Short-range automobile wireless communication device

In a short-range on-vehicle radio communication system having in-car radio communication function, a two-way transmission antenna for radio communication is arranged at a panel portion outside a sheet metal chassis of a car navigation unit. As a result, the communication area becomes large, signals including those from a cellular phone of low radio communication output power are easily received, and it is possible to save manufacturing cost.

This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/JP00/08561 which has an International filing date of Dec. 4, 2000, which designated the United States of America.

FIELD OF THE INVENTION

The present invention relates to an arrangement of a two-way transmission antenna for radio communication in a short-range on-vehicle radio communication system having in-car radio communication function.

BACKGROUND ART

FIG. 15is a block diagram of a conventional hands-free system. A cellular phone base station1and a cellular phone2are connected via a public radio communication line. Signals are transmitted and received between the cellular phone2and a hands-free unit3via a connector2C prepared in the cellular phone2. A microphone4and a speaker5are connected to the hands-free unit3. Telephone signals gathered by the microphone4arranged in a vehicle are transmitted to the cellular phone base station1via the hands-free unit3and the cellular phone2. Then, the telephone signals are transmitted from the cellular phone base station1to a person on the other end of a so-called public telephone line. On the other hand, telephone signals from the person on the other end of the line are transmitted to the hands-free unit3through the reverse path. The speaker5reproduces the telephone signals from the person on the other end of the line inputted to the hands-free unit3. The cellular phone2, the hands-free unit3, the microphone4and the speaker5are arranged in the vehicle6.

The hands-free system shown inFIG. 15has a disadvantage in the aspect that it is necessary to connect the connector2C of the cellular phone2every time a cellular phone user gets on the vehicle. Moreover, it is also necessary to disconnect the connector2C when the user gets out of the vehicle. Furthermore, when the connector2C is not connected, the connector2C with cable should be put away to avoid the car room from looking not neat.

FIG. 16is a block diagram of another conventional hands-free system, which overcomes the mentioned disadvantage of the hands-free system ofFIG. 15. The hands-free system shown inFIG. 16is disclosed, for example, in the Japanese Patent Publication (unexamined) No. 276261/1998 titled “Radio Communication System”. A difference from the hands-free system ofFIG. 15consists in that the, instead of the cable system, the cellular phone2aand the hands-free unit3aare connected by a radio system (or an infrared system). Flow of the telephone signals is the same as in the case ofFIG. 15, and further description thereof is omitted herein. In another known system disclosed in the Japanese Patent Publication (unexamined) No. 331064/1999 titled “Hands-free Telephone System”, a radio (FM) circuit portion built in the cellular phone2abody is formed into a radio module7and is arranged at a connector portion of the cellular phone2a.

The disadvantages of connecting and disconnecting the connector of the cellular phone2aand looking not neat are overcome by using the radio system (or the infrared system) in connecting the cellular phone2aand the hands-free unit3a. In the case of the infrared system, it is necessary that infrared transmitting and receiving sections of the cellular phone2aand the hands-free unit3aare arranged opposite to each other. On the contrary, in the case of the radio system, it is not necessary that the transmitting and receiving sections of the cellular phone2aand the hands-free unit3aare arranged opposite to each other.

As a short-range radio communication system of such type, Bluetooth SIG has proposed a “Bluetooth” system. (BLUETOOTH is a registered trademark of Telefonaktiebolaget L M Ericsson, Sweden.)

The mentioned system has the following characteristics:(1) Cordless communication between one apparatus and another is achieved.(2) Radio circuit of this system is applicable to a variety of apparatuses such as personal computers, printers, cellular phones, personal digital assistants, etc.(3) The global standard is adopted.(4) Both voice and data communication can be transmitted and received.(5) A frequency band of 2.4 GHz called an ISM band is adopted.(6) It is not necessary to arrange the communication apparatuses opposite to each other unlike those in the infrared communication.(7) Communication between apparatuses is available in the form of not only 1 to 1 like the infrared communication but also 1 to 7 (maximum).(8) It is possible to choose any output power convenient for each apparatus because there are following three types of outputs:Class 1: 100 mW (+20 dBm) max;Class 2: 2.5 mW (+4 dBm) max; andClass 3: 1 mW (0 dBm) max.

