JP5631374B2 - antenna - Google Patents

Description

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

  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.

  It has been proposed to use LCX as an antenna when it is desired to avoid a large and conspicuous antenna in a space that is not so long, such as a conference room or a waiting room (see Patent Document 1). In particular, in order to make the presence of the antenna as unknown as possible, a small diameter LCX having an outer diameter of about 10 mm or less is used. Further, it has been studied to use the small diameter LCX as an antenna even in a communication device which is a narrower space (see Non-Patent Document 2).

  In the small-diameter LCX used in Patent Document 1 and Non-Patent Document 2, a zigzag slot is provided in an outer conductor having an outer diameter of about 5 mm. The length in the longitudinal direction of the slot (slot length) is Ls, the length of the slot in the circumferential direction of the LCX is Lsc, and the length of the slot in the axial direction of the LCX is Lsl. The angles with respect to the axial direction of the slots are γ and (180−γ).

  The slot is equivalent to a tear present in the outer conductor. Therefore, when the circumferential length Lsc is increased, when an external force such as bending is applied to the LCX, the external conductor breaks and becomes a fatal situation in which it cannot operate as an antenna. On the other hand, if the axial length Lsl is too long, the slot is distorted by an external force applied to the LCX and the intensity of the radiated wave from the LCX may become unstable. Empirically, the circumferential length Lsc is desirably 50% or less of the outer conductor circumferential length. If the outer conductor has an outer diameter of 5 mm, the circumference is about 16 mm. Therefore, the circumferential length Lsc of the slot needs to be 8 mm or less. Further, the axial length Lsl is empirically desirably 15 mm or less.

  For an outer conductor having an outer diameter of 5 mm, a slot having a circumferential length Lsc of 3 mm to 8 mm and an axial length Lsl of 15 mm or less has a slot length Ls of about 6 mm to about 16 mm and an angle γ of about 20 degrees. ~ 30 degrees. The slot shape actually applied to LCX is selected according to the required radiation intensity. For example, when increasing the radiation intensity, the slot length Ls is increased to about 16 mm and the angle γ is set to about 30 degrees. When reducing the radiation intensity, the slot length Ls is shortened to about 6 mm and the angle γ is set to about 30 degrees.

  As described above, in the outer conductor having an outer diameter of 5 mm, the slot length Ls is from several mm to about 16 mm at the longest. In LCX, electromagnetic waves leaking from individual slots form radiated waves. However, if the wavelength of the electromagnetic wave used is too long for the slot length Ls, the electromagnetic wave leaking from the slot is reduced. This corresponds to the fact that electromagnetic waves with a short wavelength (high frequency) are transmitted through the wire mesh with respect to the mesh of the wire mesh, and electromagnetic waves with a long wavelength (low frequency) with respect to the mesh cannot be transmitted through the wire mesh and reflected. In an electromagnetic wave with a frequency of 450 MHz (wavelength 666 mm), when the slot length Ls is several tens of mm or less, that is, about 5% or less of the wavelength, radiation from the LCX is very small (see Non-Patent Document 1 and FIG. 2).

  In a small-diameter LCX having an outer conductor with an outer diameter of 5 mm, the slot length Ls is about 16 mm at the longest. Therefore, the electromagnetic wave that can be emitted has a wavelength of about 320 mm or less, that is, a frequency of about 940 MHz or more. Accordingly, the small diameter LCX having an outer conductor with an outer diameter of 5 mm is difficult to use at a frequency of 940 MHz or less and is not practical at 450 MHz or less.

  There is a sleeve antenna as a small antenna that can be used at a low frequency (see Non-Patent Document 3). The sleeve antenna has a 1/4 wavelength center conductor exposed at the end of the coaxial cable, and a 1/4 wavelength long conductor is folded and connected around the outer conductor from the base of the exposed portion. Yes. However, the outer diameter of the sleeve antenna is as small as 25 mm, which is not suitable for use in a minute and complicated space.

JP 2011-244194 A

Toshihiko Kishimoto and Shin Sasaki “LCX Communication System” The Institute of Electronics and Communication, published on August 20, 1982 Oka, et al. “A Study on the Use of LCX in Small and Complex Spaces” 2012 IEICE General Conference, B-5-155 “Antenna Engineering Handbook” The Institute of Electronics, Information and Communication Engineers, 2nd edition, Ohmsha, published July 25, 2008, p. 136-137

  In view of the above problems, an object of the present invention is to provide an antenna that can be reduced in diameter and used at a low frequency.

