Antenna device

Abstract: An antenna system is made up by a main antenna (1) and a sub antenna (2) which is adjacent the main antenna (1) in a direction not perpendicular to the direction of beam scan of the main antenna and which has a beam axis coincident with the beam axis of the main antenna (1). Antenna beams of the antenna system are scanned by a scan device (3). Received signals of both the antennas are subjected to a multiplication process under a phase coherent condition in a phase-coherent multiplying device (4). A signal corresponding to a maximum frequency component in the output signal of the phase-coherent multiplying device is output by a frequency discriminating device (5) and then subjected to a signal conversion process in a signal converting device (6). Information about a radio wave source only in the direction of the antenna beams is thus obtained.

Description: The present invention relates to antenna devices intended for receiving radio waves, and more particularly to antenna devices which can precisely extract information about a radio wave source without increasing the size of the antenna, e.g. as known from EP-A-0 358 342. Generally, the beam width and the side lobe level are typical indices representing the quality of antenna patterns of receiving antennas and the like. The smaller the beam width and the side lobe level, the higher is the performance of the antenna. Thus, when seeking to receive radio waves from radio wave sources, the smaller the beam width and the side lobe level, the more a received pattern approaches the distribution of the radio wave source. Accordingly, if there was an antenna having a beam width of one-to-infinity and a side lobe level of 0, a received pattern in perfect match with the distribution of radio wave sources could be obtained. As is apparent from the theory on electromagnetic fields, however, it is impossible to realize such an antenna having a beam width of one-to-infinity and a side lobe level of 0. Because the beam width is inversely proportional to the antenna length, the beam width can be made very small by increasing the antenna length to a very large extent. But it is known that even if the antenna length is increased considerably, the side lobe level cannot be equal to or smaller than the minimum value defined on the basis of Chebychev's distribution. When receiving a radio wave by an actual antenna, therefore, variations of a radio wave source whose length is not larger than the length corresponding to the beam width cannot be detected, and the radio wave is received even at the side lobe level, resulting in a detected value different from the true intensity of the radio wave in the beam direction. While there have been conventionally known several techniques adapted to narrow the beam width or lower the side lobe level, no techniques have yet been proposed which can reduce both the beam width and the side lobe level simultaneously, or can realize an equivalent effect to that resulting from a simultaneous reduction in both the indices. The present invention has been made with a view to reducing the above problem found with conventional antennas, and its object is to provide an antenna device which can extract precise information about a radio wave source without increasing the size of an antenna. To achieve the above object, an antenna device of the present invention comprises an antenna system made up by a main antenna for receiving a radio wave and at least one sub antenna which is adjacent said main antenna in a direction not perpendicular to the direction of beam scan of said main antenna and which has a beam axis coincident with the beam axis of said main antenna; means for scanning antenna beams of said antenna system; the antenna device being characterised by means for executing a phase-coherent multiplication process of received signals of said main antenna and said sub antenna to output a phase term produced depending on the length of a component of the center-to-center spacing between said main antenna and said sub antenna in the direction of beam scan, the wavelength of a received radio wave and the angle of a radio wave source with respect to the direction of the antenna beams; and by means for outputting a signal corresponding to the maximum one of frequencies in turn corresponding to time-dependent fluctuations of said phase term output from said means for executing a phase-coherent multiplication process. In the antenna device thus arranged, by executing the phase-coherent multiplication process of the received signals of the main antenna and the sub antenna, there is produced a phase term  expressed by the following Equation (1) when the angle of a radio wave source with respect to the direction of antenna beams of the antenna system is  (rad), on the assumption that the length of a component of the center-to-center spacing between the main antenna and the sub antenna in the direction of beam scan is d (m) and the wavelength of a received radio wave is λ (m):  = (2π/λ) ·d·sin A description will now be given of the process of deriving the above Equation (1). As shown in Fig. 1, assuming that the length of a component of the center-to-center spacing between a main antenna a and a