Surface wave antenna and wireless power transmission system

Abstract

This surface wave antenna for performing power transmission or power reception using a surface wave includes: a resonance part composed of a radiation electrode and a conductor for supporting the radiation electrode; a ground electrode; a short stub provided on a back surface side of the surface wave antenna; and a power supply point connected to the surface wave antenna in parallel with the short stub.

Description

Surface wave antenna and wireless power transmission system

[0001] The present invention relates to wireless power transmission technology.

[0002] In recent years, various wireless power transmission technologies for wirelessly transmitting power have been proposed. As one of the wireless power transmission technologies, Non-Patent Document 1 discloses an antenna that transmits power wirelessly by propagating surface waves (Zenneck waves) of an electromagnetic field along a metal surface.

[0003] Experimental Realization of Zenneck Type Wave-based Non-Radiative, Non-Coupled Wireless Power Transmission (Scientific Reports 2020 Jan 22)

[0004] Regarding the wireless power transmission method using surface waves of an electromagnetic field disclosed in Non-Patent Document 1, the surface waves of an electromagnetic field propagate so as to be confined near the surface of a metal and do not diffuse into space, and therefore have higher transmission efficiency than wireless power transmission methods using radio waves that propagate while spreading through space.

[0005] However, because the antenna's radiation directivity is isotropic and transmits surface waves in all directions along the metal surface, power is transmitted in directions other than the desired direction of the power receiving device, reducing transmission efficiency. Furthermore, surface waves radiated in unintended directions are reflected by the edges of the metal, interfering with the desired wave and creating null points on the metal where power reception efficiency is extremely low.

[0006] The present invention has been made in view of the above points, and has an object to provide a technique for improving transmission efficiency in a technique for transmitting power using surface waves.

[0007] According to the disclosed technology, there is provided a surface wave antenna that transmits or receives power using surface waves, comprising: a resonator portion configured by a radiation electrode and a conductor supporting the radiation electrode; a ground electrode; a short stub provided on the back side of the surface wave antenna; and a feed point connected to the surface wave antenna in parallel with the short stub.

[0008] According to the disclosed technology, it is possible to improve the transmission efficiency in a technology that transmits power using surface waves.

[0009] 1 is a diagram for explaining a problem; FIG. 2 is a cross-sectional view of the surface wave antenna 100 of the first embodiment; FIG. 3 is a perspective view of the surface wave antenna 100 of the first embodiment; FIG. 4 is a diagram for explaining the operation of the surface wave antenna 100; FIG. 5 is a diagram for explaining the operation of the surface wave antenna 100; FIG. 6 is a configuration diagram of a wireless power transmission system; FIG. 7 is a cross-sectional view of the surface wave antenna 100 of the second embodiment; FIG. 8 is a perspective view of the surface wave antenna 100 of the second embodiment; FIG. 9 is a cross-sectional view of the surface wave antenna 100 of the third embodiment; FIG. 10 is a perspective view of the surface wave antenna 100 of the third embodiment.

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The embodiment described below is merely an example, and the embodiment to which the present invention is applied is not limited to the following embodiment.

[0011] (About the Issues) A ​​wireless power transmission method using an electric field resonance antenna is known. While this wireless power transmission method can achieve high transmission efficiency over short distances, the generated electric field rapidly attenuates in the air, making it difficult to transmit power over long distances. Therefore, by propagating electromagnetic field waves along the surface of a conductor (metal), the energy is confined near the metal surface rather than diffusing into the air, making it possible to transmit power over longer distances. This wave is called a surface wave (Zenneck wave).

[0012] However, conventional antennas that generate surface waves have no directionality, so the surface waves propagate in all directions along the metal plate. This is shown in Figure 1. Figure 1 shows that the surface waves transmitted from a power transmitting antenna 1 propagate in all directions along the metal plate 2.

[0013] (Outline of the embodiment) In order to efficiently transmit power from a power transmitting antenna to a power receiving antenna, it is desirable to transmit surface waves only in the direction of the power receiving antenna without propagating power in unnecessary directions.

[0014] Therefore, in this embodiment, a directional surface wave antenna (surface wave antenna 100) that transmits power only in the front direction of the antenna is used. Examples 1 to 3 will be described below as specific configuration examples of the surface wave antenna 100.

[0015] (Surface wave antenna 100 according to the first embodiment) Fig. 2 shows a cross-sectional view of the surface wave antenna 100 according to the first embodiment as viewed from the side. Fig. 3 shows a perspective view of the surface wave antenna 100 according to the first embodiment.

