore, conventionally, as shown in FIG. 16, when the distance between the antennas is reduced, the antennas have directivity in opposite directions. In the present invention, even if the distance between the antennas is reduced, Has good directivity in the first direction, so that there is a high possibility that radio waves from the first direction can be satisfactorily transmitted and received by at least one of the antennas even in a multipath environment, and has sufficiently high performance as a space diversity system. It will be.

  Therefore, according to the diversity antenna structure described in claim 7, it is possible to achieve miniaturization and space saving while giving each antenna good directivity in the same specific direction (first direction). It becomes possible.

  Strictly speaking, the element length here is the length in the direction of standing from the ground plane, that is, the length of the main polarization direction at each antenna. For example, each antenna is vertically erected from the ground plane. If the polarization direction is vertical, the element length of each antenna is the length (height) from the ground plane to the tip. Since the diversity antenna structure of the present invention is a space diversity system, the main polarization direction is the same. Therefore, the effects of the present invention described above can be obtained by making the lengths in the main polarization direction different from each other.

  There are various ways of specifically setting the first element length L1 and the second element length L2, respectively. For example, as described in claim 8, within the frequency band of the reception target radio wave by the diversity antenna structure. The basic element length L is a quarter length of a wavelength corresponding to a predetermined center frequency f0 (hereinafter also referred to as “reception wavelength λ”), and the first element length L1 is equal to or less than the basic element length L. The two element length L2 may be set to be not less than the basic element length L.

  Thus, with reference to the length L (basic element length) of ¼ of the reception wavelength λ, one is shorter than the basic element length L and the other is longer than the basic element length L, so that the first direction The reception target radio wave with the center frequency f0 coming from can be satisfactorily received by both antennas.

  By setting the element lengths L1 and L2 of each antenna so as to satisfy L1 <L2, L1 ≦ L, and L2 ≧ L, as in the diversity antenna structure according to claim 8 described above, the above-described sufficient Although the effect is obtained, the element lengths L1 and L2 are more preferably set as described in claim 9.

  That is, the first element length L1 is set so that the resonance frequency f1 of the first antenna is within a range from the center frequency f0 to a frequency f0a higher than the center frequency f0 by a predetermined frequency width fa. The element length L2 is set so that the resonance frequency f2 of the second antenna falls within the range from the center frequency f0 to the frequency f0b that is lower than the center frequency f0 by the frequency width fa.

  In this way, by setting the resonance frequency of each antenna to be within the range of f0 ± fa with respect to the center frequency f0 of the reception target radio wave, the reception target radio wave from the first direction can be obtained by both antennas. It can be received well.

  Furthermore, as described in claim 10, both the first antenna and the second antenna are configured such that the voltage standing wave ratio in the frequency band of the reception target radio wave is equal to or less than a preset threshold value. For example, the reception performance of each antenna can be further improved.

  Next, the invention according to claim 11 is the diversity antenna structure according to any one of claims 7 to 10, wherein the second antenna is provided with respect to the position of the feeding point of the second antenna. A length longer than the second element length L2 at a position a predetermined distance away from the position of the feeding point of the antenna 1 (that is, the side opposite to the first direction, hereinafter also referred to as “second direction”). The reflecting plate is erected so that the plate surface faces the second antenna.

  By making the first element length L1 shorter than the second element length L2 as described above, both antennas can have good directivity in the first direction. By providing the reflector on the second direction side of the antenna, the directivity in the first direction can be further improved.

  This reflector functions as a reflector (reflector) for both antennas, but is provided on the second direction side with respect to the second antenna, so that it is particularly a reflector for the second antenna. The action of is great. Therefore, in particular, the directivity of the second antenna is greatly affected, the directivity of the second antenna in the first direction is improved, and the gain in the first direction is further increased. And as a whole diversity antenna structure, the reception performance with respect to the reception object radio wave from the first direction can be further improved.

  Next, the invention according to claim 12 is configured such that radio waves can be received by a spatial diversity system by at least two antennas, the first antenna and the second antenna, and each of the two antennas is provided on the ground plane. A diversity antenna structure that is erected apart from each other in a direction parallel to the plate surface, the first element length L1 being the element length of the first antenna, and the second element being the element length of the second antenna The length L2 is the same length. Then, a reflection having a length longer than the second element length L2 at a predetermined distance away from the position of the feeding point of the first antenna on the ground plate on the side opposite to the position of the feeding point of the first antenna. The plate is erected so that the plate surface faces the second antenna.

  That is, in the diversity antenna structure according to the twelfth aspect, the first antenna and the second antenna have the same element length. However, since the reflection plate is erected on the second direction side with respect to the second antenna, the reflection plate is provided for each antenna in the same manner as the diversity antenna structure according to claim 11 described above. It functions as a reflector and can direct the directivity of each antenna in the first direction.

  Therefore, according to the diversity antenna structure according to claim 12, even if the element length of each antenna is the same by providing the reflector, each antenna is excellent in the same specific direction (first direction). It is possible to achieve downsizing and space saving while maintaining a high directivity.

  Here, in order to make the function of the reflection plate work effectively, it is ideal that the distance between the second antenna and the reflection plate is, for example, about λ / 4. However, the present invention originally has one purpose of further reducing the interval between the antennas. For example, each antenna has good directivity in the first direction even if it is reduced to about λ / 6 to λ / 8. Making it possible. For this reason, providing the reflector at a position approximately λ / 4 away from the second antenna increases the size of the diversity antenna structure as a whole, which is contrary to the purpose of downsizing and space saving. Therefore, it is desirable that the reflector is as close as possible to the second antenna.

  However, when the reflecting plate is brought close to the second antenna, the coupling between the reflecting plate and the second antenna becomes strong, the directivity of the second antenna is lost, and good directivity in the first direction is obtained. It becomes impossible.

  Therefore, in order to maintain the directivity of the second antenna in the first direction by reducing the coupling between the two as much as possible while keeping the reflector close to the second antenna, as described in claim 13, A slit may be formed in the reflector. That is, a slit having a predetermined length is formed in the reflector in a range facing the second antenna in the length direction.

  Thus, by providing a slit in the reflecting plate, a loop-shaped current flows around the slit, thereby reducing the current in the portion where the slit is formed in the current flowing in the reflecting plate. Therefore, the coupling between the reflector and the second antenna can be weakened.

  As a result, even if the reflector is provided in the very vicinity of the second antenna (for example, a distance of λ / 15 to λ / 20), the function as a reflector by the reflector is sufficiently exhibited for each antenna. A high gain in the first direction can be realized by directing the directivity of each antenna (particularly the directivity of the second antenna) in the first direction.

Although how to set the length of a slit can be set suitably, it is good to set it as 1/3 or more of the length of a reflecting plate as described in Claim 14, for example.
In this way, since the current is small in the range of at least 1/3 of the total length of the reflecting plate where the slit is formed, the coupling between the two is sufficient even when the distance to the second antenna is short. Can be suppressed.

  Moreover, although it can also determine suitably in which part slit is formed in a reflecting plate, it is good to form from the lower end of a reflecting plate, for example as described in Claim 15. That is, the slit is formed from the lower end of the reflecting plate on the ground plate side, so that the reflecting plate is formed so that the lower end side is cut into a concave shape.

  In the antenna and the reflector plate erected on the ground plane, a larger current flows on the ground plane side. Therefore, the coupling with the second antenna can be more effectively suppressed if the current on the side close to the ground plane in the entire length of the reflecting plate can be reduced as much as possible.

  Therefore, as described in claim 15, if a slit is provided in a concave shape from the lower end side of the reflector, the loop current around the slit flows through the ground plane on the lower side of the reflector (that is, the ground plane is also a loop current). Therefore, the coupling with the second antenna can be more effectively suppressed.

