Abstract:
A side-looking array antenna system is disclosed in which the inclination angle of the antenna main beam is determined depending on the intervals of a plurality of antenna elements forming an array. A switch is provided to selectively connect one of the ports of an antenna feeder to a signal source or the like. The switch is used to select one of the ports of the antenna feeder as an excitation port, thereby reversing the direction of inclination of the antenna main beam. The antenna main beam can thus be directed in either of the two directions by the operation of the switch. Also, by using this antenna system, a Doppler radar vehicle speedometer with small calculation error, automotive radar and the like may be simply configured.

Description:
BACKGROUND OF THE INVENTION 
     1. FIELD OF THE INVENTION 
     The present invention relates to an antenna system comprising a side-looking array antenna, or more in particular to an antenna system which is capable of spatial beam scanning with a simple construction. 
     2. DESCRIPTION OF THE RELATED ART 
     It is often necessary to effect a spatial high speed scanning of an antenna beam for the purpose of receiving an electromagnetic wave from various directions and sending it in various directions such as in an antenna system used for a radar. In such a case, a multiplicity of antenna elements are generally arranged in the same plane to make up an array antenna, and the phases of the electrical signals for sending electromagnetic waves, respectively, supplied to antenna elements are varied according to a predetermined rule thereby to effect an electrical beam scanning in a phased array system. When beam scanning, effected in the phased array or the like, is not provided, on the other hand, a plurality of antennas corresponding to a plurality of desired beam directions are respectively provided and switched for operation. 
     In the above-mentioned phased array system, the phase of each antenna element is varied, and therefore each antenna element is required to be connected with a phase shifter. This results in an increased connection loss or transmission loss, thereby deteriorating the antenna characteristics. Further, due to the added weight of the phase shifter, generally the whole antenna system becomes heavy and bulky. In an antenna system which does not effect beam scanning, it is necessary to provide as many antennas as the desired number of beam directions. As a result, a great number of antennas are required, thereby posing the problem of connection loss, transmission loss and the weight and bulkiness of the system as in the case of phased array system. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an antenna system in which the number of phase shifters or the number of antennas of the phased array system can be reduced. 
     According to the present invention, there is provided an antenna system comprising a side-looking array antenna in which an antenna main beam is inclined at an angle depending on the intervals of the antenna elements making up an array, and switching means for connecting a signal source or the like to one or the other of the ports of an antenna power feeder by its switching operation, the switching means being used for switching the ports of the antenna feeder as an excitation port thereby to reverse the direction of inclination of the antenna main beam for scanning space, in which the antenna main beam is capable of being directed in either of two directions by switching operation of the switching means, thereby making it possible to reduce the number of phase shifters or antennas in a phased array. 
     By utilizing the antenna system according to the present invention, it is possible to realize with a simple construction a transmission-receiving system of beam scanning type, a receiving system, a Doppler radar vehicle speedometer with small calculation error, an automotive radar with a broad detection range, etc. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a configuration diagram for explaining the basic principle of an antenna system according to the present invention. 
     FIG. 2 is a perspective view of a basic embodiment of an antenna system according to the present invention. 
     FIG. 3 and FIG. 4 are examples of a vehicle carrying a Doppler speedometer using an antenna system according to the present invention. 
     FIG. 5 is a block diagram showing a configuration of the Doppler speedometer. 
     FIG. 6 is a top plan view of an vehicle equipped with an automotive radar using an antenna system according to the present invention. 
     FIG. 7 is a configuration diagram of a transmission-receiving system using an antenna system according to the present invention. 