  According to one aspect of the present invention, a first general-purpose coaxial cable that feeds a signal, a linear center conductor that is connected to the first general-purpose coaxial cable at a start end and extends from the start end to a termination end, And a leaky coaxial cable having an outer conductor having a plurality of slots provided at a predetermined pitch along the extending direction, covering the center conductor with the insulator interposed therebetween, and setting the wavelength of the signal to 1 and As one of 2 and 3, an antenna is provided in which the length of the leaky coaxial cable in the extending direction is nλ / 4.

  In one aspect of the present invention, it is desirable to include a terminator that is connected to the termination and terminates at the characteristic impedance of the leaky coaxial cable. Moreover, it is desirable to provide the ferrite core which covers the outer periphery of a 1st general purpose coaxial cable adjacent to a starting end part. Furthermore, it is good also as a structure provided with the 2nd general purpose coaxial cable connected to the termination part, and the ferrite core which covers the outer periphery of a 2nd general purpose coaxial cable adjacent to a termination part.

  According to the present invention, it is possible to provide an antenna that can be reduced in diameter and used at a low frequency.

It is the schematic which shows an example of the antenna which concerns on embodiment of this invention. It is the schematic which shows an example of LCX used for the antenna which concerns on embodiment of this invention. It is the schematic which shows an example of the measurement system of the coupling loss distribution of the antenna which concerns on embodiment of this invention. It is a figure which shows an example of the coupling loss distribution obtained from the antenna whose LCX length is a half wavelength. It is a figure which shows an example of the coupling loss distribution obtained from the antenna whose length of LCX is 1/4 wavelength. It is a figure which shows an example of the coupling loss distribution obtained from the antenna whose length of LCX is 3/4 wavelength. It is a figure which shows an example of the coupling loss distribution obtained from the antenna whose length of LCX is 1 wavelength. It is the schematic which shows an example of the antenna which concerns on the modification of embodiment of this invention. It is a figure which shows an example of the coupling loss distribution obtained from the antenna shown in FIG. It is the schematic which shows the other example of the antenna which concerns on the modification of embodiment of this invention. It is a figure which shows an example of the coupling loss distribution obtained from the antenna shown in FIG. It is a figure which shows an example of the coupling loss distribution obtained by removing a ferrite core from the antenna 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.

  As shown in FIG. 1, the antenna according to the embodiment of the present invention includes LCX 1, terminator 3, and general-purpose coaxial cable 7. The cable length in the extending direction (z-axis direction) of LCX1 is L. The terminator 3 is connected to the LCX 1 via the connector 5 at the end of the LCX 1. The general-purpose coaxial cable 7 is connected to the LCX 1 via the connector 9 at the start end of the LCX 1. The terminator 3 is a termination resistor having the same resistance value as the characteristic impedance of the LCX 1 so that unnecessary reflection does not occur. The general-purpose coaxial cable 7 supplies a high-frequency signal to the LCX 1.

  As shown in FIG. 2, the LCX 1 includes a center conductor 10, an insulator 12, an outer conductor 14, and a sheath 16. The center conductor 10 extends in the z-axis direction from the start end where the general-purpose coaxial cable 7 is connected to the end where the terminator 3 is connected. The insulator 12 is provided so as to cover the central conductor 10. The outer conductor 14 is provided so as to cover the central conductor 10 with the insulator 12 interposed therebetween. The sheath 16 is provided so as to cover the outer periphery of the outer conductor 14. The outer conductor 14 is provided with a plurality of slots 18 at a predetermined pitch along the extending direction (z-axis direction) of the LCX 1. The slot 18 is disposed at an angle γ with respect to the z axis.

  For example, a copper wire having a diameter of 2 mm is used for the central conductor 10 and a foamed polyethylene having a diameter of 5 mm is used for the insulator 12. A copper foil having a thickness of 0.01 mm is vertically attached to the outer periphery of the insulator 12 as the outer conductor 14, and a sheath 16 having an outer diameter of 7 mm is covered on the outer periphery of the outer conductor 14. The characteristic impedance of LCX1 is 50Ω. The slot 18 has a slot length of about 12 mm and an angle with respect to the axial direction of 30 degrees. In the following description, the signal frequency supplied to LCX1 is 450 MHz. However, the signal frequency is not limited and any frequency may be used. The wavelength of the electromagnetic wave having a frequency of 450 MHz is about 666 mm, which is 50 times larger than the slot length of 12 mm.

  LCX1 with a cable length L of half the signal frequency, ¼ wavelength, ¾ wavelength, and 1 wavelength was produced. Actually, the cable length L is determined in consideration of the wavelength reduction in LCX1. Since the general wavelength shortening rate is 5%, the half wavelength, 1/4 wavelength, 3/4 wavelength, and 1 wavelength of the frequency 450 MHz are 333 mm, 167 mm, 500 mm, and 666 mm, respectively. Therefore, when the wavelength shortening rate is expected to be 5%, the cable lengths L of the half-wavelength, quarter-wavelength, quarter-wavelength, and one-wavelength LCX1 are 316 mm, 158 mm, 475 mm, and 633 mm, respectively. A 50Ω terminator 3 is attached to the end of the LCX 1 to prevent reflection.