In choosing one output power among the three types described in the foregoing characteristic (8) of the system, Class 3 of the smallest power consumption (i.e., the battery life is long) for a portable device such as cellular phone. It is preferable to adopt Class 1 from the viewpoint of giving an importance to a communication area such as home modem station where communication with each room is available on the power-saving basis.

Supposing that a radio communication is conducted using a cellular phone having a small radio communication output power function like in the foregoing system with a cellular phone brought into a vehicle, it is possible to utilize this radio communication enjoying the following advantages: p0(1) Hands-free telephone{circle around (1)} Cordless and hands-free telephone is available even when the cellular phone is put in a bag, on a rear seat, or the like.{circle around (2)} This contributes to safe driving.(2) Information and communication display{circle around (1)} Information provided by an IT-adapted cellular phone of a mobile communication company is displayed on a map display of a car navigation unit. The information is enlarged on the display and easy to see.{circle around (2)} Plural persons in the vehicle can see the display at the same time.{circle around (3)} This contributes to safe driving.

Now, environment of the radio communication is described below.(1) A power-saving type is adopted as the radio output power of the cellular phone in most cases.{circle around (1)} In the case of power-saving output power, it is considered that communication is available within a distance of several meters.(2) In the cases of the foregoing hands-free telephone (1) and information and communication display (2), a radio communication circuit is built in a car navigation unit in most cases.{circle around (1)} As the car navigation unit is composed of a sheet metal chassis, a radio communication antenna is arranged inside the sheet metal chassis of the car navigation unit, and therefore sensitivity is low.{circle around (2)} The car navigation unit is arranged at any of various places such as center console, under side of seat, and trunk.{circle around (3)} In the case of arranging the car navigation unit in the trunk, to be free from outside influence such as “pressure”, “waterdrop”, and the like from loaded cargo, the unit incorporating the communication antenna of the car navigation may be further covered with a sheet metal when required.

Accordingly, an object of the invention is to provide a short-range on-vehicle radio communication system having in-car radio communication function, capable of widening the communication area so that any signal from a cellular phone of low radio communication output power as described above may be easily received.

DISCLOSURE OF INVENTION

The invention provides a short-range on-vehicle radio communication system having an in-car radio communication function, in which a two-way transmission antenna for radio communication is arranged outside a sheet metal chassis of a main body. As a result of employing such an arrangement, communication area becomes larger and any signal from a cellular phone of low radio communication output power might be easily received.

The invention is applied to a car navigation unit, and the two-way transmission antenna for radio communication is arranged at a panel portion outside the sheet metal chassis of the car navigation unit. As a result of employing such an arrangement, communication area becomes broader, any signal from a cellular phone of low radio communication output power might be easily received, and it is further possible to save the manufacturing cost.

The invention provides the short-range on-vehicle radio communication system having in-car radio communication function, in which the two-way transmission antenna for radio communication is arranged outside the main body. As a result of employing such an arrangement, a stable and large communication area is secured irrespective of the place of locating the short-range on-vehicle radio communication system main body, and any signal from a cellular phone of low radio communication output power might be easily received.

The invention provides the system of the short-range radio communication of which radio communication output power is small, which is applied to the car navigation unit. As a result of employing such an arrangement, the problem of communication antenna being low in sensitivity is solved, and any signal from a cellular phone of low radio communication output power of several meters in communication available distance is might be easily received.

The invention provides the short-range on-vehicle radio communication system having in-car radio communication function, in which the two-way transmission antenna for radio communication is arranged at a ceiling in a car room. As a result of employing such an arrangement, influence due to the place where the person to communicate to (i.e., the cellular phone) is located in the car room, is less. Furthermore, difference in distance between each place where the main body of the short-range on-vehicle radio communication system is located in the vehicle and the two-way transmission antenna for radio communication is reduced, and therefore fluctuation in loss caused by cable length is restrained.