  Next, the invention according to claim 16 is the diversity antenna structure according to any one of claims 7 to 15, wherein the diversity antenna structure receives radio waves coming from at least the front of the vehicle. The first antenna and the second antenna are arranged so that the feeding points of these antennas are separated from each other by a predetermined distance in the front-rear direction of the vehicle, and the first antenna and the second antenna are first with respect to the second antenna. Are arranged so that the antenna is located on the front side of the vehicle.

  Thus, if the diversity antenna structure according to any one of claims 7 to 15 is used as a diversity antenna structure for being mounted on a vehicle and receiving radio waves from the front of the vehicle, the entire antenna can be reduced in size. While realizing, both the first antenna and the second antenna constituting the diversity antenna structure can receive radio waves from the front of the vehicle satisfactorily. Therefore, it is possible to provide a small and high-performance space diversity diversity antenna structure as an in-vehicle antenna for receiving radio waves from the front of the vehicle.

  The invention according to claim 17 is the diversity antenna structure according to any one of claims 7 to 16, wherein a distance between each feeding point of the first antenna and the second antenna is The wavelength is equal to or less than ¼ of the wavelength (reception wavelength λ) corresponding to a predetermined frequency within the frequency band of the reception target radio wave of the diversity antenna structure.

  To reduce the diversity antenna structure, simply reducing the distance of each antenna to ¼ or less of the reception wavelength λ, the antennas have directivities in opposite directions as described above. The function of the space diversity system for radio waves will be impaired. Therefore, when the present invention (Claims 7 to 16) is applied in such a configuration in which the distance between the antennas is as short as ¼ or less of the reception wavelength λ, the directivity of each antenna can be improved even if the distance between the antennas is short. Both can be directed in the first direction, which is more effective.

It is explanatory drawing for demonstrating the state by which the diversity antenna structure of 1st Embodiment is mounted in the vehicle. It is explanatory drawing showing schematic structure, such as a diversity antenna structure of 1st Embodiment, and its peripheral circuit. It is explanatory drawing for demonstrating the performance (directivity) of the diversity antenna structure of 1st Embodiment. It is explanatory drawing for demonstrating the performance (VSWR) of the diversity antenna structure of 1st Embodiment. It is a figure showing the structure of the diversity antenna structure of 2nd Embodiment, (a) is a schematic block diagram of the diversity antenna structure whole, (b) is explanatory drawing of the reflecting plate which comprises a diversity antenna structure, (c) is the figure It is explanatory drawing showing the electric current distribution of a reflecting plate. It is explanatory drawing for demonstrating the performance of the diversity antenna structure of 2nd Embodiment, (a) is a figure showing the directivity when there is no reflecting plate for comparison, (b) is the slit of the slit for comparison. The figure showing the directivity at the time of using the reflecting plate which does not exist, (c) is a figure showing the directivity for every length of the reflecting plate at the time of using the reflecting plate with a slit. It is explanatory drawing for demonstrating the relationship between the height Ls of a slit, and the gain of a 2nd antenna. It is a figure showing the diversity antenna structure of 3rd Embodiment, (a) is a figure showing schematic structure of a diversity antenna structure, (b) is a figure showing the directivity for every height of a reflecting plate. It is a perspective view showing the other structural example of a diversity antenna structure. It is a block diagram showing schematic structure of the antenna structure of 4th Embodiment. It is explanatory drawing for demonstrating the characteristic of the antenna structure of 4th Embodiment. It is a block diagram showing schematic structure of the antenna structure of 5th Embodiment. It is explanatory drawing for demonstrating the characteristic of the antenna structure of 5th Embodiment. It is explanatory drawing for demonstrating the relationship between the difference Ld of the height of the antenna for LTE, and the antenna for vehicles, and the characteristic of the antenna for vehicles in the antenna structure of 5th Embodiment. It is a block diagram showing the modification of an antenna structure. It is explanatory drawing for demonstrating the relationship between the distance of each antenna, and the directivity of each antenna in the diversity antenna structure of a space diversity system.

Preferred embodiments of the present invention will be described below with reference to the drawings.
[First Embodiment]
As shown in FIG. 1, the diversity antenna structure 10 of this embodiment is mounted on a roof 101 of a vehicle 100. Specifically, the diversity antenna structure 10 is configured such that the first antenna 11 and the second antenna 12 are erected on the ground plate 13 at a predetermined distance in a direction parallel to the plate surface (in this embodiment, the front-rear direction of the vehicle 100). These components are built in a resin case 9 as a whole. Each of the antennas 11 and 12 is erected in a direction perpendicular to the plate surface of the ground plane 13.

  The position where the diversity antenna structure 10 is mounted is the vehicle rear side of the roof 101, that is, the side close to the rear window glass 102. Each antenna 11, 12 is connected to a switching circuit 2 provided at a predetermined position inside the vehicle 100 from each feeding point 14, 15 by a coaxial cable for feeding.

  The diversity antenna structure 10 of the present embodiment is a diversity antenna structure based on a spatial diversity system, and more specifically, is a so-called antenna selection system diversity antenna structure that switches to one of the antennas having good reception sensitivity. However, it goes without saying that the application of the present invention is not limited to the spatial diversity of the antenna selection method, and may be applied to the spatial diversity of other methods such as the maximum ratio combining method.

  In addition, the diversity antenna structure 10 of the present embodiment is used for vehicle-to-vehicle communication in the collision prevention system in order to prevent the vehicles from colliding with each other in advance at an intersection with poor visibility, for example. Therefore, the arrival direction of the radio wave to be received (reception target radio wave) is in front of the vehicle 100, and the diversity antenna structure 10 is configured so that the radio waves arriving from the front of the vehicle can be satisfactorily received by both the antennas 11 and 12. ing. However, applying the present invention to the diversity antenna structure for vehicle-to-vehicle communication in the collision prevention system is merely an example.

  In addition, the reception target radio wave in this embodiment is a 720 MHz band. More specifically, in the present embodiment, a 10 MHz frequency band having a center frequency of 720 MHz (that is, a band of 715 to 725 MHz) is used as a use band, and each antenna 11 and 12 can receive radio waves in this use band satisfactorily. It is configured as follows.

  A more specific configuration of the diversity antenna structure 10 and its peripheral circuits will be described with reference to FIG. As shown in FIG. 2, the diversity antenna structure 10 of the present embodiment includes a first antenna 11 erected vertically on the ground plane 13, and is vertically spaced apart from the first antenna 11 toward the vehicle rear side. The second antenna 12 is provided upright.

  That is, the two antennas 11 and 12 constituting the diversity antenna structure 10 of the present embodiment are monopole antennas erected on the ground plane 13 so that the main polarization directions are the same. Since the ground plane 13 has a plane surface substantially parallel to the ground, each of the antennas 11 and 12 has a vertically polarized wave as a main polarization component and can satisfactorily receive vertically polarized radio waves coming from the front of the vehicle. It is configured as follows.

  The first element length L1 (the length from the plate surface of the ground plate 13 to the tip of the first antenna 11, in other words, the length from the feeding point 14 to the tip) is the element length of the first antenna 11. More than the second element length L2 which is the element length of the second antenna 12 (the length from the plate surface of the ground plate 13 to the tip of the second antenna 12, in other words, the length from the feed point 15 to the tip). short.

  The element length here means more strictly the length in the main polarization direction, that is, the height from the plate surface of the ground plane 13. In the present embodiment, each antenna 11 and 12 is erected vertically with respect to the ground plane 13 and the erection direction and the main polarization direction are the same, so that the total length of each antenna 11 and 12 remains as it is. Element length.

  Further, the distance between the antennas 11 and 12 (the distance between the feeding points 14 and 15) is appropriately set within the range of λ / 8 to λ / 6. Of course, this interval is merely an example. The above-described 720 MHz band, which is the frequency band of the reception target radio wave, and the center frequency are merely examples.

