Method for reducing side lobes in antenna patterns

Abstract: There is disclosed a method for reducing side lobes wherein an antenna system comprises a main antenna (1) and a sub antenna (2) which does not have an isotropic antenna pattern in a direction (X direction) of an orientation of a side lobe of the main antenna to be reduced which is arranged in such a way that the sub antenna adjoins the main antenna in the direction (Y direction) perpendicular to the X direction and the beam axis of the sub antenna coincides with the beam axis of the main antenna. An antenna beam of the antenna system is scanned in the X direction and a signal received with the main antenna is multiplied by a signal received with the sub antenna on the condition that phases of both signals coincide with each other.

Description: The present invention relates to a method for processing antenna patterns of antenna systems for receiving radio waves, more specifically, a method for reducing side lobes in antenna patterns without widening beam widths. The beam width and side lobe are indices for describing the characteristics of antenna patterns of general antennas including receiving antennas. Narrower beam widths or lower side lobes give better antenna pattern characteristics. A multiplicative array is known, for example from EP-A- 0358342 and from the paper "Beam compressed microwave scanning radiometer" by K. Arai and T. Suzuki, IGARSS' 89, 12 th Canadian Symposium on Remote Sensing, July 1989, Vancouver, Canada, IEEE No. 89CH2768-0, pages 268-270, as a method for reducing beam widths by multiplying each signal received with plural antennas. Fig. 1 of the accompanying drawings shows the constitution of such an antenna which reduces beam widths. In this figure, 101 is a main antenna such as an array antenna comprising plural radiating elements which are arranged along a straight line with equal spaces and 102 is a reference antenna. The reference antenna 102 is arranged apart from the main antenna 101 in the X direction corresponding to the orientation of the beam width to be reduced. 103 is a multiplier which multiplies the signal received with the main antenna 101 by the signal received with the reference antenna 102. In this antenna system having such a constitution, each signal received with antennas 101 and 102 is applied to the multiplier 103 with the same phase to perform the multiplication between the directional characteristic of the main antenna 101 and that of the reference antenna so that the resulting synthesized directional characteristic shows a reduction in the beam width. The beam width and the side lobe, the indices for the antenna pattern, show an antinomical relationship with each other. That is, when the beam width is reduced by the conventional method described above, the side lobes become high. On the other hand, when the side lobes are reduced the beam width becomes wide. Therefore, for example in radar antennas, when the side lobes are reduced, then the beam width is widened and the resolution or ability to distinguish targets gets degraded. As a result, the radar could misidentify plural targets as being a single target. On the other hand, when the beam width is reduced, the side lobes become high and the radar might misidentify a target which exists in a certain direction of some side lobes as if it exists in the observation direction although there is nothing actually in the observation direction. As described above, the beam width and the side lobe have characteristics conflicting with each other, so that it is impossible to optimize both of these at the same time. Therefore, compromises between the beam width and side lobes have been required such that the beam width is minimized on a certain condition of the side lobes, or the side lobes are minimized on a certain condition of the beam width, as for example in the Chebyshev distribution. It is an object of the present invention to solve the above problems of conventional antenna systems. It is a more specific object of this invention to provide a method for reducing side lobes in an antenna pattern without widening the beam width. In accordance with a first aspect of the present invention, there is provided a method for reducing side lobes in an antenna pattern obtained from an antenna system having; a main antenna for receiving radio waves; and one or more sub antennas which does/do not have an isotropic antenna pattern in a first direction coinciding with the orientation of a side lobe of said main antenna to be reduced and which is/are arranged in such a way that it/they adjoin said main antenna in a second direction which is perpendicular to said first direction and its/their beam axis/axes coincide(s) with the beam axis of said main antenna; wherein the antenna beam of said antenna system is scanned in said first direction; and wherein the signal received with said main antenna is multiplied by the signal(s) received with said one or more sub antennas on the