Retroreflective antenna, and object detecting system

Abstract

A retroreflective antenna according to an embodiment comprises an antenna main body for retroreflecting incoming radio waves, and a passive notch filter, wherein: the antenna main body includes a plurality of pairs of antenna elements, and a plurality of transmission lines provided corresponding to the plurality of pairs of antenna elements; each pair of antenna elements of the plurality of pairs of antenna elements is disposed with point symmetry about a reference point in the antenna main body; each of the plurality of transmission lines connects the corresponding pair of antenna elements, among the plurality of pairs of antenna elements; electrical lengths of the plurality of transmission lines are the same; and the passive notch filter is provided in at least one transmission line among the plurality of transmission lines.

Description

Retroreflector antenna and object detection system

[0001] This disclosure relates to a retroreflective antenna and an object detection system. This application claims priority to Japanese Patent Application No. 2022-128219, filed on August 10, 2022, and incorporates by reference all of the contents of said Japanese application.

[0002] A Van Atta array antenna is known as one type of antenna (see Patent Documents 1, 2, and 3). The Van Atta array antenna has multiple pairs of antenna elements. Each of the multiple pairs of antenna elements is arranged point-symmetrically with respect to a reference point. The pairs of antenna elements arranged point-symmetrically with respect to the reference point are connected by a transmission line. The multiple transmission lines corresponding to the multiple pairs of antenna elements have the same electrical length. The Van Atta array antenna, having the above configuration, retroreflects incoming radio waves. Because the Van Atta array antenna retroreflects incoming radio waves, the reflected power when radio waves are reflected by the Van Atta array antenna is relatively large. Therefore, it is easy to distinguish between radio wave reflection by a reflector that does not have a retroreflection function and radio wave reflection from the Van Atta array antenna. Therefore, the Van Atta array antenna can be used for object detection.

[0003] U.S. Patent No. 2,908,202 International Publication No. 00 / 59068 U.S. Patent Application Publication No. 2020 / 0194887

[0004] A retroreflection antenna according to one aspect of the present disclosure comprises an antenna body that retroreflects incoming radio waves, and a passive notch filter, wherein the antenna body has a plurality of pairs of antenna elements and a plurality of transmission lines that correspond to the plurality of pairs of antenna elements, each pair of antenna elements being arranged point-symmetrically with respect to a reference point on the antenna body, each of the plurality of transmission lines connecting a corresponding pair of antenna elements among the plurality of pairs of antenna elements, the plurality of transmission lines having the same electrical length, and the passive notch filter being provided on at least one of the plurality of transmission lines.

[0005] FIG. 1 is a schematic diagram of a retroreflective antenna according to one embodiment. FIG. 2 is a plan view of an example of a specific embodiment of the retroreflective antenna shown in FIG. 1. FIG. 3 is a side view of a substrate included in the retroreflective antenna shown in FIG. 1. FIG. 4 is a schematic diagram of an object detection system according to a second embodiment. FIG. 5 is a diagram showing the frequencies of transmitted waves and received waves in a radar device. FIG. 6 is a diagram showing the difference frequency between the transmitted wave frequency and the received wave frequency shown in FIG. 5. FIG. 7 is a schematic diagram of a retroreflective antenna according to a third embodiment. FIG. 8 is a diagram showing the difference frequency between the transmitted wave frequency and the received wave frequency obtained when the retroreflective antenna shown in FIG. 7 is applied to the object detection system shown in FIG. 4. FIG. 9 is a diagram of a retroreflective antenna according to a fourth embodiment, viewed from the front side. FIG. 10 is a diagram of the retroreflective antenna shown in FIG. 9, viewed from the back side. FIG. 11 is a side view of a substrate included in the retroreflective antenna shown in FIG. 9. FIG. 12 is a diagram showing a modified example of the retroreflective antenna.

[0006] [Problem to be Solved by the Present Disclosure] As described above, the reflected power of radio waves reflected by a Van Atta array antenna is relatively large. However, it is difficult to distinguish between reflections from a reflector that has high radio wave reflective capability (easily reflects more radio waves) even without retroreflection and reflections from a Van Atta array antenna. Therefore, for example, it is conceivable to use an input device for inputting predetermined information (e.g., identification information) related to the retroreflector antenna and superimposing the predetermined information on the radio waves retroreflected by the Van Atta array antenna. In this case, reading the predetermined information identifies the reflection as having come from a Van Atta array antenna. When such an input device is used, a battery or the like is required to power the device. As a result, problems such as increased manufacturing costs for the Van Atta array antenna, additional costs for battery replacement, and increased weight of the Van Atta array antenna arise.

[0007] The present disclosure aims to provide a retroreflector antenna that does not require a power supply and can retroreflect incoming radio waves with information superimposed on them, and an object detection system using the same.

[0008] Effect of the Present Disclosure According to the present disclosure, it is possible to provide a retroreflector antenna that does not require a power supply and that can retroreflect incoming radio waves with information superimposed on them, and an object detection system using the same.

[0009] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be listed and described.

[0010] (1) A retroreflection antenna according to one aspect of the present disclosure includes an antenna body that retroreflects incoming radio waves and a passive notch filter, wherein the antenna body has a plurality of pairs of antenna elements and a plurality of transmission lines that correspond to the plurality of pairs of antenna elements, each pair of antenna elements being arranged point-symmetrically with respect to a reference point on the antenna body, each of the plurality of transmission lines connecting a corresponding pair of antenna elements among the plurality of pairs of antenna elements, the plurality of transmission lines having the same electrical length, and the passive notch filter being provided on at least one of the plurality of transmission lines.

[0011] The retroreflecting antenna described in (1) above has an antenna main body with the above configuration, and is therefore capable of retroreflecting incoming radio waves. At least one of the multiple transmission lines of the antenna main body is provided with a passive notch filter. In this case, by designing the passive notch filter to cut a portion of the frequency of the incoming radio waves, the retroreflecting antenna described in (1) above retroreflects radio waves with attenuated reflected power for the portion of the radio waves corresponding to the cut frequency. In this case, the presence of an attenuated portion of the radio waves corresponds to information being superimposed on the retroreflected radio waves. Because this information is superimposed on the radio waves using the passive notch filter, no power supply is required. Therefore, the retroreflecting antenna described in (1) above is capable of retroreflecting radio waves with information superimposed on the incoming radio waves, without requiring a power supply.

[0012] (2) In the retroreflector antenna described in (1) above, the plurality of pairs of antenna elements may include a first antenna element group, a second antenna element group, and a third antenna element group, each of which includes N pairs of antenna elements (N is an integer greater than or equal to 1), and when one of the N pairs of antenna elements in each of the first antenna element group, the second antenna element group, and the third antenna element group is an i-th pair of antenna elements (i is greater than or equal to 1 and less than or equal to N), the i-th pair of antenna elements in the second antenna element group may be positioned at a position rotated 120 degrees from the i-th pair of antenna elements in the first antenna element group around a predetermined direction of the reference point, and the i-th pair of antenna elements in the third antenna element group may be positioned at a position rotated 240 degrees from the i-th pair of antenna elements in the first antenna element group around the predetermined direction of the reference point.

[0013] In the above configuration, the i-th pair of antenna elements in the second antenna element group and the i-th pair of antenna elements in the third antenna element group are positioned at positions rotated 120 degrees and 240 degrees around a predetermined direction of the reference point from the i-th pair of antenna elements in the first antenna element group. Therefore, the arrangement of the transmission path connecting the i-th pair of antenna elements in the second antenna element group and the third antenna element group can also be the arrangement of the transmission path connecting the i-th pair of antenna elements in the first antenna element group rotated around the reference point. In this case, by designing the multiple pairs of antenna elements in the first antenna element group and the transmission lines connecting them, the multiple pairs of antenna elements in the second antenna element group and the transmission lines connecting them and the multiple pairs of antenna elements in the third antenna element group and the transmission lines connecting them can also be determined. As a result, in the above configuration, when designing a retroreflector antenna, only one-third of the retroreflector antenna needs to be designed. This makes it easy to design a retroreflector antenna.

[0014] (3) The retroreflective antenna described in (1) or (2) above may have a plurality of the passive notch filters, the number of the passive notch filters being the same as the number of the transmission lines, the frequencies cut by the passive notch filters being the same, and each of the passive notch filters provided in the passive notch filters may be provided on each of the transmission lines.