The feeding points 14 and 15 of the antennas 11 and 12 are respectively connected to the switching circuit 2 through feeding lines, and further connected to the vehicle-to-vehicle communication wireless device 1 via the switching circuit 2.
The switching circuit 2 includes a first termination resistor 4 connected to the first antenna 11, a second termination resistor 6 connected to the second antenna 12, and a connection destination of the first antenna 11 at the time of transmission / reception for wireless communication between vehicles. The first switch 3 that switches to either the device 1 or the first termination resistor 4 and the second switch that switches the connection destination of the second antenna 12 to either the vehicle-to-vehicle communication wireless device 1 or the second termination resistor 6 during transmission / reception 5 is provided. Note that the resistance values of the first termination resistor 4 and the second termination resistor 6 are both 50Ω in this embodiment.

  Switching of the switches 3 and 5 is controlled by a switching signal from the inter-vehicle communication wireless device 1. In the present embodiment, as described above, the antenna selection method of spatial diversity is adopted, so the inter-vehicle communication radio device 1 connects one of the antennas 11 and 12 to the inter-vehicle communication radio device 1. In this case, the other is connected to a terminating resistor so as to behave as a parasitic element.

  The switching signal from the vehicle-to-vehicle communication wireless device 1 to each of the switches 3 and 5 may be transmitted using a dedicated communication line (not shown), or on a coaxial cable through which transmission / reception signals are transmitted. It is good also as a structure which makes it superimpose and transmit. In addition, since a specific operation (antenna switching method or the like) of the antenna selection method, which is one method of spatial diversity, is well known, the description thereof is omitted here.

  The inter-vehicle communication wireless device 1 detects the presence / absence of another vehicle approaching the host vehicle from the front based on the received signals from the antennas 11 and 12, and uses the detection result to drive the vehicle 100. Output to other control devices to control.

  In the diversity antenna structure 10 of the present embodiment configured as described above, the first element length L1 of the first antenna 11 on the vehicle front side is shorter than the second element length L2 of the second antenna 12 on the vehicle rear side. Thus, when viewed from the second antenna 12, the first antenna 11 behaves as a director, whereby the second antenna 12 is in the direction toward the first antenna 11 (that is, in front of the vehicle). Therefore, the gain in front of the vehicle is improved. On the other hand, when viewed from the first antenna 11, the second antenna 12 behaves as a reflector, so that the first antenna 11 has good directivity in the direction opposite to the second antenna 12 side (that is, in front of the vehicle). That is, both the first antenna 11 and the second antenna 12 have good directivity with respect to the front of the vehicle.

  Therefore, in the present embodiment, as described above, the distance between the antennas 11 and 12 is close to λ / 8 to λ / 6, but both the antennas 11 and 12 have good directivity with respect to the front of the vehicle. be able to.

  FIG. 3 shows an example of directivity of the antennas 11 and 12 when the element lengths L1 and L2 are different as in the present embodiment and when the element lengths L1 and L2 are the same as in the prior art.

  As shown in FIG. 3, when the element lengths L1 and L2 are the same, one antenna behaves as a reflector with respect to the other antenna, so that the directivity of both is biased. And although the directivity of the first antenna faces the front of the vehicle, the directivity of the second antenna faces the opposite side of the vehicle.

  On the other hand, as in the present embodiment, the element lengths L1 and L2 are different from each other, and the first element length L1 of the first antenna 11 on the vehicle front side is greater than the second element length L2 of the second antenna 12. By shortening the antennas 11 and 12, the directivity bias is eliminated and the antennas 11 and 12 are almost non-directional. That is, the second antenna 12 on the vehicle rear side also has good directivity with respect to the vehicle front, and the gain with respect to the vehicle front is improved.

  Further, by directing the directivity of the antennas 11 and 12 forward, even if noise generated from an in-vehicle noise generation source such as an air conditioner motor passes through the rear window glass 102 and goes around the antennas 11 and 12, Can be less affected.

  Here, the element lengths L1 and L2 of the antennas 11 and 12 can be appropriately determined as long as radio waves in the 720 MHz band from the front of the vehicle can be satisfactorily received, but in the present embodiment, the diversity antenna structure 10 is used. The first element length L1 is defined as a basic element length L having a quarter length of the wavelength (reception wavelength λ) corresponding to the center frequency 720 MHz in the frequency band of the reception target radio wave (715 to 725 MHz in this example). The element length L is equal to or shorter than the element length L, and the second element length L2 is set to be equal to or longer than the basic element length L.

  In this embodiment, the resonance frequency f1 of the first antenna 11 is within a range from the center frequency 720 MHz to a predetermined frequency width (corresponding to the frequency width fa of the present invention, 30 MHz in this example) to a higher frequency 750 MHz. In addition, the element lengths L1 and L2 of the antennas 11 and 12 are set so that the resonance frequency f2 of the second antenna 12 falls within the range from the center frequency 720 MHz to the frequency 690 MHz that is lower than the frequency width (30 MHz). ing.

  Furthermore, in this embodiment, both the first antenna 12 and the second antenna 12 are such that the voltage standing wave ratio (VSWR) in the frequency band (715 to 725 MHz) of the reception target radio wave is 3 or less. Element lengths L1 and L2 of the antennas 11 and 12 are set.

  In order to satisfy the various system operation conditions described above, in the present embodiment, specifically, the first element length L1 of the first antenna 11 is set to 0.85L, and the second element length of the second antenna 12 is set. L2 is set to 1.15L.

  FIG. 4 shows a characteristic example of the VSWR of each antenna 11 and 12. As shown in FIG. 4, both the first antenna 11 and the second antenna 12 have a VSWR of 3 or less within the range of 715 to 725 MHz, which is the use band.

  Considering only the improvement of the directivity toward the front of the vehicle, it is preferable to make the first antenna 11 shorter. However, the shorter the first antenna 11 is, the more the characteristic of the first antenna 11 shown in FIG. 4 is shifted to the right side (high frequency side), and the VSWR is set to 3 or less in the use band. The condition is not met. As for the second antenna 12 as well, it is preferable to make the second antenna 12 longer if only the directivity toward the front of the vehicle is improved. However, the longer the second antenna 12 is, the more the characteristic of the second antenna 12 shown in FIG. 4 is shifted to the left (low frequency side), and the VSWR is set to 3 or less in the use band. The operating conditions will not be met. Therefore, in the present embodiment, the above-described element lengths (L1 = 0.85L, L2 = 1.15L) are employed in order to make the directivity toward the front of the vehicle as good as possible while satisfying the system operation conditions. ing.

  According to the diversity antenna structure 10 of the present embodiment described above, the first antenna 11 is made to be the first antenna 11 by making the first element length L1 of the first antenna 11 shorter than the second element length L2 of the second antenna 12. It is possible to prevent the two antennas 12 from acting as reflectors and conversely act as waveguides, thereby reducing the distance between the antennas 11 and 12 to λ / 8 to λ / 6. In addition, both the antennas 11 and 12 can have good directivity in front of the vehicle.

  Therefore, it is possible to achieve downsizing and space saving of the diversity antenna structure 10 as a whole while the antennas 11 and 12 have good directivity toward the front of the vehicle. In addition, since each of the antennas 11 and 12 is a monopole antenna, the antennas 11 and 12 can be simply configured, and as a result, the overall configuration of the diversity antenna structure 10 can be simplified.

  In the present embodiment, the element lengths L1 and L2 of the antennas 11 and 12 are set not only to improve the directivity in front of the vehicle but also to satisfy the system operation conditions. Therefore, it is possible to more reliably receive radio waves from the front of the vehicle, and to provide diversity antenna structure 10 with high reception performance.

[Second Embodiment]
FIG. 5A shows a schematic configuration of the diversity antenna structure 20 of the second embodiment. The diversity antenna structure 20 of the present embodiment shown in FIG. 5 is mounted on the vehicle 100 and used as a space diversity antenna, similarly to the diversity antenna structure 10 of the first embodiment. There are two antennas, a first antenna 11 and a second antenna 12, which are arranged at a predetermined distance (λ / 8 to λ / 6) apart.

