Leakage coaxial cable
The leaky coaxial cable with a reduced outer conductor occupancy and adjusted pitch improves power transmission by increasing radiated power and maintaining circular polarization, enhancing usability for devices like RFID tags and sensors.
Patent Information
- Application Number
- JP2023221333
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2023-12-27
- Publication Date
- 2025-07-09
Smart Images
Figure 2025103731000001_ABST
Abstract
Description
Technical Field
[0001] The present disclosure relates to a leaky coaxial cable capable of radiating circularly polarized waves.
Background Art
[0002] As described in Patent Document 1, an external conductor is wound in a single helix at a constant pitch on the surface of an insulator covering the periphery of a linear center conductor, so that circularly polarized electromagnetic waves are radiated when a high-frequency signal is input. There is known a leaky coaxial cable configured as such. Further, as described in Patent Document 2, there is also known a configuration in which a slot is provided in the outer conductor of a coaxial cable so as to radiate linearly polarized electromagnetic waves from the slot.
[0003] According to the leaky coaxial cable described in Patent Document 2, since linearly polarized electromagnetic waves are radiated from the slot, it is necessary to adjust the orientation of the antenna corresponding to the polarization plane of the electromagnetic waves in order to receive the electromagnetic waves with a dipole antenna. On the other hand, since circularly polarized electromagnetic waves are radiated from the leaky coaxial cable described in Patent Document 1, it is not necessary to adjust the orientation of the antenna when receiving the electromagnetic waves with a dipole antenna. Therefore, the leaky coaxial cable described in Patent Document 1 can improve usability as compared with that described in Patent Document 2.
Prior Art Documents
Patent Documents
[0004]
Patent Document 1
Patent Document 2
Summary of the Invention
Problems to be Solved by the Invention
[0005] By the way, in the leakage coaxial cable described in Patent Document 1, the occupancy rate of the outer conductor on the insulator surface is 50% or more, preferably 69% or more, and the VSWR (voltage standing wave ratio) representing the relationship between the incident wave and the reflected wave to the leakage coaxial cable is 2.0 or less. This is because in Patent Document 1, it is premised that the leakage coaxial cable is used for communication purposes for transmitting and receiving high-frequency signals in the frequency band of 800 MHz to 2400 MHz.
[0006] However, as described in Patent Document 2, the leakage coaxial cable can also be used for power transmission purposes, for example, to supply power to an external device with a high-frequency signal in a narrow frequency band such as 850 MHz to 930 MHz. When the leakage coaxial cable is used for power transmission in this way, if the occupancy rate of the outer conductor on the insulator surface is 50% or more, the radiated power of the electromagnetic wave from the leakage coaxial cable may be suppressed, and it may not be possible to supply the desired power to the external device.
[0007] One aspect of the present disclosure aims to increase the power of the electromagnetic wave to be leaked in a leakage coaxial cable capable of radiating circularly polarized electromagnetic waves so that it can be favorably used for power transmission purposes.
Means for Solving the Problems
[0008] A leakage coaxial cable according to one aspect of the present disclosure includes an inner conductor extending along an axis, an insulator covering the inner conductor, and an outer conductor wound in a single helix with a constant pitch so as to form a gap around the insulator.
[0009] When the wavelength of the high-frequency signal to be transmitted or received is λ, the relative permittivity of the insulator at that wavelength is εr, and the pitch of the winding of the outer conductor in the direction along the axis is P, these parameters satisfy the relationship shown by the following formula.
[0010]
Equation
[0011] Moreover, the occupancy rate on the surface of the insulator of the outer conductor is less than 50% and not less than 20%, and the return loss representing the attenuation amount of the power reflected at the termination and reaching the input end with respect to the power input from the input end is 8 dB or more.
[0012] According to the leaky coaxial cable of the present disclosure configured as described above, compared with the one described in Patent Document 1, the radiated power of electromagnetic waves can be increased, and it can be favorably used for power transmission applications.
Brief Description of the Drawings
[0013]
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Embodiments for Carrying Out the Invention
[0014] Hereinafter, embodiments of the present disclosure will be described. The leaky coaxial cable 10 of the present embodiment is a leaky coaxial cable used for power transmission applications that operates these components by supplying power to RFID tags and various sensors.