[0015] In this case, the portion of the radio waves corresponding to the same frequency is cut off for all radio waves propagating through multiple transmission lines. Therefore, in the retroreflected radio waves, the reflected power of the portion corresponding to the frequency cut off by the passive notch filter is attenuated more significantly. As a result, the information superimposed on the radio waves is easier to detect.

[0016] (4) In the retroreflective antenna described in (1) or (2) above, it may include a plurality of notch filter groups, each of which has a plurality of the passive notch filters that cut different frequencies, the number of the plurality of notch filter groups is the same as the number of the plurality of transmission lines, and each of the plurality of notch filter groups may be provided on each of the plurality of transmission lines.

[0017] In this case, each transmission line is provided with multiple passive notch filters that cut off different frequencies. Therefore, the reflected power of the retroreflected radio waves corresponding to different frequencies is attenuated. Therefore, the retroreflecting antenna described in (4) above can superimpose a lot of information onto the retroreflected radio waves.

[0018] (5) An object detection system according to another aspect of the present disclosure includes a retroreflective antenna attached to an object, the retroreflective antenna being any one of the retroreflective antennas described in (1) to (3) above, and a radar device that transmits radio waves modulated using an FMCW method and receives the radio waves retroreflected by the retroreflective antenna, wherein the frequencies cut by the passive notch filter of the retroreflective antenna are frequencies within the frequency band of the radio waves.

[0019] In the object detection system described in (5) above, the radar device transmits radio waves and receives radio waves retroreflected by a retroreflector antenna. The radar device transmits radio waves modulated using the FMCW method. Therefore, the frequency of the radio waves transmitted from the radar device is linearly modulated over time. Therefore, the frequency of the received radio waves also changes over time. The retroreflector antenna provided in the object detection system described in (5) above is the retroreflector antenna described in any one of (1) to (3). Therefore, in the radio waves retroreflected by the retroreflector antenna, the reflected power of the portion of the radio waves corresponding to the frequency cut by the passive notch filter is attenuated. Therefore, the portion of the radio waves transmitted from the radar device corresponding to the frequency cut by the passive notch filter is received by the radar device with reduced reception strength. Therefore, the reception state of the radio waves at the radar device determines whether or not the radio waves are reflected from the retroreflector antenna. As a result, an object to which the retroreflector antenna is attached is detected.

[0020] (6) The object detection system described in (5) above may include a plurality of notch filter groups, each of which has a plurality of the passive notch filters that cut off different frequencies, the number of the plurality of notch filter groups being the same as the number of the plurality of transmission lines, and each of the notch filter groups in the plurality of notch filter groups being provided on each of the plurality of transmission lines.

[0021] In this case, each transmission line of the retroreflective antenna is provided with multiple passive notch filters that cut off different frequencies. Therefore, the reflected power of each portion of the retroreflected radio wave corresponding to a different frequency is attenuated, allowing the retroreflective antenna to superimpose a large amount of information onto the retroreflected radio wave. Therefore, the object detection system described in (6) above makes it easy to distinguish an object to which a retroreflective antenna is attached from other objects. In this case, for example, it is possible to manage objects using the object detection system.

[0022] [Details of the embodiments of the present disclosure] Specific examples of the embodiments of the present disclosure will be described below with reference to the drawings. The present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. In the description of the drawings, the same elements are given the same reference numerals, and duplicate explanations will be omitted.

[0023] First Embodiment Fig. 1 is a schematic diagram of a retroreflector antenna 2 according to one embodiment. The retroreflector antenna 2 according to one embodiment will be outlined with reference to Fig. 1 .

[0024] The retroreflecting antenna 2 is an antenna that reflects (i.e., retroreflects) radio waves 4 arriving from a predetermined direction in the opposite direction along the predetermined direction. The radio waves 4 are, for example, millimeter waves. The frequency band of the millimeter waves is 30 GHz to 300 GHz. In the first embodiment, the radio waves 4 arriving at the retroreflecting antenna 2 are radio waves from a transmission source. In the first embodiment, the transmission source is a sensor (e.g., a radar device) for object detection. An example of the radio waves 4 is radio waves modulated by FMCW (Frequency Modulation Continuous Wave) method.

[0025] The retroreflection antenna 2 includes an antenna body 10 that retroreflects the incoming radio waves 4 and three notch filters 21, 22, and 23.

[0026] The antenna body 10 has three pairs of antenna elements 111a, 111b, 112a, 112b, 113a, and 113b, and three transmission lines 121, 122, and 123. In this embodiment, the antenna body 10 has six antenna elements 111a, 111b, 112a, 112b, 113a, and 113b. Hereinafter, the three pairs of antenna elements 111a, 111b, 112a, 112b, and 113a and 113b may be referred to as "pairs of antenna elements 111a, 111b, etc.", and the six antenna elements 111a, 111b, 112a, 112b, 113a, and 113b may be referred to as "antenna elements 111a, 111b, etc."

[0027] Each of the six antenna elements 111 a, 111 b, etc. is configured to receive radio waves 4 and to emit radio waves 4.

[0028] The six antenna elements 111a, 111b, etc. are arranged in the order of antenna element 111a, antenna element 112a, antenna element 113a, antenna element 113b, antenna element 112b, and antenna element 111b on the dotted-dash line shown in Figure 1 for convenience of explanation.

[0029] The three pairs of antenna elements 111a, 111b, etc. (in other words, six antenna elements 111a, 111b, etc.) are arranged so as to satisfy the following condition I: [Condition I] The pair of antenna elements is arranged point-symmetrically with respect to a reference point of the antenna body.

[0030] Therefore, the pair of antenna elements 111a, 111b are arranged point-symmetrically with respect to the reference point C of the antenna body 10. The pair of antenna elements 112a, 112b are arranged point-symmetrically with respect to the reference point C. The pair of antenna elements 113a, 113b are arranged point-symmetrically with respect to the reference point C.

[0031] Transmission line 121, transmission line 122, and transmission line 123 are lines that transmit radio waves 4. Transmission line 121 connects antenna element 111a and antenna element 111b. Transmission line 122 connects antenna element 112a and antenna element 112b. Transmission line 123 connects antenna element 113a and antenna element 113b. Transmission line 121, transmission line 122, and transmission line 123 have the same electrical length. Transmission line 121, transmission line 122, and transmission line 123 are not connected to each other.

[0032] The notch filter 21 is provided on the transmission line 121. The notch filter 22 is provided on the transmission line 122. The notch filter 23 is provided on the transmission line 123. In the first embodiment, the notch filters 21, 22, and 23 are filters that cut off a portion of the frequencies in the frequency band of the radio waves 4. The frequencies cut off by the notch filters 21, 22, and 23 are the same. The notch filters 21, 22, and 23 are passive notch filters.

[0033] The retroreflector antenna 2 will be further described with reference to Figures 2 and 3. Figure 2 is a plan view of an example of a concrete embodiment of the retroreflector antenna 2 shown in Figure 1. Figure 3 is a side view of the substrate 13 used in the retroreflector antenna 2 shown in Figure 1.

[0034] One form of the retroreflector antenna 2 shown in Fig. 2 is a concrete form that uses a microstrip structure to embody the retroreflector antenna 2 conceptually shown in Fig. 1. Therefore, the retroreflector antenna 2 shown in Fig. 2 has an antenna body 10 and notch filters 21, 22, and 23. In the retroreflector antenna 2, the antenna body 10 and the notch filters 21, 22, and 23 are mounted on a substrate 13.

[0035] 3, the substrate 13 has a dielectric layer 131 and a ground conductor layer 132. Examples of materials for the dielectric layer 131 include Teflon (registered trademark) and aluminum oxide. The ground conductor layer 132 is formed on the back surface 131b of the dielectric layer 131. The material for the ground conductor layer 132 is a metal (for example, copper, silver, tungsten, molybdenum, etc.).

[0036] The antenna body 10 is formed on the surface 131 a of the dielectric layer 131 .

[0037] Specifically, the six antenna elements 111a, 111b, etc. of the antenna body 10 are formed in the shape of rectangular (or square) patches. Each of the six antenna elements 111a, 111b, etc. is realized, for example, as a conductive film. An example of the material of the conductive film is metal (e.g., copper, silver, tungsten, molybdenum, etc.). The size of the six antenna elements 111a, 111b, etc. is set so that they can transmit and receive radio waves 4.