  The diversity antenna structure 20 of the present embodiment is different from the diversity antenna structure 10 of the first embodiment, particularly for the purpose of further improving the forward directivity of the second antenna 12. That is, the reflecting plate 21 is erected at a position (a side opposite to the first antenna 11) at a predetermined distance. 5A shows a state in which the reflecting plate 21 is viewed from the side surface (from the left-right direction of the vehicle), and FIG. 5B shows the reflecting plate 21 from the front side (front of the vehicle). To) Shows the state of viewing.

  The reflecting plate 21 is made of a conductor, and the lower end thereof is electrically connected to the ground plate 13. The reflecting plate 21 is erected so that its plate surface is parallel to a plane perpendicular to the vehicle front-rear direction (that is, a straight line connecting the feeding points 14 and 15 of the antennas 11 and 12). . That is, the normal direction of the reflecting plate 21 coincides with the front-rear direction of the vehicle, and the reflector 21 is erected so as to face the second antenna 12.

  The distance between the second antenna 12 and the reflecting plate 21 (that is, the distance between the feeding point 15 of the second antenna 12 and the standing position of the reflecting plate 21) is an extremely short distance. In the present embodiment, for example, λ / 15 to λ / 20. Further, the length (height from the standing position of the ground plate 13) Lh of the reflecting plate 21 is longer than the second element length L2 of the second antenna 12, and Lh = 1.02L2 (that is, as an example in the present embodiment) It is set to 1.02 times the second element length L2.

  Here, in order to effectively operate the reflector 21 as a so-called reflector, it is generally necessary to set the distance from the second antenna 12 to about λ / 4. As a result, the reception efficiency of the second antenna 12 decreases and the gain deteriorates as a whole. Moreover, the directivity in the forward direction of the vehicle is not improved much. However, if the distance between the reflecting plate 21 and the second antenna 12 is set to about λ / 4, the diversity antenna structure 20 as a whole cannot be increased in size and is not practical.

  Therefore, while making the distance between the second antenna 12 and the reflector 21 close to each other for miniaturization, the function as a reflector is improved by suppressing the coupling between them, and the directivity in the front of the vehicle is excellent. In order to achieve this, a slit 22 is formed in the reflecting plate 21.

  As shown in FIG. 5 (b), the slit 22 is within the range facing the second antenna 12 in the reflector 21, that is, the same height as the second element length L 2 of the second antenna 12 from the plate surface of the ground plane 13. In this range, it is formed at a predetermined height (length) Ls from the lower end (base plate 13 side). Thereby, the reflecting plate 21 has a shape in which the lower end side is cut out in a concave shape as a whole.

  The height Ls of the slit 22 is set to 1/3 of the height Lh of the reflection plate 21. The height of the slit 22 is an example and can be determined as appropriate. However, as described later, it is preferable that the height of the slit 22 is 1/3 or more of the height Lh of the reflector 21.

  The reflecting plate 21 is arranged so that a straight line connecting the feeding point 14 of the first antenna 11 and the feeding point 15 of the second antenna 12 passes through the lower end side of the reflecting plate 21, that is, the straight line extends in the horizontal direction of the reflecting plate 21. Although it is preferable to stand on the ground plate 13 so as to pass through the range of the width, in this embodiment, the reflector 21 is particularly set up so as to pass through the center in the range of the width in the left-right direction. That is, the straight line passing through each of the feeding points 14 and 15 passes through the center position in the left-right direction of the reflector 21, in other words, passes through the center position in the left-right direction of the slit 22. The same applies to each of the diversity antenna structures 30 and 40 (see FIGS. 8 and 9) described later.

  By forming the slits 22 in the reflection plate 21 in this way, a loop-shaped current flows between the reflection plate 21 and the ground plate 13, thereby weakening the coupling with the second antenna 12. FIG. 5C shows an example of the current distribution of the reflecting plate 21.

  When there is no slit 22, the current distribution of the reflecting plate 21 is such that the current value increases as it approaches the ground plate 13, and the current value decreases as it approaches the tip. Therefore, in this case, the coupling with the second antenna 12 becomes strong. In contrast, when the slit 22 is formed, a loop-shaped current flows around the slit 22 as shown in FIG. The current in the formed part becomes small.

  For this reason, the coupling between the reflector 21 and the second antenna 12 can be weakened. As a result, even if the reflector 21 is provided in the very vicinity of the second antenna 12 (distance of λ / 15 to λ / 20), it is reflected. The function as a reflector by the plate 21 can be sufficiently exhibited. Therefore, compared to the diversity antenna structure 10 of the first embodiment without the reflector 21, the directivity of the antennas 11 and 12 toward the front of the vehicle (particularly the directivity of the second antenna 12) can be obtained more reliably. A higher gain with respect to the front of the vehicle can be obtained.

  Here, the effect obtained by providing the reflecting plate 21 will be described more specifically with reference to each directivity diagram of FIG. FIG. 6A shows the directivity of each of the antennas 11 and 12 when the reflection plate 21 is not provided for comparison. This is the same as the directivity (see FIG. 3) of the antennas 11 and 12 of the diversity antenna structure 10 of the first embodiment.

  As shown in FIG. 6 (a), each of the antennas 11 and 12 has a difference in the element lengths L1 and L2, thereby providing almost omnidirectional directivity. It can receive radio waves from the front of the vehicle.

  On the other hand, when a reflector is provided at a very close position (λ / 15 to λ / 20) on the rear side of the second antenna 12, the directivities of the antennas 11 and 12 are as shown in FIG. As shown in FIG.

  Of these, FIG. 6B shows the directivity when a reflector without slits is used for comparison. As shown in FIG. 6B, the directivity toward the front of the vehicle is further enhanced by the action of the reflector for the first antenna 11 simply by providing a reflector without slits. In this case, the directivity toward the front of the vehicle is weakened due to the coupling with the reflecting plate, while the directivity toward the rear side is increased, and the function as the diversity antenna structure is impaired. The reason why the directivity of the second antenna 12 is biased to the rear side as described above is that the reflection plate is erected at a very close position as described above and the coupling with the reflection plate becomes strong. Yes.

  On the other hand, FIG. 6C shows the directivity when the reflecting plate 21 of the present embodiment having the slits 22 is used, and the directivity when the height Lh of the reflecting plate 21 is changed to three types. Showing sex. Note that the height Ls of the slit 22 is 1/3 of the height Lh of the reflector 21.

  As shown in FIG. 6C, although the first antenna 11 has a slight difference depending on the height Lh of the reflector 21, the directivity with respect to the vehicle front as a whole is the case of the first embodiment (FIG. 6). 6 (a) see the left side).

  On the other hand, the directivity of the second antenna 12 varies greatly depending on its length (height) Lh even if a reflector 21 longer than the second element length L2 of the second antenna 12 is provided. As shown in the figure, when the height Lh of the reflecting plate 21 is relatively short, 1.01L2, although the slit 22 is formed, the influence of the coupling with the reflecting plate 21 is large, and the directivity is on the rear side. It is biased to.

  However, if the reflecting plate 21 is set to 1.02L2 which is higher than this, the directivity of the second antenna 12 becomes the vehicle front direction due to the effect of the slit 22. In addition, even when compared with the case of the first embodiment without the reflecting plate 21 (see the right side of FIG. 6A), it can be seen that the directivity behind the vehicle is weakened and the directivity ahead of the vehicle is increased.

  When the reflector 21 is set to 1.03 Lh having a higher height, the directivity of the second antenna 12 is directed toward the front side of the vehicle as a whole, and reception performance with respect to the front side of the vehicle is further improved. From this, it can be seen that the height Lh of the reflecting plate 21 is preferably 1.02 times or more the second element length L2 of the second antenna 12.