[0015] Therefore, during power supply, a transmitter that generates a high-frequency signal of a predetermined frequency for power supply is connected to one end of the leaky coaxial cable 10 via a transmission line (not shown) composed of a plug or a coaxial cable. The other end of the leaky coaxial cable 10 is terminated or open with a dummy resistor having a resistance value corresponding to the characteristic impedance of the coaxial cable.
[0016] In this embodiment, as the high-frequency signal for power supply, for example, a narrow-band high-frequency signal in the 920 MHz band such as 918 MHz or 919.2 MHz is used.
[0017] [Configuration] As shown in FIG. 1, the leaky coaxial cable 10 has a linear inner conductor 20 extending along the axis 12. For example, a copper wire is used for the inner conductor 20, and the outer peripheral surface of the inner conductor 20 is covered by a cylindrical insulator 22 provided concentrically. And an outer conductor 24 is provided on the outer peripheral surface of the insulator 22.
[0018] As the material of the insulator 22, for example, polyethylene, polytetrafluoroethylene, vinyl chloride, or foams thereof can be used. Also, the inner diameter of the outer conductor 24 (in other words, the outer diameter of the insulator 22) is set according to the standards of coaxial cables such as 5C, 8C, 5D, 8D...
[0019] Also, the outer diameter of the inner conductor 20 is appropriately adjusted in consideration of the relative permittivity εr of the insulator 22 so that the characteristic impedance of the leaky coaxial cable 10 becomes 75 Ω or 50 Ω, which is the characteristic impedance corresponding to the coaxial cable standards 5C, 8C or 5D, 8D.
[0020] Also, the outer peripheral surface of the outer conductor 24 is covered by an electrically insulating sheath 26. As the material of the sheath 26, for example, polyethylene, vinyl chloride, or a non-halogen-based flame retardant can be used.
[0021] In this embodiment, as the external conductor 24, a metal tape with an adhesive applied to a single strip of metal foil is wound around the insulator 22 in a single-layer spiral at a constant pitch P. As the metal foil constituting the external conductor 24, for example, copper, aluminum, or silver can be used. Further, this metal foil has a thickness of, for example, 50 μm or more and 300 μm or less.
[0022] In this way, by spirally attaching a strip of metal foil to the surface of the insulator 22 using an adhesive as the external conductor 24, even if the leaky coaxial cable 10 is deformed during transportation of the leaky coaxial cable 10, it is possible to suppress a change in the pitch P of the external conductor 24.
[0023] Note that when spirally winding the external conductor 24 around the surface of the insulator 22, it is not always necessary to attach the external conductor 24 to the insulator 22 using an adhesive. For example, after winding the external conductor 24 around the surface of the insulator 22, the external conductor 24 may be fixed to the surface of the insulator 22 by pressing the external conductor 24 from the outside with a sheath 26 or the like.
[0024] Here, the width (external conductor width) Wm of the external conductor 24 is the length of the metal foil in the direction along the axis 12 of the internal conductor 20 (axial direction). The pitch P is the winding period of the metal foil in the axial direction of the leaky coaxial cable 10, and is the length that the metal foil advances in the axial direction of the internal conductor 20 when the metal foil makes one turn around the insulator 22.
[0025] In this embodiment, the pitch P of the external conductor 24 is larger than the external conductor width Wm, and the external conductor 24 is wound around the outer peripheral surface of the insulator 22 while forming a single-layer spiral groove (gap) 30. The gap width Wg is the interval between adjacent external conductors 24 in the direction orthogonal to the side edge of the external conductor 24 when the gap 30 is projected onto a plane including the axis 12.
[0026] In the leakage coaxial cable 10 of the present embodiment, the relationship expressed by the following formula (1) holds. In formula (1), εr is the relative permittivity of the insulator 22, λ is the wavelength (design wavelength) of the applied high-frequency signal, and P is the pitch of the winding of the outer conductor 24. Note that the relative permittivity εr of the insulator 22 has frequency dependence, and naturally, it is the value at the frequency of the applied high-frequency signal.