[0038] The arrangement of the six antenna elements 111a, 111b, etc. is the same as that described with reference to Fig. 1. That is, each of the pair of antenna elements 111a, 111b, the pair of antenna elements 112a, 112b, and the pair of antenna elements 113a, 113b is arranged point-symmetrically with respect to the reference point C of the antenna body 10.

[0039] The transmission line 121 is formed as a linear conductive film connecting the pair of antenna elements 111a and 111b. The transmission line 122 is formed as a linear conductive film connecting the pair of antenna elements 112a and 112b. The transmission line 123 is formed as a linear conductive film connecting the pair of antenna elements 113a and 113b. Examples of materials for the conductive films functioning as the transmission lines 121, 122, and 123 are the same as those for the antenna element 111a. The transmission lines 121, 122, and 123 in the retroreflector antenna 2 shown in FIG. 2 are microstrip lines. In the first embodiment, the transmission lines 121, 122, and 123 are made of the same material, have the same thickness, and have the same width. The electrical lengths of the transmission lines 121, 122, and 123 are the same as described above. The electrical lengths of the transmission lines 121, 122, and 123 are adjusted, for example, by their paths.

[0040] The notch filter 21 is formed as a stub S that branches off continuously from a portion of the transmission line 121. The notch filter 22 is formed as a stub S that branches off continuously from a portion of the transmission line 122. The notch filter 23 is formed as a stub S that branches off continuously from a portion of the transmission line 122. The stubs S provided on the transmission lines 121, 122, and 123 are each formed from the same conductive film as the transmission lines 121, 122, and 123.

[0041] The length L of the stubs S provided on each of the transmission lines 121, 122, and 123 is the length from the end of the stub S on the corresponding transmission line 121, 122, or 123 side to the free end of the stub S. The length L of the stub S is set by the following equation 1: L=(1 / 4)×λ+n×(λ / 2) (Equation 1) In equation 1, λ is the wavelength corresponding to the frequency to be cut by the stubs S that are notch filters 21, 22, and 23. n is an integer equal to or greater than 0.

[0042] The retroreflector antenna 2 shown in Fig. 2 can be manufactured, for example, as follows: First, the substrate 13 is prepared. Then, the antenna body 10 and the stub S are formed on the surface 131a of the dielectric layer 131 by, for example, printing technology. In this way, the retroreflector antenna 2 is manufactured.

[0043] The configuration of the antenna body 10 in the retroreflecting antenna 2 shown in Figures 1 and 2 corresponds to the configuration of a Van Atta array antenna. Therefore, the antenna body 10 can retroreflect the incoming radio waves 4.

[0044] For example, consider a case where radio waves 4 from a transmission source are incident on the retroreflecting antenna 2 at an angle, and the wavefronts of the radio waves 4 reach the antenna elements 111a, 112a, 113a, 113b, 112b, and 111b in this order. In this case, the radio waves 4 incident on the antenna elements 111a, 112a, and 113a in this order propagate through the transmission lines 121, 122, and 123, respectively, and are radiated from the antenna elements 111b, 112b, and 113b that are paired with the antenna elements 111a, 112a, and 113a, respectively. Similarly, following incidence of radio wave 4 on antenna element 113a, the radio wave 4 incident on antenna elements 113b, 112b, and 111b propagates through transmission lines 123, 122, and 121, respectively, and is paired with antenna elements 113b, 112b, and 111b, respectively, before being radiated from antenna elements 113a, 112a, and 111a. Because the electrical lengths of transmission lines 121, 122, and 123 are the same, the time required for radio wave 4 to propagate through transmission lines 121, 122, and 123 is the same. In this case, radio wave 4 incident on antenna elements 111a, 112a, 113a, 113b, 112b, and 111b in this order is radiated from antenna elements 111b, 112b, 113b, 113a, 112a, and 111a in this order after a certain delay time. This allows the radio waves 4 arriving at the retroreflection antenna 2 to be reflected in the opposite direction along the direction of arrival (i.e., towards the transmission source).

[0045] In the retroreflecting antenna 2, notch filters 21, 22, and 23 (stubs S in FIG. 2 ) are provided on the transmission line 121 connecting the pair of antenna elements 111a and 111b, the transmission line 122 connecting the pair of antenna elements 112a and 112b, and the transmission line 123 connecting the pair of antenna elements 113a and 113b. Therefore, when radio waves 4 propagate through the transmission lines 121, 122, and 123, the portions of the radio waves 4 corresponding to the frequencies cut by the notch filters 21, 22, and 23 are attenuated. Therefore, in the radio waves 4 retroreflected from the retroreflecting antenna 2, the reflected power of the portions corresponding to the frequencies cut by the notch filters 21, 22, and 23 is reduced. In this way, the notch filters 21, 22, and 23 can change the state of the radio waves 4. This change in the radio waves 4 corresponds to predetermined information. Therefore, the retroreflection antenna 2 can retroreflect the incoming radio waves 4 with predetermined information (for example, identification information of the retroreflection antenna 2) superimposed on the radio waves 4.

[0046] The notch filters 21, 22, and 23 are passive filters. Therefore, unlike active filters, there is no need to supply power to the notch filters 21, 22, and 23. Therefore, the retroreflection antenna 2 can retroreflect the incoming radio waves 4 with predetermined information superimposed thereon without requiring a power supply.

[0047] Since the retroreflective antenna 2 does not require a power supply, the retroreflective antenna 2 does not require a space for mounting a battery for power supply. This allows the retroreflective antenna 2 to be made smaller (or thinner). As described above, since a battery is not required, the manufacturing cost of the retroreflective antenna 2 is reduced. Furthermore, there is no need for costs for continuing to use the retroreflective antenna 2 (such as the cost of purchasing additional batteries).

[0048] For example, automobiles are equipped with sensors (such as millimeter wave radar sensors) that use radio waves to detect bicycles, people (pedestrians), and the like.

[0049] The retroreflective antenna 2 can retroreflect the incoming radio waves 4 with predetermined information (e.g., identification information of the retroreflective antenna 2) superimposed on the radio waves 4 based on the frequencies cut by the notch filters 21, 22, and 23. Therefore, when a vehicle uses a sensor to detect an object as described above, it is easy to distinguish between reflections from objects with relatively high reflected power (e.g., signs) and reflections from the retroreflective antenna 2. The sensor mounted on the vehicle transmits radio waves 4 modulated by the FMCW method, i.e., radio waves whose frequency is linearly modulated over time. Therefore, when the radio waves 4 from the sensor are retroreflected by the retroreflective antenna 2, the predetermined information based on the frequencies cut by the notch filters 21, 22, and 23 is more reliably superimposed on the radio waves 4. Therefore, the retroreflective antenna 2 is effective when the incoming radio waves 4 are modulated by the FMCW method.

[0050] In the retroreflecting antenna 2, notch filters 21, 22, and 23 are provided on the transmission lines 121, 122, and 123. Therefore, in the radio waves 4 propagating through the transmission lines 121, 122, and 123, the portions corresponding to the frequencies cut by the notch filters 21, 22, and 23 are attenuated. As a result, in the radio waves 4 retroreflected from the retroreflecting antenna 2, the portions of the reflected power corresponding to the frequencies cut by the notch filters 21, 22, and 23 are largely attenuated. As a result, the predetermined information superimposed on the retroreflected radio waves 4 can be easily detected.

[0051] Second Embodiment In the second embodiment, an object detection system using the retroreflector antenna 2 described in the first embodiment will be described. Fig. 4 is a schematic diagram of an object detection system 1 according to the second embodiment.

[0052] In the second embodiment, a case where the retroreflective antenna 2 is attached to (or built into) an object 5 will be described. The object 5 is, for example, a reflector attached to a bicycle, a hat, a bag (including a backpack), a reflective vest worn by a worker at a construction site, or the like.

[0053] The object detection system 1 includes a retroreflecting antenna 2 and a radar device (sensor device) 3. The retroreflecting antenna 2 is the same as the retroreflecting antenna 2 shown in Fig. 2. Therefore, a description of the retroreflecting antenna 2 will be omitted.

[0054] The radar device 3 includes a radar main body 3 a and a control device 3 b. The radar device 3 is mounted on, for example, an automobile. The radar device 3 detects objects around the radar device 3.