The invention provides the short-range on-vehicle radio communication system having in-car radio communication function, in which the two-way transmission antenna for radio communication is arranged in an interior light module on the ceiling in the car room, on a surface of the module, or around the module. As a result of employing such an arrangement, the two-way transmission antenna for radio communication is out of sight, and wiring can be performed together with wiring for the interior light. The antenna can be arranged on the ceiling together with the interior light module. Further, the influence due to the place where the person to communicate to (i.e., the cellular phone) is located in the car room is reduced. Difference in distance between each place where the main body of the short-range on-vehicle radio communication system is arranged in the vehicle and the two-way transmission antenna for radio communication is less, and therefore fluctuation in loss caused by the cable length is restrained.

The invention provides the short-range on-vehicle radio communication system having in-car radio communication function, in which the two-way transmission antenna for radio communication is arranged on the front seat side. As a result of employing such an arrangement, the two-way transmission antenna for radio communication is located in the vicinity of the cellular phone brought into the vehicle by a driver who uses the hands-free telephone in most cases, which increases reliability in transmitting and receiving through the radio communication.

The invention provides the short-range on-vehicle radio communication system having in-car radio communication function, in which the two-way transmission antenna for radio communication is arranged on an outer circumference of the main body. As a result of employing such an arrangement, the antenna cable is shortened and loss due to cable length is less. Further the antenna mounting work becomes easy.

The invention provides the short-range on-vehicle radio communication system having in-car radio communication function, in which plural two-way transmission antennas for radio communication are arranged outside the sheet metal chassis of the main body in the vehicle. As a result of employing such an arrangement, the communication-available area becomes highly reliable, which enables development for large-sized vehicles such as buses.

The invention provides the short-range on-vehicle radio communication system having in-car radio communication function, in which among the plural two-way transmission antennas for radio communication, one is arranged at a front portion inside the vehicle and another one is arranged at a rear portion inside the vehicle.

The invention provides the short-range on-vehicle radio communication system having in-car radio communication function, in which among the plural two-way transmission antennas for radio communication, one is arranged at a panel portion outside the sheet metal chassis and another one is arranged outside the main body.

Furthermore, the invention provides the short-range on-vehicle radio communication system having in-car radio communication function, in which an antenna output of the highest reception level is selected among the plural two-way transmission antennas for radio communication. As a result of employing such an arrangement, when the person to communicate to uses a communication apparatus of small communication output power, the communication is secured even when the person to communicate to moves. This enables development for large-sized vehicles such as buses.

BEST MODE FOR CARRYING OUT THE INVENTION

In the case of performing a radio communication between a cellular phone and an on-vehicle radio communication system (a car navigation unit), a power-saving type output power is adopted as the radio output power of the cellular phone in most cases.

On the other hand, the on-vehicle radio communication system (the car navigation unit) is covered with a sheet metal chassis except for a panel face.

Taking account of the foregoing conditions, Embodiment 1 of the invention is hereinafter described with reference toFIG. 1.FIG. 1is a schematic diagram showing an arrangement according to Embodiment 1 of the invention. It is supposed herein that a short-range radio communication is performed between a cellular phone2band a car navigation unit8(a short-range on-vehicle radio communication system) each having short-range radio communication function. In this case, if a two-way transmission antenna9afor short-range radio communication in the can navigation unit8is arranged inside a sheet metal chassis portion8b, radiation pattern characteristic of the two-way transmission antenna9afor radio communication is restricted, and an area where communication is available becomes narrow. In order to relax the restriction on the radiation pattern characteristic of the two-way transmission antenna9afor radio communication, the two-way transmission antenna9afor radio communication is arranged outside the sheet metal chassis portion8bof the can navigation unit8, i.e., at a panel portion8c.

As a result, the communication area becomes large, and this improves reception of signals including those from the cellular phone2bof low short-range radio communication output power. It is possible to save the manufacturing cost by arranging the two-way transmission antenna9afor radio communication at the panel portion8coutside the sheet metal chassis portion8bof the car navigation unit8.

