Beam compression process for antenna patterns

Abstract: An antenna system is made up by arranging a sub antenna (2) to be adjacent a main antenna (1) in the direction where the beam width of the main antenna (1) is to be compressed and to have a beam axis coincident with the beam axis of the main antenna. Antenna beams of the antenna system are scanned in the direction (X) of compression of the beam width. The received signals of the main antenna and the sub antenna are subjected to the steps of signal processing and multiplication in a signal processing and multiplying circuit (3). In these steps, only when the received pattern waveform of the main antenna and the received pattern waveform of the sub antenna are both upwardly convex, is the received signal of the sub antenna output and subjected to the multiplication process for preventing the occurrence of distortion in the received pattern.

Description: The present invention relates to a beam compression process for antenna patterns which eliminates distortions of a received pattern produced during a process of antenna pattern compression to thereby improve performance of the beam compression. Generally, the beam width is one of the indices which represents the quality of the antenna patterns of receiving antennas and the like. The smaller the beam width, the higher is the performance of the antenna pattern. However, the beam width and the size (length) of an antenna are inversely proportional to each other. Thus, a reduction in the beam width increases the antenna size, while a reduction in the antenna size increases the beam width. In an attempt to double the power of object discrimination, i.e., resolution, in a radar antenna, for example, the beam width must be halved, which results in doubling of the antenna size. The doubled antenna size raises various drawbacks such as an increase in not only the area occupied by the antenna, but also the weight of the antenna and the dimensions of the antenna support structure. Conversely, if the antenna size is halved, the beam width is doubled and the discriminating power deteriorates down to half level. It is well known that the beam width and the antenna size are mutually contradictory, as mentioned above. Since actual antennas are subjected to limitations in the area occupied by the antenna and other factors in most cases, a point of compromise is found in practical use at some extent of the beam width under such limitations. For the purpose of improving the above problem, there has been conventionally known a beam compressing process using the principle of a multiplicative array wherein the beam width is reduced by multiplying the received signals of a plurality of antennas by each other, for example as known from EP-A-0 358 342. Fig.1 is a diagram showing an antenna arrangement for carrying out such beam compression. Denoted by reference numeral 101 is a main antenna constituted by, for example, an array antenna comprising a plurality of radiation elements arrayed in a rectilinear form with equal intervals, and 102 is a sub antenna. The sub antenna 102 is arranged at a position spaced from the main antenna 101 in the X direction, i.e., the direction in which the beam width is to be compressed. 103 is a multiplying circuit for multiplying the received signal of the main antenna 101 by the received signal of the sub antenna 102. In the antenna device thus arranged, the signals received by the antennas 101,102 are input in phase to the multiplying circuit 103 and subjected to a multiplication process. As a result, the directional characteristic of the main antenna and the directional characteristic of the sub antenna are multiplied to give a synthetic directional characteristic with the beam width compressed therein. However, the above-explained conventional beam compression process for an antenna pattern has the problem that although an identical effect to that resulting from beam compression can be obtained, the grating lobe of the sub antenna forms a received pattern as if a radio wave source is present, in spite of the absence of any radio wave source, i.e., the problem that distortions are caused in the received pattern. The present invention has been made with a view of solving the above problem in the conventional beam compression process, and its object is to seek to provide a beam compression process for an antenna pattern by which no distortions are caused in the received pattern, while enabling compression of beam width. To achieve the above object, a beam compression process for an antenna pattern according to the present invention comprises the steps of providing an antenna system made up by a main antenna for receiving a radio wave and at least one sub antenna which is disposed adjacent said main antenna in the direction in which the beam width of said main antenna is to be compressed and which has a beam axis coincident with the beam axis of said main antenna; scanning the antenna beams of said antenna system in said direction of compression of the beam width; and executing an in-phase multiplication process of the received signals of said main antenna and said sub antenna, wherein the received signal of said sub antenna is output and subjected to said multiplication process for beam compression only when the received pattern waveform of said main antenna and the received pattern waveform of said sub antenna are both upwardly convex. In such a beam compression process for an antenna pattern, the antenna beams of the antenna system are scanned in the direction of compression of the beam width, and the received signals of the