sub antenna b in the direction of beam scan is d, the beam direction of an antenna system made up by both the antennas a, b is c, and the direction of a radio wave source is s, a radio wave arriving at the main antenna a from the radio wave source is delayed behind a radio wave arriving at the sub antenna b from the radio wave source by time corresponding to the length L in the illustrated case. Since the length L is expressed by L = d·sin, the Equation (1) is derived by normalizing it with the wavelength λ and further multiplying it by 2π for conversion into units of radians. Since the antenna beam is to be scanned in the present invention, the angle  is expressed by the following Equation (2) on condition that the scan speed is ω (rad/sec) and time is t (sec):  = ωt Putting Equation (2) into Equation (1) results in the following Equation (3):  = (2π/λ) ·d·sinωt It is thus found that the phase term  is a function of time. When the phase is a function of time, its differential over time corresponds to frequency. The following Equation (4), obtained by differentiating Equation (3) over time, represents frequencies corresponding to time-dependent fluctuations of the phase: d/dt = (2π/λ) ·d·ω·cosωt Putting Equation (2) into Equation (4) results in the following Equation (5): d/dt = (2π/λ) ·d·ω·cos From this Equation (5), it is found that frequency components due to time-dependent fluctuations of the phase term vary depending on the angle . The case is now considered of receiving a radio wave of frequency ω 1 . In this case, it is understood that information about the radio wave source in the beam direction of the antenna system, i.e., information about the radio wave source at  = 0, is contained in the frequency expressed by the following Equation (6) derived from Equation (5): ω 1 + (2π/λ) ·d·ω·cos 0 = ω 1 + (2π/λ) ·d·ω It is also understood that information about radio wave sources in other directions than  = 0 is contained in the frequencies expressed by the following Equation (7): ω 1 + (2π/λ) ·d·ω·cos Accordingly, as with Equation (5), it is found that the received radio wave frequency varies depending on the angle . In theory, therefore, by using the means for outputting a signal corresponding to the maximum one of the frequencies expressed by Equation (7) to thereby select only the radio wave of the frequency expressed by Equation (6) (i.e., the maximum one of the fluctuating frequencies), information about the radio wave source only in the direction of antenna beams of the antenna system can be extracted. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is an illustration for explaining the occurrence of a phase difference between radio waves arriving at a main antenna and a sub antenna; Fig. 2 is a conceptual diagram of a schematic arrangement showing one embodiment of an antenna device according to the present invention; Figs. 3A and 3B are graphs showing the simulation results of a received power pattern when a main antenna and a sub antenna of the antenna device are assumed as a single antenna and the final received power pattern of the antenna device according to the present invention, respectively; Fig.4 is a diagrammatic view showing one practical example of an antenna device according to the present invention; and Fig.5 is a diagrammatic view showing another practical example of an antenna device according to the present invention. In Fig.2, denoted by reference numeral 1 is a main antenna for receiving a radio wave which may be of any type or size, such as a dipole antenna, a horn antenna and an array antenna so long as it can receive a target radio wave. 2 is a sub antenna which may also be of any type or size, such as a dipole antenna, a horn antenna and an array antenna. The sub antenna 2 is arranged adjacently to the main antenna 1 in a direction not perpendicular to the direction of beam scan of the main antenna 1, and has a beam axis coincident with the beam axis of the main antenna 1. 3 is a beam scanning device such as an antenna rotating device for scanning both antenna beams of the main antenna 1 and a sub antenna 2 at a constant speed simultaneously. Denoted by 4 is a phase-coherent multiplying device for executing a multiplication process of high-frequency received signals of the main antenna 1 and the sub antenna 2 under a phase coherent condition, and outputting a multiplication process signal which produces a phase term determined depending on the length of a component of the center-to-center spacing between the main antenna 1 and the sub antenna 2 in the direction of the beam scan, the wavelength of a received radio wave and the angle of a radio wave source with respect to the direction of the antenna beams. 5 is a frequency discriminating device for executing a frequency analysis of the signal output from the phase-coherent multiplying device 4, and outputting only a signal corresponding to a maximum frequency component in the analyzed signal. 