Conformal array antenna
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a conformal array antenna for use with a radar system. 
     2. Description of the Prior Art 
     FIG. 1 illustrates a block diagram of a prior art antenna system. In the figure, the reference numeral 1 designates a conformal array antenna including a structural base body 2 assuming a semi-spherical configuration and a number n of antenna units 3 1  to 3 n  arrayed on the structural base body 2. A number n of signal lines 4 1  to 4 n  interconnect the antenna units 3 1  to 3 n  and a microwave beam forming circuit 5. Each of the antenna units 3 1  to 3 n  which constitute the conformal array antenna 1 is an independent unitary antenna device. 
     Next, the operation of the prior art antenna system will be described. A microwave power is received by the antenna units 3 1  to 3 n  arrayed on the semi-spherical structural base body 2 of the conformal array antenna 1, and is transmitted via the signal lines 4 1  to 4 n  to the microwave beam forming circuit 5 where the microwave signals are synthesized to form a multiplicity of beams by making use of microwave phase shifters, microwave variable attenuators, microwave switches and microwave couplers. 
     In the thus constructed conventional antenna system, the antenna beams can be arbitrarily formed over the semisphere. In the case of forming a multiplicity of beams by employing microwave devices such as a phase shifter, an attenuator, a switch, a coupler and a distributor, however, the configuration loss becomes larger and only a limited number of beams can be formed concurrently. Supposing that a beam is oriented in a desired direction when used as a part of the radar system, the shadowed units among the antenna units 3 1  to 3 n  when viewing the conformal array antenna 1 from the desired direction cannot be effectively utilized. Especially when a scanning angle approximates to 90° from the zenith, almost half of the elements are not available for use. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to eliminate the problems described above. 
     It is an object of the present invention to provide an antenna system capable of simultaneously synthesizing a plurality of beams and constantly utilizing all the antenna units in an effective manner. 
     In order to accomplish the above object, an antenna system according to the present invention comprises a plurality of antenna units each of which is adapted to convert outputs from an element antenna into a digital signal, and a digital beam forming circuit. The digital beam forming circuit effects a parallel process for synthesizing digital signals including phase and amplitude information supplied from the respective antenna units. It is, therefore, possible to concurrently synthesize the digital signals to form a multiplicity of beams, which permits effective utilization of all the antenna units. Additionally, the problems that are caused by cross polarization can be eliminated. Moreover, a considerable improvement in performance is provided with respect to multi-target processing, expansion of the antenna beam scanning range, interconnection with other signal processing systems based on digital processing, and miniaturization of the antenna system. 
     It is another object of the invention to provide an antenna system capable of simultaneously synthesizing digital signals to form a multiplicity of beams, utilizing all the antenna units effectively and reducing the electromagnetic interference between signal lines interconnecting the antenna units and a digital beam forming circuit. 
     In order to achieve this object, an antenna system according to the present invention comprises a plurality of antenna units each having photo-modulator means. The output from the photo-modulator means is sent by optical fibers to photo-demodulator means which convert the light signals to the corresponding electrical signals. These electrical signals are in a digital form and are supplied to a digital beam forming circuit. The digital beam forming circuit is capable of processing the digital signals including phase amplitude information by effecting a parallel process for synthesizing such digital signals. It is, therefore, possible to concurrently form a multiplicity of beams, which permits effective utilization of all the antenna units. Because the optical fibers are employed for transmission of the signals, the problem caused by the electromagnetic interference is greatly reduced. 
     It is still another object of the present invention to provide an antenna system capable of simultaneously synthesizing a multiplicity of beams, utilizing all the antenna units in an effective manner, and solving the problems that are caused by electromagnetic interference and cross polarization attributed to the difference in polarization between the antenna units. 
     In order to achieve this object, an antenna system of the present invention comprises a plurality of antenna units each including a transmitting section, a receiving section and a TR switch. The transmitting sections include a phase controller and are connected to a microwave power distributor, while the receiving sections include a low-noise amplifier and the received signals are converted to digital signals and fed to a digital beam forming circuit. The digital beam forming circuit serves to process the digital signals including phase-amplitude information for arbitrarily synthesizing these signals to form multiple beams simultaneously, and to enable all the antenna units to be utilized effectively. Moreover, because the transmitting section and the receiving section are incorporated to use the same element antenna, the problems caused by cross polarization are eliminated. If the signals are transmitted through optical fibers, a remarkable reduction in the electromagnetic interference can be expected and the signal transmission lines can be miniaturized. 