The car navigation unit8has hands-free function, and to which the microphone4and the speaker5is connected. The cellular phone2b, the car navigation unit8, the microphone4, and the speaker5are all located in the vehicle6. Telephone signals gathered by the microphone4arranged in the vehicle6are transmitted to the cellular phone base station1via the car navigation unit8and the cellular phone2b. On the other hand, telephone signals from the person on the other end of the line are transmitted to the car navigation unit8through the reverse path. The speaker5reproduces telephone signals of the person on the other end of the line inputted to the car navigation unit8. It is a matter of course that the system shown inFIG. 1is also applicable to a case of displaying data other than the telephone signals from the cellular phone2bon the map display (not shown) for the car navigation unit8, and to a case of transmitting data other than the telephone signals from the car navigation unit8to the cellular phone2b.

FIG. 2is a schematic diagram showing an arrangement according to Embodiment 2 of the invention. In this Embodiment 2, the two-way transmission antenna9afor short-range radio communication of the can navigation unit8ais arranged outside the car navigation unit8a. This two-way transmission antenna9afor short-range radio communication is covered with a resin case or the like that does not restrict the radiation pattern characteristic of the antenna9a.

As a result of employing such arrangement, it is obvious that the communication area is not restricted by the sheet metal chassis8band the short-range radio communication environment is further improved. Furthermore even in the case that the car navigation unit8ais arranged at a place where the radio environment is not desirable, it is possible to achieve a favorable short-range radio communication with the cellular phone2b. Particularly in the case that the car navigation unit8ais arranged in the trunk, or even in the case that the unit8ais covered with a protective sheet metal for protection from “external pressure” “water drop”, arranging the two-way transmission antenna for short-range radio communication outside the car navigation unit8amakes it possible to stably secure the communication-available area irrespective of the place where the car navigation unit8amain body is arranged.

FIG. 3is a schematic view showing an arrangement according to Embodiment 3 of the invention.FIG. 3shows Example 1 of the aspect of arranging the two-way transmission antenna9afor short-range radio communication of the car navigation unit8aoutside the car navigation unit8a. In this Example 1 showing the aspect of arranging the two-way transmission antenna9afor short-range radio communication, the antenna9ais arranged at the ceiling in the car room (Place A). As a result, the radio environment between the two-way transmission antenna9afor short-range radio communication and the cellular phone2blocated at any of points α, β, γ, and δ in the car room is good in the same manner. Influence due to the place of the person to talk to (i.e., place of the cellular phone) in the car room is minimum.

In the case that the car navigation unit8ais arranged at any of the points a, b, and c in the vehicle, the distance between the two-way transmission antenna9afor short-range radio communication and the car navigation unit8ais almost equal, and fluctuation in loss caused by a cable length is restrained. Furthermore, even in the case that the car navigation unit8ais arranged in the trunk and the unit is covered with a protective sheet metal10for protection from “external pressure” “water drop”, it is possible to keep radio environment good between the two-way transmission antenna9afor short-range radio communication and the cellular phone2b.

FIG. 4is a schematic view showing an arrangement according to Embodiment 4 of the invention.FIG. 4shows Example 2 of the aspect of arranging the two-way transmission antenna9afor short-range radio communication of the car navigation unit8aoutside the car navigation unit8a. In this Example 2 showing the aspect of arranging the two-way transmission antenna9afor short-range radio communication is arranged, the antenna9ais arranged in an interior light module (Place B) in the car room, on a surface of the module or around the module. As a result, the following advantages are obtained in addition to those described in Embodiment 3.

First, in the case of arranging the two-way transmission antenna9afor short-range radio communication in the interior light module, the two-way transmission antenna9afor short-range radio communication is out of sight, and the car room looks very neat. Furthermore, the two-way transmission antenna9afor short-range radio communication is wired together with wiring of the interior light module, thereby work efficiency being improved.

In the case of arranging the two-way transmission antenna9afor short-range radio communication on the surface of the interior light module or in the vicinity thereof, not only the two-way transmission antenna9afor short-range radio communication is wired together with wiring of the interior light module, but also the two-way transmission antenna9afor short-range radio communication and the interior light module can be installed at the same time, thereby work efficiency being improved.

