JP5622881B2 - Leaky coaxial cable - Google Patents

Description

  The present invention relates to a leaky coaxial cable used as an antenna.

  The leaky coaxial cable (LCX) is one in which a plurality of slots are provided in the outer conductor of a normal coaxial cable (see Non-Patent Document 1). Through such a slot, an electromagnetic wave signal inside the cable can be emitted to the outside, or an electromagnetic wave signal outside the cable can be taken into the cable. That is, LCX is a cable type antenna and can be said to be a special long and narrow transmitting / receiving antenna.

  LCX is effective as an antenna in an environment with a long communication area. In particular, an electromagnetic wave insensitive zone is likely to occur in a winding tunnel or a place where many metal bodies exist. In such a place, if a general antenna is used, a large number of antennas must be installed. However, since each slot works as an antenna in LCX, a large number of antennas are arranged along LCX. Therefore, it is difficult to generate an electromagnetic wave insensitive zone even in a long and narrow communication area by laying only one LCX. The laying work can be carried out very simply by simply extending the LCX, without the need to attach a general antenna individually for electrical wiring.

  In the normally used LCX, the slots are arranged in a zigzag manner. Such electromagnetic wave radiation modes from LCX include Eφ polarized waves and Ez polarized waves in the circumferential direction and the axial direction of the electric field. The radiation angles of the Eφ polarization and the Ez polarization are determined from the operating frequency, the slot pitch, and the relative dielectric constant of the insulator between the inner conductor and the outer conductor (see Non-Patent Document 1).

  In the conventional LCX, a radiation mode having a single Eφ polarization is adopted in order to ensure the stability of the intensity of the radiated electromagnetic wave. In this case, it is often used as a backfire antenna in which the radiation angle of Eφ polarized wave is negative with respect to the direction toward the terminal side of the LCX, that is, toward the power feeding side. Since LCX emits electromagnetic waves from each of a plurality of slots, a stable communication area can be realized along LCX. Moreover, if the input power to the LCX is appropriately weakened, a stable communication area can be obtained only in the vicinity of the LCX. Therefore, the communication target area can be limited only to the periphery of the LCX main body, and it is used as an antenna for preventing information leakage and ensuring communication security. In addition, it has been proposed that the LCX is arranged vertically with respect to a desk such as a conference room so that wireless communication is limited to a narrow space (see Patent Document 1). In the proposed method, the radiation direction of the radiation wave from the LCX is directed to the desk side so that only a person near the desk can perform wireless communication.

JP 2012-209636 A

Toshihiko Kishimoto and Shin Sasaki “LCX Communication System” The Institute of Electronics and Communication Engineers, Corona Publishing, August 20, 1982

  However, when the vicinity of the end side of the LCX was examined in detail, it was found that Ez-polarized radiation was actually present. Such unintended emission of Ez polarization near the LCX termination side is unnecessary, and it is necessary to prevent information leakage and ensure communication security.

  In view of the above problems, an object of the present invention is to provide a leaky coaxial cable capable of suppressing a radiated wave on the terminal side.

According to one aspect of the present invention, there is provided a leaky coaxial cable used as an antenna installed vertically on a top plate of a desk , covering a linear center conductor through which a high-frequency signal having a constant frequency propagates, and the center conductor An insulator, an outer conductor that covers the insulator and has a plurality of slots arranged at a constant pitch along the axial direction of the center conductor, and a sheath that covers the outer periphery of the outer conductor, and the longitudinal direction of the plurality of slots is Are arranged at the same angle with respect to the axial direction, the propagation wavelength of the high-frequency signal is λg, the dielectric constant of the insulator is εr, and [(εr) ^1/2 / {1+ (εr) ^1/2 }] is A As described above, a leaky coaxial cable having a pitch P in the range of Aλg <P <λg is provided.

  In one embodiment of the present invention, the pitch P is desirably in the range of 0.6λg to 0.9λg. Further, the longitudinal directions of the plurality of slots may be arranged at an angle inclined with respect to the axial direction, or may be arranged at an angle perpendicular to the axial direction.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the leaky coaxial cable which can suppress the radiation wave by the side of a termination | terminus.

