Passive antenna panel

Abstract

To provide a passive antenna panel capable of improving the performance of an SAR.SOLUTION: The passive antenna panel includes a substrate, a feeding waveguide provided on a surface of the substrate, and an RF signal input section configured to input an RF signal into the feeding waveguide. Multiple coupling slots are formed in the feeding waveguide. Waveguide center lines of the multiple coupling slots are positioned between a panel center line of the substrate and a quarter line of the substrate. The substrate includes a first region located between the panel center line and the quarter line, and a second region located between an edge in the elevation direction and the quarter line. The number N of rows of radiation slot pairs formed in the first region is smaller than the number (N+p) of rows of radiation slot pairs formed in the second region.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a passive antenna panel. [Background technology]

[0002] Development of SAR satellites has been ongoing for some time. SAR stands for Synthetic Aperture Radar. SAR satellites irradiate electromagnetic waves (such as microwaves) towards the Earth's surface and use the returned electromagnetic waves to image the condition of the Earth's surface. SAR sometimes uses passive antenna panels.

[0003] Patent Document 1 discloses an invention of a passive antenna panel equipped with a feed waveguide. This feed waveguide has a structure combining a series feeding and a corporate feeding. A τ-shaped branch circuit is provided in a portion of the feed waveguide located at the center of the passive antenna panel. Each panel can be considered to be divided into two sub-panels, with the center of the panel as the boundary. Generally, each panel is formed with a row of radiating slot pairs for radiating signals. The number of rows of radiating slot pairs is symmetrical with respect to the center line of the sub-panel (the quadrant of the panel). In other words, the number of rows of radiating slot pairs arranged between the center line of the panel and the center line of the sub-panel is generally the same as the number of rows of radiating slot pairs arranged between the center line of the sub-panel and the edge of the panel. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent No. 1,130,9623 Summary of the Invention [Problem to be solved by the invention]

[0005] When evaluating the performance of a SAR, the antenna gain and aperture efficiency for each frequency band are important. As a result of extensive research by the inventors of this application, it was found that the performance of the SAR can be improved by devising the arrangement of the radiation slot pairs.

[0006] The present invention has been made in consideration of the above circumstances, and has an object to provide a passive antenna panel that can improve the performance of SAR. [Means for solving the problem]

[0007] In order to solve the above problem, a passive antenna panel according to one embodiment of the present invention comprises a substrate, a feed waveguide provided on the surface of the substrate, and an RF signal input section for inputting an RF signal to the feed waveguide, wherein a plurality of coupling slots are formed in the feed waveguide, and the waveguide center lines of the plurality of coupling slots are located between a panel center line of the substrate and a quadrant of the substrate, the substrate has a first region between the panel center line and the quadrant, and a second region between an edge in an elevation angle direction and the quadrant, and the number N of rows of inner radiating slot pairs formed in the first region is less than the number N+p of rows of outer radiating slot pairs formed in the second region. [Effects of the Invention]

[0008] According to the above-described aspects of the present invention, it is possible to provide a passive antenna panel that can improve the performance of SAR. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is an overall view of a SAR satellite according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing the passive antenna panel group of FIG. 1 in a folded state. [Figure 3] FIG. 2 is an overall view of the passive antenna panel group of FIG. 1. [Figure 4] 3A and 3B are diagrams illustrating an example of the structure of the substrate of each passive antenna panel. [Figure 5]1A and 1B are diagrams illustrating an example of the structure of a passive antenna panel. [Figure 6] FIG. 6 is an enlarged view of a portion V1 in FIG. 5. [Figure 7] 10 is a graph showing the relationship between the position of the passive antenna panel in the Y-axis direction and the excitation amplitude. [Figure 8] 10 is a graph showing the relationship between the position of the passive antenna panel in the Y-axis direction and the excitation phase. [Figure 9] 1 is a graph showing the relationship between frequency and directivity. [Figure 10] 10 is a graph showing the relationship between frequency and aperture efficiency. [Figure 11] 1 is a graph showing the relationship between elevation angle and signal strength. [Figure 12] 10A and 10B are diagrams illustrating an example of a primary slot and a secondary slot according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] (First embodiment) A SAR satellite equipped with a passive antenna panel of a first embodiment will be described below with reference to the drawings. As shown in Fig. 1, the SAR satellite 1 comprises a satellite main body 2 and a passive antenna panel group 3. The passive antenna panel group 3 includes a plurality of passive antenna panels 3a to 3g. In this embodiment, there are seven passive antenna panels 3a to 3g, but this number may be changed. Each of the passive antenna panels 3a to 3g is provided with a solar panel S.