main antenna and the sub antenna are multiplied in-phase by each other, whereby the beam width is compressed based on the principle of a multiplicative array. In the step of multiplying the received signals, only when the received pattern waveform of the main antenna and the received pattern waveform of the sub antenna are both upwardly convex, is the received signal of the sub antenna output and subjected to the multiplication process. As a result, distortions of the received pattern are removed. More specifically, in the case of targeting a radio wave source with discrete distribution like a radar, for example, the received pattern waveform of antennas is generally upwardly convex near the angle at which the radio wave source is present, but not upwardly convex near the angle at which the radio wave source is absent. With the beam compression process based on the principle of a multiplicative array, there occurs a large grating lobe because the sub antenna is arranged at a position spaced from the rotary axis for the scan. Accordingly, even near the angle at which the radio wave source is absent, the received pattern waveform becomes upwardly convex due to the radio wave received by the grating lobe. For that reason, if the received signals of the main antenna and the sub antenna are simply subjected to the multiplication process, distortions would be caused in the received pattern. However, when no radio wave sources are present in the direction of main beam of the main antenna and a radio wave source is present in the direction of the grating lobe of the sub antenna, the received pattern waveform of the sub antenna is upwardly convex, but the received pattern waveform of the main antenna is not upwardly convex. In view of the foregoing, the present invention is arranged to output the received signal of the sub antenna and multiply it by the received signal of the main antenna, only when the received pattern waveform of the main antenna is upwardly convex and the received pattern waveform of the sub antenna is also upwardly convex. It is thus possible to remove distortions of the received pattern. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig.1 is a conceptual diagram showing a conventional antenna device adapted for beam compression; Fig.2 is a conceptual diagram showing the schematic arrangement of an antenna device to explain one embodiment of a beam compression process for an antenna pattern according to the present invention; Figs. 3A and 3B are graphs showing the simulation result of a synthetic pattern obtained by the antenna device used in the embodiment of the present invention and the simulation result of a synthetic pattern obtained by the antenna device used in the conventional beam compression, respectively; Fig.4 is a perspective view showing one example of a practical arrangement of an antenna system of the antenna device used for practising the present invention; and Fig.5 is a block diagram showing one example of the practical arrangement of a signal processing and multiplying circuit. In Fig.2, denoted by reference numeral 1 is a main antenna for receiving a radio wave which comprises a horn antenna, an array antenna or the like. 2 is a sub antenna which may be any type of antenna, 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 the X direction, i.e., the direction in which the beam width of the pattern of the main antenna 1 is to be compressed, and has a beam axis coincident with the beam axis of the main antenna 1. 3 is a signal processing and multiplying circuit for executing a double differential process on the received signal of the main antenna 1 and the received signal of the sub antenna 2, determining from the resultant sign whether received pattern waveforms are upwardly convex or not, and then multiplying the received signals by each other only when the received pattern waveforms of the main antenna and the sub antenna are both upwardly convex. 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 received outputs are subjected in phase to the steps of signal processing and multiplication processing in the signal processing and multiplying circuit 3. By obtaining the output of the multiplying circuit as a final output, as explained hereinbefore in the "Summary of the Invention", the output due to the grating lobe of the sub antenna is removed and the beam is compressed. As a result, there can be provided an output with no distortions. 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 a main antenna comprising a rectangular antenna of uniform distribution with an antenna length in the scan direction which is ten times the received wavelength, and a sub antenna comprising a rectangular antenna of uniform distribution with an antenna length in the scan direction which is one times the received wavelength, when the same spot radio wave source is present in each of the directions of + 15 degrees, + 5 degrees, - 5 degrees and - 15 degrees about an antenna's rotary axis at points spaced from the rotary axis by an equal distance. It should be noted that the graphs indicate only the region of + direction because the spot radio wave sources are laterally symmetrical about 0 degrees. Fig.3A shows the pattern obtained by the beam compression process of the present invention, and Fig.3B shows the pattern obtained by the conventional beam compression process. In these graphs, one-dot-chain lines represent the received power pattern of the main antenna, broken lines represent the received power pattern of the sub antenna, and solid lines represent the received power pattern obtained after multiplying the