  First, as shown in FIG. 3, an antenna having a half-wavelength LCX1 cable was installed on the floor of the anechoic chamber 30, and the coupling loss distribution was measured. In the anechoic chamber 30, the axial direction of LCX1 is z, and the height direction is x. In the central part of the anechoic chamber 30, the LCX1 is arranged so that the start end of the LCX 1 is located at z = 1 m. A power feeding unit 20 provided outside the anechoic chamber 30 is connected to the start end of the LCX 1 via the general-purpose coaxial cable 7. As the receiving antenna 24, for example, a half-wave standard dipole antenna is disposed directly above the antenna. The receiving antenna 24 is connected to a receiving unit 28 provided outside the anechoic chamber 30 via an approach cable 26. The radiated wave from the antenna shown in FIG. 1 is Ez polarized wave.

  A signal having an input power Pt with a frequency of 450 MHz is supplied from the power feeding unit 20 to the starting end of the LCX 1, and a radiated wave from the antenna is received by the receiving antenna 24. The reception unit 28 detects the reception power Pr of the radiated wave. The coupling loss Lc is calculated by the following equation.

Lc = −10 log (Pr / Pt) (dB) (1)

FIG. 4 shows the result of measuring the distribution of the coupling loss Lc while changing the height (x) of the receiving antenna 24 in the range of 0.25 m to 1.5 m and the position (z) in the range of 0 to 3 m. . As shown in FIG. 4, it can be seen that strong radiation is obtained above LCX1. Wireless communication in a narrow space in the 450 MHz band can be realized by using LCX1 having a cable length of half wavelength.

  Next, an antenna having an LCX1 cable length of ¼ wavelength was installed on the floor of the anechoic chamber 30 shown in FIG. 3, and the coupling loss distribution was measured. FIG. 5 shows the result of measuring the distribution of the coupling loss Lc. As shown in FIG. 5, it can be seen that strong radiation is obtained above LCX <b> 1 though the radiation is weak as a whole as compared with the case where LCX having a half-wavelength is used. In this way, wireless communication in a narrow space in the 450 MHz band can be realized even when the cable length is LCX1 having a quarter wavelength.

  Further, an antenna having a LCX1 cable length of 3/4 wavelength was installed on the floor surface of the anechoic chamber 30 shown in FIG. 3, and the coupling loss distribution was measured. FIG. 6 shows the result of measuring the distribution of the coupling loss Lc. As shown in FIG. 6, it can be seen that strong radiation is obtained above LCX1. As described above, even when the cable length is 3/4 wavelength LCX1, wireless communication in a narrow space in the 450 MHz band can be realized.

  Further, an antenna having a LCX1 cable length of one wavelength was installed on the floor surface of the anechoic chamber 30 shown in FIG. 3, and the coupling loss distribution was measured. FIG. 7 shows the result of measuring the distribution of the coupling loss Lc. As shown in FIG. 7, in the axial direction, it can be confirmed that there is a region where the radiation rapidly decreases above the center of LCX1. The cause of the decrease in the radiation is that the cable length of the LCX 1 is one wavelength, the current on the outer conductor 14 is the same magnitude from the central part of the LCX 1 and is directed in the opposite direction to the start and end parts and radiates above the central part. This is because a weak region is generated. Thus, it can be seen that when LCX1 having a cable length of one wavelength is used, stable radiation cannot be obtained around the antenna.

  As described above, it is desirable to use the LCX1 having a cable length of 1/4 wavelength and 3/4 wavelength as an antenna used in a narrow space in the 450 MHz band. Further, when the cable length of the LCX1 is 1/4 wavelength and 3/4 wavelength, the current distribution on the outer conductor 14 is symmetric when the cable length is half wavelength, and the coupling loss distribution Lc above the LCX1 is also Become symmetrical. Therefore, in the LCX1 having a cable length of half wavelength, more stable radiation can be obtained around the antenna. Thus, from the viewpoint of coupling loss and radiation stability, the half-wavelength is most desirable as the cable length of LCX1, and then 3/4 wavelength is desirable. Further, the LCX1 having a cable length of ¼ wavelength is most suitable for miniaturization.

(Modification)
As shown in FIG. 8, the antenna according to the modification of the embodiment of the present invention includes LCX 1, terminator 3, general-purpose coaxial cable 7, and ferrite core 40. The ferrite core 40 covers the outer periphery of the general-purpose coaxial cable 7 adjacent to the start end of the LCX 1.