  Further, in the present embodiment, the height of the slit 22 is set to 1/3 of the height Lh of the reflector 21 as described above, but this is shorter than 1/3 of the height Lh of the reflector 21. This is because the gain in the vehicle front direction of the second antenna 12 cannot be obtained sufficiently. FIG. 7 shows the relationship between the height Ls of the slit 22 and the gain of the second antenna 12.

  As shown in FIG. 7, when the height Ls of the slit 22 is low, the gain of the second antenna 12 is low, but when the height of the second antenna 12 is 1/3 or more of the height Lh of the second antenna 12, The gain on the front (front of the vehicle) is improved and becomes a sufficient value. This is because the current distribution of the reflecting plate 21 is divided into a loop-shaped current flowing around the slit 22 and a current flowing in the height direction (linear direction) of the reflecting plate 21 by the slit 22. This is because the coupling between the antenna 12 and the reflector 21 is weakened. Therefore, in the present embodiment, the height Ls of the slit 22 is set to 1/3 of the height Lh of the reflecting plate 21.

  According to the diversity antenna structure 20 of the present embodiment described above, the reflector 21 is further provided behind the second antenna 12 with respect to the diversity antenna structure 10 of the first embodiment. The 12 directivities can be more directed toward the front of the vehicle, and the gain ahead of the vehicle can be further increased. Therefore, the diversity antenna structure 20 as a whole can further improve the reception performance for radio waves from the front of the vehicle.

  In addition, a slit 22 is formed in the reflecting plate 21 so that the coupling with the second antenna 12 is weakened. For this reason, the reflector 21 can be disposed in the very vicinity of the second antenna while maintaining the directivity of the antennas 11 and 12 in front of the vehicle, so that higher performance diversity can be achieved while keeping the size down. An antenna structure 20 can be provided.

[Third Embodiment]
FIG. 8A shows a diversity antenna structure 30 according to the third embodiment. The diversity antenna structure 30 of the present embodiment shown in FIG. 8A includes two antennas, a first antenna and a second antenna, as well as the diversity antenna structure 20 of the second embodiment, and is located behind the second antenna. Although the reflector is erected in a very close position, it differs from the second embodiment in that the element length of each antenna is the same.

  That is, in the diversity antenna structure 30 of the present embodiment, the first antenna 31 and the second antenna 32 are erected on the ground plane 33. The distance between the antennas 31 and 32 (that is, the distance between the feeding points 34 and 35) is set to be λ / 8 to λ / 6 as in the above embodiment. In addition, a reflector 36 in which a slit 37 is formed is erected at a very close position on the rear side of the second antenna 32.

  The configuration and shape of the reflector 36 and the positional relationship with the antennas 31 and 32 are the same as those of the reflector 21 of the second embodiment shown in FIG. Further, the height Lh of the reflecting plate 36 is made higher than the height (second element length L2) of the second antenna 32, and the height Ls of the slit 37 is set to 1/3 of the height Lh of the reflecting plate 36. This is also the same as the diversity antenna structure 20 of the second embodiment.

  In the diversity antenna structure 30 of the present embodiment, the first element length L1 of the first antenna 31 and the second element length L2 of the second antenna 32 are equal in length, both of which are 1 / of the reception wavelength λ. 4, that is, the same length as the basic element length L.

  When the element lengths L1 and L2 are equal, when the antennas 31 and 32 are arranged close to each other as in the present embodiment, as described with reference to FIG. Becomes stronger, the directivity of each antenna collapses, and the directivity of the second antenna 32 peels away from the rear of the vehicle.

  However, in this embodiment, since the reflecting plate 36 is erected on the rear side of the second antenna 32, the directivity of the second antenna 32 can be directed to the front of the vehicle by the reflecting plate 36. That is, even if the element lengths L1 and L2 are the same, the directivity and gain of the antennas 31 and 32 in the front of the vehicle can be improved by providing the reflector 36, and the diversity antenna structure 30 as a whole has desired characteristics. It can be realized. The principle that the directivity of the second antenna 32 can be directed to the front of the vehicle by the reflector 36, that is, the action of the reflector 36 on the second antenna 32, is exactly the same as the diversity antenna structure 20 of the second embodiment. .

  Here, changes in the directivity of the antennas 31 and 32 with respect to the height Lh of the reflecting plate 36 will be described with reference to FIG. FIG. 8B shows the directivity when the height Lh of the reflecting plate 36 is changed to three types.

  As shown in FIG. 8B, the first antenna 31 has directivity in front of the vehicle as a whole regardless of the height Lh of the reflector 36, but the height Lh of the reflector 36 is increased. As a result, the directivity is more directed toward the front of the vehicle, and the gain ahead of the vehicle is further increased.

  On the other hand, for the second antenna 32, even if the reflector 36 longer than the second element length L2 (= L1 = L) of the second antenna 32 is provided, the directivity depends on the length (height) Lh. It will be very different. As shown in the figure, when the height Lh of the reflecting plate 36 is 1.04L, the slit 37 is formed, but the effect of coupling with the reflecting plate 36 is large, and the directivity is still biased toward the rear side. ing.

  However, when the reflecting plate 36 has a height of 1.05 L, which is higher than this, the directivity (gain) in front of the vehicle becomes larger due to the effect of the slit 37. When the reflector 36 is set to a higher height of 1.06Lh, the directivity of the second antenna 32 as a whole is directed toward the front side of the vehicle, and the reception performance with respect to the front side of the vehicle is further enhanced. Therefore, when the element lengths L1 and L2 of the antennas 31 and 32 are equal as in the present embodiment, the height Lh of the reflector 36 is 1.05 of the second element length L2 of the second antenna 32. It turns out that it is preferable to set it as 2 times or more.

  Therefore, according to the diversity antenna structure 30 of the present embodiment, by providing the reflector 36, even if the element lengths L1 and L2 of the antennas 31 and 32 are the same, the vehicle is connected to both the antennas 31 and 32. It is possible to achieve downsizing and space saving while giving good directivity to the front.

[Fourth Embodiment]
FIG. 10 shows a schematic configuration of the antenna structure 50 of the present embodiment.
The antenna structure 50 of the present embodiment is configured such that a printed circuit board 56 in which two antennas 51 and 52 are formed by strip conductors on one plate surface is erected vertically on a ground plate 53.

  The printed circuit board 56 is erected so that the plate surface thereof is parallel to the longitudinal direction of the vehicle. The first antenna 51 is formed of a strip conductor on the end side in the vehicle front direction on one surface, and the second antenna 52 is formed of a strip conductor on the end side in the vehicle rear direction. Yes. When viewed from the ground plane 53, each of the antennas 51 and 52 is in a state of being erected perpendicularly to the plate surface of the ground plane 53. Therefore, it can be said that each of the antennas 51 and 52 is a monopole antenna erected vertically on the ground plane 53 as in the first embodiment.

  In the antenna structure 50 of this embodiment, the antennas 51 and 52 are used for different communication systems. That is, instead of the diversity antenna structure as in the first to third embodiments described above, the antennas 51 and 52 transmit and receive radio waves in different frequency bands. For example, the first antenna 51 transmits and receives radio waves of 730 to 760 MHz, and the second antenna 52 transmits and receives radio waves of 715 to 725 MHz.

  Therefore, the first resonance length λ1 (λ1 = v / f1, v is the speed of light), which is the wavelength (resonance length) corresponding to the resonance frequency f1 of the first antenna 51, corresponds to the resonance frequency f2 of the second antenna 52. The wavelength (resonance length) is shorter than the second resonance length λ2. That is, each of the antennas 51 and 52 is configured to resonate at a predetermined center frequency in the frequency band of radio waves transmitted and received by itself (so that the center frequency becomes the resonance frequency). The horizontal distance (distance between the feeding points) of each antenna is ¼ or less of the second resonance length λ2 in this example, and more specifically, for example, λ2 / 8 to λ2 / 6. Yes.