     Other features and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a conventional conformal array antenna system; 
     FIG. 2 is a block diagram of a first embodiment of a conformal array antenna system according to the present invention; 
     FIG. 3 is a block diagram of an antenna unit of the conformal array antenna system shown in FIG. 2; 
     FIG. 4 shows in detail the structure of the conformal array antenna system shown in FIG. 2; 
     FIG. 5 is a schematic diagram of the DPSD shown in FIG. 4; 
     FIG. 6 is a block diagram of a second embodiment of a conformal array antenna system according to the present invention; 
     FIG. 7 is a block diagram of an antenna unit of the conformal array antenna system shown in FIG. 6; 
     FIG. 8 is a modified form of the second embodiment; 
     FIG. 9 is a block diagram of a third embodiment of a conformal array antenna system according to the present invention; 
     FIG. 10 shows the structure of the antenna unit shown in FIG. 9; 
     FIG. 11 is a block diagram of a fourth embodiment of a conformal array antenna system according to the present invention; and; 
     FIG. 12 is a modified form of the fourth embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 shows the first embodiment of the present invention which is embodied as a receiving antenna system or a passive detection antenna system for use with a separate transmitting antenna system. 
     In FIG. 2, a conformal array antenna 10 includes a structural base body 11 which assumes a semi-spherical configuration and a number n of antenna units 12 1  to 12 n  arrayed on the structural base body 11. A number n of signal lines 13 1  to 13 n  interconnect the antenna units 12 1   to 12 n  and a digital beam forming circuit 14. The antenna units 12 1  to 12 n  have the same structure. FIG. 3 shows a schematic diagram of the antenna unit 12 1  as an example. The antenna unit 12 1  comprises an element antenna 12 11 , a low-noise amplifier 12 12  and an A/D converter 12 13 . 
     Next, the operation of the antenna system will be explained with reference to FIGS. 2 and 3. Microwave signals are received by the element antennas 12 11  to 12 n .sbsb.1 of the antenna units 12 1  to 12 n  which are fixed to the structural base body 11 of the conformal array antenna 10. The received microwave signals are then amplified by the low-noise amplifiers 12 12  to 12 n .sbsb.2, the outputs of which are, directly or after being converted into the IF signals, supplied to A/D converters 12 13  to 12 n .sbsb.3 which convert the supplied microwave signals to digital signals including phase and amplitude information. The digital signals are transmitted via the signal lines 13 1  to 13 n  to the digital beam forming circuit 14, in which the signals are synthesized as the digital signals to form multiple-beams by employing known techniques such as discrete Fourier transformation, fast Fourier transformation and Winograd Fourier transformation. Hence, it is feasible to digitally effect a parallel process of a plurality of signals transmitted from the antenna units 12 1  to 12 n  in accordance with arbitrary beam configurations. Pieces of information sent from all the antenna units 12 1  to 12 n  can be processed at any time in an effective manner, thereby enabling the information arriving from all directions in the semi-sphere to be obtained. 
     Generally speaking, the amplitudes and phases at the antenna aperture of each of the antenna units 12 1  to 12 n  are different from each other in correspondence with the position of the antenna units and the direction of the incoming waves. Accordingly, the signal e i  received by the element antenna 12 i .sbsb.1 of the antenna unit 12 i  is expressed as follows: 
     
         e.sub.i =g.sub.i e.sup.jφ i 
    
     i=1, 2, . . . , n 
     wherein g i  is an element pattern of the element antenna 12 i .sbsb.1 and is a complex amount that depends on the position of the element antenna, and φ i  represents an electrical length which is equivalent to the difference between the mutual distances of the respective element antenna, the received signal e i  thus being a complex number. 