[0011] (direction definition) In this embodiment, the positional relationship of each component will be described using an XYZ Cartesian coordinate system. The X axis indicates the azimuth direction. The Y axis indicates the elevation direction. The Z axis is a direction perpendicular to both the X and Y axes.

[0012] The passive antenna panels 3a to 3g can be changed between a position in which they are aligned in a row in the X-axis direction (FIG. 1) and a folded position (FIG. 2). Adjacent passive antenna panels 3a to 3g in the X-axis direction are rotatably connected to each other by hinges. That is, each passive antenna panel 3a to 3g rotates around the hinge, changing between the position shown in FIG. 1 and the position shown in FIG. 2.

[0013] Passive antenna panel 3d, located in the center in the X-axis direction, is fixed to the satellite main body 2. Three passive antenna panels 3a to 3c are connected to the -X side of passive antenna panel 3d. Three passive antenna panels 3e to 3g are connected to the +X side of passive antenna panel 3d.

[0014] As shown in FIG. 3, the passive antenna panel 3a is provided with a first feed waveguide 4a and a second feed waveguide 5a. Similarly, the passive antenna panels 3b to 3g are provided with first feed waveguides 4b to 4g and second feed waveguides 5a to 5g, respectively. The feed waveguides 4a to 4g and 5a to 5g are, for example, metal tubes and are configured to be able to propagate RF signals. The feed waveguides 4a to 4g adjacent to each other in the X-axis direction are configured to be connected to each other when the passive antenna panels 3a to 3g are unfolded (FIG. 1). The feed waveguides 4a to 4g are connected to form a first waveguide path 4. Similarly, the feed waveguides 5a to 5g are connected to form a second waveguide path 5. The first waveguide path 4 and the second waveguide path 5 are symmetrical with respect to the panel center line C. The panel center line C is a line that passes through the center of each of the passive antenna panels 3a to 3g in the Y-axis direction.

[0015] RF signals are input to the waveguide paths 4 and 5 from a plurality of RF signal input units 6. The waveguide paths 4 and 5 are provided with a plurality of branch units (τ-shaped branch circuits). The RF signal input from the RF signal input unit 6 propagates along the waveguide paths 4 and 5 while branching at each branch unit. The RF signal also propagates so as to spread within the plane of the passive antenna panels 3a to 3g. The RF signal is then radiated from each of the passive antenna panels 3a to 3g.

[0016] The following describes the structure of the passive antenna panels 3a to 3g for radiating RF signals. The passive antenna panels 3a to 3g differ from one another in terms of the shapes of the feed waveguides 4a to 4g and 5a to 5g. However, apart from the shapes of the feed waveguides 4a to 4g and 5a to 5g, the passive antenna panels 3a to 3g have the same basic structure. Therefore, in the following description, the structure of the passive antenna panel 3a will be described as a representative of the seven passive antenna panels 3a to 3g. In other words, the following description also applies to the passive antenna panels 3b to 3g.

[0017] As shown in Figure 4, the passive antenna panel 3a has a substrate 10. The substrate 10 has a first metal plate 11, a first adhesive layer 12, a honeycomb core 13, a second adhesive layer 14, and a second metal plate 15. The first adhesive layer 12 bonds and fixes the first metal plate 11 to the honeycomb core 13. The second adhesive layer 14 bonds and fixes the second metal plate 15 to the honeycomb core 13. The first metal plate 11 has a plurality of radiation slot pairs formed therein.