6 is a signal converting device for converting a unit of the output signal of the frequency discriminating device 5 from [power 2 ] into [power]. It should be noted that when the phase-coherent multiplying device 4 executes a multiplication process of electric fields, the output signal of the frequency discriminating device 5 is obtained in a unit of [power] and, therefore, the signal converting device 6 can be dispensed with. In the antenna device thus arranged, when a radio wave arrives during the period in which the antenna beams are scanned, the main antenna 1 and the sub antenna 2 output received signals depending on the respective antenna patterns. These outputs are subjected to the multiplication process under a phase coherent condition in the phase-coherent multiplying device 4 and then applied to the frequency discriminating device 5. The frequency discriminating device 5 outputs the received signal corresponding to a maximum frequency component in the applied signal for applying it to the signal converting device 6. As explained hereinbefore in the "Summary of the Invention", the output signal of the frequency discriminating device 5 contains information about the radio wave source only in the direction of the antenna beams. The signal converting device 6 derives the square root of the signal in units of [power 2 ] resulting from the phase-coherent multiplication process of the received power signal of the main antenna 1 and the received power signal of the sub antenna 2 in the phase-coherent multiplying device 4, followed by outputting the resulting signal as a final output. It will be noted that when the multiplying device 4 executes a multiplication process of received electric field signals, the process executed by the signal converting device 6 is not required. Figs. 3A and 3B are graphs each showing the simulation result of a received power pattern obtained by scanning the antenna system which is made up by the main antenna 1 comprising a rectangular antenna of uniform distribution with an antenna length in the scan direction which is 10 times the received wavelength, and the sub antenna 2 comprising the same rectangular antenna as the main antenna 1 , the main antenna 1 and the sub antenna 2 being arranged adjacent each other in the scan direction with the center-to-center interval therebetween set to 10 times the received wavelength, when radio wave sources are present which have constant phase, have an amplitude ranging from 0.1 to 2.1, and fluctuate with a period of 3 degrees. It should be noted that the graphs indicate only the region of + direction because the radio wave sources are laterally symmetrical about 0 degrees. Fig. 3A shows the received power pattern obtained by assuming the main antenna and the sub antenna to be a single antenna, i.e., by using a rectangular antenna of uniform distribution with an antenna length in the scan direction which is 20 times the received wavelength, and Fig. 3B shows the received power pattern obtained by an antenna device according to the present invention. In the graph of Fig. 3A , a broken line represents the distribution pattern of the radio wave sources. It will be noted that these patterns are plotted with their maximum values set as a reference. It will be found from the simulation results shown in Figs. 3A and 3B that the antenna device according to the present invention can provide the received power pattern in match with the distribution pattern of the radio wave sources without being affected by a finite beam width and a side lobe level larger than 0. Next, one example of a practical arrangement of the antenna device according to the present invention will be described with reference to Fig. 4. In this example, the antenna system is made up by using a circular patch array antenna as each of the main antenna 11 and the sub antenna 12. The sub antenna 12 is arranged at a position spaced from the main antenna 11 in the direction of the beam scan, i.e., in the X direction. The antenna beam scanning device is implemented by an electronic scan method using phase shifters 13, 14 to change the phases of the received signals of the main antenna 11 and the sub antenna 12. A phase-coherent multiplying device 15 is constituted by a local oscillator 16 and mixers 17, 18, 19. A frequency discriminating device 20 may be of analog or digital configuration. In the case of an analog configuration, a frequency discriminating circuit or a filter can be used to output only the target frequency. In the case of digital configuration, only the target signal of maximum frequency can be output by converting the output of the mixer 19 to a digital signal by an A/D converter and applying the digital signal to a digital filter or such a process as FFT (Fast Fourier Transform). When the frequency discriminating device 20 is of analog configuration, a signal converting device 21 can be constituted by an A/D converter and a computer so that the analog output of the frequency discriminating device 20 is converted into a digital signal and this digital signal is subjected to a signal conversion process in the computer. When the frequency discriminating device 20 is of digital configuration, a signal converting device 21 can be constituted by a computer to execute the signal conversion process therein. The foregoing parts of each device can be all implemented by using known techniques. It will be noted that, as mentioned hereinbefore, when the multiplying device 15 executes a multiplication process of electric field signals, the signal converting device 21 is not required. Fig. 5 is a block diagram showing another practical example. In this example, a rotary table 23 for performing a mechanical scan is used as the beam scanning device of the main antenna 21 and the sub antenna 22. Received signals of both the antennas 21, 22 are output through slip rings or rotary joints 24, 25, respectively. Denoted by 26 is a local oscillator for supplying a phase reference signal to receivers 27, 28. The receivers 27, 28 demodulate the signals received by the antennas for outputting the respective amplitudes and phases of the received signals. 