  Among the antennas 51 and 52, the first antenna 51 has an element (strip conductor) standing vertically from a feeding point 54 on the plate surface of the ground plate 53, and its element length L1 (the plate of the ground plate 53). The height in the vertical direction from the surface to the antenna tip is λ1 / 4 (however, the shortening rate and the like are not considered).

  On the other hand, although the second antenna 52 is different from the first antenna 51 in use frequency and resonance frequency, the length of the entire antenna (the height in the vertical direction from the plate surface (feed point 55) of the ground plane 53 to the tip of the antenna). Is the same L1 as the first antenna 51. However, more specifically, the second antenna 52 is provided with a matching circuit 62 in the vicinity of the feeding point 55 as shown in an enlarged view in the lower part of FIG. That is, power is supplied from the feed point 55 to the element 61 via the matching circuit 62.

  Therefore, although the height (physical length) of the entire antenna including the second antenna 52 is L1 like the first antenna 51, the resonance length λ2 of the entire antenna including the matching circuit 62 and the element 61 is the first antenna 51. Longer than the resonance length λ1.

  The matching circuit 62 includes a conductor 71 connected to the feeding point 55, a coil 72 connected between the conductor 71 and the element 61, one end connected to the conductor 71, and the other end connected to the conductor 74. And a conductor 74 connected to the other end of the capacitor 74 is connected to the ground plane 53. However, such a configuration is merely an example, as long as it has a function as the matching circuit 62, such as using a different circuit configuration or using a matching circuit made up of one chip element. A specific configuration is not particularly limited.

  The operation and the like of the antenna structure 50 according to this embodiment including the matching circuit 62 will be described with reference to FIG. In the case of a conventional antenna structure not including the matching circuit 62 (the length of each antenna 111, 112 is L1) as shown in the upper left part of FIG. 11, as shown in the middle part of the left side of FIG. The resonance frequency of 112 is the same f1. Therefore, as shown in the lower left part of the figure, the directivities of the antennas 111 and 112 have the same characteristics as those of the conventional diversity antenna structure shown in the lower part of FIG. In other words, although the first antenna 111 exhibits good directivity characteristics in front of the vehicle, the second antenna 112 cannot obtain good directivity with respect to the front of the vehicle.

  On the other hand, as shown in the upper right part of FIG. 11, when the second antenna 52 has a matching circuit 62 (details are shown in FIG. 10), the resonance frequency of the first antenna 51 is shown in the middle part of the right side of the figure. Is f1, whereas the resonance frequency of the second antenna 52 is f2 larger than f1. That is, the resonance length λ1 of the first antenna 51 is shorter than the resonance length λ2 of the second antenna 52.

  Therefore, when viewed from the second antenna 52, the first antenna 51 behaves as a director, and as a result, as shown in the lower right side of FIG. It has good directivity in front of the vehicle, and the gain in front of the vehicle is improved. On the other hand, when viewed from the first antenna 51, the second antenna 52 behaves as a reflector, whereby the first antenna 51 has good directivity in the direction opposite to the second antenna 52 side (that is, in front of the vehicle). That is, both the first antenna 51 and the second antenna 52 have good directivity with respect to the front of the vehicle.

  That is, even if the length of the whole antenna is the same, by providing a matching circuit on one side to make the resonance lengths different, as a result, each antenna interacts in the same way as in the first embodiment. The same effect as in the first embodiment can be obtained.

  For this reason, in this embodiment, although the distance between the antennas 11 and 12 is close to λ2 / 8 to λ2 / 6 as described above, both antennas 51 and 52 have good directivity with respect to the front of the vehicle. On the other hand, it is possible to reduce the size and space of the entire antenna structure 50.

[Fifth Embodiment]
FIG. 12 shows a schematic configuration of the antenna structure 80 of the present embodiment.
Similar to the diversity antenna structure 10 of the first embodiment, the antenna structure 80 of the present embodiment is mounted on the roof 101 of the vehicle 100 in a state of being built in the resin case 9. The antenna structure 80 is configured such that a printed circuit board 87 in which three antennas 81, 82, 83 are formed by strip conductors on one plate surface is erected vertically on a ground plane 88.

  The printed circuit board 87 is erected so that the plate surface thereof is parallel to the longitudinal direction of the vehicle. The first antenna 81 is formed of a strip conductor on the end side in the vehicle front direction on one surface, and the third antenna 83 is formed of a strip conductor on the end side in the vehicle rear direction. A second antenna 82 is formed of a strip conductor between these antennas 81 and 83 (substantially intermediate position).

  When viewed from the ground plane 88, each of the antennas 81, 82, 83 is in a state of being erected perpendicular to the plate surface of the ground plane 88. Therefore, it can be said that each of the antennas 81, 82, 83 is a monopole antenna erected vertically on the ground plane 88 as in the first embodiment.

  The antenna structure 80 of the present embodiment is used for two communication systems having different adjacent frequency bands. That is, the first antenna 81 and the second antenna 82 arranged on the front side are used for MIMO (Multiple Input Multiple Output) in LTE (Long Term Evolution), and the two antennas 81 and 82 are used, for example, 730. Transmits and receives radio waves in the frequency band of ˜760 MHz. On the other hand, the third antenna 83 disposed on the rear side is used for inter-vehicle communication as in the first embodiment, and transmits and receives radio waves of, for example, 715 to 725 MHz. In the following description, the first antenna 81 is also referred to as an LTE first antenna, the second antenna 82 is also referred to as an LTE second antenna, and the third antenna 83 is also referred to as an inter-vehicle antenna.

  The antenna structure 80 of the present embodiment is also configured such that the radiation and arrival directions of the radio waves to be transmitted and received are in front of the vehicle 100 and the radio waves arriving from the front of the vehicle can be satisfactorily received by the antennas 81, 82, and 83. .

Further, the inter-vehicle antenna 83 having a low use frequency band is disposed behind the antennas 81 and 82 for LTE having a high use frequency band.
The first element length L1 that is the element length of each of the antennas 81 and 82 for LTE (the length from the plate surface (feeding points 84 and 85) of the ground plate 88 to the tip of each of the antennas 81 and 82 for LTE) is The second element length L2, which is the element length of the vehicle-to-vehicle antenna 83 (the length from the plate surface (feed point 86) of the base plate 88 to the tip of the vehicle-to-vehicle antenna 83), is shorter.

  That is, the resonance frequency of each of the antennas 81 and 82 for LTE is a predetermined center frequency f1 in the frequency band (730 to 760 MHz) of the radio wave transmitted and received by itself, and the resonance length corresponding to the center frequency is the first resonance length λ1. . The first element length L1 of each of the LTE antennas 81 and 82 is about ¼ of the first resonance length λ1. On the other hand, the resonance frequency of the vehicle-to-vehicle antenna 83 is a predetermined center frequency f2 in the frequency band (715 to 725 MHz) of radio waves transmitted and received by the vehicle-to-vehicle antenna 83, and the resonance length corresponding to the center frequency is the second resonance length λ2. The second element length L2 of the inter-vehicle antenna 83 is about ¼ of the second resonance length λ2.

  That is, in this embodiment, the method of using a matching circuit as in the fourth embodiment is not used, and the resonance length of each of the antennas 81 to 83 is set to the wavelength of its own transmission / reception radio wave by simply changing the element length. (Center wavelength). The horizontal distance (distance between the feeding points) of each antenna is ¼ or less of the second resonance length λ2 in this example, and more specifically, for example, λ2 / 8 to λ2 / 6. Yes.