[0016] 2 and 3, the surface wave antenna 100 includes a radiation electrode 50, a ground electrode 60, and a short stub 30, with a dielectric 20 provided on the short stub 30. A feed point 40 is provided to connect the ground electrode 60 to a metal wire 90 extending from the radiation electrode 50 toward the short stub 30. The feed point 40 is connected in parallel with the short stub 30 to the radiation electrode 50 and the ground electrode 60. The feed point may also be referred to as a power feeding section. The portion between the radiation electrode 50 and the ground electrode 60 constitutes a resonator section 10. The role (operation) of each section will be described later.

[0017] A metal wire 70 extends from the surface of the radiation electrode 50 toward the front, and a metal wire 80 extends from the end of the short stub 30 toward the rear. The radiation electrode 50 and the short stub 30 are connected by a metal wire 90. The metal wire 70, the metal wire 90, and the metal wire 80 form a single metal wire.

[0018] 2 and 3, the area of ​​the ground electrode 60 when viewed from the front is larger than the area of ​​the radiation electrode 50. However, this is not limitative.

[0019] The metal wires 70 and 80 extending in front of and behind the surface wave antenna 100 are metals that act as guides for the propagation of surface waves. The surface waves radiated from the surface wave antenna 100 propagate along the surfaces of the metal wires 70, thereby transmitting power efficiently.

[0020] <Operation Mechanism of Surface Wave Antenna 100> The operation mechanism of the surface wave antenna 100 will be described with reference to Figures 4 and 5. As shown in Figure 4 (and Figures 2 and 3), a short stub 30 (also referred to as "stub 30") having a length of a quarter wavelength is installed on the rear side of the surface wave antenna 100 (rear side of the ground electrode 60). Here, the stub 30 is a coaxial cable having a length of a quarter wavelength, and the ground and the core wire (metal wire 90) are short-circuited at its end.

[0021] At this time, a standing wave is generated inside the stub 30, and an open state is created at the position of the antenna's feed point 40, i.e., at the front of the quarter-wavelength short stub 30. Therefore, even if high-frequency power is supplied from the feed point 40, no current flows backward from this point. Furthermore, the electric field generated in the air from the radiation electrode 50 is reflected by the ground electrode 60, so the electric field does not leak out to the back. This makes it possible to prevent surface waves from being generated in the metal wire 80 located toward the back of the surface wave antenna 100.

[0022] Next, with reference to FIG. 5, a mechanism by which the surface wave antenna 100 generates a surface wave forward will be described.

[0023] 5, a resonator 10 that resonates with an electrical length of a quarter wavelength is configured in front of a feed point 40 of a surface wave antenna 100. A radiating electrode 50 is provided at the tip of the resonator 10, and the height is reduced in the same manner as a capacitance-loaded monopole antenna.

[0024] When high-frequency power is supplied from the power supply point 40, the resonator 10 resonates, and the voltage amplitude at the radiation electrode 50 increases, generating a strong electric field wave near the radiation electrode 50, which then jumps to the metal wire 70 in front of the radiation electrode 50 and becomes a surface wave.

[0025] Thereafter, as shown in FIG. 6, the electric field and magnetic field waves propagate along the metal wire 70 near the surface of the metal wire 70 as surface waves while exchanging energy.

[0026] The surface waves transmitted from the surface wave antenna 100 (also called the power transmitting antenna 100 on the power transmitting side) can be received by a power receiving antenna 200 having the same structure as the power transmitting antenna 100, as shown in FIG.

[0027] 7 , a power transmitting circuit 110 and a power receiving circuit 210 are connected to the feed point of the power transmitting antenna 100 and the feed point of the power receiving antenna 200, respectively. The power transmitting circuit 110 includes a power supply device that supplies power, an inverter that converts the power into high-frequency power, and an impedance matching circuit. The power receiving circuit 210 includes an impedance matching circuit, a converter that converts high-frequency power into direct current, and a load that uses or stores the received power.

[0028] (Surface wave antenna 100 of Example 2) In the above-described Example 1, the surface wave antenna 100 is fixed to the metal wire that serves as a transmission path for the surface waves. Although power transmission using surface waves requires a metal wire that serves as a guide for transmitting the surface waves, the power itself is a wireless power transmission that is transmitted by electromagnetic waves, and therefore convenience is further improved if the power transmitting antenna 100 and the power receiving antenna 200 can move freely relative to the metal wire.

[0029] Therefore, a surface wave antenna 100 in which the antenna can move freely relative to the metal wire will be described as Example 2. Fig. 8 shows a cross-sectional view of the surface wave antenna 100 of Example 2 as seen from the side. Fig. 9 shows a perspective view of the surface wave antenna 100.

