Surface wave antenna and wireless power transmission system

Abstract

This surface wave antenna uses surface waves to transmit or receive power and is provided with: a resonance unit constituted of a radiating electrode and a pillar section supporting the radiating electrode; a ground electrode; a power feed point provided between the ground electrode and the pillar section; and a shield that is connected to the ground electrode and installed so as to surround the periphery of the pillar section.

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 resonance portion configured by a radiation electrode and a pillar portion supporting the radiation electrode; a ground electrode; a power supply point provided between the ground electrode and the pillar portion; and a shield connected to the ground electrode and installed so as to surround the periphery of the pillar portion.

[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 basic configuration. FIG. 1 is a diagram for explaining a mechanism of power propagation in the basic configuration. FIG. 2 is a diagram for explaining a mechanism of power propagation in the basic configuration. FIG. 3 is a diagram for explaining a mechanism of power propagation in the basic configuration. FIG. 4 is a diagram for explaining a mechanism of power propagation in the basic configuration. FIG. 5 is a diagram for explaining a problem in the basic configuration. A cross-sectional view of a surface wave antenna 100 of a first embodiment. A perspective view of the surface wave antenna 100 of the first embodiment. A diagram for explaining a problem in the first embodiment. A cross-sectional view of a surface wave antenna 100 of a second embodiment. A perspective view of the surface wave antenna 100 of the second embodiment. A configuration diagram of a wireless power transmission system.

[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] A wireless power transmission method using an electric field resonance antenna is known. While this method can achieve high transmission efficiency over short distances, it cannot transmit power over long distances because the generated electric field rapidly attenuates in the air. 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.

[0013] (Basic Configuration) 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. Therefore, in this embodiment, a directional surface wave antenna (surface wave antenna 100) is used that transmits power only in the front direction of the antenna.

[0014] 1 and 2 show the basic configuration of a surface wave antenna 100. Fig. 1 is a perspective view of the surface wave antenna 100, and Fig. 2 is a cross-sectional view of the surface wave antenna 100 as seen from the side.

[0015] 1 and 2 , 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 a pillar portion 70 extending from the radiation electrode 50 toward the short stub 30 to the ground electrode 60. 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 feed section. The portion between the radiation electrode 50 and the ground electrode 60 constitutes a resonance section 10.

[0016] 1 and 2, 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.

[0017] In the surface wave antenna 100, the pillar 70 and the radiation electrode 50 are designed to have a size that allows them to resonate electrically at a quarter wavelength at the frequency of the high-frequency power that carries the power, and when high-frequency power of this frequency is input to the feed point 40, they resonate and generate a strong electric field in the surrounding area.

[0018] The short stub 30 is a quarter-wavelength transmission line with a shorted tip, and can be made from, for example, a coaxial line. Because it has an open end at the feed point 40, it serves to prevent current from flowing to the back of the antenna. Charges of the same magnitude but opposite signs alternately accumulate in the radiation electrode 50 and ground electrode 60, generating electric field waves in the surrounding area.

[0019] An example of the generated electric field is shown by the electric field lines in Figure 2. At this time, longitudinal waves of the electric field oscillating parallel to the direction of propagation are generated in the forward direction of the radiation electrode 50, and transverse waves of the electric field oscillating perpendicular to the direction of propagation are generated in the lateral direction of the columnar portion 70.

[0020] (Issues in the basic configuration) Surface waves of the electromagnetic field propagate along the metal while confining energy near the surface of the metal, without diffusing into space, and therefore have higher transmission efficiency than wireless power transmission methods that use radio waves, which propagate by spreading throughout space. The electromagnetic field that forms the surface wave is called the near field because it is concentrated near the metal or antenna, while an electromagnetic field that propagates to infinity like radio waves is called the far field.

[0021] When transmitting power using surface waves, it is sufficient for the antenna to radiate only the near field, but the antenna with the basic configuration shown in Figures 1 and 2 generates both the near field and the far field. As a result, radio waves in the far field that do not contribute to power transmission may have an undesirable effect on the surrounding area, making it impossible to increase the transmission output of the antenna, and limiting the amount of power that can be transmitted using surface waves.