29, 30 are A/D converters for converting amplitude signals of the outputs of the receivers 27, 28 from analog into digital form. 31, 32 are A/D converters for converting phase signals of the outputs of the receivers 27, 28 from analog into digital form. 33 is a computer for executing a phase-coherent multiplication process, an FFT process and then a signal conversion process based on the amplitude signals and the phase signals from the A/D converters 29, 30, 31, 32. While the above embodiment and examples are illustrated as using a single sub antenna, the sub antenna may be provided plural in number. Additionally, each of these sub antennas may be of any type, such as a dipole antenna, a horn antenna and an array antenna. In the case of using plural sub antennas, the received signals of the plural sub antennas are all added together and, thereafter, the phase-coherent multiplication process is executed on the resulting sum and the received signal of the main antenna. In this case, the total received power of the sub antennas is increased, which results in a higher antenna gain and S/N ratio than using the single sub antenna. According to the present invention, as explained above in conjunction with the described embodiment, since the antenna device is arranged to output a signal corresponding to a maximum frequency component in the signal output from the means for executing the phase-coherent multiplication process, it is possible to extract precise information about a radio wave source only in the direction of antenna beams of the antenna system without increasing the antenna size.

Claims: An antenna device comprising: an antenna system made up by a main antenna for receiving a radio wave and at least one sub antenna which is adjacent said main antenna in a direction not perpendicular to the direction of beam scan of said main antenna and which has a beam axis coincident with the beam axis of said main antenna; means for scanning antenna beams of said antenna system; the antenna device being characterised by means for executing a phase-coherent multiplication process of received signals of said main antenna and said sub antenna to output a phase term produced depending on the length of a component of the centre-to-centre spacing between said main antenna and said sub antenna in the direction of beam scan, the wavelength of a received radio wave and the angle of a radio wave source with respect to the direction of the antenna beams; and by means for outputting a signal corresponding to the maximum one of frequencies in turn corresponding to time-dependent fluctuations of said phase term output from said means for executing the phase-coherent multiplication process, whereby information about the radio wave source only in the direction of the antenna beams is obtained. An antenna device according to claim 1, wherein said means for scanning antenna beams of said antenna system is constituted by phase shifters to scan the antenna beams electronically. An antenna device according to claim 1, wherein said means for scanning antenna beams of said antenna system is constituted by a rotary table to scan the antenna beams mechanically. An antenna device according to claim 1, wherein said antenna system comprises a plurality of said sub antenna, and said means for executing a phase-coherent multiplication process executes the addition of the received signals of said sub antennas, and then the phase-coherent multiplication process of the added total received signal of said sub antennas and the received signal of said main antenna. An antenna device according to claim 2, wherein said antenna system comprises a plurality of said sub antenna, and said means for executing a phase-coherent multiplication process executes the addition of the received signals of said sub antennas, and then the phase-coherent multiplication process of the added total received signal of said sub antennas and the received signal of said main antenna. An antenna device according to claim 3, wherein said antenna system comprises a plurality of said sub antenna, and said means for executing the phase-coherent multiplication process executes the addition of the received signals of said sub antennas, and then the phase-coherent multiplication process of the added total received signal of said sub antennas and the received signal of said main antenna.

Inventors: SEZAI TOSHIHIRO, SEZAI, TOSHIHIRO, SEZAI, TOSHIHIRO, C/O NATIONAL SPACE DEVELOPMENT
Applicants: NAT SPACE DEV AGENCY, NATIONAL SPACE DEVELOPMENT AGENCY OF JAPAN
Application year: 1993
Main classification: H01Q3
IPC codes: H01Q3, G01S7, H01Q21