condition that the phases of both said signals coincide with each other. In accordance with a further aspect of the present invention, there is provided a method for making a reduction in a beam width in addition to a reduction in side lobes in an antenna pattern obtained from an antenna system having; a main antenna for receiving radio waves; and one or more sub antennas which do not have an isotropic antenna pattern in a first direction coinciding with the orientation of the side lobe of said main antenna to be reduced and which is/are arranged in such a way that said one or more sub antennas adjoin said main antenna in a second direction perpendicular to said first direction and the beam axis/axes of said one or more sub antennas coincide with the beam axis of said main antenna; wherein the antenna beam of said antenna system is scanned in a direction other than either the first direction or the second direction; and wherein the signal received with said main antenna is multiplied by the signal(s) received with one or more said sub antennas, on the condition that the phases of both said signals coincide with each other. In order to implement the above methods, an antenna system may be used which comprises a main antenna for receiving radio waves and one or more sub antennas which do not have an isotropic antenna pattern in a first direction of the orientation of a side lobe to be reduced of the main antenna, which is arranged in such a way that the sub antenna adjoins the main antenna in a second direction perpendicular to the first direction and the beam axis of the sub antenna coincides with the beam axis of the main antenna; wherein an antenna beam of the antenna system is scanned in the first direction and a signal received with the main antenna is multiplied by a signal received with the sub antennas by means of multiplication in the condition that the phases of both signals coincide with each other. In general cases, except for the special isotropic case, an antenna pattern shows the maximum magnitude in the beam axis direction. When the maximum magnitude is 1 (that is, the magnitude is normalized with respect to the maximum magnitude), then any magnitude, except for in the beam axis direction, is less than 1. Therefore, if, as described above, the signal received with the main antenna is multiplied by the signal received with the sub antenna which does not have an isotropic antenna pattern, in the condition that the phase of these signals are same, then the resultant multiplied values become less than values of the main antenna except for in the beam axis direction because the main antenna pattern is multiplied by the sub antenna pattern which has a value less than 1, except for in the beam axis direction. In this way, side lobes can be reduced without widening the beam width. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a schematic diagram of a conventional antenna system for reducing beam width; Fig. 2 is a conceptual diagram of a low side-lobe antenna for explanation of a method for reducing side lobes in antenna pattern in accordance with the present invention; Figs. 3A to 3C are graphs showing patterns of each antenna in the system of Fig. 2 and the resultant synthesized pattern after multiplication; Figure 4 is a graph showing the pattern of a sub antenna of a conventional antenna system with a reduced beam width; Figure 5 is a schematic diagram of a low side-lobe antenna system embodying the present invent ion; and Figure 6 is a block diagram of a multiplier which may be used in connection with the present invention. Referring first to Fig. 2, there is shown a conceptual diagram of a low side-lobe antenna system showing a method for reducing side lobes in an antenna pattern according to the present invention. In this figure, 1 is a main antenna for receiving radio waves such as a horn antenna or an array antenna. 2 is a sub antenna arranged in such a way that it adjoins the main antenna 1 in the Y direction perpendicular to the X direction and that its beam axis coincides with the beam axis of the main antenna 1 so that the side lobe in the X direction in the antenna pattern of the main antenna 1 is reduced. Any kind of antennas, except those which have an isotropic antenna pattern in the X direction, can be utilized as a sub antenna 2,such as a dipole antenna, a horn antenna and an array antenna. 