It is the schematic which shows an example of LCX which concerns on embodiment of this invention. It is a side schematic diagram showing an example of a measurement system of radiation from LCX. It is the schematic which shows an example of the conventional zigzag type LCX. It is a figure which shows an example of the measurement result of the radiation mode and radiation direction of the conventional zigzag type LCX. It is a figure which shows an example of the measurement result of the radiation mode and radiation direction of LCX which concerns on embodiment of this invention. It is a figure which shows an example of the coupling loss distribution of LCX shown in FIG. It is a figure which shows an example of the coupling loss distribution of LCX in P <0.5 (lambda) g. It is a figure which shows an example of the coupling loss distribution of LCX in 0.5 (lambda) g <P <A (lambda) g. It is a figure which shows an example of the coupling loss distribution of LCX in 1 (lambda) g <P. It is the schematic which shows the other example of LCX which concerns on embodiment of this invention. It is a figure which shows an example of the coupling loss distribution of LCX shown in FIG.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description of the drawings, the same or similar parts are denoted by the same or similar reference numerals. However, it should be noted that the drawings are schematic, and the relationship between the thickness and the planar dimensions, the ratio of the thickness of each layer, and the like are different from the actual ones. Therefore, specific thicknesses and dimensions should be determined in consideration of the following description. Moreover, it is a matter of course that portions having different dimensional relationships and ratios are included between the drawings.

  The following embodiments of the present invention exemplify apparatuses and methods for embodying the technical idea of the present invention. The technical idea of the present invention is based on the material and shape of component parts. The structure, arrangement, etc. are not specified below. The technical idea of the present invention can be variously modified within the technical scope described in the claims.

  The LCX 1 according to the embodiment of the present invention includes a center conductor 3, an insulator 5, an outer conductor 7, and a sheath 9, as shown in FIG. The central conductor 3 extends in the axial direction (z-axis direction) from the power supply side to which the high-frequency signal is supplied toward the terminal side. The insulator 5 is provided so as to cover the central conductor 3. The outer conductor 7 is provided so as to cover the insulator 5. The sheath 9 is provided so as to cover the outer periphery of the outer conductor 7. The external conductor 7 is provided with a plurality of slots 10 at a constant pitch P along the axial direction of the LCX. The slot 10 is an oblique slot arranged with its longitudinal direction inclined at an angle γ with respect to the z-axis direction.

  For example, the LCX 1 includes a central conductor 3 made of copper wire having a diameter of 2 mm, an insulator 5 made of polyethylene foam having a relative dielectric constant εr of 1.5 and an outer diameter of 5 mm, and an outer conductor made of copper foil having a thickness of 0.01 mm. 7 and a sheath 9 having an outer diameter of 7 mm are used. The characteristic impedance of LCX1 is 50Ω, for example. The slot 10 has a length of 12 mm and a width of 2 mm, and an angle γ with respect to the z-axis direction is 30 degrees.

  In the description of the embodiment, as shown in FIG. 2, a configuration is used in which LCX 1 is installed vertically on a top board 30 such as a conference desk as a communication area. As a coordinate system, the axial direction of LCX1 is z, and the circumferential direction about LCX1 is φ. The radiation angle θ of the radiated wave from the LCX 1 is negative (−) when going to the power supply side and positive (+) when going to the terminal side, with the orthogonal direction x of the z-axis being 0 degree. A power feeding unit 14 that supplies a signal of the input power Pin is connected to the power feeding side of the LCX 1 via the coaxial cable 16. A termination unit 12 having a characteristic impedance of LCX1, for example, 50Ω, is connected to the termination side. The radiated wave from the LCX 1 is received by a receiving antenna 20 such as a half-wave standard dipole antenna. The receiving antenna 20 is connected via a coaxial cable 22 to a receiving unit 24 that detects the received power Pout of the radiated wave.

  As described in Non-Patent Document 1, a radiation wave from LCX has a plurality of radiation modes. Each radiation mode is called an m-th order radiation mode. Here, m is a negative integer. The radiation angle θm of the m-th order radiation mode is calculated from the slot pitch, the relative dielectric constant of the LCX insulator, and the wavelength of the frequency used.

  As shown in FIG. 3, in the general LCX 1a, a plurality of slots 10a and 10b are provided in a zigzag manner at a constant pitch P along the z-axis. The slot 10a is disposed with its longitudinal direction inclined at an acute angle α with respect to the z-axis direction. The slot 10b adjacent to the slot 10a is provided with its longitudinal direction inclined at an obtuse angle β with respect to the z-axis direction. The angles α and β are complementary to each other.