     FIG. 8 is a configuration diagram of a receiving system including an antenna system according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a configuration diagram showing a basic embodiment of the antenna system according to the present invention. In FIG. 1, reference numeral 1 designates a side-looking array antenna. As shown in FIG. 2, the array antenna 1 includes a dielectric substrate 2 made of Teflon or ceramics, a plurality of slots 3 making up an antenna element in the surface of the dielectric substrate 2, a conductive layer 4 having the slots 3, and a power feeder 5 on the back of the dielectric substrate 2 for supplying power to the slots 3 through the dielectric substrate 2, thereby making up what is called the microstrip slot array antenna. The direction of the main beam used for scanning space in the side-looking array antenna depends on the interval l of the slots 3 arranged along the feeder making up the array of the antenna elements. In this embodiment, the antenna main beam is directed along the direction of φ=π/2, θ=θ 0  in the coordinate system of FIGS. 1 and 2. The interval l is determined in the manner described below. Specifically, if the antenna main beam is placed in the direction of φ=π/2, θ=θ 0 , the electric field from the slots 3 is added in the same phase in this direction, and therefore the equation below is established. 
     
         k.sub.0 lsinθ.sub.0 -βgl=2pπ(p=0, ±1, ±2, . . . ) 
    
     where 
     k 0  =2π/λ 0   (λ 0  : Free space wavelength) 
     βg=2π/λg (λg: Propagation wavelength along strip line) 
     As a result, l is determined in a manner to satisfy the above-mentioned equation. In FIG. 1, numeral 6 designates a change-over switch for selecting one of the terminals 5a and 5b of the feeder 5 of the array antenna an an excitation port. If the terminal 5a is selected as an excitation port, for instance, a signal source 7 is connected to the terminal 5a, and a matching load 8 is connected to the terminal 5b as shown by the solid line in FIG. 1. If the other terminal 5b is selected as an excitation port, on the other hand, the signal source is connected to the terminal 5b, and the matching load 8 is connected to the terminal 5a as shown by the dashed line. 
     In this configuration, assuming that the terminal 5a of the feeder 5 of the side-looking array antenna 1 is selected as an excitation port by the switch 6, the antenna main beam is placed along the direction of φ=π/2, θ=-θ 0  (shown by the solid line in FIG. 1) in accordance with above-mentioned equation. On the other hand, assuming that the switch 6 is turned to use the other terminal 5b of the feeder 5 as an excitation port, the antenna main beam is placed along the direction of φ=π/2, θ=θ 0  (shown by the dashed line in FIG. 1). In this way, by switching the excitation ports of the feeder 5 by the switch 6, the direction of inclination of the antenna main beam is reversed, thereby attaining substantially the same state as if the array antenna 1 were rotated by 180° within the installation plane. Specifically, it is possible to direct the antenna main beam in either of two different directions using a single array antenna 1, without moving the antenna proper and without phase scanning by a phase shifter. This rotation is effected by switching the signal input ports by the switch 6. For this reason, in spatial scanning of the antenna main beam by the antenna system according to this embodiment, the scanning of substantially one half of the angle is required as compared with the prior art by switching the excitation ports, and therefore the number of phase shifters for phase scanning, the number of antennas are reduced by one half, and the scanning angle is reduced to one half of the scanning angle for a mechanical scanning. At the same time, the connection loss, transmission loss and the weight and size of the antenna system are reduced. 
     In the aforementioned embodiment, the switch 6 may be operated manually, or alternatively, automatically at high speed by use of a computer or the like. Also, the switch 6 may be realized by a switching element or switching circuit operable at high speed by use of a semiconductor switch or the like as well as a mechanically-constructed switch. 
     In the aforementioned embodiment, with the increase in the slots 3 providing antenna elements arranged along the feeder 5, the power absorbed into the matching load 8 is reduced greatly. Therefore, the above-mentioned functions are not affected if the matching load 8 is not connected to the terminal of the feeder 5, thereby leaving it open or short. 
     Further, according to the present invention, the above-described microstrip slot array antenna may be replaced with equal effect by a waveguide slot array antenna or a crank-type microstrip array antenna which satisfies the relationship between l and θ 0 . 
     Now, a specific embodiment of the antenna system according to the present invention will be described. FIGS. 3 and 4 show an example of the antenna system according to the present invention applied to an automotive speedometer using the Doppler effect. The antenna system used in this case is the same as the antenna system comprising a transmitter and a receiver as illustrated in FIG. 7. 