In the foregoing description, the antenna is arranged in the interior light module in the car room, on the surface of the module or around the module. The same advantages as in the interior light module are also obtained by arranging the two-way transmission antenna9afor short-range radio communication integrally with a rearview mirror (room mirror) in the car room or by arranging the two-way transmission antenna9afor short-range radio communication around the rearview mirror.

In other words, in the case that the antenna9ais arranged integrally with the rearview mirror (room mirror), the car room looks neat. In the case that the antenna9ais arranged around the rearview mirror (room mirror), a mounting member for mounting the rearview mirror (room mirror) and a mounting member for mounting the two-way transmission antenna.9afor short-range radio communication can be combined, and the rearview mirror (room mirror) and the two-way transmission antenna9afor short-range radio communication can be arranged at the same time, thereby work efficiency being improved.

FIG. 5is a schematic view showing an arrangement according to Embodiment 5 of the invention.FIG. 5shows Example 3 of the aspect of arranging the two-way transmission antenna9afor short-range radio communication of the car navigation unit8aoutside the car navigation unit8a. In this Example 3 showing the aspect of arranging the two-way transmission antenna9afor short-range radio communication, the antenna9ais arranged on the front seat side in the car room (Place C: the two-way transmission antenna9afor short-range radio communication is arranged under the assistant driver's seat inFIG. 5). As a result, there is a possibility that the antenna9ais located in the vicinity of the cellular phone brought into the vehicle by the driver who is supposed to use the mentioned hands-free telephone in most cases, thereby reliability on transmitting and receiving in radio communication being improved.

FIG. 6is a schematic view showing an arrangement according to Embodiment 6 of the invention.FIG. 6shows Example 4 of the aspect of arranging the two-way transmission antenna9afor short-range radio communication of the car navigation unit8aoutside the car navigation unit8a. In this Example 4 showing the aspect of arranging the two-way transmission antenna9afor short-range radio, the antenna9ais arranged on a surface of a dashboard in the car room, inside the dashboard or around the dashboard (Place D). The portion indicated by the half-tone dot meshing inFIG. 7shows the Place D in an image of actual vehicle.

As a result, in the case that the car navigation unit8ais arranged either at a point a (center console) or at a point b (under the front seat) in the vehicle, the distance between the two-way transmission antenna9afor short-range radio communication and the car navigation unit8ais shorter than that in the foregoing Embodiment 3 or 4, and loss caused by the cable length is reduced. Further, mounting the two-way transmission antenna9afor short-range radio communication is easier than that in the foregoing Embodiment 3 or 4.

Furthermore, arranging the cellular phone2beither at the point γ or at the point δ in the car room does not cause any significant deterioration in radio environment between the two-way transmission antenna9afor short-range radio communication and the cellular phone2b.

FIG. 8is a perspective view showing an arrangement according to Embodiment 7 of the invention.FIG. 8shows Example 5 of the aspect of arranging the two-way transmission antenna9afor short-range radio communication of the car navigation unit8aoutside the car navigation unit8a. In this Example 5 showing the aspect of arranging the two-way transmission antenna9afor short-range radio communication, the antenna9ais arranged on a surface of the center console or floor console (the portion indicated by half-tone dot meshing inFIG. 8) in the car room, inside the console or around the console. As a result, there is a possibility that the antenna9ais located in the vicinity of the cellular phone brought into the vehicle by the driver who is supposed to use the mentioned hands-free telephone in most cases, thereby reliability on transmitting and receiving in radio communication being improved.

Consequently, in the case that the car navigation unit8ais arranged either at a point a (center console) or at a point b (under the front seat) in the vehicle, particularly at the point a (the center console), the distance between the two-way transmission antenna9afor short-range radio communication and the car navigation unit8ais shorter than that in the foregoing Embodiment 3 or 4, and loss caused by the cable length is reduced. Further, mounting the two-way transmission antenna9afor short-range radio communication is easier than in the foregoing Embodiment 3 or 4.