[0027]
Number
[0028] Also, in the leakage coaxial cable 10 of the present embodiment, the occupancy rate of the outer conductor 24 (in other words, the metal foil constituting the outer conductor 24) on the outer peripheral surface of the insulator 22 is set to be less than 50% and 20% or more. Note that the occupancy rate (%) of the outer conductor 24 on the outer peripheral surface of the insulator 22 can be obtained as "(Wm / P)×100" by dividing the outer conductor width Wm by the pitch P.
[0029] Also, the pitch P of the outer conductor 24 is set such that the return loss, which represents the attenuation amount of the power of the high-frequency signal reflected at the end of the leakage coaxial cable 10 and reaching the input end with respect to the power of the high-frequency signal input from the input end of the leakage coaxial cable 10, is 8 dB or more. The return loss can be adjusted, for example, by the occupancy rate of the outer conductor 24 and the pitch P.
[0030] [Effect] The leakage coaxial cable 10 of the present embodiment configured as described above can be easily manufactured, for example, by spirally attaching an outer conductor 24 formed of a strip-shaped metal tape to the outer peripheral surface of an insulator 22 covering the inner conductor 20 and covering the periphery with a sheath 26.
[0031] And according to the leakage coaxial cable 10 of the present embodiment, when a high-frequency signal is input to the inner conductor 20 by satisfying the condition of formula (1), an electromagnetic wave of circular polarization corresponding to the spiral winding direction of the outer conductor 24 is stably radiated.
[0032] Further, by setting the occupancy rate of the outer conductor 24 on the outer peripheral surface of the insulator 22 to less than 50% and 20% or more, and setting the return loss to 8 dB or more, the power of the electromagnetic wave leaking (in other words, radiated) from the leaky coaxial cable 10 can be increased. Therefore, according to the present embodiment, a leaky coaxial cable 10 suitable for power supply to external devices such as RFID tags and sensors can be provided for power supply use.
[0033] On the other hand, on the side of the external device receiving power supply from the leaky coaxial cable 10, stable power supply can be received from the leaky coaxial cable 10 without adjusting the orientation of the antenna of the power receiving unit receiving power supply from the leaky coaxial cable 10. Therefore, according to the leaky coaxial cable 10 of the present embodiment, the usability of the external device receiving power supply from the leaky coaxial cable 10 can be improved.
[0034] In addition, since the occupancy rate of the outer conductor 24 on the outer peripheral surface of the insulator 22 of the leaky coaxial cable 10 of the present embodiment can be made smaller than that of the leaky coaxial cable described in Patent Document 1, the material cost of the outer conductor 24 can be suppressed and the manufacturing cost can be reduced.
[0035] [Explanation of differences from Patent Document 1] The basic configuration of the leaky coaxial cable 10 of the present embodiment is the same as that described in Patent Document 1. The difference from that described in Patent Document 1 is that the occupancy rate of the outer conductor 24 on the surface of the insulator 22 is less than 50% and 20% or more, and the return loss is 8 dB or more. In Patent Document 1, the VSWR representing the relationship between the incident wave and the reflected wave to the leaky coaxial cable is set to 2.0 or less. When this is converted to return loss, it is 9.54 dB or more.
[0036] Hereinafter, this difference will be described. First, when the leaky coaxial cable 10 is used for power transmission rather than for communication, the frequency band of the electromagnetic wave radiated from the leaky coaxial cable 10 can be made narrower than when it is used for communication.
[0037] Also, when the leaky coaxial cable 10 is used for power transmission applications, it is known that the return loss is smaller than when it is used for communication applications, and it may be set to 8 dB or more (see Patent Document 2).
[0038] Also, in order to increase the amount of power supplied from the leaky coaxial cable 10 to an external device, the occupancy rate of the outer conductor 24 on the surface of the insulator 22 may be reduced to make it easier for high-frequency signals to leak (in other words, radiate).
[0039] Therefore, when radiating a narrowband high-frequency signal used for power transmission applications from the leaky coaxial cable 10, the inventors of the present application considered reducing the occupancy rate of the outer conductor 24 while ensuring a return loss of 8 dB or more for the high-frequency signal with the leaky coaxial cable 10.
[0040] This investigation was carried out by changing the occupancy rate of the outer conductor 24, the pitch P, the outer conductor width Wm, and the gap width Wg in 5D-size and 8D-size coaxial cables used as leaky coaxial cables, and measuring (simulating) the return loss and directivity.