[0018] As shown in FIG. 5, the feed waveguide 4a has a first waveguide 41, a second waveguide 42, and a third waveguide 43. A connection portion 41a is provided at an end of the first waveguide 41. As shown in FIG. 3, a signal is input to the feed waveguide 4a from the feed waveguide 4b of the passive antenna panel 3b adjacent to the passive antenna panel 3a via the connection portion 41a. As shown in FIG. 5, the feed waveguide 5a has a first waveguide 51, a second waveguide 52, and a third waveguide 53. A connection portion 51a is provided at an end of the first waveguide 51. As shown in FIG. 3, a signal is input to the feed waveguide 5a from the feed waveguide 5b of the passive antenna panel 3b via the connection portion 51a.

[0019] As shown in FIG. 5, the passive antenna panel 3a has a symmetrical structure with respect to the panel center line C. In the following description, the line that bisects the panel center line C and the edge E of the passive antenna panel 3a is referred to as the "quadrant C'." The quadrant C' is one of the three lines that divide the passive antenna panel 3a into four equal parts in the Y-axis direction, and is the other two lines other than the panel center line C. In FIG. 5, the quadrant C' is omitted for clarity. The region of the passive antenna panel 3a between the panel center line C and the quadrant C' is referred to as the "first region A1." The region of the passive antenna panel 3a between the quadrant C' and the edge E is referred to as the "second region A2." The passive antenna panel 3a has two first regions A1 and two second regions A2.

[0020] The feed waveguide 4a and the feed waveguide 5a have a symmetrical structure with respect to the panel center line C. Therefore, in the following, the structure of the feed waveguide 4a will be described as a representative of the two feed waveguides 4a and 5a of the passive antenna panel 3a.

[0021] In the feed waveguide 4a, the first waveguide 41, the second waveguide 42, and the third waveguide 43 extend in the X-axis direction. The second waveguide 42 is located closer to the edge E than the first waveguide 41. The third waveguide 43 is located closer to the edge E than the second waveguide 42. That is, the first waveguide 41, the second waveguide 42, and the third waveguide 43 are arranged in this order from the panel center line C toward the edge E. The first waveguide 41 and the second waveguide 42 are connected by a τ-shaped branching circuit. The second waveguide 42 and the third waveguide 43 are also connected by a τ-shaped branching circuit. An RF signal input from the connection portion 41a branches and propagates in the order of the first waveguide 41, the second waveguide 42, and the third waveguide 43.

[0022] FIG. 6 is an enlarged view of portion V1 in FIG. 5. As shown in FIG. 6, a plurality of coupling slots 43a are formed in the third waveguide 43. The center line of the third waveguide 43 is referred to as the "waveguide center line L." The plurality of coupling slots 43a are arranged on the waveguide center line L. The waveguide center line L extends parallel to the X-axis. Each coupling slot 43a is a long hole that is parallel to the waveguide center line L, i.e., parallel to the X-axis. The plurality of coupling slots 43a are also arranged side by side along the X-axis.

[0023] As shown in FIG. 6, the waveguide center line L is offset toward the edge E in the Y-axis direction with respect to the quadrant C'. The distance in the Y-axis direction between the waveguide center line L and the quadrant C' is referred to as the "offset amount d." A row of multiple radiation slot pairs is formed on the first metal plate 11 of the substrate 10. As shown in FIG. 6, the radiation slot pairs arranged in the first region A1 are referred to as the "inner radiation slot pair R1c." The radiation slot pairs arranged in the second region A2 are referred to as the "outer radiation slot pair R1e." In other words, the inner radiation slot pair R1c is arranged between the quadrant C' and the panel center line C, and the outer radiation slot pair R1e is arranged between the quadrant C' and the edge E. Note that the position of the quadrant C' in FIG. 6 is merely an example when p = 0. The position of the quadrant C' changes depending on the value of p. However, even in this case, the offset amount d is defined as the distance between the waveguide center line L and the quadrant C'.