[0030] 8 and 9, the basic structure of the surface wave antenna 100 of the second embodiment is the same as the structure of the surface wave antenna 100 of the first embodiment. Here, differences from the first embodiment will be described.

[0031] As shown in Figures 8 and 9, in the second embodiment, a hole is provided in the central axis of the surface wave antenna 100, and a metal wire 70 serving as a transmission path for the surface wave passes through the hole. This allows the position of the surface wave antenna 100 to be freely moved along the metal wire 70. In the example shown in Figures 8 and 9, the radiation electrode 50 and the bottom of the short stub 30 are connected by a cylindrical metal portion 91, but this is just one example. The portion 91 may have any shape as long as it has a hole through which the metal wire 70 can pass.

[0032] In the second embodiment, similarly to the one shown in FIG. 7, a wireless power transmission system can be configured using a power transmitting antenna 100 and a power receiving antenna 200 having the same structure as the surface wave antenna 100.

[0033] (Surface wave antenna 100 according to embodiment 3) In embodiments 1 and 2, the case where the surface wave transmission medium is a one-dimensional metal wire has been described. The surface wave transmission mechanism described in embodiments 1 and 2 can also be applied to the case where the surface wave transmission medium is a two-dimensional metal plate. In embodiment 3, the case where the surface wave transmission medium is a two-dimensional metal plate will be described.

[0034] A surface wave antenna 100 according to the third embodiment is shown in Fig. 10 and Fig. 11. Fig. 10 is a cross-sectional view of the surface wave antenna 100 according to the third embodiment as viewed from the side, and Fig. 11 is a perspective view of the surface wave antenna 100 according to the third embodiment.

[0035] 10 and 11 , the radiation electrode 50 and the short stub 30 are connected by a metal rod 92, and a feed point 40 is provided between the ground electrode 60 and the metal rod 92. A resonance unit 10 is formed between the ground electrode 60 and the radiation electrode 50. The surface wave antenna 100 is placed on a metal plate 75. Note that the surface wave antenna 100 may be held in any manner as long as it is placed on the metal plate 75 as shown in FIG. 10 . Furthermore, the bottom of the surface wave antenna 100 may be in contact with the metal plate 75 or may be separated from the metal plate 75.

[0036] The mechanism of propagation of the surface wave in the third embodiment is the same as that described in the first embodiment. In Fig. 10, the surface wave propagates from the radiation electrode 50 to the right.

[0037] In the third embodiment, similarly to the one shown in FIG. 7, a wireless power transmission system can be configured using a power transmitting antenna 100 and a power receiving antenna 200 having the same structure as the surface wave antenna 100.

[0038] (Application Examples) Examples 1 and 2 below are specific applications in which the surface wave antenna 100 described in this embodiment is used to transmit power using surface waves propagating on a metal surface.

[0039] Example 1: Space elevator A space elevator is a concept for an elevator with an orbit that extends from the surface of a planet or other object to geostationary orbit or beyond. Carbon nanotubes are considered a promising material for the pillars that support the elevator, but when supplying power to the elevator via wire, ultra-long-distance power transmission using cables involves large losses, so it would be more effective to supply power using surface waves, which transmit power using the low-loss space near the surface of the carbon nanotubes as a medium.

[0040] Example 2: Transport of renewable energy The distance from the construction site of a wind power plant or other natural energy power plant to the connection point to the power grid is often far, so the challenge is how to transport the generated electricity at low cost. While conventional power distribution methods use two electric wires, a method that transmits power using surface waves requires only one electric wire, making it possible to build a power grid at half the cost.

[0041] Summary of the Embodiment As described above, the surface wave antenna 100 according to the present embodiment includes the resonator 10 , the ground electrode 60 , the short stub 30 , and the feed point 40 .

[0042] The resonator 10 is composed of a radiation electrode 50 and conductors (e.g., 90, 91, 92) that support it, and resonates with an electrical length of a quarter wavelength. The short stub 30 is provided on the rear side of the surface wave antenna 100 and has a length of a quarter wavelength. The feed point 40 is connected in parallel with the short stub 30 to the radiation electrode 50 and the ground electrode 60.

[0043] A metal wire serving as a medium for transmitting surface waves is provided on the central axis of the surface wave antenna 100. For example, the surface wave antenna 100 has a hole penetrating the central axis of the surface wave antenna 100, and a metal wire serving as a medium for transmitting surface waves passes through the hole. The surface wave antenna 100 can be moved along the metal wire.

[0044] The surface wave antenna 100, which includes the resonator 10, the ground electrode 60, the short stub 30, and the feed point 40, may be installed on a metal plate. In this case, the surface wave radiated from the surface wave antenna 100 propagates on the plane of the metal plate. Furthermore, the surface wave antenna 100 can move on the plane of the metal plate.