  The modification of the embodiment is different from the embodiment in that the ferrite core 40 is provided adjacent to the starting end portion of the LCX1. Other configurations are the same as those in the embodiment, and thus redundant description is omitted.

  In the coupling loss distribution shown in FIGS. 4 to 6, it can be seen that there is radiation also on the general-purpose coaxial cable 7 side. This is because the current on the outer conductor 14 of the LCX 1 slightly propagates to the general-purpose coaxial cable 7 side.

  For example, an antenna in which the ferrite core 40 is mounted on the general-purpose coaxial cable 7 adjacent to the start end of the LCX 1 using the LCX 1 having a cable length of half wavelength was manufactured. The produced antenna was installed on the floor of the anechoic chamber 30 shown in FIG. 3, and the coupling loss distribution was measured. FIG. 9 shows the result of measuring the distribution of the coupling loss Lc. As shown in FIG. 9, it can be seen that radiation waves are concentrated above LCX1. Thus, the radiation toward the general-purpose coaxial cable 7 can also be reduced, and an antenna having ideal radiation directivity can be realized. Further, the same effect can be obtained even when the LCX 1 having a quarter wavelength and a quarter wavelength is used.

  Note that a communication area may be set in the middle of the general-purpose coaxial cable. In such a case, as shown in FIG. 10, a general-purpose coaxial cable (first general-purpose coaxial cable) 7 is connected to the start end side of LCX1, and a general-purpose coaxial cable (second general-purpose coaxial cable) 7a is connected to the end side of LCX1. It becomes the composition which was done. The general-purpose coaxial cable 7 is provided with a ferrite core 40 adjacent to the start end of the LCX1, and the general-purpose coaxial cable 7a is provided with a ferrite core 42 adjacent to the end of the LCX1.

  For example, an antenna in which a general-purpose coaxial cable 7a having a length of 1 m and a characteristic impedance of 50Ω was connected to a terminal portion using LCX1 having a cable length of half wavelength was manufactured. A 50Ω terminator 3 is attached to the end of the general-purpose coaxial cable 7a to prevent reflection. The produced antenna was installed on the floor of the anechoic chamber 30 shown in FIG. 3, and the coupling loss distribution was measured. Further, the ferrite cores 40 and 42 were removed from the antenna shown in FIG. 10, and the coupling loss distribution was measured. The measurement results are shown in FIGS. As shown in FIG. 11, when ferrite cores 40 and 42 are used, it can be seen that radiation is concentrated above LCX1. On the other hand, as shown in FIG. 12, when a ferrite core is not used, there is radiation from LCX1, but radiation with an unstable distribution may also be present above general-purpose coaxial cable 7a connected to the end side of LCX1. Recognize.

  As described above, when the general-purpose coaxial cables 7 and 7a are connected to the start and end portions of the LCX 1, it is desirable to provide the ferrite cores 40 and 42 in the general-purpose coaxial cables 7 and 7a, respectively. By providing the ferrite cores 40 and 42, the current leaking from the outer conductor 14 of the LCX 1 to the general-purpose coaxial cables 7 and 7a can be suppressed. As a result, radiation toward the general-purpose coaxial cable 7 can also be reduced, and an antenna having ideal radiation directivity can be realized.

(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)
3. Terminator 7. General-purpose coaxial cable (first general-purpose coaxial cable)
7a ... General purpose coaxial cable (second general purpose coaxial cable)
DESCRIPTION OF SYMBOLS 10 ... Center conductor 12 ... Insulator 14 ... Outer conductor 16 ... Sheath 18 ... Slot 40 ... Ferrite core 42 ... Ferrite core

Claims (4)

A first general purpose coaxial cable for feeding a signal;
A linear center conductor connected to the first general-purpose coaxial cable via a connector at the start end, and extending from the start end to the end, an insulator covering the center conductor, and the center conductor sandwiching the insulator And a leaky coaxial cable having an outer conductor provided with a plurality of slots at a predetermined pitch along the extending direction,
The antenna is characterized in that the wavelength of the signal is λ, n is any one of 1, 2, and 3, and the length of the leaky coaxial cable in the extending direction is nλ / 4.   The antenna according to claim 1, further comprising a terminator connected to the termination unit and terminating at a characteristic impedance of the leaky coaxial cable.   The antenna according to claim 1, further comprising a ferrite core that covers an outer periphery of the first general-purpose coaxial cable adjacent to the start end portion. A second universal coaxial cable connected to the end portion;
The antenna according to claim 1, further comprising a ferrite core that covers an outer periphery of the second general-purpose coaxial cable adjacent to the terminal end.

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