[0024] The dimensions shown in FIG. 6 are defined as follows: d: Offset amount (distance between the waveguide center line L and the quadrant C') d1+d: Distance from the center of the radiating slot pair R1c closest to the waveguide center line L to the waveguide center line L d2+d: Distance from the center of the radiating slot pair R1c farther from the waveguide center line L to the waveguide center line L d1-d: distance from the center of the radiating slot pair R1e closest to the waveguide center line L to the waveguide center line L d2-d: Distance from the center of the radiating slot pair R1e farther from the waveguide center line L to the waveguide center line L

[0025] The wavelength at the center frequency of the RF signal propagating between the first metal plate 11 and the second metal plate 15 in the substrate 10 is set to λ. In this case, λ may be equal to d1+d2. Note that λ=d1+d2 does not have to be satisfied.

[0026] Next, the number of rows of radiating slot pairs R1c, R1e will be described. The number of rows of radiating slot pairs R1c in one first area A1 is defined as N. The number of rows of radiating slot pairs R1e in one second area A2 is defined as N+p, where N and p are both natural numbers. The passive antenna panel 3a has two first areas A1 and two second areas A2. Therefore, the total number of rows of radiating slot pairs R1c that the first metal plate 11 has is 2×N. The total number of rows of radiating slot pairs R1e that the first metal plate 11 has is 2×(N+p).

[0027] As described above, the passive antenna panel 3a of this embodiment includes a substrate 10, a feed waveguide 4a provided on the surface of the substrate 10, and an RF signal input unit 6 that inputs an RF signal to the feed waveguide 4a. A plurality of coupling slots 43a are formed in the feed waveguide 4a. The waveguide center lines L of the plurality of coupling slots 43a are located between the panel center line C of the substrate 10 and a quadrant C' of the substrate 10. The substrate 10 has a first region A1 between the panel center line C and the quadrant C' and a second region A2 between an edge E in the elevation angle direction (Y-axis direction) and the quadrant C'. The number (N) of rows of inner radiating slot pairs R1c formed in the first region A1 is less than the number (N+p) of rows of outer radiating slot pairs R1e formed in the second region A2. N and p are natural numbers.

[0028] Below, the results of confirming the advantageous effects obtained by this embodiment will be described. In the following description, "without offset" refers to the case where the offset amount d is zero. In the case where "with offset", the offset amount d is set to an appropriate value. Furthermore, in Fig. 7 etc., "N" is the number of columns of the inner radiation slot pair R1c, and "p" is the difference between the number of columns of the outer radiation slot pair R1e and the number of columns of the inner radiation slot pair R1c.

[0029] In the prior art structure, the waveguide centerline L was positioned to overlap the quadrant line C', and the rows of radiating slot pairs were positioned symmetrically with respect to the quadrant line C'. In other words, in the prior art, the number of rows of radiating slot pairs R1c between the panel centerline C and the quadrant line C' was the same as the number of rows of radiating slot pairs R1e between the quadrant line C' and the edge E.

[0030] In contrast, in this embodiment, the number (N) of rows of inner radiating slot pairs R1c is smaller than the number (N+p) of rows of outer radiating slot pairs R1e. In this embodiment, the uniform amplitude between the waveguide center line L and the panel center line C is 1 / N, which is larger than the uniform amplitude between the waveguide center line L and the edge E, 1 / (N+p). Having fewer rows of radiating slot pairs R1c than the rows of radiating slot pairs R1e can improve the aperture efficiency of the SAR. This is because the propagation distance of the RF signal from the waveguide center line L to the center of the passive antenna panel 3a (panel center line C) is shorter, making it possible to suppress ripples. In addition, the directivity and the operable frequency band can be improved.

[0031] When the number of rows (N) of the inner radiating slot pair R1c is different from the number of rows (N+p) of the outer radiating slot pair R1e, a phase shift occurs between the first area A1 and the second area A2. Specifically, the phase in the first area A1 becomes higher and the phase in the second area A2 becomes lower. Such a phase shift causes a phase jump at the branching section, which undesirably reduces the antenna's directivity and aperture efficiency, increases side lobes, and so on.