[0055] The radar main body 3a transmits radio waves 4 and receives radio waves 4 transmitted from the radar main body 3a and reflected by surrounding objects. In one embodiment, the radar main body 3a has a transmitting antenna that transmits the radio waves 4 and a receiving antenna that receives the radio waves 4. In one embodiment, the transmitting antenna and the receiving antenna may be a common antenna. The radio waves 4 transmitted by the radar main body 3a are, for example, millimeter waves.

[0056] The radar main body 3a transmits radio waves 4 modulated by the FMCW method. In this case, the radar main body 3a transmits radio waves 4 whose frequency is linearly modulated over time, as shown by the solid line in Fig. 5. Hereinafter, the radio waves 4 transmitted by the radar main body 3a will sometimes be referred to as "transmitted waves," and the radio waves received by the radar main body 3a will sometimes be referred to as "received waves."

[0057] The control device 3b controls the radar main body 3a. Specifically, the control device 3b has a function of switching between transmitting and not transmitting radio waves 4 from the radar main body 3a, and a function of controlling the radar main body 3a so that the radar main body 3a transmits radio waves 4 modulated by the FMCW method. The control device 3b detects the object 5 using at least the received waves. The control device 3b is, for example, a computer. The control device 3b may be a device dedicated to the radar device 3, or a computer included in a device (an automobile in the second embodiment) on which the radar device 3 is mounted may be made to function as the control device 3b by executing an object detection program in the computer.

[0058] An example of the detection principle of an object 5 in the object detection system 1 will be described with reference to Figures 5 and 6. Figure 5 is a diagram showing the frequencies of the transmitted wave and the received wave in the radar device 3. The horizontal axis in Figure 5 represents time, and the vertical axis represents frequency. The frequency f represented by the solid line in Figure 5 ｓ (t) indicates the frequency of the transmission wave, and the frequency f ｄ (t) indicates the frequency of the received wave.

[0059] FIG. 6 shows the difference frequency f between the frequency of the transmission wave and the frequency of the reception wave shown in FIG. Ｌ 6 is a diagram showing the difference frequency (t). The horizontal axis in FIG. 5 represents time, and the vertical axis represents the difference frequency. Times t1, t2, etc. in FIG. 6 are the same as times t1, t2, etc. in FIG. 5.

[0060] When the control device 3b receives the received wave, the control device 3b calculates the difference frequency f Ｌ (t) is calculated using the following formula 2: f Ｌ (t) = f ｓ (t)-f ｄ (t)...Formula 2

[0061] The frequency f shown by the dashed line in FIG. ｄ The received wave corresponding to (t) is the radio wave 4 reflected by the retroreflecting antenna 2. Therefore, the received wave is received by the radar device 3 a certain delay time (τ) after the transmitted wave is transmitted from the radar device 3. The radio wave 4 transmitted from the radar main body 3 a is modulated by the FMCW method, and the frequency changes linearly over time. Therefore, the frequency of the radio wave 4 reflected by the retroreflecting antenna 2 also changes over time.

[0062] Since the received wave is the radio wave 4 reflected by the retroreflection antenna 2, as shown in FIG. ｄ At (t), the frequency cut by the stub S (notch filter) is lost. Therefore, as shown in FIG. 5, the received wave is cut off during the period when the part of the transmitted wave (radio wave 4) corresponding to the frequency cut by the stub S (notch filter) returns to the radar main body 3a (between time t2 and time t3 in FIG. 5). Therefore, the difference frequency f ＬThe control device 3b calculates the difference frequency f ｄ By detecting the absence of (t) (or the discontinuity of the received wave), the radar device 3 detects that the received wave is a wave received from the retroreflecting antenna 2. This allows the radar device 3 to selectively detect an object 5 to which the retroreflecting antenna 2 is attached.

[0063] 1 , the first embodiment has been described in terms of a configuration in which one notch filter 21 is provided on the transmission line 121, one notch filter 22 is provided on the transmission line 122, and one notch filter 23 is provided on the transmission line 123. However, a plurality of notch filters may be provided on each of the transmission lines 121, 122, and 123. As the third embodiment, a configuration in which a plurality of notch filters is provided on each of the transmission lines 121, 122, and 123 will be described.

[0064] 7 is a schematic diagram of a retroreflection antenna 2A according to the third embodiment. The retroreflection antenna 2A has an antenna body 10 and a group of notch filters 24, 25, and 26.

[0065] The configuration of the antenna body 10 is the same as that of the first embodiment, so a description of the antenna body 10 will be omitted.

[0066] The notch filter group 24 is provided on the transmission line 121. The notch filter group 24 includes three notch filters 24a, 24b, and 24c. The notch filters 24a, 24b, and 24c are passive notch filters. The notch filters 24a, 24b, and 24c cut off different frequencies. Here, the frequency cut off by the notch filter 24a is referred to as the first frequency, the frequency cut off by the notch filter 24b is referred to as the second frequency, and the frequency cut off by the notch filter 24c is referred to as the third frequency. In this case, the notch filter group 24 cuts off portions of the radio waves 4 propagating through the transmission line 121 that correspond to the first frequency, the second frequency, and the third frequency.

[0067] The notch filter group 25 is provided on the transmission line 122. The notch filter group 25 has three notch filters 25a, 25b, and 25c. The notch filters 25a, 25b, and 25c are passive notch filters. The frequency cut by the notch filter 25a is the first frequency. The frequency cut by the notch filter 25b is the second frequency. The frequency cut by the notch filter 25c is the third frequency. In this case, the notch filter group 25 cuts off portions of the radio waves propagating through the transmission line 122 that correspond to the first frequency, the second frequency, and the third frequency.

[0068] The notch filter group 26 is provided on the transmission line 123. The notch filter group 26 has three notch filters 26a, 26b, and 26c. The notch filters 26a, 26b, and 26c are passive notch filters. The frequency cut by the notch filter 26a is the first frequency. The frequency cut by the notch filter 26b is the second frequency. The frequency cut by the notch filter 26c is the third frequency. In this case, the notch filter group 26 cuts off portions of the radio waves propagating through the transmission line 123 that correspond to the first frequency, the second frequency, and the third frequency.

[0069] When the antenna body 10 and the notch filter group 24, 25, and 26 are mounted on the substrate 13 shown in Figure 3, the notch filters 24a, 24b, 24c, 25a, 25b, 25c, 26a, 26b, and 26c can be formed as stubs S, as in the case of the retroreflective antenna 2 shown in Figure 2.

[0070] In the retroreflection antenna 2A, the radio waves 4 arriving at the retroreflection antenna 2A are retroreflected by the antenna body 10.

[0071] The transmission lines 121, 122, and 123 of the retroreflective antenna 2A are provided with notch filter groups 24, 25, and 26. As described above, the notch filter groups 24, 25, and 26 attenuate the portions of the radio waves 4 propagating through the transmission lines 121, 122, and 123 corresponding to the first, second, and third frequencies. Therefore, the retroreflective antenna 2A can retroreflect the radio waves 4 while superimposing predetermined information defined by the first, second, and third frequencies onto the radio waves arriving at the retroreflective antenna 2A. In this case, the predetermined information is defined based on three frequencies rather than one frequency. Therefore, more detailed information can be superimposed on the radio waves 4 as the predetermined information. As a result, the object to which the retroreflective antenna 2A is attached can be easily identified by the radio waves 4 retroreflected from the retroreflective antenna 2A.

[0072] The retroreflecting antenna 2A can be applied to the object detection system described in the second embodiment. In this case, the retroreflecting antenna 2A is used instead of the retroreflecting antenna 2 shown in FIG.

[0073] When the retroreflection antenna 2A is applied to the object detection system described in the second embodiment, when the radar device 3 receives the received wave, the difference frequency f Ｌ That is, in the radio wave 4 retroreflected from the retroreflection antenna 2A, the parts corresponding to the first frequency, the second frequency, and the third frequency are attenuated, so that the differential frequency f Ｌ In (t), three missing portions occur. Therefore, for example, if the missing portions are considered to be "0" and the rest of the signal is considered to be "1", a binary signal corresponding to the retroreflective antenna 2A can be obtained. This binary signal can be changed by adjusting the multiple frequencies cut by the notch filter groups 24, 25, and 26. Therefore, for example, by adjusting the first frequency, second frequency, and third frequency cut by the notch filter groups 24, 25, and 26 depending on the object to which the retroreflective antenna 2A is attached, it is possible to distinguish and detect the object to which the retroreflective antenna 2A is attached.