[0041] As a result, it was found that in power transmission applications, if the occupancy rate of the outer conductor 24 on the surface of the insulator 22 is 20% or more, the return loss can be set to 8 dB or more by appropriately adjusting the pitch P.
[0042] Therefore, according to the present embodiment, by setting the occupancy rate of the external operation to less than 50% and 20% or more as described above, the radiated power of electromagnetic waves can be increased compared to that described in Patent Document 1, and the leaky coaxial cable 10 that can be used for power transmission applications can be provided.
[0043] Hereinafter, the simulation results of the return loss and directivity in the leaky coaxial cable 10 will be described. In Simulations 1 and 2 described below, the length of the leaky coaxial cable was set to 10 m and the cable end was left open, and the simulation software Femtet (registered trademark) manufactured by Murata Software was used.
[0044] [Simulation 1] For Samples A and B in which the pitch P, the width Wm of the outer conductor, and the gap width Wg were set as shown in Table 1 so that the occupancy rate of the outer conductor 24 was 25% in the 5D-size and 8D-size coaxial cables, the return loss and the directivity were calculated by simulation. The return loss in Table 1 is the minimum value of the return loss (a numerical value rounded down to the second decimal place) when a high-frequency signal in the 920 MHz band is input.
[0045]
Table 1
[0046] The simulation results are shown in FIGS. 2 and 3. In Simulations 1 and 2, the inner conductor 20 and the outer conductor 24 of the leaky coaxial cable 10 were assumed to be perfect conductors. Also, the insulator 22 had a relative permittivity of 1.35 and tan δ of 0 for the 5D-size coaxial cable, and a relative permittivity of 1.32 and tan δ of 0 for the 8D-size coaxial cable. Further, the sheath 26 was made of a flame-retardant material, and had a relative permittivity of 4.5 and tan δ of 0 for both the 5D and 8D sizes.
[0047] FIG. 2 shows the simulation results of Sample A, which is a leaky coaxial cable with a 5D size and an outer conductor 24 occupancy rate of 25%, and FIG. 3 shows the simulation results of Sample B, which is a leaky coaxial cable with an 8D size and an outer conductor 24 occupancy rate of 25%.
[0048] As shown in FIG. 2, in Sample A, the return loss of the high-frequency signal with a frequency f1 of 918 MHz was 16.834 dB, and the return loss of the high-frequency signal with a frequency f2 of 920 MHz was 12.542 dB.
[0049] Also, as shown in FIG. 3, for sample B, the return loss of the high-frequency signal with a frequency f1: 918 MHz was 11.994 dB, and the return loss of the high-frequency signal with a frequency f2: 920 MHz was 11.239 dB.
[0050] Therefore, it can be seen that in the leaky coaxial cable with an external conductor occupancy of 25%, a return loss of 8 dB or more can be ensured for any high-frequency signal in the 920 MHz band. In FIGS. 2 and 3, since the return loss is described as S11 of the S parameter, a negative sign is assigned. Also, in FIGS. 4 and 5 described later, the return loss is similarly described.
[0051] Next, in the directivity simulations of samples A and B, the radiation angle dependencies of the intensities of the left-handed circular polarization and right-handed circular polarization shown in FIGS. 2 and 3 were obtained. Specifically, when a high-frequency signal of 920 MHz was input to samples A and B, the intensities of the left-handed circular polarization and right-handed circular polarization radiated from the leaky coaxial cable 10 were calculated by simulation in an angular range of -90 to +90 [deg], with the direction orthogonal to the axis 12 being 0 [deg].
[0052] As a result, as shown in FIG. 2, among the electromagnetic waves radiated from the 5D-sized sample A, the intensity of the left-handed circular polarization that becomes the main mode peaked at a radiation angle θ1: -52 [deg]. Also, as shown in FIG. 3, among the electromagnetic waves radiated from the 8D-sized sample B, the intensity of the left-handed circular polarization that becomes the main mode peaked at a radiation angle θ2: -34 [deg]. Therefore, according to the leaky coaxial cable 10 of the present embodiment, it can be seen that the intensity of the left-handed circular polarization becomes maximum at a predetermined radiation angle, and the maximum power can be supplied to the external device at that radiation angle. Note that the left-handed circular polarization is used as the main mode because the external conductor 24 of the leaky coaxial cable 10 of the present embodiment is right-handed and easily radiates electromagnetic waves of the left-handed circular polarization.