[0032] Therefore, in this embodiment, the waveguide center line L of the coupling slot 43a is positioned between the panel center line C and the quadrant C' of the substrate 10. In other words, the waveguide center line L is shifted toward the edge E with respect to the quadrant C' by the offset amount d. This makes it possible to cancel the phase shift that occurs between the first region A1 and the second region A2, and as a result, it is possible to suppress a decrease in the antenna's directivity and aperture efficiency, an increase in side lobes, etc.

[0033] Furthermore, according to the configuration of this embodiment, a tapered, step-like excitation amplitude can be obtained, as shown in the graph of N=7, p=2 in FIG. 7. The horizontal axis in FIG. 7 represents the position (radiating slot pair number) in the Y-axis direction of the passive antenna panel. The vertical axis in FIG. 7 represents the excitation amplitude. In conventional technology, tapering of the excitation amplitude is achieved at the expense of the main lobe width, resulting in reduced antenna directivity and aperture efficiency. According to the tapering method of this embodiment, by setting p / N to be smaller than a predetermined value, it is possible to achieve the effect of reducing side lobes while maintaining antenna directivity and aperture efficiency (see FIG. 11).

[0034] The horizontal axis in Fig. 8 represents the position in the Y-axis direction of the passive antenna panel (the number of the radiation slot pair). The vertical axis in Fig. 8 represents the excitation phase. In the data for N=8, p=2 (before offset), the excitation phase changes depending on the position in the Y-axis direction. In contrast, in the data for N=8, p=2 (after offset), the excitation phase is constant regardless of the position in the Y-axis direction. In this way, it was confirmed that by introducing the "offset amount d," the excitation phase can be kept constant even after changing the number of radiation slot pair rows in the first area A1 and the second area A2.

[0035] 9 to 11, "Simulation" indicates the simulation results, and "Measurement" indicates the actual measurement results. The horizontal axis in Figure 9 is frequency (GHz). The vertical axis in Figure 9 is directivity (dBi). As shown in Figure 9, when p = 2, the average directivity in the frequency band of 9.3 to 9.9 GHz was significantly improved compared to when p = 0. The higher the average directivity, the better the signal-to-noise ratio (SNR), which is an important index of SAR image quality. As the data for "N = 7 p = 2 Measurement" shows, the actual measurement results were even better than the simulation results.

[0036] The horizontal axis in Figure 10 represents frequency (GHz). The vertical axis in Figure 10 represents aperture efficiency (%). As shown in Figure 10, when N = 7 and p = 2, the average aperture efficiency in the 9.3-9.9 GHz frequency band is significantly improved compared to when N = 8 and p = 0. A higher average aperture efficiency means that a smaller antenna can achieve performance equivalent to that of a larger antenna. This reduces satellite weight and launch costs. Furthermore, a higher average aperture efficiency also means that the maximum power required to ensure the SNS ratio and SAR image quality is lower. This reduces the cost of using a high-power amplifier and the risk of thermal throttling. Generally, the larger the antenna size, the greater the antenna loss and the lower the aperture efficiency. Therefore, in Figure 10, the aperture efficiency when N = 8 and p = 0 is lower than when N = 7 and p = 0. However, when using the new configuration N = 7 and p = 2, the aperture efficiency is almost the same as when N = 7 and p = 0. As the data from N = 7 p = 2 measurements show, the actual measurements yielded even better results than the simulations.

[0037] The horizontal axis in Figure 11 is the elevation angle (deg). The vertical axis in Figure 11 is the signal strength (dB) with the value at elevation angle = 0 (deg) as the reference. The frequency is 9.6 GHz. As shown in Figure 11, when p = 2, the side lobes were suppressed more than when p = 0. As the data for "N = 7 p = 2 Measurement" shows, good results were obtained not only from simulation but also from actual measurements. Note that a slight asymmetry appears in the side lobes in "N = 7 p = 2 Measurement" in Figure 11. This asymmetry is caused by an error in installing the antenna during measurement.