[0022] (Mechanism of Power Propagation in Basic Configuration) In order to clearly explain the problems associated with the basic configuration described above, the mechanism of power propagation in the basic configuration will be described.

[0023] As shown in Fig. 3, when a metal wire 80 is placed in front of the radiation electrode 50 in the basic configuration, longitudinal waves of the electric field generated by the antenna jump to the surface of the metal wire 80 and propagate as surface waves along the metal wire 80. A current flows in the metal wire 80 along the vibration direction of the electric field as indicated by the dashed arrow in Fig. 3, and this current attracts the surface waves of the electric field to the surface of the metal wire 80, propagating power.

[0024] Here, a cross section of the metal wire 80 taken at position A in Fig. 3 is shown in Fig. 4. As shown in Fig. 4, electric field lines extend radially around the metal wire 80, and these transmit power as surface waves.

[0025] On the other hand, Fig. 5 shows the electric field around the antenna when a metal wire 90 is placed laterally of the antenna. Fig. 6 shows the cross section of the metal wire 90 at position B in Fig. 5. In this case, the electric field lines intersect the metal wire 90 perpendicularly, so no current flows through the metal wire 90. The positive and negative charges generated on the surface of the metal wire 90 cancel each other out and become zero, so no surface waves are generated on the surface of the metal wire 90.

[0026] This shows that surface waves propagating on the surface of the metal occur only in the forward direction of the antenna.

[0027] Next, we will explain how far-field radio waves that interfere with high-power power transmission are generated from the antenna.

[0028] The radio waves are transverse waves in which the electric field vibrates perpendicularly to the direction of propagation, and are generated in the transverse direction of the antenna by the current flowing through the resonating pillar 70 as indicated by the dashed arrow in FIG.

[0029] Below, we will explain, using first and second examples, antenna configurations that solve the above-mentioned problems in the basic configuration and reduce the radiation of unnecessary radio waves that have a negative impact on the surrounding area without interfering with the generation of surface waves that are responsible for power transmission.

[0030] (First embodiment) A surface wave antenna 100 according to a first embodiment is shown in Figures 8 and 9. Figure 8 is a cross-sectional side view of the surface wave antenna 100 according to the first embodiment, and Figure 9 is a perspective view of the surface wave antenna 100 according to the first embodiment. For convenience, the surface wave antenna 100 may also be referred to as an antenna.

[0031] 8 and 9, in the surface wave antenna 100 of the first embodiment, in addition to the basic configuration, a shield 110 is provided to surround the periphery of the pillar portion 70. The shield 110 is also connected to the ground electrode 60. This makes it possible to confine radio waves generated laterally from the pillar portion 70 inside the shield and prevent them from radiating to the outside. Since no surface waves are generated in the lateral direction, there is no effect on power transmission using surface waves. In other words, the first embodiment has the effect of confining radio waves generated laterally inside the shield.

[0032] (Second Example) As described above, the first example is more effective. However, in the configuration of the first example, when the radiation electrode 50 and the ground electrode 60 are located in different planes and a step is created, as shown in FIG. 10 , some of the electric field lines leaking laterally from the antenna become radio waves, and it is not possible to completely suppress the radiation of unnecessary radio waves.

[0033] Therefore, in the second embodiment, as shown in FIGS. 11 and 12, a further improved antenna structure is used to reduce the radiation of radio waves.

[0034] Fig. 11 is a cross-sectional side view of the surface wave antenna 100 according to the second embodiment, and Fig. 12 is a perspective view of the surface wave antenna 100 according to the second embodiment. As shown in Figs. 11 and 12, in the second embodiment, a second ground electrode 62 is provided in the same plane as the radiation electrode 50, and the first ground electrode 61 and the second ground electrode 62 are connected with a shield 110 sandwiched therebetween.

[0035] The first ground electrode 61 is located in the same position as the ground electrode 60 in the basic configuration ( FIGS. 1 and 2 ) and the first embodiment ( FIGS. 8 and 9 ), and is connected to the feed point 40. However, in the second embodiment, the first ground electrode 61 is connected to a shield 110 that surrounds the periphery of the pillar portion 70, and the shield 110 is connected to the second ground electrode 62.

[0036] The configuration of the second embodiment may be considered to correspond to the first embodiment (FIGS. 8 and 9) in which the portion of the ground electrode 60 outside the shield 110 is moved to a position in the same plane as the radiation electrode 50.