  In the zigzag LCX 1a, assuming that the dielectric constant εr of the insulator 5 and the signal frequency are constant, no radiation wave exists when the pitch P is short, and radiation occurs as the pitch P becomes longer. When the pitch P is lengthened, first, an Eφ, −1 mode of polarization with an electric field of φ direction is generated as a lowest order mode from a radiation angle θ of −90 degrees. When the pitch P is increased, the radiation wave is perpendicular to the z axis, that is, a radiation angle θ of 0 degree. When the pitch P is further increased, the polarization of the Eφ, −1 mode is directed in the direction where the radiation angle θ is +90 degrees. As pitch P increases, Ez, -2 mode with an electric field in the z direction is generated as the second radiation mode, and then higher-order radiation modes such as Eφ, -3 mode are generated as the third radiation mode. To do.

When there are a plurality of radiation modes in the same polarization, the radiation intensity becomes unstable due to interference between the radiation waves of each mode. Therefore, it is desirable to use in the range of the pitch P which becomes a single radiation mode region. The conventionally used single radiation mode region is a region where only the Eφ, −1 mode exists before the Ez, −2 mode is generated. Here, when the propagation wavelength of the signal in the LCX is λg, the dielectric constant of the insulator 5 is εr, and [(εr) ^1/2 / {1+ (εr) ^1/2 }] is A, the single radiation mode Is a range of 0.5Aλg <P <Aλg. For example, when used in the configuration shown in FIG. 2, the radiation angle θ is negative, that is, the communication area can be limited to the top plate 30 of the desk by using it as a backfire antenna. For the backfire type, the pitch P may be set in the range of 0.5Aλg <P <0.5λg. In the vicinity of P≈0.5λg where the radiation angle θ is about 0 °, resonance is caused by the slots 10a and 10b, and the reflected wave on the power feeding side increases, so that it is not used.

  However, when the zigzag type LCX1a in the range of the pitch P, which was conventionally considered to be a single radiation mode region of Eφ, -1, unexpected radiation is generated in the vertical direction of the LCX1a from the terminal side and the power feeding side. It has been confirmed that. Radiation from the terminal side and the power feeding side is unnecessary radiation, which may cause leakage of communication information. FIG. 4 shows the result of measuring the radiated wave by changing the pitch P of the slots 10a and 10b of the LCX 1a with the configuration shown in FIG.

  As shown in FIG. 4, Eφ, -1 mode radiation is P <0.5 Aλg on the power feeding side, 0.5 Aλg <P <0.5λg, radiation angle θ is −90 degrees to 0 degrees, P> 0. At 5λg, the radiation angle θ is 0 ° to + 90 °. Resonance occurs when the pitch P is 0.5λg and 1λg. Ez, -2 mode radiation is P <0.5λg, termination side, 0.5λg <P <Aλg, power supply side, Aλg <P <1λg, radiation angle θ is −90 degrees to 0 degrees, and P> 1λg. The radiation angle θ is 0 degree to +90 degrees. Eφ, -3 mode radiation has no radiation when P <1.5 Aλg, and radiation angle θ is −90 degrees to 0 degrees when P> 1.5 Aλg. Thus, even in the range of 0.5 Aλg <P <Aλg, which has been conventionally referred to as a single radiation mode region, radiation exists on the termination side and the power feeding side. In particular, in the range of 0.5 Aλg <P <0.5λg used as a backfire antenna, there is radiation toward the termination side. As shown in FIG. 4, it can be seen that the zigzag LCX 1a cannot realize a single-radiation mode region backfire antenna, and there is unnecessary radiation from the terminal side.

  FIG. 5 shows the result of measuring the radiated wave by changing the pitch P of the slot 10 with the configuration shown in FIG. 2 using the LCX 1 having the oblique slots shown in FIG. As shown in FIG. 5, since the slots 10 have the same shape, only even mode radiation exists. In LCX1 having an oblique slot as shown in FIG. 5, the radiation of Ez, -2 mode and Eφ, -2 mode is terminated at P <0.5λg, feed side at 0.5λg <P <Aλg, and Aλg < When P <1λg, the radiation angle θ is −90 degrees to 0 degrees, and when P> 1λg, the radiation angle θ is 0 degrees to +90 degrees. Resonance occurs when the pitch P is 1λg. Ez, -4 mode and Eφ, -4 mode radiation has no radiation when P <2Aλg, and a radiation angle θ of −90 degrees to 0 degrees when P> 2Aλg. Note that resonance occurs when the pitch P is 2λg.

  As described above, by using the oblique slots whose longitudinal directions are arranged at the same angle with respect to the z-axis, the Ez, -2 mode and the Eφ, -2 mode are within the range of Aλg <P <1λg. It is possible to realize a backfire antenna with a polarized wave. In the case of an oblique slot, Ez, -2 mode and Eφ, -2 mode polarizations are mixed, but no interference occurs because the directions are orthogonal to each other.