3 is a multiplier for performing multiplication between the signal received with the main antenna 1 and the signal received with the sub antenna 2. In order to scan the beam of the antenna system comprising the main antenna 1 and the sub antenna 2 in the X direction, the antenna system can be mechanically moved on a rotary table, or otherwise, phased-array antennas are used for each antenna and the feeding phases at each antenna are controlled so that the beam is scanned electronically with no mechanical motion. When radio waves arrive at such an antenna system whose beam is scanned in the X direction, the main antenna 1 and the sub antenna 2 output the received signals depending on each antenna pattern. These output signals are multiplied together by the multiplier 3 on the condition that the phase of these signals coincide with each other, and the output of the multiplier gives the final output corresponding to the main antenna pattern with reduced side lobes. Figure 3A to 3C show simulated characteristics of an antenna system comprising a main antenna whose X-direction length is about twelve times as large as the wavelength of received radio waves and a sub antenna having a length of a third of the main-antenna length. Figure 3A shows the pattern M() of the main antenna, and Figure 3B shows the pattern r() of the sub antenna. Figure 3C shows the synthesized pattern (solid line) resulting from multiplication between pattern M() of the main antenna and pattern r() of the sub antenna, and also the pattern of the main antenna (broken line). In Figure 3C, the magnitude c of a side lobe in the synthesized pattern is the product of the magnitude a of the side lobe in pattern M() of the main antenna and the magnitude b of the side lobe in pattern r() of the sub antenna. Here, the magnitude of b is always less than 1 except at 0 degree (the beam axis direction) because the pattern r() of the sub antenna is not isotropic. Therefore, the magnitude c of the side lobe in the synthesized pattern is always less than the magnitude a of the side lobe in pattern M() of the main antenna. Hence, reduction in side lobes can be achieved without widening the beam width. As can be seen from Figure 3C, the present invention also makes some contribution to the reduction in the beam width. On the other hand, in the case of a conventional antenna system shown in Figure 1, in which a sub antenna is arranged apart from a main antenna in the X direction, the pattern R() of the sub antenna in the X direction is described by the pattern r() of the sub antenna of the present invention multiplied by cos(k·d·sin), where k=2π/λ, d is a distance between the center of the main antenna and the center of the sub antenna,  is the angle relative to the X direction, and λ is the wavelength of the received radio wave. That is, in the present invention, the sub antenna does not have any separation from the main antenna in the X direction while it has some separation in the Y direction, but, in contrast, the conventional antenna system has separation d in the X direction. As a result, the pattern R() of the sub antenna of the conventional antenna system is represented by r() multiplied by cos(k·d·sin), as described above, and it has short periodicity as shown in Figure 4. Consequently when the pattern M() of the main antenna is multiplied by this pattern R() of the sub antenna, the reduction in beam width can be achieved but large side lobes arise. Referring to Figure 5, an example of the constitution of a practical embodiment of the antenna system is described. In this example, circular patch array antennas are utilized for both of a main antenna 11 and the sub antenna 12, and the sub antenna 12 is arranged apart from the main antenna 11 in the Y direction. With regard to the multiplication circuit, when the multiplication is carried out in an analogue way, common multiplier circuits or frequency modulators can be used. In order to perform multiplication digitally, known means are available such as means that, after the received signal is converted to a digital signal by an analogue-to-digital converter, digital multiplication is carried out. An example of such a means is shown in Figure 6. In this figure, 21 is a main antenna, 22 is a sub antenna, 23 and 24 are receivers for receiving the radio waves caught by each antenna, 25 and 26 are analogue-to-digital converters which convert the output signal of the receiver 23 and 24 to digital signals, and 27 is a multiplier for establishing the products of the output of analogue-to-digital converter 25 and the output of analogue-to-digital converter 26. In the multiplication circuit of such a constitution, the radio waves received with the main antenna 21 and the sub antenna 22 are applied to the receiver 23 and 24 to output the DC signals corresponding to the power of the received radio waves. These receiver output signals are applied to the analogue-to-digital converter 25 and 26 to convert the signals into digital data. Furthermore, these digital data are multiplied together by the multiplier 27 and the resultant multiplied data are output. In the above embodiment, a single sub antenna is utilized. However, plural sub antennas can be used as long as they do not have isotropic directional characteristics. In such a case, there are two following methods available for achieving multiplication of the signal received with the main antenna. In the first method, after adding the outputs of the plural sub antennas, the resultant added output is multiplied by the output of the main antenna. In this case, the sub antennas