[0074] By including the notch filters 24, 25, and 26, the retroreflective antenna 2A can retroreflect radio waves 4 with predetermined information related to the retroreflective antenna 2A (or related to the object to which the retroreflective antenna 2A is attached) superimposed on the radio waves 4 based on three different frequencies, as described above. Therefore, the retroreflective antenna 2A is, for example, a reflector on which an information code indicating the predetermined information is written. When such a retroreflective antenna 2A is applied to the object detection system described in the second embodiment, the radar device 3 corresponds to a device that reads the information code. In this case, for example, the retroreflective antenna 2A and the object detection system using it can be used to manage objects.

[0075] (Fourth embodiment) In the first embodiment, a configuration in which a plurality of antenna elements of a retroreflecting antenna are arranged linearly (one-dimensionally) has been described. The plurality of antenna elements may be arranged two-dimensionally. In the fourth embodiment, a configuration in which a plurality of antenna elements are arranged two-dimensionally will be described.

[0076] Fig. 9 is a diagram showing a retroreflector antenna 2B according to a fourth embodiment as viewed from the front side. Fig. 10 is a diagram showing the retroreflector antenna 2B shown in Fig. 9 as viewed from the back side. Fig. 11 is a side view of a substrate 33 of the retroreflector antenna 2B shown in Fig. 9.

[0077] 9 and 10, the retroreflection antenna 2B has an antenna body 30 and nine stubs (notch filters) S. The antenna body 30 and the stubs S are mounted on a substrate 33.

[0078] First, the substrate 33 will be described. As shown in Fig. 11, the substrate 33 has a dielectric layer 331, a dielectric layer 332, and a conductor layer 333. The dielectric layer 332, the conductor layer 333, and the dielectric layer 331 are stacked in this order. Examples of materials for the dielectric layer 331 and the dielectric layer 332 are the same as those for the dielectric layer 131 shown in Fig. 3. Examples of materials for the conductor layer 333 are the same as those for the ground conductor layer 132 shown in Fig. 3.

[0079] Next, the antenna body 30 and the stub S will be described.

[0080] As shown in Figures 9 and 10, the antenna body 30 has nine pairs of antenna elements, namely, a pair of antenna elements 311a, 311b, a pair of antenna elements 312a, 312b, a pair of antenna elements 313a, 313b, a pair of antenna elements 314a, 314b, a pair of antenna elements 315a, 315b, a pair of antenna elements 316a, 316b, a pair of antenna elements 317a, 317b, a pair of antenna elements 318a, 318b, and a pair of antenna elements 319a, 319b.

[0081] Therefore, the antenna body 30 has 18 antenna elements 311a, 311b, 312a, 312b, 313a, 313b, 314a, 314b, 315a, 315b, 316a, 316b, 317a, 317b, 318a, 318b, 319a, and 319b.

[0082] Hereinafter, the nine pairs of antenna elements may be referred to as "pairs of antenna elements 311a, 311b, etc.", and the eighteen antenna elements may be referred to as "antenna elements 311a, 311b, etc.".

[0083] The positional relationship between the pair of antenna elements 311a, 311b is the same as the positional relationship between the pair of antenna elements 111a, 111b in the first embodiment. That is, the pair of antenna elements 311a, 311b are arranged point-symmetrically with respect to the reference point C of the antenna body 30. This also applies to the pair of antenna elements 312a, 312b, the pair of antenna elements 313a, 313b, the pair of antenna elements 314a, 314b, the pair of antenna elements 315a, 315b, the pair of antenna elements 316a, 316b, the pair of antenna elements 317a, 317b, the pair of antenna elements 318a, 318b, and the pair of antenna elements 319a, 319b. Therefore, the positional relationship between the pair of antenna elements 311a, 311b, etc. satisfies the above condition I.

[0084] The nine antenna elements 311a, 311b, etc. are formed on the surface 331a of the dielectric layer 331. The shapes of the antenna elements 311a, 311b, etc. and the materials of the antenna elements 311a, 311b, etc. are the same as those of the antenna element 111a in the first embodiment. The nine antenna elements 311a, 311b, etc. are formed by, for example, printing technology.

[0085] As shown in Fig. 9, the nine antenna elements 311a, 311b, etc. are arranged in a hexagonal shape with the reference point C at the center. In the configuration shown in Fig. 9, the antenna elements 311a, 312a, 317b, 318b, 314a, 315a, 311b, 312b, 317a, 318a, 314b, and 315b are arranged in a hexagonal shape. The antenna elements 311a, 312a, 317b, 318b, 314a, 315a, 311b, 312b, 317a, 318a, 314b, and 315b are arranged in this order clockwise with respect to the reference point C. An imaginary hexagon formed by antenna elements 311a, 312a, 317b, 318b, 314a, 315a, 311b, 312b, 317a, 318a, 314b, and 315b is referred to as a first hexagon.

[0086] Antenna elements 311a, 317b, 314a, 311b, 317a, and 314b are arranged at the corners of a first hexagon. Antenna element 312a is arranged at the center between antenna elements 311a and 317b. Antenna element 318b is arranged at the center between antenna elements 317b and 314a. Antenna element 315a is arranged at the center between antenna elements 314a and 311b. Antenna element 312b is arranged at the center between antenna elements 311b and 317a. Antenna element 318a is arranged at the center between antenna elements 317a and 314b. Antenna element 315b is arranged at the center between antenna elements 314b and 311a.

[0087] Antenna elements 313a, 319b, 316a, 313b, 319a, and 316b are arranged in a hexagonal shape inside the first hexagon. The antenna elements 313a, 319b, 316a, 313b, 319a, and 316b are arranged in this order clockwise with respect to reference point C. The imaginary hexagon formed by the antenna elements 313a, 319b, 316a, 313b, 319a, and 316b is referred to as a second hexagon. The antenna elements 313a, 319b, 316a, 313b, 319a, and 316b are arranged at the corners of the second hexagon.

[0088] As described above, in the antenna main body 30, 18 antenna elements 311a, 311b, etc. are arranged on imaginary first and second hexagons of different sizes centered on the reference point C.

[0089] In a configuration in which 18 antenna elements 311 a, 311 b, etc. are arranged in a hexagonal shape, nine pairs of antenna elements 311 a, 311 b, etc. can be divided into three antenna element groups. In the fourth embodiment, the antenna body 30 has a first antenna element group G1, a second antenna element group G2, and a third antenna element group G3.

[0090] The first antenna element group G1 includes a pair of antenna elements 311a and 311b, a pair of antenna elements 312a and 312b, and a pair of antenna elements 313a and 313b. The second antenna element group G2 includes a pair of antenna elements 314a and 314b, a pair of antenna elements 315a and 315b, and a pair of antenna elements 316a and 316b. The third antenna element group G3 includes a pair of antenna elements 317a and 317b, a pair of antenna elements 318a and 318b, and a pair of antenna elements 319a and 319b.

[0091] In Fig. 9, the antenna elements belonging to the second antenna element group G2 are all hatched with the same pattern. In Fig. 9, the antenna elements belonging to the third antenna element group G3 are all hatched with the same pattern. The hatching applied to the antenna elements belonging to the third antenna element group G3 is different from the hatching applied to the antenna elements belonging to the second antenna element group G2. The antenna elements belonging to the first antenna element group G1 are not hatched. In Fig. 9, as described above, the antenna elements belonging to the first antenna element group G1, the second antenna element group G2, and the third antenna element group G3 are clearly indicated by the presence or absence of hatching and the difference in hatching.

[0092] As described above, each of the first antenna element group G1, the second antenna element group G2, and the third antenna element group G3 has three pairs of antenna elements. If one of the three pairs of antenna elements included in each of the first antenna element group G1, the second antenna element group G2, and the third antenna element group G3 is referred to as the i-th pair of antenna elements (i is any one of 1 to 3), the three pairs of antenna elements included in the first antenna element group G1, the second antenna element group G2, and the third antenna element group G3 satisfy the following condition II: [Condition II] The i-th pair of antenna elements in the second antenna element group G2 are located at a position obtained by rotating the i-th pair of antenna elements in the first antenna element group G1 by 120 degrees around a predetermined direction of the reference point C, and the i-th pair of antenna elements in the third antenna element group G3 are located at a position obtained by rotating the i-th pair of antenna elements in the first antenna element group G1 by 240 degrees around the predetermined direction of the reference point C.