FIG. 9is a perspective view showing an arrangement according to Embodiment 8 of the invention. Numeral2dis a cellular phone holder arranged at the place indicated by the arrow.FIG. 9shows Example 6 of the aspect of arranging the two-way transmission antenna9afor short-range radio communication of the car navigation unit8aoutside the car navigation unit8a. In this Example 6 showing the aspect of arranging the two-way transmission antenna9afor short-range radio communication, the antenna9ais arranged in the vicinity of the cellular phone holder in the car room (the portion indicated by half-tone dot meshing inFIG. 9). As a result, the antenna9ais located in the vicinity of the cellular phone brought into the vehicle, thereby reliability on transmitting and receiving in radio communication being improved.

Consequently, in the case that the car navigation unit8ais arranged either at a point a (center console) or at a point b (under the front seat) in the vehicle, particularly at the point a (the center console), the distance between the two-way transmission antenna9afor short-range radio communication and the car navigation unit8ais shorter than that in the foregoing Embodiment 3, 4 or 6, and loss caused by the cable length is reduced. Further, mounting the two-way transmission antenna9afor short-range radio communication is easier than in the foregoing Embodiment 3 or 4.

FIG. 10is a schematic view showing an arrangement according to Embodiment 9 of the invention.FIG. 10shows Example 7 of the aspect of arranging the two-way transmission antenna9afor short-range radio communication of the car navigation unit8aoutside the car navigation unit8a. In this Example 7 of the aspect of arranging the two-way transmission antenna9afor short-range radio communication, the antenna9ais arranged on a surface of a rear tray in the car room, inside the rear tray or around the rear tray (Place E). As a result, in the case that the car navigation unit8ais arranged at a point c in the vehicle, the distance between the two-way transmission antenna9afor short-range radio communication and the car navigation unit8ais shorter than in Embodiment 3 or 4, and the loss caused by the cable length is reduced. Further, mounting two-way transmission antenna9afor short-range radio communication is easier than in the foregoing Embodiment 3 or 4.

Furthermore, arranging the cellular phone2beither at the point α or at the point β does not cause any significant deterioration in radio environment between the two-way transmission antenna9afor short-range radio communication and the cellular phone2b.

FIG. 11is a schematic view showing an arrangement according to Embodiment 10 of the invention.FIG. 11shows Example 8 of the aspect of arranging the two-way transmission antenna9afor short-range radio communication of the car navigation unit8aoutside the car navigation unit8a. In this Example 8 of the aspect of arranging the two-way transmission antenna9afor short-range radio communication, the antenna9ais arranged in the trunk of the vehicle. (i.e., Place F: the two-way transmission antenna9afor short-range radio communication is arranged on the rear side of the rear tray inFIG. 11). As a result, in the case that the cellular phone is arranged in a bag accommodated in the trunk, the antenna9ais located in the vicinity of the cellular phone, thereby reliability on transmitting and receiving in radio communication being improved.

Further, in the case that the car navigation unit8ais arranged in the trunk as shown inFIG. 11, the distance between the car navigation unit8aand the two-way transmission antenna9afor short-range radio communication becomes shorter. Thus, loss caused by the antenna cable is reduced, thereby efficiency in mounting work being improved. When the driver makes hands-free telephone talk using the cellular phone, it is obvious that the hands-free telephone talk is achieved just by connecting the microphone for the driver arranged in the vicinity of the driver's seat and the speaker for reproducing the voice of the person on the other end of the line to the car navigation unit8aarranged in the trunk.

FIG. 12is a schematic view showing an arrangement according to Embodiment 11 of the invention.FIG. 12shows Example 9 of the aspect of arranging the two-way transmission antenna9afor short-range radio communication of the car navigation unit8aoutside the car navigation unit8a. In this Example 9 of the aspect of arranging the two-way transmission antenna9afor short-range radio communication, the antenna9ais arranged on the outer circumference of the car navigation unit8a(Place G). In this Embodiment 11, the car navigation unit8abody is arranged under the front seat. As a result, the distance between the two-way transmission antenna9afor short-range radio communication and the car navigation unit8abecomes extremely short, loss caused by the cable length is extremely small, and mounting work is easy.