received signal of the main antenna by the received signal of the sub antenna, respectively, with maximum values of the respective patterns set as a reference. It will be found from the graphs of Figs. 3A and 3B that the present beam compression process can remove distortion of the pattern which has been caused in the conventional process, and can provide the received power pattern corresponding to the antenna pattern with the beam width compressed narrower than that of the main antenna. Next, one example of a practical arrangement of the antenna device 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 a main antenna 11 and a sub antenna 12. The sub antenna 12 is arranged at a position spaced from the main antenna 11 in the X direction. A signal processing and multiplying circuit 13 can be implemented in an either digital or analog manner. In the case of adopting an analog manner, the double differential process can be executed by known techniques such as a differentiating circuit using a capacitor and a resistor. Determination of the double differential value can be executed by known techniques using a comparator. The multiplication process can be executed by known techniques such as a multiplying circuit or a frequency modulation circuit. In the case of adopting a digital manner, the signal processing and multiplying circuit 13 can be formed of such means based on known techniques as converting the received signals into digital signals by A/D converters and executing the double differentiation, the determination of the double differential value, and the multiplication by a computer. One example of the digital circuit is shown in Fig. 5. Referring to Fig. 5, denoted by 21 is a main antenna, 22 is a sub antenna, 23, 24 are receivers for receiving radio waves caught by the antennas 21, 22, respectively, 25, 26 are A/D converters for converting outputs of the receivers 23, 24 into digital signals, respectively, and 27 is a computer for executing the double differentiation, the determination of the double differential value, and the multiplication for outputs of the A/D converters 25, 26. In the circuit thus arranged, the radio waves received by the main antenna 21 and the sub antenna 22 are input to the receivers 23,24 which output the respective powers of the received radio waves in the form of DC signals. These outputs of the receivers 23,24 are applied to the A/D converters 25,26 for conversion into digital values which are then subjected to the double differentiation, the determination of the double differential value, and the multiplication in the computer 27, followed by outputting a multiplied value. While the above embodiment is 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. The multiplication process in the case of using plural sub antennas can be executed by either of the two methods below. With the first method, the outputs of the plural sub antennas are all added together and, thereafter, the resulting sum is multiplied by the output 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. With the second method, the outputs of the plural sub antennas are multiplied by the output of the main antenna, successively. This method achieves not only compression of the beam width, but also a reduction in the side lobe. According to the present invention, as explained above in conjunction with the described embodiment, in the step of multiplying the received signals of the main antenna and the sub antenna by each other in phase, only when the received pattern waveform of the main antenna and the received pattern waveform of the sub antenna are both upwardly convex, the received signal of the sub antenna is output and subjected to the multiplication process. As a result, it is possible to remove distortions of the received pattern which has been caused in the conventional beam compression, and also to compress the beam width of the main antenna.

Claims: A beam compression process for antenna patterns comprising the steps of providing an antenna system made up by a main antenna for receiving a radio wave and at least one sub antenna which is disposed adjacent said main antenna in the direction in which the beam width of said main antenna is to be compressed and which has a beam axis coincident with the beam axis of said main antenna; scanning the antenna beams of said antenna system in said direction of compression of the beam width; and executing an in-phase multiplication process of the received signals of said main antenna and said sub antenna, wherein the received signal of said sub antenna is output and subjected to said multiplication process only when the received pattern waveform of said main antenna and the received pattern waveform of said sub antenna are both upwardly convex. A beam compression process for antenna patterns according to claim 1, wherein said step of providing said antenna system comprises providing said sub antenna plural in number, and said multiplication process step comprises the steps of adding the received signals of said sub antennas together, and multiplying the added total received signal of said sub antennas by the received signal of said main antenna. A beam compression process for antenna patterns according to claim 1, wherein said step of providing said antenna system comprises providing said sub antenna plural in number, and said multiplication process step comprises the step of multiplying the received signals of said sub antennas by the received signal of said main antenna successively.

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: H01Q21
IPC codes: H01Q3, G01S7, H01Q21, G01K11