[0093] 9, the ith pair of antenna elements in each of the first antenna element group G1, the second antenna element group G2, and the third antenna element group G3 corresponds to the pairs of antenna elements 311a, 311b, etc., in the first antenna element group G1, the second antenna element group G2, and the third antenna element group G3, as shown in Table 1. G1, G2, and G3 in Table 1 correspond to the first antenna element group G1, the second antenna element group G2, and the third antenna element group G3.

[0094]

[0095] Condition II above will be specifically explained based on the correspondence shown in Table 1. Here, the arrangement relationship of the pair of antenna elements 311a, 311b, the pair of antenna elements 314a, 314b, and the pair of antenna elements 317a, 317b, which are the first pair of antenna elements of the first antenna element group G1, the second antenna element group G2, and the third antenna element group G3, will be explained.

[0096] The antenna element 314a of the pair of antenna elements 314a, 314b (first pair of antenna elements) belonging to the second antenna element group G2 is disposed at a position rotated by an angle θ1 clockwise (around a predetermined direction) with respect to the reference point C from the antenna element 311a of the pair of antenna elements 311a, 311b (first pair of antenna elements) belonging to the first antenna element group G1. The angle θ1 is 120 degrees.

[0097] The pair of antenna elements 311a and 311b are point-symmetric with respect to the reference point C, and the pair of antenna elements 314a and 314b are point-symmetric with respect to the reference point C. Therefore, the antenna element 314b is also disposed at a position rotated clockwise by an angle θ1 from the antenna element 311b with respect to the reference point C.

[0098] Therefore, the pair of antenna elements 314a and 314b are arranged at positions obtained by rotating the pair of antenna elements 311a and 311b clockwise (around the predetermined direction) with respect to the reference point C by an angle θ1.

[0099] The antenna element 317a of the pair of antenna elements 317a, 317b (first pair of antenna elements) belonging to the third antenna element group G3 is disposed at a position obtained by rotating the antenna element 311a of the pair of antenna elements 311a, 311b (first pair of antenna elements) belonging to the first antenna element group G1 by an angle θ2 clockwise (around a predetermined direction) with respect to the reference point C. The angle θ2 is 240 degrees.

[0100] The pair of antenna elements 311a and 311b are point-symmetric with respect to the reference point C, and the pair of antenna elements 317a and 317b are point-symmetric with respect to the reference point C. Therefore, the antenna element 317b is also disposed at a position rotated clockwise by an angle θ2 from the antenna element 311b with respect to the reference point C.

[0101] Therefore, the pair of antenna elements 317a and 317b are arranged at positions obtained by rotating the pair of antenna elements 311a and 311b clockwise with respect to the reference point C by an angle θ2.

[0102] The second pair of antenna elements and the third pair of antenna elements of the first antenna element group G1, the second antenna element group G2 and the third antenna element group G3 are similar to the first pair of antenna elements.

[0103] 9 and 10, the antenna body 30 has transmission lines 321, 322, 323, 324, 325, 326, 327, 328, and 329 for connecting the antenna elements 311a, 311b, etc. The transmission lines 321, 322, 323, 324, 325, 326, 327, 328, and 329 have the same electrical length. In the retroreflection antenna 2C, the antenna elements 311a, 311b, etc. are connected using the back surface of the substrate 33 (the surface opposite to the surface on which the antenna elements 311a, 311b, etc. are arranged).

[0104] The transmission line 321 connects the antenna element 311a and the antenna element 311b (a pair of antenna elements). The transmission line 321 has a portion formed on the back surface 332a of the dielectric layer 332 and portions formed on the front surface 331a of the dielectric layer 331 and connected to the pair of antenna elements 311a and 311b. The material of the portion of the transmission line 321 formed on the front surface 331a and the portion formed on the back surface 332a is the same as that of the transmission line 121 in the first embodiment.

[0105] The portion of the transmission line 321 connected to the antenna element 311a and the portion formed on the back surface 332a are connected via a via V1a. The portion of the transmission line 321 connected to the antenna element 311b and the portion formed on the back surface 332a are connected via a via V1b. The vias V1a and V1b are formed by filling through-holes that penetrate the substrate 33 in the thickness direction with a conductive material. In the fourth embodiment, the conductive material of the vias V1a and V1b is the same as the material of the portion of the transmission line 321 formed on the back surface 332a. The vias V1a and V1b may also be part of the transmission line 321.

[0106] The transmission line 322 connects the antenna element 312a and the antenna element 312b (a pair of antenna elements). The explanation of the transmission line 322 is the same as that of the transmission line 321, except that the transmission line 321, the antenna element 311a, the antenna element 311b, the via V1a, and the via V1b are replaced with the transmission line 322, the antenna element 312a, the antenna element 312b, the via V2a, and the via V2b.

[0107] The transmission line 323 connects the antenna element 313a and the antenna element 313b (a pair of antenna elements). The explanation of the transmission line 323 is the same as that of the transmission line 321, except that the transmission line 321, the antenna element 311a, the antenna element 311b, the via V1a, and the via V1b are replaced with the transmission line 323, the antenna element 313a, the antenna element 313b, the via V3a, and the via V3b.

[0108] The transmission line 324 connects the antenna element 314a and the antenna element 314b (a pair of antenna elements). The explanation of the transmission line 324 is the same as that of the transmission line 321, except that the transmission line 321, the antenna element 311a, the antenna element 311b, the via V1a, and the via V1b are replaced with the transmission line 324, the antenna element 314a, the antenna element 314b, the via V4a, and the via V4b.

[0109] The transmission line 325 connects the antenna element 315a and the antenna element 315b (a pair of antenna elements). The explanation of the transmission line 325 is the same as that of the transmission line 321, except that the transmission line 321, the antenna element 311a, the antenna element 311b, the via V1a, and the via V1b are replaced with the transmission line 325, the antenna element 315a, the antenna element 315b, the via V5a, and the via V5b.

[0110] The transmission line 326 connects the antenna element 316a and the antenna element 316b (a pair of antenna elements). The explanation of the transmission line 326 is the same as that of the transmission line 321, except that the transmission line 321, the antenna element 311a, the antenna element 311b, the via V1a, and the via V1b are replaced with the transmission line 326, the antenna element 316a, the antenna element 316b, the via V6a, and the via V6b.

[0111] The transmission line 327 connects the antenna element 317a and the antenna element 317b (a pair of antenna elements). The explanation of the transmission line 327 is the same as that of the transmission line 321, except that the transmission line 321, the antenna element 311a, the antenna element 311b, the via V1a, and the via V1b are replaced with the transmission line 327, the antenna element 317a, the antenna element 317b, the via V7a, and the via V7b.

[0112] The transmission line 328 connects the antenna element 318a and the antenna element 318b (a pair of antenna elements). The explanation of the transmission line 328 is the same as that of the transmission line 321, except that the transmission line 321, the antenna element 311a, the antenna element 311b, the via V1a, and the via V1b are replaced with the transmission line 328, the antenna element 318a, the antenna element 318b, the via V8a, and the via V8b.

[0113] The transmission line 329 connects the antenna element 319a and the antenna element 319b (a pair of antenna elements). The explanation of the transmission line 329 is the same as that of the transmission line 321, except that the transmission line 321, the antenna element 311a, the antenna element 311b, the via V1a, and the via V1b are replaced with the transmission line 329, the antenna element 319a, the antenna element 319b, the via V9a, and the via V9b.

[0114] The transmission lines 321, 322, 323, 324, 325, 326, 327, 328, and 329 (hereinafter referred to as "transmission lines 321, 322, etc.") have the same configuration on the surface 331a. The vias V1a, V1b, V2a, V2b, V3a, V3b, V4a, V4b, V5a, V5b, V6a, V6b, V7a, V7b, V8a, V8b, V9a, and V9b also have the same configuration.

[0115] Therefore, the electrical lengths of the transmission lines 321, 322, etc. are adjusted by the paths of the portions formed on the back surface 332 a of the transmission lines 321, 322, etc. In other words, the portions formed on the back surface 332 a of the transmission lines 321, 322, etc. are formed so that the electrical lengths of the transmission lines 321, 322, etc. are the same.

[0116] In the fourth embodiment, a pair of antenna elements 311a, 311b, etc. are arranged so as to satisfy condition II.