     The Doppler speedometer operates in such a manner that an electromagnetic wave is transmitted from an automotive vehicle toward the road surface along the direction of car travel at a depression angle of θ, and the difference between the transmitting and receiving frequencies thereof, that is, the Doppler frequency, is used to calculate the speed of the vehicle relative to the ground. Generally, this system is configured as shown in FIG. 5. In this Doppler speedometer, when the vehicle body is inclined while running, the propagation angle of the electromagnetic wave to the road surface changes, thereby causing an error in vehicle speed calculation. In order to prevent such an error of vehicle speed calculation due to the inclination of the vehicle body, the Janus configuration in which beams are radiated in two opposite directions is generally used. Specifically, in FIG. 3, when the automotive vehicle is inclined in the longitudinal direction against the travel direction shown by arrow F, the error in the calculation of the vehicle speed attributable to the vehicle inclination angle δ is reduced by radiating beams at the depression angle θ in two directions, that is, forward and rearward of vehicle travel (indicated by A and B in the drawing), and by calculating the vehicle speed from the Doppler frequencies for the two directions. In the prior-art configuration of the Doppler radar vehicle speedometer with the Janus configuration, an antenna system is required with antenna beams directed in two predetermined directions, thus necessitating two antennas. According to the antenna system of the present invention, by contrast, the antenna 9 is capable of radiating beams either in the forward or the rearward directions at the depression angle θ as shown in FIG. 3, and therefore the Janus configuration becomes possible with a single antenna, thereby considerably reducing the error in the calculation of the vehicle speed caused by the vehicle inclination angle δ. Also, if the antenna system 10 according to the present invention is installed in such a manner as to direct the antenna beam leftward and rightward of vehicle travel as shown in FIG. 4, the calculation error of the vehicle speed caused by lateral vehicle inclination angle δ&#39; against the direction of travel may be reduced. 
     FIG. 6 shows an antenna system according to the present invention as applied to an automotive radar system. An antenna 11 is installed at the front center of a vehicle, and the antenna main beam is directed rightward or leftward diagonally in the forward direction by operating a changeover switch not shown. In similar fashion, an antenna 12 is installed at the rear center of the vehicle and the antenna main beam thereof is directed rightward or leftward diagonally in rearward direction. In this way, by applying the antenna system of the present invention to the automotive radar, the antenna 11 may be used for leftward or rightward search diagonally in forward direction and the antenna 12 for leftward or rightward search diagonally in rearward direction. As a result, unlike the conventional automotive radar requiring an antenna for each direction of search, the number of antennas is reduced by one half. Although the present embodiment has the antennas 11 and 12 installed at the front and rear centers respectively of the vehicle, the antennas are not necessarily installed at the center, but may be installed at the right or left end at the front and rear parts of the vehicle with equal effect. 
     FIG. 7 shows a transmission-receiving system configured by use of an antenna system according to the present invention. In FIG. 7, the same component elements as those in FIG. 1 are designated by the same reference numerals as in FIG. 1 respectively. A switch 6, which is the same as the switch 6 described above with reference to FIG. 2, is connected with a transmitter 72 and a receiver 73 through a circulator 71. A transmission signal produced from the transmitter 72 is applied through the circulator 71 and the switch 6 to the antenna 1 for the purpose of transmission by the antenna. The electromagnetic wave received by the antenna 1 is supplied through the switch 6 and the circulator 71 to the receiver 73. If the switch 6 is configured in the manner described, above in FIG. 2, the directivity of the antenna 1 can be switched, by switching the switch 6 whereby a tranmission/receiving operation in two directions becomes possible. 
     FIG. 8 shows a system used only for receiving which is configured of an antenna according to the present invention. In this case, a receiver 73 is connected to the switch 6. In this configuration, the connection of the switch 6 is turned over so that the directivity of the antenna 1 may be switched in two directions, whereby the receiver is capable of receiving the electromagnetic wave from two directions. 
     The switch 6 in the above-mentioned Doppler vehicle speedometer, vehicle radar, transmission-receiving system and the receiving system may be operated either manually or automatically as already explained. Further, the antenna system according to the present invention may be applied generally to vehicles in addition to the automotive vehicles.