     Referring now to FIG. 4, there is shown in schematic form the structure of the conformal array antenna system as shown in FIG. 2. As shown in FIG. 4, the digital beam forming circuit 14 includes a number n of serial-to-parallel converters 14 11  to 14 n .sbsb.1 connected respectively to the signal lines 13 1  to 13 n , a number n of digital phase sensitive detectors 14 12 , to 14 n .sbsb.2 connected to the corresponding serial-to-parallel converters, and a digital beam forming unit 15 for producing a plurality of output signals at output port P 1  to P n . The signal lines 13 1  to 13 n  carry m-bit digital signals from the analogue-to-digital converters 12 13  to 12 n .sbsb.3 to the serial-to-parallel converters 14 11  to 14 n .sbsb.1. 
     An explanation will be made by giving instances of the procedure of processing the microwave signal impinging on the antenna unit 12 i . 
     The microwave reflected by a target and received by the element antenna 12 i  is an analogue signal. The analogue signal thus received is in turn amplified by the low-noise amplifier 12 i .sbsb.2, with the relative relationship between the amplitude and the phase maintained. The amplified signal is fed to the analogue-to-digital converter 12 i .sbsb.3 in which the signal is sampled and quantized to form an m-bit digital signal. The m-bit signal is transmitted through the signal line 13 i  to the serial-to-parallel converter 14 i .sbsb.1 in the digital beam forming circuit 14. 
     In the digital beam forming circuit, the m-bit serial signal from the line 13 i  is converted to an m-bit parallel signal by the serial-to-parallel converter 14 i .sbsb.1. The parallel signal is sent every sampling time to the digital phase sensitive detector (DPSD) 14 i .sbsb.2, which converts the input signal to an I-signal and a Q-signal having the following relation: 
     
         e.sub.i =I.sub.i +jQ.sub.i 
    
     FIG. 5 shows an example of the DPSD. The input signal to the DPSD 14 i .sbsb.2, is divided into two portions which are multiplied by the sine and cosine waves, respectively, to output two separate signals I i  and Q i  which are to be supplied to the digital beam forming unit 15. Similar to this, the signals received by the remaining antenna units are processed and sent to the digital beam forming unit 15. The digital beam forming unit is well-known as a discrete Fourier transform (DFT) beamformer, a fast Fourier transform (FFT) beamformer or a Winograd transform beamformer. Accordingly, the output signals corresponding respectively with n directions θ 1  to θ n  are obtained from the output port P 1  to P n . For example, the output signal E i  at the port P i  is expressed as follows: 
     
         E.sub.i =I.sub.i +jQ.sub.i 
    
     
         |E.sub.i |=(I.sub.i.sup.2 +Q.sub.i.sup.2).sup.1/2 
    
     
         &lt;E.sub.i =tan.sup.-1 (Q.sub.i /I.sub.i) 
    
     Turning now to FIG. 6, the second embodiment of the present invention is shown. In FIG. 6, identical components and elements are designated by the same numerals as those used in FIGS. 2 through 5. A number n of antenna units 20 1  to 20 n  arrayed on the structural base body 11 are connected through optical fibers 21 1  to 21 n  to a number n of photo-demodulators 22 1  to 22 n  which are, for example, photoelectric converters. The outputs from the photodemodulators are fed to the digital beam forming circuit 14 for synthesis. The antenna units 20 1  to 20 n  are of the same structure. FIG. 7 shows a block diagram of the antenna unit 20 1  as an example. As shown in the figure, the antenna unit 20 1  comprises an element antenna 20 11 , a low-noise amplifier 20 12 , connected to the element antenna 20 11 , an analogue-to-digital converter 20 13 , connected to the low-noise amplifier 20 12  and a photo-modulator 20 14  connected to the analogue-to-digital converter 20 13 . The photo-modulator may be a conventional electro-photo converter. 