[0037] Furthermore, "the second ground electrode 62 is provided in the same plane as the radiation electrode 50" means that, for example, as shown in Figures 11 and 12, the surface of the radiation electrode 50 (the surface as viewed from the right side in Figure 11) and the surface of the second ground electrode 62 (the surface as viewed from the right side in Figure 11) are on the same plane. This "surface" is, for example, a "flat surface." Note that this "flat surface" does not have to be a strictly flat surface.

[0038] Furthermore, the "same plane" does not necessarily have to be the same plane in a strict sense. For example, the plane including the surface of the radiation electrode 50 (referred to as plane 1) and the plane including the surface of the ground electrode 62 (referred to as plane 2) may not completely coincide with each other, and there may be a small distance between plane 1 and plane 2.

[0039] As shown in FIG. 11 , with the configuration of the second embodiment, radio waves generated from the pillar portion 70 are reflected by the shield 110 and do not leak to the outside, and the electric field lines generated from the radiation electrode 50 are absorbed by the second ground electrode 62 located in the same plane, canceling out the lateral electric field, thereby suppressing the radiation of radio waves originating from the electric field lines.

[0040] If a metal wire is placed in front of the antenna, a surface wave is excited on the metal wire by the longitudinal wave of the near-field electric field generated in front of the radiation electrode 50, making it possible to transmit power using the surface wave.

[0041] As described above, the surface wave antenna 100 of the first and second embodiments can realize an antenna that generates surface waves for transporting power while suppressing radiation of far-field radio waves that may have undesirable effects on the surrounding area. This reduces the impact on surrounding devices, etc., and allows the output of the power transmitting side to be increased until the required amount of power can be delivered to the power receiving side.

[0042] (Power Transmission System) Fig. 13 shows a configuration example of a power transmission system using the surface wave antenna 100 of the second embodiment. Note that Fig. 13 uses the surface wave antenna 100 of the second embodiment, but this is not limiting, and the surface wave antenna 100 of the first embodiment may also be used.

[0043] 13, the power transfer system includes a power transmitting antenna 200 and a receiving antenna 300. Both the power transmitting antenna 200 and the receiving antenna 300 are the surface wave antenna 100 of the second embodiment.

[0044] A metal wire 120 is provided between the power transmitting antenna 200 and the receiving antenna 300. The surface wave transmitted from the power transmitting antenna 200 propagates along the surface of the metal wire 120 and can be received by the power receiving antenna 300, which has the same structure as the power transmitting antenna 200.

[0045] 13 , a power transmitting circuit 210 and a power receiving circuit 310 are connected to the feed points of the power transmitting antenna 200 and the power receiving antenna 300, respectively. The power transmitting circuit 210 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 310 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.

[0046] (Application Examples) Examples 1 and 2 below are specific applications of the surface wave antenna 100 described in this embodiment for transmitting power using surface waves propagating on a metal surface.

[0047] 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, carbon nanotubes do not have the necessary conductivity. Therefore, it is more effective to supply power via surface waves, which transmit power using the space near the surface of the carbon nanotubes as a medium.

[0048] 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.

[0049] (Summary of the embodiment) As described above, the surface wave antenna 100 according to the present embodiment includes the resonance portion 10, the short stub 30, the radiation electrode 50, the ground electrode 60, the pillar portion 70, the feed point 40, and the shield 110. Note that the "ground electrode 60" may be interpreted as a collective term for the first ground electrode 61 and the second ground electrode 62.

[0050] The resonator 10 is composed of a radiation electrode 50 and a support pillar 70, and resonates at an electrical length of a quarter wavelength. The feed point 40 is provided between the end of the pillar 70 opposite the radiation electrode 50 and the ground electrode 60. The shield 110 is connected to the ground electrode 60 and is installed so as to surround the pillar 70.

[0051] As described in the second embodiment, the surface wave antenna 100 may be configured to include a second ground electrode 62 in the same plane as the radiation electrode 50, and the first ground electrode 61 and the second ground electrode 62 may be connected via a shield 110.

[0052] For example, as shown in FIG. 13, a metal wire 120 serving as a medium for transmitting surface waves may be provided on the central axis of a surface wave antenna 100 .