  FIG. 6 shows the result of measuring the coupling loss around the LCX1 by placing the LCX1 having a pitch P of 80 mm and a total length of 1 m vertically on the center of the top plate 30 of the desk having a width of about 3 m. Show. As shown in FIG. 2, a signal of input power Pin is supplied from the power feeding unit 14, and a radiated wave from the LCX 1 is received by the receiving antenna 20. The reception unit 24 detects the reception power Pout of the radiated wave. The coupling loss Lc is calculated by the following equation.

Lc = -10 log (Pout / Pin) (dB) (1)

For example, when the frequency is 2.4 GHz and the signal of the input power Pin is supplied from the feeding part of the LCX1, when the relative dielectric constant εr of the insulator 5 is 1.5, A is 0.55 and λg is 102 mm. The pitch P is in the range of Aλg <P <1λg.

  As shown in FIG. 6, the radiated wave is radiated toward the installation surface, that is, the top plate 30 at a radiation angle θ of −20 degrees. In the radiated wave, Ez polarization and Eφ polarization are mixed. These radiated waves are Ez polarization of Ez, -2 mode and Eφ polarization of Eφ, -2 mode, respectively. The coupling loss Lc of the Ez polarization and the Eφ polarization is about 65 dB and about 70 dB, respectively, at a position about 1.5 m away from the LCX 1 on the top plate 30.

  In addition, Ez polarized waves and Eφ polarized waves are also observed from the terminal side. The coupling loss Lc of the Ez polarization and Eφ polarization on the termination side is about 75 dB and about 85 dB, respectively, at a position about 1.5 m away from the terminator 12.

  Next, with the same configuration of FIG. 6, the radiation on the terminal side of the LCX 1 is confirmed with the pitch P in the range of P <0.5λg. For example, the coupling loss distribution around the LCX 1 was measured with the pitch P being P = 0.4λg, that is, 41 mm. As shown in FIG. 7, it can be seen that the radiated wave is radiated in the x-axis direction, which is the radial direction of LCX1, from the terminal side. In the radiated wave, Ez polarization and Eφ polarization are mixed.

  Similarly, the radiation on the feeding side of the LCX 1 is confirmed with the pitch P in the range of 0.5λg <P <Aλg. For example, when the pitch P is P = 0.53λg, that is, 54 mm, the coupling loss distribution around the LCX 1 is measured. As shown in FIG. 8, it can be seen that the radiated wave is radiated in the x-axis direction, which is the radial direction of LCX1, from the power supply side. In the radiated wave, Ez polarization and Eφ polarization are mixed.

  Furthermore, it is confirmed that the radiation angle θ from the LCX 1 is emitted in the direction of 0 degree <θ <90 degrees with the pitch P in the range of 1λg <P. For example, the coupling loss distribution around the LCX 1 was measured by setting the pitch P to P = 1.2λg, that is, 122 mm. As shown in FIG. 9, it can be seen that the radiated wave is radiated from LCX1 at a radiation angle θ of +12 degrees. In the radiated wave, Ez polarization and Eφ polarization are mixed.

  As described above, the LCX 1 using the slanted slot can be used as a backfire antenna that suppresses unnecessary radiation from the termination side in a range where the pitch P is Aλg <P <1λg. As a result, a narrow space on the top board 30 such as a conference desk can be limited to the communication area, and communication security can be ensured. In addition, in LCX1 using an oblique slot, the polarization of Ez, -2 mode and Eφ, -2 mode is mixed, so that the degree of freedom of the receiving antenna to be used can be increased.

  The slot 10 is an oblique slot whose angle γ with respect to the z-axis direction in the longitudinal direction is 30 degrees. The angle γ is not limited to 30 degrees, and may be an arbitrary inclination angle. Further, as shown in FIG. 10, a slot 10 having an angle γ in the longitudinal direction perpendicular to the z-axis direction may be used. FIG. 5 shows the result of measuring the radiated wave using the LCX 1 having the vertical slot 10 and changing the pitch P of the slot 10 with the configuration shown in FIG. As shown in FIG. 5, since the slots 10 have the same vertical shape, only even-mode Ez polarized radiation exists. Ez, -2 mode radiation is P <0.5λg, termination side, 0.5λg <P <Aλg, power supply side, Aλg <P <1λg, radiation angle θ is −90 degrees to 0 degrees, and P> 1λg. The radiation angle θ is 0 degree to +90 degrees. Resonance occurs when the pitch P is 1λg. Ez, -4 mode radiation has no radiation when P <2Aλg, and radiation angle θ is −90 degrees to 0 degrees when P> 2Aλg. Note that resonance occurs when the pitch P is 2λg. Thus, in the LCX 1 having the vertical slot 10, a single-radiation mode backfire antenna with only Ez polarization can be realized in the range of Aλg <P <1λg.