[0045] Furthermore, a wireless power transmission system can be configured by using the surface wave antenna 100 on the power transmitting side as a power transmitting antenna and the surface wave antenna 100 on the power receiving side as a power receiving antenna. That is, in the wireless power transmission system, coupling electrodes of two or more surface wave antennas 100 that transmit power using surface waves of an electromagnetic field propagating along the surface of a metal wire or a metal plate face each other with the metal transmission medium sandwiched therebetween, and power can be exchanged between the surface wave antennas 100.

[0046] (Effects of the Technique According to the Embodiments) According to the surface wave antenna 100 of the present embodiment, when transmitting power along a metal surface using surface waves of an electromagnetic field, it is possible to transmit power only in the front direction of the surface wave antenna 100 and not in the rear direction, thereby enabling highly efficient transmission of power toward the power receiving antenna.

[0047] According to the surface wave antenna 100 of this embodiment, power is not transmitted in an unnecessary direction (towards the rear) and power arriving from an unintended direction (towards the rear) is not received, thereby preventing interference between surface waves arriving from both the front and rear, which would result in an extreme drop in received power.

[0048] Furthermore, by suppressing unnecessary radiation of electromagnetic waves, the risk of causing electromagnetic interference to surrounding electronic devices can be reduced.

[0049] Normally, when transmitting power via a wire, a pair of electric wires is required, but when transmitting power using surface waves, only one cable is required, which cuts the cost in half.

[0050] Furthermore, when transmitting power using surface waves, the power itself does not move inside the conductor that guides the surface waves, but propagates in the space near the conductor (such as a cable) with low loss, making it possible to transmit power with low loss (high efficiency).

[0051] The following additional notes are provided regarding the above-described embodiments.

[0052] <Additional Notes> (Additional Item 1) A surface wave antenna that transmits or receives power using surface waves, comprising: a resonator unit configured with a radiation electrode and a conductor supporting the radiation electrode; a ground electrode; a short stub provided on the back side of the surface wave antenna; and a feed point connected to the surface wave antenna in parallel with the short stub. (Additional Item 2) The surface wave antenna according to Additional Item 1, wherein the resonator unit resonates with an electrical length of a quarter wavelength, and the length of the short stub is the quarter wavelength. (Additional Item 3) The surface wave antenna according to Additional Item 1 or 2, wherein a metal wire serving as a medium for transmitting surface waves is provided on the central axis of the surface wave antenna. (Additional Item 4) The surface wave antenna according to any one of Additional Items 1 to 3, wherein the surface wave antenna has a hole penetrating the central axis thereof. (Additional Item 5) The surface wave antenna according to Additional Item 1 or 2, wherein the surface wave antenna is provided on a metal plate serving as a medium for transmitting surface waves. (Supplementary Item 6) A wireless power transmission system including a surface wave antenna according to any one of Supplementary Items 1 to 5 on a power transmitting side and another surface wave antenna according to any one of Supplementary Items 1 to 5 on a power receiving side.

[0053] Although the present embodiment has been described above, the present invention is not limited to such a specific embodiment, and various modifications and changes are possible within the scope of the gist of the present invention described in the claims.

[0054] REFERENCE SIGNS LIST 10 Resonator 20 Dielectric 30 Short stub 40 Feeding point 50 Radiation electrode 60 Ground electrode 70, 80, 90 Metal wire 75 Metal plate 91 Cylindrical portion 92 Metal rod 100 Surface wave antenna 110 Power transmitting circuit 200 Power receiving antenna 210 Power receiving circuit

Claims

1. A surface wave antenna that transmits or receives electricity using surface waves, comprising: a resonator portion consisting of a radiation electrode and a conductor supporting the radiation electrode; a ground electrode; a short stub provided on the rear side of the surface wave antenna; and a power feed point connected in parallel to the short stub with respect to the surface wave antenna.

2. The surface wave antenna according to claim 1, wherein the resonator resonates with an electrical length of a quarter wavelength, and the length of the short stub is a quarter wavelength.

3. A surface wave antenna according to claim 1 or 2, wherein a metal wire serving as a medium for transmitting surface waves is provided on the central axis of the surface wave antenna.

4. The surface wave antenna according to claim 1 or 2, having a hole passing through the central axis of the surface wave antenna.

5. The surface wave antenna according to claim 1 or 2, wherein the surface wave antenna is provided on a metal plate that serves as a medium for transmitting surface waves.

6. A wireless power transmission system comprising a surface wave antenna according to claim 1 or 2 on the power transmitting side, and another surface wave antenna according to claim 1 or 2 on the power receiving side.

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