[0117] In other words, if the three pairs of antenna elements each having in the first antenna element group G1, the second antenna element group G2 and the third antenna element group G3 are referred to as the ith pair of antenna elements, the ith pair of antenna elements in the second antenna element group G2 is positioned at a position rotated 120 degrees around a predetermined direction of the reference point C from the ith pair of antenna elements in the first antenna element group G1, and the ith pair of antenna elements in the third antenna element group G3 is positioned at a position rotated 240 degrees around the predetermined direction of the reference point C from the ith pair of antenna elements in the first antenna element group G1.

[0118] In this case, the transmission line 321 etc. can be formed so as to satisfy condition III. [Condition III] The path of the transmission line connecting the ith pair of antenna elements in the second antenna element group G2 is a path obtained by rotating the path of the transmission line connecting the ith pair of antenna elements in the first antenna element group G1 by 120 degrees around the reference point C.

[0119] The path of the transmission line connecting the i-th pair of antenna elements in the third antenna element group G3 is a path rotated 240 degrees around the reference point C from the path of the transmission line connecting the i-th pair of antenna elements in the first antenna element group G1.

[0120] 10 (portions of the transmission line 321, etc. on the rear surface 322a) are formed to satisfy condition III. This point will be explained based on the correspondence between pairs of antenna elements 311a, 311b, etc. belonging to the first antenna element group G1, the second antenna element group G2, and the third antenna element group G3, and the first pair of antenna elements, the second pair of antenna elements, and the third pair of antenna elements, as shown in Table 1.

[0121] Based on the correspondence shown in Table 1, the first pairs of antenna elements in the first antenna element group G1, the second antenna element group G2 and the third antenna element group G3 are a pair of antenna elements 311a, 311b, a pair of antenna elements 314a, 314b and a pair of antenna elements 317a, 317b.

[0122] 10, the path of the transmission line 324 connecting the pair of antenna elements 314a and 314b is a path obtained by rotating the transmission line 321 connecting the pair of antenna elements 311a and 311b by 120 degrees around the reference point C. The path of the transmission line 327 connecting the pair of antenna elements 317a and 317b is a path obtained by rotating the transmission line 321 by 240 degrees around the reference point C. Since FIG. 10 is a diagram of the substrate 33 as viewed from the back side, the rotation direction of the transmission line 321 is counterclockwise.

[0123] Based on the correspondence relationships in Table 1, the second pairs of antenna elements of the first antenna element group G1, the second antenna element group G2 and the third antenna element group G3 are a pair of antenna elements 312a, 312b, a pair of antenna elements 315a, 315b and a pair of antenna elements 318a, 318b.

[0124] 10, the path of the transmission line 325 connecting the pair of antenna elements 315a and 315b is a path obtained by rotating the transmission line 322 connecting the pair of antenna elements 312a and 312b by 120 degrees around the reference point C. The path of the transmission line 328 connecting the pair of antenna elements 318a and 318b is a path obtained by rotating the transmission line 322 by 240 degrees around the reference point C. Since FIG. 10 is a diagram of the substrate 33 as viewed from the back side, the rotation direction of the transmission line 322 is counterclockwise.

[0125] Based on the correspondence relationships in Table 1, the third pairs of antenna elements of the first antenna element group G1, the second antenna element group G2 and the third antenna element group G3 are a pair of antenna elements 313a, 313b, a pair of antenna elements 316a, 316b and a pair of antenna elements 319a, 319b.

[0126] 10, the path of the transmission line 326 connecting the pair of antenna elements 316a and 316b is a path obtained by rotating the transmission line 323 connecting the pair of antenna elements 313a and 313b by 120 degrees around the reference point C. The path of the transmission line 329 connecting the pair of antenna elements 319a and 319b is a path obtained by rotating the transmission line 323 by 240 degrees around the reference point C. Since FIG. 10 is a diagram of the substrate 33 as viewed from the back side, the rotation direction of the transmission line 323 is counterclockwise.

[0127] A stub (notch filter) S is provided on each of the transmission lines 321, 322, 323, 324, 325, 326, 327, 328, and 329. As shown in Fig. 10, in the fourth embodiment, the stub S is provided on a portion of the rear surface 332a of the transmission line 321, etc. The arrangement of the stub S and the conditions that the stub S must satisfy are the same as those of the stub S described in the first embodiment, and therefore a description thereof will be omitted.

[0128] The transmission lines 321, 322, etc. and the stub S may also be formed by, for example, printing techniques.

[0129] As described above, the pair of antenna elements 311a, 311b, etc. satisfy condition I, and the electrical lengths of the transmission lines 321, etc. connecting the pair of antenna elements 311a, 311b, etc. are the same. Therefore, the antenna body 30 of the retroreflective antenna 2B also functions as a Van Atta array antenna. Therefore, the retroreflective antenna 2B can also retroreflect radio waves arriving at the retroreflective antenna 2B. Furthermore, the transmission lines 321, etc. are provided with stubs S, which are passive notch filters. Therefore, as in the first embodiment, it is possible to superimpose information corresponding to the retroreflective antenna 2B on the radio waves retroreflected by the retroreflective antenna 2B. Therefore, the retroreflective antenna 2B has the same effects as the retroreflective antenna 2.

[0130] In the fourth embodiment, the rear surface side of the substrate 33 is used for connecting the pair of antenna elements 311a, 311b, etc. by the transmission lines 321, 322, etc. Therefore, even if the pair of antenna elements 311a, 311b, etc. are arranged two-dimensionally, the degree of freedom in designing the paths of the transmission lines 321, etc. is improved.

[0131] In the retroreflector antenna 2B, the positional relationship between the pair of antenna elements 311a, 311b, etc., satisfies condition II. Therefore, the transmission lines 321, 322, etc. can also be formed to satisfy condition III. In this case, the retroreflector antenna 2B is easy to design. This point will be explained.

[0132] When designing the retroreflection antenna 2B, the antenna elements 311a, 311b, 312a, 312b, 313a, and 313b belonging to the first antenna element group G1 and the transmission lines 321, 322, and 323 connecting them are designed. Next, the pair of antenna elements 311a and 311b, the pair of antenna elements 312a and 312b, and the pair of antenna elements 313a and 313b, as well as the transmission lines 321, 322, and 323, are rotated 120 degrees and 240 degrees around the reference point C so as to satisfy the above conditions II and III. This also determines the configuration and arrangement of the antenna elements 314a, 314b, 315a, 315b, 316a, 316b, 317a, 317b, 318a, 318b, 319a, 319b and transmission lines 324, 325, 326, 327, 328, 329 belonging to the second antenna element group G2 and the third antenna element group G3.

[0133] Therefore, when designing the retroreflective antenna 2B, it is possible to practically design the retroreflective antenna 2B by designing one-third of the retroreflective antenna 2B. In this way, since the retroreflective antenna 2B is easy to design, it is also easy to manufacture the retroreflective antenna 2B.

[0134] The retroreflection antenna 2B can be applied to the object detection system 1 described in the second embodiment.

[0135] Although various embodiments of the present disclosure have been described above, the present disclosure is not limited to the exemplified embodiments and various modifications are possible.

[0136] The notch filter is not limited to a stub, as long as it is a passive notch filter and is configured to cut off a portion of the frequency band of the radio waves arriving at the retroreflector antenna (or the frequency modulation width if the radio waves are frequency modulated). For example, the notch filter may be configured using passive elements such as a coil, a capacitor, and a resistor.

[0137] The radio waves reflected by the retroreflecting antenna and the radio waves transmitted by the radar device described in the second embodiment are not limited to millimeter waves, and may be, for example, microwaves or submillimeter waves.

[0138] The number of pairs of antenna elements, transmission lines, and notch filters that the retroreflector antenna has is not limited to the exemplified numbers. In the fourth embodiment, the number of pairs of antenna elements that each of the first antenna element group, the second antenna element group, and the third antenna element group has is not limited to three. That is, when the first antenna element group, the second antenna element group, and the third antenna element group each have N pairs of antenna elements, N may be 1 or 2, or may be 4 or more. When the first antenna element group, the second antenna element group, and the third antenna element group each have N pairs of antenna elements, i in the i-th pair of antenna elements is an integer between 1 and N.