[0038] Next, the relationship between the values ​​of N and p will be described. It is preferable that p / N, which is the ratio of p to N, be 0.45 or less. In the example of FIG. 7 etc., when N=7 and p=2, p / N=0.29. If p / N is greater than 0.45, the tapering effect can further reduce the side lobe level, but the aperture efficiency also decreases, which may be undesirable when improving aperture efficiency is given a higher priority than reducing side lobes. One method for achieving the desired aperture efficiency is to perform amplitude correction in the coupling slot 43a. An example of amplitude correction will be described below in the second embodiment.

[0039] (Second embodiment) The second embodiment has the same basic structure as the first embodiment. Therefore, a description of the similar configuration will be omitted and differences will be mainly described. In the second embodiment, as shown in FIG. 12 , a coupling slot 43a is formed by a primary slot 43a1 and a secondary slot 43a2. One of the primary slot 43a1 and secondary slot 43a2 that form one coupling slot 43a is positioned on the waveguide center line L, and the other is positioned away from the waveguide center line L in the Y-axis direction. Furthermore, when focusing on a pair of coupling slots 43a adjacent to each other in the X-axis direction, the primary slots 43a1 and secondary slots 43a2 are alternately positioned along the waveguide center line L.

[0040] More specifically, the coupling slots 43a arranged in the X-axis direction will be described by assigning slot numbers such as (n=1, 2, 3, ...). When the slot number is odd (n=1, 3, 5, ...), the primary slot 43a1 is arranged on the waveguide center line L, and the secondary slot 43a2 is arranged at a position shifted in the Y-axis direction relative to the primary slot 43a1. Conversely, when the slot number is even (n=2, 4, 6, ...), the secondary slot 43a2 is arranged on the waveguide center line L, and the primary slot 43a1 is arranged at a position shifted in the Y-axis direction relative to the secondary slot 43a2. The length of the secondary slot 43a2 in the X-axis direction is shorter than that of the primary slot 43a1.

[0041] In this way, amplitude correction can be performed by providing primary slot 43a1 and secondary slot 43a2 and adjusting the distance between primary slot 43a1 and secondary slot 43a2. The configuration of this embodiment is suitable for cases where increasing the p / N value reduces aperture efficiency. However, even if the p / N value is 0.45 or less, providing primary slot 43a1 and secondary slot 43a2, as in this embodiment, is not essential.

[0042] The technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. Furthermore, the components in the above-described embodiments can be replaced with well-known components as appropriate, and the above-described embodiments and modifications can be combined as appropriate, without departing from the spirit of the present invention. [Explanation of symbols]

[0043] 3a to 3g... Passive antenna panel 4a to 4g... Feeding waveguide 6... RF signal input section 10... Substrate 43a... Coupling slot 43a1... Primary slot 43a2... Secondary slot A1... First region A2... Second region C... Panel center line C'... Quadrant line R1c... Inner radiation slot pair R1e... Outer radiation slot pair

Claims

1. A substrate; a feed waveguide provided on a surface of the substrate; an RF signal input unit that inputs an RF signal to the feed waveguide, a plurality of coupling slots are formed in the feed waveguide; The waveguide centerlines of the plurality of coupling slots are located between a panel centerline of the substrate and a quadrant of the substrate; the substrate has a first region between the panel center line and the quadrant line, and a second region between an edge in an elevation angle direction and the quadrant line; A passive antenna panel, wherein the number N of rows of inner radiating slot pairs formed in the first region is less than the number N+p of rows of outer radiating slot pairs formed in the second region.

2. 2. The passive antenna panel according to claim 1, wherein p / N, the ratio of p to N, is 0.45 or less.

3. 3. The passive antenna panel according to claim 1, wherein the coupling slots include a primary slot and a secondary slot that is shorter than the primary slot.

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