  In FIG. 11, an LCX 1 having a length of 5 mm, a vertical shape having a width of 2 mm, and a slot 10 having a pitch P of 80 mm and having a total length of 1 m is vertically installed in the center of a top plate 30 of a desk having a width of about 3 m. And the result of having measured the coupling loss around LCX1 is shown. As shown in FIG. 11, the radiated wave is radiated toward the installation surface, that is, the top plate 30 at a radiation angle θ of −20 degrees. The radiated wave is mainly Ez, -2 mode Ez polarized wave, and Eφ polarized wave is weak. The coupling loss Lc of Ez polarization is about 73 dB at a position about 1.5 m away from LCX1 on the top plate 30.

  In addition, Ez polarized waves and Eφ polarized waves are also observed from the terminal side. The coupling loss Lc of the Ez polarization and Eφ polarization on the termination side is about 85 dB and about 100 dB, respectively, at a position about 1.5 m away from the terminator 12. Thus, the LCX 1 using the vertical slot 10 can be used as a backfire antenna that suppresses unnecessary radiation from the terminal side. As a result, a narrow space on the top board 30 such as a conference desk can be limited to the communication area, and communication security can be ensured.

  In the above description, the length of LCX1 is 1 m, the radiation angle θ is about −20 degrees, and a communication area of about 3 m is secured, but the length of LCX1 and the radiation angle θ are not limited. For example, the length of LCX1 is practically in the range of 0.5 m to 3 m. Further, as a communication area on a desk such as a conference room, a space of about 1 m to about 6 m may be considered. In such a case, the radiation angle θ may be in the range of about −10 degrees to about −80 degrees. The radiation angle θm of the m-th order radiation mode polarization is calculated from the pitch P, the relative dielectric constant εr of the insulator 5 and the propagation wavelength λg of the signal in the LCX 1 by the following equation.

sin θm = mλg (εr) ^1/2 / (2P) + (εr) ^1/2 (2)

The pitch P is expressed by the following equation.

P = [m (εr) ^1/2 λg / {(εr) ^1/2 −sin θm}] / 2 (3)

For example, regarding the polarization of Ez, -2 mode and Eφ, -2 mode, when the relative dielectric constant εr of the insulator 5 is 1.5, the radiation angle θ-2 is in the range of −10 degrees to −80 degrees. Therefore, the pitch P may be set in the range of 0.6λg to 0.9λg according to the equation (3).

  Moreover, although the rounded rectangle is used as the shape of the slot 10, the shape is not limited. For example, the slot 10 may be rectangular, or may be circular, elliptical, or square.

(Other embodiments)
Although the embodiments of the present invention have been described as described above, it should not be understood that the descriptions and drawings constituting a part of this disclosure limit the present invention. From this disclosure, various alternative embodiments, examples and operational techniques will be apparent to those skilled in the art. Accordingly, the technical scope of the present invention is defined only by the invention specifying matters according to the scope of claims reasonable from the above description.

1 ... Leaky coaxial cable (LCX)
DESCRIPTION OF SYMBOLS 3 ... Center conductor 5 ... Insulator 7 ... Outer conductor 9 ... Sheath 10 ... Slot 12 ... Terminator

Claims (4)

Leakage coaxial cable used as an antenna installed vertically on the top of the desk,
A linear center conductor through which a high-frequency signal having a constant frequency propagates;
An insulator covering the central conductor;
An outer conductor covering the insulator and having a plurality of slots arranged at a constant pitch along the axial direction of the central conductor;
A sheath covering the outer periphery of the outer conductor,
The longitudinal directions of the plurality of slots are arranged at the same angle with respect to the axial direction;
When the propagation wavelength of the high-frequency signal is λg, the relative dielectric constant of the insulator is εr, and [(εr) ^1/2 / {1+ (εr) ^1/2 }] is A, the pitch P is Aλg <P < Leakage coaxial cable characterized by being in the range of λg.   The leaky coaxial cable according to claim 1, wherein the pitch P is in a range of 0.6λg to 0.9λg.   The leaky coaxial cable according to claim 1 or 2, wherein longitudinal directions of the plurality of slots are arranged at an angle inclined with respect to the axial direction.   The leaky coaxial cable according to claim 1, wherein longitudinal directions of the plurality of slots are arranged at an angle perpendicular to the axial direction.

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