[0139] In the above embodiment, a passive notch filter is provided on all of the multiple transmission lines of the retroreflective antenna. However, it is sufficient if a passive notch filter is provided on at least one of the multiple transmission lines of the retroreflective antenna. For example, in a configuration in which a passive notch filter is provided on one of the multiple transmission lines, a portion of the radio waves propagating through the transmission line on which the passive notch filter is provided can be attenuated. As a result, information based on the passive notch filter can be superimposed on the radio waves retroreflected from the retroreflective antenna. As described in the above embodiment, in a configuration in which passive notch filters are provided on all of the multiple transmission lines of the retroreflective antenna, the reflected power of the portion of the radio waves cut by the passive notch filter can be more significantly attenuated. Therefore, the retroreflective antenna or an object to which the retroreflective antenna is attached can be detected with high accuracy.

[0140] In the fourth embodiment, the configuration in which the multiple antenna elements are arranged to form two hexagons of different sizes has been described. However, depending on the number and sizes of the antenna elements included in the retroreflective antenna, the multiple antenna elements may be arranged to form one hexagon or three or more hexagons of different sizes.

[0141] When multiple antenna elements are arranged two-dimensionally, the arrangement of the multiple antenna elements is not limited to the hexagonal arrangement as described in the fourth embodiment. For example, the multiple antenna elements may be arranged in a quadrangular shape.

[0142] Figure 12 is a schematic diagram of another embodiment of a retroreflective antenna. The retroreflective antenna 2C shown in Figure 12 differs from the retroreflective antennas shown in Figures 9 and 10 in that it does not have a stub S. Other than this difference, the configuration of the retroreflective antenna 2C is the same as that of the retroreflective antenna 2B. Therefore, the retroreflective antenna 2C also has a first antenna element group G1, a second antenna element group G2, and a third antenna element group G3. Furthermore, the arrangement of the multiple antenna elements belonging to the first antenna element group G1, the second antenna element group G2, and the third antenna element group G3 satisfies condition II. Therefore, the design of the retroreflective antenna 2C is as easy as that of the retroreflective antenna 2B. As a result, the retroreflective antenna is easy to manufacture.

[0143] Thus, from the viewpoint of ease of design, a retroreflector antenna includes an antenna body that retroreflects incoming radio waves, the antenna body having a plurality of pairs of antenna elements and a plurality of transmission lines provided corresponding to the plurality of pairs of antenna elements, each pair of antenna elements among the plurality of pairs of antenna elements being arranged point-symmetrically with respect to a reference point on the antenna body, each of the plurality of transmission lines connecting a corresponding pair of antenna elements among the plurality of pairs of antenna elements, the plurality of transmission lines having the same electrical length, the at least one passive notch filter being provided on at least one transmission line among the plurality of transmission lines, and the plurality of pairs of antenna elements being virtually divided into a first antenna element group, a second antenna element group, and a third antenna element group. The first antenna element group, the second antenna element group, and the third antenna element group each include N pairs of antenna elements (N is an integer greater than or equal to 1), and when one of the N pairs of antenna elements in each of the first antenna element group, the second antenna element group, and the third antenna element group is the i-th pair of antenna elements (i is greater than or equal to 1 and less than or equal to N), the i-th pair of antenna elements in the second antenna element group are positioned at a position rotated 120 degrees around a predetermined direction of the reference point from the i-th pair of antenna elements in the first antenna element group, and the i-th pair of antenna elements in the third antenna element group are positioned at a position rotated 240 degrees around the predetermined direction of the reference point from the i-th pair of antenna elements in the first antenna element group.

[0144] The various embodiments and modifications described above may be combined as appropriate without departing from the spirit of the present disclosure.

[0145] REFERENCE SIGNS LIST 1...Object detection system 2, 2A, 2B, 2C...Retroreflector antenna 3...Radar device 3a...Radar main body 3b...Control device 4...Radio waves 5...Object 10...Antenna main body 111a...Antenna element 111b...Antenna element 112a...Antenna element 112b...Antenna element 113a...Antenna element 113b...Antenna element 121...Transmission line 122...Transmission line 123...Transmission line 13...Substrate 131...Dielectric layer 132...Ground conductor layer 131b...Back surface 131a...Front surface 21...Notch filter (passive notch filter) 22...Notch filter (passive notch filter) 23...Notch filter (passive notch filter) 24...Notch filter group 24a...Notch filter (passive notch filter) 24b...Notch filter (passive notch filter) 24c...Notch filter (passive notch filter) 25...Notch filter group 25a...Notch filter (passive notch filter) 25b...Notch filter (passive notch filter) 25c...Notch filter (passive notch filter) 26...Notch filter group 26a...Notch filter (passive notch filter) 26b...Notch filter (passive notch filter) 26c...Notch filter (passive notch filter) 30...Antenna body 311a...Antenna element 311b...Antenna element 312a...Antenna element 312b...Antenna element 313a...Antenna element 313b...Antenna element 314a...Antenna element 314b...Antenna element 315a...Antenna element 315b...Antenna element 316a...Antenna element 316b...Antenna element 317a...Antenna element 317b...Antenna element 318a...antenna element 318b...antenna element 319a...antenna element 319b...antenna element 321...transmission line 322...transmission line 323...transmission line 324...transmission line 325...transmission line 326...transmission line 327...transmission line 328...transmission line 329...transmission line 33...substrate 331...dielectric layer 331a...surface 332...dielectric layer 322a...back surface 333...conductor layer C...reference point G1...first antenna element group G2...second antenna element group G3...third antenna element group L...lengthS...Stub (passive notch filter) V1a...Via V1b...Via V2a...Via V2b...Via V3a...Via V3b...Via V4a...Via V4b...Via V5a...Via V5b...Via V6a...Via V6b...Via V7a...Via V7b...Via V8a...Via V8b...Via V9a...Via V9b...Via θ1...Angle θ2...Angle

Claims

1. A retroreflector antenna comprising: an antenna body that retroreflects incoming radio waves; and a passive notch filter, wherein the antenna body has a plurality of pairs of antenna elements; and a plurality of transmission lines provided corresponding to the plurality of pairs of antenna elements, wherein each pair of antenna elements among the plurality of pairs of antenna elements is arranged point-symmetrically with respect to a reference point on the antenna body, wherein each of the plurality of transmission lines connects a corresponding pair of antenna elements among the plurality of pairs of antenna elements, wherein the plurality of transmission lines have the same electrical length, and wherein the passive notch filter is provided on at least one of the plurality of transmission lines.

2. The retroreflective antenna according to claim 1, wherein the plurality of pairs of antenna elements include a first antenna element group, a second antenna element group, and a third antenna element group, each of the first antenna element group, the second antenna element group, and the third antenna element group includes N pairs of antenna elements (N is an integer equal to or greater than 1), and when one of the N pairs of antenna elements in each of the first antenna element group, the second antenna element group, and the third antenna element group is the i-th pair of antenna elements (i is equal to or greater than 1 and equal to or less than N), the i-th pair of antenna elements in the second antenna element group is located at a position obtained by rotating the i-th pair of antenna elements in the first antenna element group by 120 degrees around the specified direction of the reference point, and the i-th pair of antenna elements in the third antenna element group is located at a position obtained by rotating the i-th pair of antenna elements in the first antenna element group by 240 degrees around the specified direction of the reference point.

3. A retroreflective antenna as described in claim 1 or claim 2, comprising a plurality of the passive notch filters, the number of the passive notch filters being the same as the number of the transmission lines, the frequencies cut by the passive notch filters being the same, and each of the passive notch filters comprised by the passive notch filters being provided on each of the transmission lines comprised by the plurality of transmission lines.

4. A retroreflective antenna as described in claim 1 or claim 2, comprising a plurality of notch filter groups, each of which has a plurality of the passive notch filters that cut off different frequencies, the number of the plurality of notch filter groups is the same as the number of the plurality of transmission lines, and each of the notch filter groups in the plurality of notch filter groups is provided on each of the plurality of transmission lines.

5. An object detection system comprising: a retroreflective antenna attached to an object, the retroreflective antenna being the antenna described in claim 1 or claim 2; and a radar device that transmits radio waves modulated by the FMCW method and receives the radio waves retroreflected by the retroreflective antenna, wherein the frequency cut by the passive notch filter of the retroreflective antenna is a frequency within the frequency band of the radio waves.

6. An object detection system as described in claim 5, comprising a plurality of notch filter groups, each of which has a plurality of the passive notch filters that cut off different frequencies, the number of the plurality of notch filter groups is the same as the number of the plurality of transmission lines, and each of the notch filter groups in the plurality of notch filter groups is provided on each of the plurality of transmission lines.