     Next, the operation of the antenna system will be described. Microwave signals are received by the element antennas 20 11  to 20 n .sbsb.1 of the antenna units 20 1  to 2O n  and then amplified by the low-noise amplifiers 20 12  to 20 n .sbsb.2. The thus amplified microwave signals are, directly or after being converted into the IF signals, supplied to the A/D converters 20 13 , to 20 n .sbsb.3 to be converted to digital signals including the phase and amplitude information. The digital signals are then converted into photo-signals by the photomodulators 20 14  to 20 n .sbsb.4 and transmitted via the optical fibers 21 1  to 21 n  to the photo-demodulators 22 1  to 22 n . The digital electric signals thus demodulated by the photodemodulators 22 1  to 22 n  are supplied to the digital beam forming circuit 14 which synthesizes the digital signals by employing known techniques such as discrete Fourier transformation, fast Fourier transformation and Winograd Fourier transformation. Also in the second embodiment, it is feasible to digitally effect a parallel process of a plurality of the signals received by the antenna units 20 1  to 20 n  according to arbitrary antenna beam configurations. Pieces of information received by the antenna units 21 1  to 21 n  can be processed in an effective manner, thereby obtaining the information from all directions in the semi-sphere. Because the optical fibers are used as transmission lines, no problem of electromagnetic interference can happen. Also, the signal lines can be miniaturized. 
     The A/D converters 20 13  to 20 n .sbsb.3 are inserted between the low-noise amplifiers and the photo-modulators in FIG. 7, but each A/D converter may, as illustrated in FIG. 8, be disposed between the photo-demodulator and the digital beam forming circuit. In this case, the photo-modulators 20 14  to 20 n .sbsb.4 convert the microwave signals, directly or after being converted into the IF signals, into the photo-signals. The thus converted photo-signals are transmitted via the optical fibers 21 1  to 21 n  to the photo-demodulators 22 1  to 22 n  to be demodulated to the electrical signals. The demodulated electrical signals are converted, directly or after being converted into the IF signals, into the digital signals by means of the A/D converters 20 13  to 20 n .sbsb.3. 
     The two embodiments described above relate to receiving antenna systems. On the other hand, the third and fourth embodiments shown in FIGS. 9 through 12 are systems capable of transmitting and receiving microwave signals. In these figures, identical elements and components are designated by the same reference numerals as those used in FIGS. 1 through 8. 
     Referring now to FIG. 9, a number n of antenna units 30 1  to 30 n  arranged on the semi-spherical body 11 of the conformal array antenna 10 are connected through a number n of sending lines 31 1  to 31 n  to a microwave power distributor 32 that is receiving microwave power from a transmitting signal generator 33. The antenna units 30 1  to 30 n  are also connected through a number n of receiving lines 34 1  to 34 n  to the digital beam forming circuit 14 which synthesizes input digital signals to form a multiplicity of beams. 
     FIG. 10 is a more detailed illustration of the conformal array antenna system shown in FIG. 9. As seen in FIG. 10, all the antenna units 30 1  to 30 n  have the same circuit structures. Element antennas 30 11  to 30 n .sbsb.1 are connected through TR switches 30 12 , to 30 n .sbsb.2 to transmitting sections 30 13  to 30 n .sbsb.3 and to receiving sections 30 14  to 30 n .sbsb.4. These TR switches 30 12  to 30 n .sbsb.2, may be conventional circulators or diode switches. The transmitting sections 30 13  to 30 n .sbsb.3 include high power amplifiers 30 15  to 30 n .sbsb.5 and phase controllers 30 16  to 30 n .sbsb.6, while the receiving sections 30 14  to 30 n .sbsb.4 include low-noise amplifiers 30 17  to 30 n .sbsb.7, and analogue-to-digital converters 30 18  to 30 n .sbsb.8. 