Furthermore, even in the case that the cellular phone2bis arranged at any of the points α, β, γ and δ, the radio environment between the two-way transmission antenna9afor short-range radio communication and the cellular phone2bis improved as compared with the panel portion of built-in antenna type described in the foregoing Embodiment 1.

FIG. 13is a schematic view showing an arrangement according to Embodiment 12 of the invention.FIG. 13shows Example 10 of the aspect of arranging the two-way transmission antennas9afor short-range radio communication of the car navigation unit8aoutside the car navigation unit8a. In this Example 10 of the aspect of arranging the two-way transmission antennas9afor short-range radio communication, plural two-way transmission antennas9afor short-range radio communication are arranged (at Places D and H). In the drawing, Place D is located on the dashboard described in Embodiment 6, and Place H is located on the rear side of the rear tray described in Embodiment 9.

As a result, in the case that the cellular phone2bis located at any of the points α, β, γ, and δ in the car room, a signal of the two-way transmission antenna9afor short-range radio communication of Place D is adopted, and in the case that the cellular phone2bis located at a point ε in the trunk (in a bag in this case), a signal of the two-way transmission antenna9afor short-range radio communication of Place H is adopted, thus the communication-available space for the cellular phone2bbecomes large. This further increases reliability on the communication-available area. This Embodiment 12 is particularly effective when the short-range radio communication system is built in a large-sized vehicle.

In the foregoing description, the plural two-way transmission antennas9afor short-range radio communication are arranged, and the antennas9aare arranged at the front and the rear sides of the vehicle. As another Example 1 of arranging the plural two-way transmission antennas9afor short-range radio communication, it is preferable that at least one of the plural two-way transmission antennas9afor short-range radio communication is arranged on the right side of the longitudinal center line of the vehicle, and at least another one of them is arranged on the left side of the longitudinal center line of the vehicle. As a result of employing such an arrangement, it is possible to reduce influence of any obstacle that shuts out in the longitudinal direction of the vehicle such as backrest of seat or human body in the vehicle.

As a further Example 2 of arranging the plural two-way transmission antennas9afor short-range radio communication, it is preferable that at least one of the plural two-way transmission antennas9afor short-range radio communication is arranged on the right side of the seating center line of the driver's seat, and at least another one of them is arranged on the left side of the seating center line of the driver's seat. As a result of employing such an arrangement, it is possible to greatly reduce influence of the human body of the driver who uses most frequently the system of the invention such as hands-free telephone talk system. The mentioned two-way transmission antennas9afor short-range radio communication arranged in this manner does not cause any significant deterioration in short-range radio communication environment at any other place than the driver's seat.

As a further Example 3 of arranging the plural two-way transmission antennas9afor short-range radio communication, it is preferable that at least one of the plural two-way transmission antennas9afor short-range radio communication is arranged on the front side of the seating center line of the driver's seat, and at least another one of them is arranged on the rear side of the seating center line of the driver's seat. As a result of employing such an arrangement, it is possible to greatly reduce influence of the human body of the driver who uses most frequently the system of the invention such as hands-free telephone talk system. The two-way transmission antennas9afor short-range radio communication arranged in this manner does not cause any significant deterioration in short-range radio communication environment even in any other place than the driver's seat.

As a further Example 4 of arranging the plural two-way transmission antennas9afor short-range radio communication, it is preferable that at least one of the plural two-way transmission antennas9afor short-range radio communication is “arranged at the panel portion8cof the short-range on-vehicle radio communication system (the car navigation unit8)” described in the foregoing Embodiment 1, and at least another one of them is arranged at a place away from the short-range on-vehicle radio communication system. As a result of employing such an arrangement, only one of the two-way transmission antennas9afor short-range radio communication is arranged outside the short-range on-vehicle radio communication system, and therefore the mounting work is easy. The two-way transmission antennas9afor short-range radio communication arranged in this manner obviously improves the short-range radio communication environment as compared with the two-way transmission antenna9afor short-range radio communication arranged only at the panel portion8cof the short-range on-vehicle radio communication system (the car navigation unit8) as described in the foregoing Embodiment 1.