  With this configuration, when viewed from the vehicle-to-vehicle antenna 83, the two antennas 81 and 82 for LTE both behave as a director, so that the vehicle-to-vehicle antenna 83 is good in front of the vehicle. It has directivity, and the gain ahead of the vehicle is improved. On the other hand, when viewed from the LTE antennas 81 and 82, the inter-vehicle antenna 83 behaves as a reflector, so that the LTE antennas 81 and 82 also have good directivity in front of the vehicle. That is, each of the LTE antennas 81 and 82 and the inter-vehicle antenna 83 has good directivity with respect to the front of the vehicle.

  It will be described with reference to FIG. 13 that the directivity of each antenna 81, 82, 83 differs when the arrangement order (arrangement order in the front-rear direction) of each antenna 81, 82, 83 differs. FIG. 13B shows the antenna structure 80 of the present embodiment shown in FIG. 12, that is, the antennas 81, 82, 83 are in order from the front of the vehicle, the first LTE antenna 81, the second LTE antenna 82, and the inter-vehicle use. The antenna structure arrange | positioned in order of the antenna 83 is shown. On the other hand, in FIG. 13A, for comparison, the antennas 81, 82, and 83 are arranged in the order of the first antenna for LTE 81, the inter-vehicle antenna 83, and the second antenna 82 for LTE in order from the front of the vehicle. 1 shows an antenna structure in which a vehicle-to-vehicle antenna 83 is arranged between LTE antennas 81 and 82.

  In the comparative example of FIG. 13 (a), although the LTE first antenna 81 improves the front gain of the vehicle, the LTE second antenna 82 has the front inter-vehicle antenna 83 acting as a reflector and moves forward. The gain decreases. Furthermore, since the inter-vehicle antenna 83 is also provided with LTE antennas 81 and 82 on the front and rear sides, radiation cancellation in the front-rear direction occurs, resulting in a decrease in gain. .

  On the other hand, in the antenna structure 80 of the present embodiment shown in FIG. 13B, the gain of the vehicle-to-vehicle antenna 83 is as follows due to the above-described action (the LTE antennas 81 and 82 behave as reflectors). It becomes higher than the comparative example of FIG.

  Therefore, even with the antenna structure 80 of the present embodiment, the distance between the antennas 11 and 12 is close to λ2 / 8 to λ2 / 6, but the antennas 81, 82, and 83 have good directivity toward the front of the vehicle. Thus, the entire antenna structure 80 can be reduced in size and saved in space.

  Here, in the antenna structure 80 of the present embodiment, the difference Ld that is the difference between the first element length L1 of each of the LTE antennas 81 and 82 and the second element length L2 of the inter-vehicle antenna 83 is determined as appropriate. However, it is preferable that it is larger than 0 and within a range of 10% or less of the second element length L2.

  The relationship between the difference Ld and the performance of the third antenna (vehicle-to-vehicle antenna) 83 is shown in FIG. FIG. 14A shows the antenna structure 80 of the present embodiment shown in FIG. 12, and FIG. 14B shows the F / V of the vehicle-to-vehicle antenna 83 when the difference Ld is changed in the antenna structure 80. An example of a change in the B ratio is shown.

  In FIG. 14B, the ratio of the difference Ld to the second element length L2 (Ld / L2) is changed to 1%, 3%, 5%, 7%, 9%, 10%, and 11%. The F / B ratio of the vehicle-to-vehicle antenna 83 is shown. As is apparent from FIG. 3B, when the difference Ld is within 10% of the second element length L2, the F / B ratio of the vehicle-to-vehicle antenna 83 is 2 [dB] or less. Conversely, when the difference Ld is greater than 10% of the second element length L2, the F / B ratio of the vehicle-to-vehicle antenna 83 is greater than 2 [dB]. FIG. 14C shows an example of directivity of the inter-vehicle antenna 83 when the difference Ld is 3% of the second element length L2 (that is, when the F / B ratio is about 0.8 [dB]). A good gain is obtained in the front-rear direction.

From these facts, it can be said that the ratio (Ld / L2) of the difference Ld to the second element length L2 is preferably 10% or less.
[Modification]
Although the embodiments of the present invention have been described above, the embodiments of the present invention are not limited to the above-described embodiments, and can take various forms as long as they belong to the technical scope of the present invention. Needless to say.

  For example, in the above-described embodiment, an example in which the two antennas constituting the diversity antenna structure are both dipole antennas is shown. However, such a configuration using a dipole antenna is merely an example, and other antennas are used. Also good.

  Specifically, for example, various antennas such as an inverted F antenna, an inverted L antenna, and a helical antenna can be used. Among these, a configuration example of a diversity antenna structure when an inverted L-type antenna is used. As shown in FIG.

  In the diversity antenna structure 40 shown in FIG. 9, the antennas 41 and 42 are arranged such that the first antenna 41 is in front of the second antenna 42 with respect to the front-rear direction of the vehicle, as in the above embodiments. A reflector 46 is erected on the rear side of the antenna 42. The distance between the antennas 41 and 42 and the distance between the second antenna 42 and the reflection plate 46 (interval based on the reception wavelength λ) are also the same as those in the above embodiments. Moreover, the slit 47 is formed in the reflecting plate 46 similarly to the reflecting plate 21 of the said 2nd Embodiment.

  Even with the diversity antenna structure 40 configured as described above, if the height of the first antenna 41 (the length in the direction perpendicular to the ground plane 43) is configured to be shorter than the height of the second antenna 42, the diversity according to the second embodiment. If the same operation and effect as the antenna structure 20 (see FIG. 5) are obtained and the heights of the antennas 41 and 42 are the same, the diversity antenna structure 30 (see FIG. 8) of the third embodiment can be obtained. Similar actions and effects can be obtained.

  Further, in the antenna structure 50 (see FIG. 10) of the fourth embodiment, the length (physical length) of the entire antenna is set to the first antenna 51 by configuring the second antenna 52 to include the matching circuit 62. Although the resonance length λ2 of the second antenna 52 is set to be larger than the resonance length λ1 of the first antenna 51, the antenna 51 further includes the matching circuit 62 in this way. , 52 may have different lengths (physical lengths), that is, the first antenna 51 may be shorter than the second antenna 52.

  In the antenna structure 80 (see FIG. 12) of the fifth embodiment, the physical length (second element length L2) of the inter-vehicle antenna 83 is approximately the resonance length λ2 of the center frequency f2 of the used frequency band. Resonance was achieved at the center frequency f2 by setting the frequency to 1/4, but the physical length of the entire antenna was the same as the first element length L1, and the matching length was used to set the resonance length to the first length. The second resonance length λ2 may be longer than the resonance length.

  That is, like the antenna structure 90 illustrated in FIG. 15, the element lengths of the LTE antennas 81 and 82 and the overall length of the vehicle-to-vehicle antenna 91 are all the same L1. However, in more detail, the inter-vehicle antenna 91 has the same configuration as the second antenna 52 in the antenna structure 50 of the fourth embodiment shown in FIG. 10, that is, a matching circuit provided on the feeding point 86 side. 93 and an element 92 connected to the matching circuit 93. The antenna structure 90 configured in this way can also obtain directivity characteristics and forward gain equivalent to those of the antenna structure 80 of the fifth embodiment shown in FIG.

  In the fourth embodiment, the first antenna 51 is not provided with a matching circuit. However, the first antenna 51 is maintained while the lengths (physical lengths) of the antennas 51 and 52 are the same. Also, a matching circuit may be provided. That is, even if the physical length of the antenna as a whole is the same, the value of each element constituting each matching circuit is made different, resulting in an antenna structure having the same characteristics as the antenna structure 50 of the fourth embodiment. it can. Specifically, for example, the configuration of the matching circuit provided in each of the antennas 51 and 52 is the same as that of the matching circuit 62 shown in FIG. 10, and the inductance value of the coil 72 is made different (for example, in the matching circuit of the first antenna 51). By setting the inductance value of the coil 72 to a value larger than the inductance value of the coil 72 in the matching circuit of the second antenna 52, the same characteristics as the antenna structure 50 of the fourth embodiment can be realized.