     Next, the operation of the antenna system of FIG. 10 will be explained. A microwave signal received from the signal generator 33 and input to the microwave power distributor 32 is distributed to a number n of outputs each having a desired amplitude and phase. These output signals are transmitted via the sending lines 31 1  to 31 n  to the transmitting sections 31 13  to 31 n .sbsb.3 of the antenna units 30 1  to 30 n . In the transmitting sections, the microwave signals undergo phase changes in the phase controllers 30 16  to 30 n .sbsb.6 so as to form desired antenna beams. Then the phase-controlled microwave signals are amplified by the high power amplifiers 30 15  to 30 n .sbsb.5, pass through the TR switches 30 12  to 30 n , and are then emitted from the element antennas 30 11  to 30 n .sbsb.1 into space. The microwave signals which have been emitted into space are reflected by a target and received by the element antennas 30 11  to 30 n .sbsb.1. Subsequently, the received microwave signals are transmitted via the TR switches 30 12  to 30 n .sbsb.2 to the receiving sections 30 14  to 30 n .sbsb.4 of the antenna units. The microwave signals input to the receiving sections 30 14  to 30 n .sbsb.4 are amplified by the low-noise amplifiers 30 17  to 30 n .sbsb.7. The thus amplified microwave signals are fed, directly or after being converted into the IF signals, to the analogue-to-digital converters 30 18  to 30 n .sbsb.8 which in turn convert the input analogue signals into digital signals including phase and amplitude information. These digital signals are transmitted via the receiving lines 34 1  to 34.sub. n to the digital beam forming circuit 14 in which the signals are synthesized to form multiple beams by employing known techniques such as discrete Fourier transformation, fast Fourier transformation and Winograd Fourier transformation. Hence, it is possible to digitally effect a parallel process of the signals sent from the antenna units 30 1  to 30 n  in accordance with arbitrary beam configurations. Furthermore, the information from all the antenna units can be processed unfailingly in an effective manner, thereby constantly obtaining information from all directions in the semi-sphere. 
     When antenna units 30 1  to 30 n .sbsb.1 which are adapted for a linearly polarized wave are employed, the polarization of the transmitted signal is the same as that of the signal received after being reflected by the target, if consideration is given to the individual element antennas 30 11  to 30 n .sbsb.1. The signals reflected by and coming from the target are converted into digital signals including phase-amplitude information, and the digital signals are synthesized by the digital beam forming circuit 14, so the problem of cross polarization caused by the difference in polarization between the antenna units is solved. 
     The same operation as the third embodiment may be expected even when light signals are utilized for transmission of signals between the antenna units 31 1  to 31 n  and the microwave power distributing circuit 32 and the digital beam forming circuit 14. FIG. 11 shows the fourth embodiment of the present invention which uses light signals for transmission of signals. In comparison with the third embodiment, the antenna units 40 1  to 40 n  of the fourth embodiment include photo-modulators 40 12  to 40 n .sbsb.2 and photo-demodulators 40 11  to 40 n .sbsb.1. The outputs from the microwave distributing circuit S2 are converted into light signals by the photomodulators 41 1  to 41 n  and are then transmitted via optical fibers 42 1  to 42 n  to photo-demodulators 40 11  to 40 n .sbsb.1 added the transmitting section 40 13  to 4O n .sbsb.3 of the antenna units. In the photo-demodulators, the light signal are converted into microwave signals to be transmitted. In reception, the digital signals are converted into light signals by means of the photo-modulators 40 12   to 40 n .sbsb.2 added to the receiving section 40 14  to 40 n .sbsb.4 of the antenna units. The thus converted light signals are transmitted via optical fibers 43 1  to 43 n  to photo-demodulators 44 1  to 44 n  to provide electrical signals to the digital beam forming circuit 14. In the fourth embodiment shown in FIG. 11, the light signals are employed for the transmission of signals between the devices and hence the problem caused by electromagnetic interference between the signal lines is obviated, and the signal lines are of diminished size by virtue of the provision of the optical fibers. 
     FIG. 12 is a modification of the fourth embodiment shown in FIG. 11. In this case, the analogue-to-digital converters 30 18  to 30 n .sbsb.8 of the receiving sections are positioned between the photo-demodulators 44 1  to 44 n  and the digital beam forming circuit 14. It can be expected that operation and effects similar to those achieved in the fourth embodiment will be exhibited. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. For example, the shape of the conformal array antenna system according to the present invention is need not be limited to the semi-sphere, but may be made to be fitted to the shape of certain structures such as ships, airplanes, missiles, vehicles, satellites and ground radar sites, or may be a portion of a cylinder, sphere or cone, or a portion or portions of a shape made as a combination of any two or three of a cylinder, a sphere and a cone. Further, the conformal array antenna system of the present invention can utilize not only linearly polarized waves but also circularly polarized waves.