[0053] [Simulation 2] In the 5D-size and 8D-size coaxial cables, for Samples C and D where the pitch P, the width Wm of the outer conductor, and the gap width Wg were set as shown in Table 2 so that the occupancy rate of the outer conductor 24 was 20%, the return loss and the directivity were calculated by simulation. The return loss in Table 2 is the minimum value of the return loss (a numerical value rounded down to the second decimal place) when a high-frequency signal in the 920-MHz band is input.
[0054]
Table 2
[0055] The simulation results are shown in FIGS. 4 and 5. FIG. 4 shows the simulation results of Sample C, which is a leaky coaxial cable with a 5D size and an occupancy rate of the outer conductor 24 of 20%. FIG. 5 shows the simulation results of Sample D, which is a leaky coaxial cable with an 8D size and an occupancy rate of the outer conductor 24 of 20%.
[0056] As shown in FIG. 4, in Sample C, the return loss of the high-frequency signal with a frequency f1 of 918 MHz was 10.462 dB, and the return loss of the high-frequency signal with a frequency f2 of 920 MHz was 9.905 dB.
[0057] Also, as shown in FIG. 5, in Sample D, the return loss of the high-frequency signal with a frequency f1 of 918 MHz was 10.920 dB, and the return loss of the high-frequency signal with a frequency f2 of 920 MHz was 12.983 dB.
[0058] From these simulation results, it can be seen that in the 5D- and 8D-size leaky coaxial cables 10, even if the occupancy rate of the outer conductor 24 is changed from 25% to 20%, a return loss of 8 dB or more can be ensured for high-frequency signals in the 920-MHz band.
[0059] Next, in the simulations of the directivities of Samples C and D, the radiation angle dependencies of the intensities of the left-handed circularly polarized wave and the right-handed circularly polarized wave shown in FIGS. 4 and 5 were obtained. Specifically, when a high-frequency signal of 920 MHz was input to Samples C and D, the intensities of the left-handed circularly polarized wave and the right-handed circularly polarized wave radiated from the leaky coaxial cable 10 were calculated by simulation in the angular range of -90 to +90 [deg], with the direction orthogonal to the axis 12 being 0 [deg].
[0060] As a result, as shown in FIG. 4, among the electromagnetic waves radiated from the 5D-sized Sample C, the intensity of the left-handed circularly polarized wave that becomes the main mode peaked at the radiation angle θ3: -32 [deg]. Further, as shown in FIG. 5, among the electromagnetic waves radiated from the 8D-sized Sample D, the intensity of the left-handed circularly polarized wave that becomes the main mode peaked at the radiation angle θ2: -34 [deg]. Therefore, according to the leaky coaxial cable 10 of the present embodiment, it can be seen that in the leaky coaxial cables 10 of 5D and 8D sizes, even when the occupancy rate of the outer conductor 24 is changed from 25% to 20%, the intensity of the left-handed circularly polarized wave becomes maximum at a predetermined radiation angle, and the maximum power can be supplied to the external device at that radiation angle.
[0061] [Modification Example] Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above embodiments and can be implemented with various modifications.
[0062] For example, in the above embodiment, the leaky coaxial cable 10 has been described as radiating a high-frequency signal in the 920 MHz band shown in Table 3. However, the technology of the present disclosure can be applied in the same manner as the above embodiment to any leaky coaxial cable that radiates a high-frequency signal used for power transmission applications.
[0063] In addition, as high-frequency signals used for power transmission applications, as shown in Table 3, in addition to the 920 MHz band, high-frequency signals in the 2.4 GHz band or the 5.7 GHz band are also known. Although the usage conditions of these high-frequency signals, such as the radiated power from the leaky coaxial cable 10, are restricted in consideration of the impact on the human body and the like, the leaky coaxial cable can be configured in the same manner as in the above-described embodiment and thus can be used to supply power to an external device.
Table 3
[0064] In the above-described embodiment, a strip-shaped metal tape with an adhesive applied to a metal foil is used for the outer conductor 24, and the leaky coaxial cable 10 is described as being configured by spirally winding this metal tape around the surface of the insulator 22. In contrast, the outer conductor 24 may be configured, for example, by forming a plurality of metal wires into a strip shape and spirally winding them around the surface of the insulator 22.