  Whether or not a matching circuit is provided for each of the antennas 81, 82, 83 constituting the antenna structure 80 of the fifth embodiment and the two antennas 81, 82 on the front side constituting the antenna structure 90 of FIG. It can be determined as appropriate.

Further, the method of increasing the resonance length by using the matching circuit in this way can also be adopted for the diversity antenna structures of the first to third embodiments.
In addition, as a specific method for increasing the resonance length without changing the entire length of the antenna, the method using the matching circuit described above is merely an example. For example, the matching circuit is used by configuring an antenna using a dielectric material. The resonance length may be increased as in the case.

  Further, the antenna structure 50 (FIG. 10) of the fourth embodiment, the antenna structure 80 (FIG. 12) of the fifth embodiment, and the antenna structure 90 shown in FIG. (Strip conductor) was formed, but such a configuration is merely an example, and each element is mounted on a ground plane as an antenna element of an air layer (that is, without a printed circuit board as in the first embodiment). The same effect can be obtained even in a standing configuration.

Each antenna does not necessarily have to have the same shape, and different antennas may be used.
In the above-described embodiment, the ground plate is provided separately from the roof 101 of the vehicle 100. However, since the vehicle body is generally made of metal (conductor), the roof 101 may be used as the ground plate. .

  In addition, the diversity antenna structures of the first to third embodiments are not limited to use as space diversity. For example, even when antennas of a plurality of communication systems using different adjacent frequencies are arranged close to each other. It is valid. In other words, in the case where a plurality of antennas are arranged close to each other, even if the plurality of antennas arranged close to each other are not used as space diversity, each is used in a different communication system. Even if it exists, it is effective to apply the technical idea of the present invention.

  DESCRIPTION OF SYMBOLS 1 ... Radio | wireless apparatus for vehicle-to-vehicle communication, 2 ... Switching circuit, 3 ... 1st switch, 4 ... 1st termination resistance, 5 ... 2nd switch, 6 ... 2nd termination resistance, 9 ... Resin case 10, 20, 30, 40 ... Diversity antenna structure, 11, 31, 41, 51, 81, 111 ... First antenna, 12, 32, 42, 52, 82, 112 ... Second antenna, 13, 33, 43, 53, 88 ... Base plate, 14, 15, 34, 35, 44, 45, 54, 55, 84, 85, 86 ... feeding point, 21, 36, 46 ... reflector, 22, 37, 47 ... slit, antenna structure ... 50, 80 , 90, 61, 92 ... element, 56, 87 ... printed circuit board, 62, 93 ... matching circuit, 71, 74 ... conductor, 72 ... coil, 73 ... capacitor, 83, 91 ... third antenna, 100 ... vehicle, 101 ... Roof, 102 Rear window glass

Claims (17)

Each of the at least two antennas, the first antenna and the second antenna, is configured to be able to transmit or receive radio waves, and the two antennas are respectively provided on the ground plane and separated from each other in a direction parallel to the plate surface. An antenna structure,
The first resonance length λ1 that is a wavelength corresponding to the resonance frequency of the first antenna is shorter than the second resonance length λ2 that is a wavelength corresponding to the resonance frequency of the second antenna. . The antenna structure according to claim 1,
The first element length L1 that is the element length of the first antenna is shorter than the second element length L2 that is the element length of the second antenna. The antenna structure according to claim 1 or 2,
At least the second antenna is configured by connecting a matching circuit to one end of the power feeding side of the element,
The antenna structure, wherein the second resonance length λ2 is a wavelength corresponding to a resonance frequency of the entire second antenna including the element and the matching circuit. The antenna structure according to claim 2,
At least the second antenna is configured by connecting a matching circuit to one end of the power feeding side of the element,
The second resonance length λ2 is a wavelength corresponding to the resonance frequency of the entire second antenna including the element and the matching circuit,
The first element length L1 that is the element length of the first antenna and the second element length L2 that is the element length of the second antenna have the same length. The antenna structure according to any one of claims 1 to 4,
Each of the first antenna and the second antenna is configured to transmit and receive radio waves of different frequency bands, and the frequency band of radio waves transmitted and received by the first antenna is transmitted to the second antenna. An antenna structure characterized by being higher than the frequency band of radio waves transmitted and received. The antenna structure according to any one of claims 1 to 4,
An antenna structure, wherein the first antenna and the second antenna are configured to transmit and receive radio waves in the same frequency band by a spatial diversity method. At least two antennas, the first antenna and the second antenna, are configured to receive radio waves by a spatial diversity method, and the two antennas are each standing on the ground plane in a direction parallel to the plate surface. A diversity antenna structure,
The diversity antenna structure according to claim 1, wherein a first element length L1 that is an element length of the first antenna is shorter than a second element length L2 that is an element length of the second antenna. The diversity antenna structure according to claim 7,
As a basic element length L, a quarter length of a wavelength corresponding to a predetermined center frequency f0 within a frequency band of a reception target radio wave by the diversity antenna structure
The first element length L1 is not more than the basic element length L,
The diversity antenna structure according to claim 1, wherein the second element length L2 is not less than the basic element length L. The diversity antenna structure according to claim 8,
The first element length L1 is set so that the resonance frequency f1 of the first antenna is in a range from the center frequency f0 to a frequency f0a higher than the center frequency f0 by a predetermined frequency width fa.
The second element length L2 is set so that the resonance frequency f2 of the second antenna is in a range from the center frequency f0 to a frequency f0b that is lower than the center frequency f0 by the frequency width fa. A feature diversity antenna structure. The diversity antenna structure according to claim 8 or 9, wherein
Both the first antenna and the second antenna are configured such that a voltage standing wave ratio in the frequency band of the reception target radio wave is equal to or less than a preset threshold value. Antenna structure. The diversity antenna structure according to any one of claims 7 to 10,
A length longer than the second element length L2 at a position away from the position of the feeding point of the second antenna by a predetermined distance on the opposite side to the position of the feeding point of the first antenna. The diversity antenna structure is characterized in that the reflector plate is erected so that the plate surface faces the second antenna. At least two antennas, the first antenna and the second antenna, are configured to receive radio waves by a spatial diversity method, and the two antennas are each standing on the ground plane in a direction parallel to the plate surface. A diversity antenna structure,
The first element length L1 that is the element length of the first antenna and the second element length L2 that is the element length of the second antenna are the same length,
A length longer than the second element length L2 at a position away from the position of the feeding point of the second antenna by a predetermined distance on the opposite side to the position of the feeding point of the first antenna. The diversity antenna structure is characterized in that the reflector plate is erected so that the plate surface faces the second antenna. The diversity antenna structure according to claim 11 or 12,
A diversity antenna structure, wherein a slit having a predetermined length is formed in the reflector in a range facing the second antenna in the length direction. The diversity antenna structure according to claim 13,
The length of the slit is 1/3 or more of the length of the reflector. The diversity antenna structure according to claim 13 or 14,
The diversity antenna structure according to claim 1, wherein the reflecting plate has a shape in which the lower end side is cut into a concave shape by forming the slit from the lower end on the ground plate side of the reflecting plate. The diversity antenna structure according to any one of claims 7 to 15,
The diversity antenna structure is mounted on the vehicle in order to receive radio waves coming from at least the front of the vehicle,
The first antenna and the second antenna are arranged such that a feeding point of each antenna is separated by a predetermined distance in the front-rear direction of the vehicle, and the first antenna is disposed with respect to the second antenna. A diversity antenna structure, wherein the diversity antenna structure is arranged to be positioned in front of the vehicle. The diversity antenna structure according to any one of claims 7 to 16,
The distance between each feeding point of the first antenna and the second antenna is ¼ or less of a wavelength corresponding to a predetermined frequency within a frequency band of a reception target radio wave of the diversity antenna structure. Diversity antenna structure.