As a further Example 5 of arranging the plural two-way transmission antennas9afor short-range radio communication, it is preferable that at least one of the plural two-way transmission antennas9afor short-range radio communication is “arranged on the outer circumference of the short-range on-vehicle radio communication system (the car navigation unit8)” described in the foregoing Embodiment 11, and at least another one of them is arranged at a place away from the short-range on-vehicle radio communication system. As a result of employing such an arrangement, only one of the two-way transmission antennas9afor short-range radio communication is arranged outside the short-range on-vehicle radio communication system, and the mounting work is easy. The two-way transmission antennas9afor short-range radio communication arranged in this manner obviously improves the short-range radio communication environment as compared with the two-way transmission antenna9afor short-range radio communication arranged only on the outer circumference of the short-range on-vehicle radio communication system (the car navigation unit8) as described in the foregoing Embodiment 11.

FIG. 14is a block diagram of an antenna output selective circuit showing an essential part of Embodiment 13 of the invention. This Embodiment 13 is an example in which two two-way transmission antennas for radio communication are used, and in which connection is made to the radio communication two-way transmission antenna output of which reception level is higher. InFIG. 14, numeral8ais the car navigation unit having a radio communication function, and numerals9a-1and9a-2are the two-way transmission antennas for radio communication arranged outside the body of the car navigation unit8a. Numerals30-1and30-2are duplexers for switching whether to receive reception signals from the two-way transmission antennas9a-1and.9a-2for radio communication or transmit transmission signals to the two-way transmission antennas9a-1and9a-2for radio communication. Numerals11-1and11-2are band-pass filters for passing frequency bands of the reception signals received by the two-way transmission antennas9a-1and9a-2for radio communication. Numerals12-1and12-2are low-noise amplifiers for amplifying the reception signals that have passed through the band-pass filters11-1and11-2. Numeral13is a level comparator for comparing a level of the reception signal of the two-way transmission antenna9a-1for radio communication and a level of the reception signal of the two-way transmission antenna9a-2for radio communication, and numerals14-1and14-2are antenna switches for switching the connection to either of the two-way transmission antennas9a-1and9a-2for radio communication. Numeral15is an A/D converter for converting a received analog signal into a digital signal, and numeral16is a demodulation circuit for demodulating the reception signal converted into the digital signal and outputting the reception signal R. Numeral17is a modulation circuit for modulating a transmission signal T, numeral18is a D/A converter for converting the modulated transmission signal from a digital signal into an analog signal, numeral19is a band-pass filter for limiting the transmission signal converted into the analog signal to a signal of a predetermined frequency band, and numeral20is an amplifier for amplifying the transmission signal.

Operation is hereinafter described. The signals received by the two-way transmission antennas9a-1and9a-2for radio communication are inputted to the level comparator13via the duplexers30-1and30-2, the band-pass filters11-1and11-2, and the low-noise amplifiers12-1and12-2. The level comparator13compares level of the two inputted signals (receiving signals), and the antenna switches14-1and14-2are controlled based on the result of the comparison. The reception signal selected by the antenna switch14-1passes through the A/D converter15and the demodulation circuit16, and is processed by a signal processor (not shown) in a later stage.

On the other hand, a transmitting signal is inputted to the antenna switch14-2via the modulation circuit17, the D/A converter18, the band-pass filter19, and the amplifier20. As described above, the antenna switch14-2is controlled to select the two-way transmission antenna for radio communication of the higher receiving signal level. Therefore, the transmission signal is transmitted from the two-way transmission antenna for radio communication of the higher reception level.

Timing for switching from one of the duplexers30-1and30-2to the other is decided on the basis of a transmission-reception switch signal S to prevent a signal from missing at the moment of switching. Further, timing for switching from one of the antenna switches14-1and14-2for selecting either of the two two-way transmission antennas for radio communication to the other switch is also decided on the basis of the transmission-reception switch signal S inputted to the level comparator13.

INDUSTRIAL APPLICABILITY

The present invention is applicable to installation of a short-range on-vehicle radio communication system having in-car radio communication function in a large-sized vehicle such as bus or train, a passenger car, or a commercial car.