[0053] 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 face each other across the metal wire 120, and power can be exchanged between the surface wave antennas 100.

[0054] (Effects of the Technique According to the Embodiment) The surface wave antenna 100 according to the embodiment can transmit power efficiently. Furthermore, in a system that transmits power along a metal surface using surface waves in an electromagnetic field, it is possible to generate surface waves that propagate power while suppressing radiation of unnecessary far-field radio waves.

[0055] According to the present surface wave antenna 100, the output of the power transmitting antenna can be increased because the influence on the outside of the antenna is small, and as a result, a large amount of power can be delivered to the power receiving device.

[0056] In addition, not emitting unnecessary radio waves means that the antenna gain for unnecessary radio waves is low, i.e., radio waves are not received, so the effects of interference caused by unintended arrival of radio waves can also be suppressed.

[0057] Normally, power transmission via cable requires a pair of electric wires, but power transmission using surface waves requires only one cable, halving construction costs. Furthermore, optical fiber cables with metal tension members that have already been laid underground can be reused for power transmission, promoting the reuse of existing infrastructure.

[0058] The surface wave antenna 100 of this embodiment is an example of an antenna. This antenna includes not only a wireless power transmission antenna but also a communication antenna. In other words, the surface wave antenna 100 may be used not only as a wireless power transmission antenna but also as a communication antenna.

[0059] The following additional notes are provided regarding the above-described embodiments.

[0060] <Additional Notes> (Additional Item 1) A surface wave antenna that transmits or receives power using surface waves, comprising: a resonance unit configured with a radiation electrode and a pillar portion supporting the radiation electrode, a ground electrode, a feed point provided between the ground electrode and the pillar portion, and a shield connected to the ground electrode and installed to surround the pillar portion. (Additional Item 2) The surface wave antenna according to Additional Item 1, further comprising: a short stub connected to the pillar portion on the back side of the surface wave antenna, the resonance unit resonating with an electrical length of a quarter wavelength, and the length of the short stub is a quarter wavelength. (Additional Item 3) The surface wave antenna according to Additional Item 1 or 2, wherein the ground electrode has a first ground electrode and a second ground electrode, the feed point is provided between the first ground electrode and the pillar portion, and the first ground electrode and the second ground electrode are connected via the shield. (Supplementary Item 4) The surface wave antenna according to Supplementary Item 3, wherein the radiation electrode and the second ground electrode are provided in the same plane. (Supplementary Item 5) The surface wave antenna according to any one of Supplementary Items 1 to 4, wherein a metal wire serving as a medium for transmitting surface waves is provided on a central axis of the surface wave antenna. (Supplementary Item 6) A wireless power transmission system including the 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.

[0061] 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.

[0062] REFERENCE SIGNS LIST 10 Resonator 20 Dielectric 30 Short stub 40 Feeding point 50 Radiation electrode 60 Ground electrode 61 First ground electrode 62 Second ground electrode 80, 90, 120 Metal wire 100 Surface wave antenna 110 Shield 200 Power transmitting antenna 210 Power transmitting circuit 300 Power receiving antenna 310 Power receiving circuit

Claims

1. A surface wave antenna that transmits or receives power using surface waves, comprising: a resonance section consisting of a radiation electrode and a pillar section that supports the radiation electrode; a ground electrode; a power supply point provided between the ground electrode and the pillar section; and a shield that is connected to the ground electrode and is installed so as to surround the periphery of the pillar section.

2. The surface wave antenna according to claim 1, further comprising a short stub connected to the pillar portion on the rear side of the surface wave antenna, the resonator portion resonating with an electrical length of a quarter wavelength, and the length of the short stub being a quarter wavelength.

3. The surface wave antenna according to claim 1, wherein the ground electrode comprises a first ground electrode and a second ground electrode, the feed point is provided between the first ground electrode and the pillar portion, and the first ground electrode and the second ground electrode are connected via the shield.

4. The surface wave antenna according to claim 3, wherein the radiation electrode and the second ground electrode are provided in the same plane.

5. A surface wave antenna according to any one of claims 1 to 4, wherein a metal wire serving as a medium for transmitting surface waves is provided on the central axis of said surface wave antenna.

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

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