receive more radio power than the single sub antenna, whereby better antenna gain and better signal-to-noise ratio are obtained. In the second method, each output of the plural sub antennas is sequentially multiplied by the output of the main antenna. In this case, it is possible to achieve more reduction in the side lobes than single sub antenna system. In the above embodiments, the antenna beam of the antenna system comprising a main antenna and sub antenna is scanned in the X direction, but the beam can be also scanned in another directions. When the beam is scanned in a direction other than either X or Y direction, then the simultaneous reduction in the beam width in the Y direction can be achieved while the degree of the reduction in side lobes in the X direction becomes slightly degraded. In this case, the plural sub antenna can also be utilized and the two methods described above can also be used for achieving multiplication of the signal received with the main antenna. As described above based on the described embodiments, the present invention provides a method in which a sub antenna which does not have an isotropic antenna pattern in the direction of the orientation of a side lobe to be reduced is arranged in such a way that it adjoins a main antenna in the direction perpendicular to the orientation of the side lobe of the main antenna to be reduced and the beam axis of the sub antenna coincides with the beam axis of the main antenna, and each signal received with each antenna is multiplied together with the same phase to reduce side lobes without widening the beam width of the main antenna.

Claims: A method for reducing side lobes in an antenna pattern obtained from an antenna system having: a main antenna (1) for receiving radio waves; and one or more sub antennas (2) which does/do not have an isotropic antenna pattern in a first direction (x) coinciding with the orientation of a side lobe of said main antenna to be reduced and which is/are arranged in such a way that it/they adjoin said main antenna (1) in a second direction (y) which is perpendicular to said first direction and its/their beam axis/axes coincide(s) with the beam axis of said main antenna (1); wherein the antenna beam of said antenna system is scanned in said first direction (x), and wherein the signal received with said main antenna (1) is multiplied by the signal(s) received with said one or more sub antennas (2) on the condition that the phases of both said signals coincide with each other. A method for reducing side lobes in an antenna pattern as claimed in claim 1, wherein said antenna system has plurality of sub antennas (2) and wherein after each signal received with each said sub antenna (2) is added together, the resultant added signal is multiplied by said signal received with said main antenna (1). A method for reducing side lobes in an antenna pattern as claimed in claim 1, wherein said antenna system has plurality of sub antennas (2) and wherein each signal received with each said sub antenna (2) is sequentially multiplied by said signal received with said main antenna (1). A method for making a reduction in a beam width in addition to a reduction in side lobes in an antenna pattern obtained from an antenna system having: a main antenna (1) for receiving radio waves; and one or more sub antennas (2) which does/do not have an isotropic antenna pattern in a first direction (x) coinciding with the orientation of the side lobe of said main antenna to be reduced and which is/are arranged in such a way that said one or more sub antennas (2) adjoin said main antenna in a second direction (y) perpendicular to said first direction (x) and the beam axis/axes of said one or more sub antennas (2) coincide with the beam axis of said main antenna (1); wherein the antenna beam of said antenna system is scanned in a direction other than either the first direction (x) or the second direction (y); and wherein the signal received with said main antenna (1) is multiplied by the signal(s) received with one or more said sub antennas (2), on the condition that the phases of both said signals coincide with each other. A method for making a reduction in the beam width in addition to a reduction is side lobes in an antenna pattern as claimed in claim 4, wherein said antenna system has a plurality of sub antennas (2) and wherein, after each signal received with each said sub antenna (2) is added together, the resultant added signal is multiplied by said signal received with said main antenna (1). A method for making a reduction in beam width in addition to a reduction in side lobes in an antenna pattern as claimed in claim 4, wherein said antenna system has a plurality of sub antennas (2) and wherein each signal received with each said sub antenna (2) is sequentially multiplied by said signal received with said main antenna (1).

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: 1992
Main classification: H01Q21
IPC codes: H01Q3, H01Q21, G01S7, G01K11