[0065] In this case as well, by applying an adhesive between the metal wire and the insulator 22, it is possible to suppress the winding position of the outer conductor 24 around the insulator 22 from shifting when the leaky coaxial cable 10 is bent and deformed.
[0066] In this way, it is possible to suppress the pitch P, the outer conductor width Wm, or the gap width Wg of the outer conductor 24 in the leaky coaxial cable 10 from deviating from the designed values and causing the desired radiation characteristics to be unobtainable.
[0067] Also, as in the leaky coaxial cable 10 shown in FIG. 6, a plurality of outer conductors 24 and a plurality of resin wires 28 may be combined using metal wires as the outer conductors 24 to form a strip shape, and this may be spirally wound around the surface of the insulator 22.
[0068] In this way, the resin wire 28 can wind the external conductor 24 with a predetermined external conductor width Wm at an arbitrary pitch P on the surface of the insulator 22 while forming a gap between the external conductors 24.
[0069] Also, in this case, the external conductor 24 composed of a metal wire can be wound around the surface of the insulator 22 with the resin wire 28 sandwiched between the external conductors 24, that is, in a state where the external conductor 24 and the resin wire 28 are in close contact. Therefore, the pitch P of the external conductor 24 can be maintained without applying an adhesive between the external conductor 24 and the insulator 22.
[0070] Moreover, according to the leaky coaxial cable 10 shown in FIG. 6, by simply changing the ratio of the number of metal wires serving as the external conductor 24 to the resin wire 28, the occupancy rate of the external conductor 24 and the external conductor width Wm on the surface of the insulator 22 can be changed. Therefore, the electromagnetic wave radiation characteristics of the leaky coaxial cable 10 can be easily adapted to a desired frequency band.
[0071] Therefore, in order to be able to manufacture leaky coaxial cables 10 with different characteristics, it is not necessary to prepare metal tapes with various external conductor widths Wm as inventory, and the manufacturing cost of the leaky coaxial cable 10 can be reduced.
[0072] Also, when manufacturing this leaky coaxial cable 10, existing manufacturing equipment used for manufacturing coaxial lines with a transverse winding shield can be diverted. Therefore, the equipment investment cost can be suppressed. As the resin wire 28, for example, polyethylene or the like can be used.
[0073] Next, a plurality of functions of one component in the above embodiment may be realized by a plurality of components, or one function of one component may be realized by a plurality of components. Further, a plurality of functions of a plurality of components may be realized by one component, or one function realized by a plurality of components may be realized by one component. Further, a part of the configuration of the above embodiment may be omitted. Further, at least a part of the configuration of the above embodiment may be added to or replaced with the configuration of another of the above embodiments.
[0074] In addition to the leaky coaxial cable 10 described above, the present disclosure can also be realized in various forms such as a system including the leaky coaxial cable 10 as a component and a manufacturing method of the leaky coaxial cable 10.
Description of Reference Numerals
[0075] 10... leaky coaxial cable, 20... inner conductor, 22... insulator, 24... outer conductor, 26... sheath, 30... gap, P... pitch.
Claims
1. an inner conductor extending along an axis; an insulator covering the inner conductor; an outer conductor wound in a single helix with a constant pitch so as to form a gap around the insulator; comprising; when the wavelength of a high-frequency signal to be transmitted or received is λ, the relative dielectric constant of the insulator at the wavelength is εr, and the pitch of winding of the outer conductor in the direction along the axis is P, the following relationship is satisfied; 【Number 1】 the occupancy rate of the outer conductor on the surface of the insulator is less than 50% and 20% or more; a leaky coaxial cable in which the return loss representing the attenuation amount of the power reflected at the termination and reaching the input end with respect to the power input from the input end is 8 dB or more.
2. The leaky coaxial cable according to claim 1, wherein the outer conductor is composed of a plurality of metal wires and is wound spirally on the surface of the insulator.
3. The leaky coaxial cable according to claim 2, wherein the plurality of metal wires constituting the outer conductor are wound spirally on the surface of the insulator with a plurality of resin wires interposed therebetween.
Citation Information
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