JP2008141566A - Transmitting antenna - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a transmitting antenna by which information other than a desired object is not read out by limiting the propagation region of radio waves. <P>SOLUTION: A pair of vertical radiowave absorbing side walls 2 and 3 which control the directionality of radio waves emitted from the antenna 8 are arranged in such a way that they are protruded from the proximity of both end edges of the antenna 8. Further, a half-value angle in plain view of the radio waves emitted form the antenna 8 is reduced by the pair of radiowave absorbing side walls 2 and 3. The reduced half-value angle after control is set to 40° or smaller. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a transmission antenna.

In recent years, systems that read information in a non-contact manner via electromagnetic waves or electromagnetic fields have been used as next-generation identification technologies, such as 2.45 GHz 5.25 GHz band wireless LAN, 5.8 GHz band used in ETC, etc. There are known transmit antennas. An antenna that transmits a radio wave for reading is conventionally known (see, for example, Patent Documents 1 and 2).
JP 2005-267077 A JP 2006-20083 A

  The conventional transmitting antenna for UHF band (800 MHz to 960 MHz) is one of the great advantages that it can read information in a non-contact manner as mentioned above. Problems such as reading information that exists in undesired places, tag information cannot be read correctly due to interference between adjacent similar systems, and reflected waves from various peripheral structures and transmission / reception antennas There is a problem that the tag becomes non-responsive at the point (Null point) where the direct waves from synthesize and particularly weaken each other. From these problems, how to control the electromagnetic environment has become a problem.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a transmission antenna that controls a radio wave propagation area so as not to read information other than a desired target.

  In order to achieve the above object, in the transmitting antenna according to the present invention, a pair of vertical radio wave absorption side walls for controlling the directivity of radio waves radiated from the antenna are arranged so as to protrude from the vicinity of both ends of the antenna. Is a thing

  Further, the half-value angle in plan view of the radio wave radiated from the antenna is narrowed by the pair of radio wave absorption side walls, and the narrowed control second half-value angle is set to 40 ° or less.

The present invention has the following remarkable effects.
Since the transmission antenna according to the present invention is provided with a pair of vertical radio wave absorption side walls that control the directivity of radio waves radiated from the antenna as protruding shapes from the vicinity of both ends of the antenna, The emitted radio wave (half-value angle) can be controlled (zone control), and reading of tag information in an undesired place can be prevented. Also, interference with radio waves radiated from another adjacent antenna can be prevented. Therefore, it is possible to reliably and accurately measure only a desired identification target.

Hereinafter, the present invention will be described in detail with reference to the drawings illustrating embodiments.
1 to 5 show an embodiment of a transmission antenna according to the present invention, and this transmission antenna includes an antenna 8 that transmits and receives radio waves in a wide range of frequencies from 300 MHz to 300 GHz.
A pair of vertical radio wave absorption side walls 2 and 3 for controlling the directivity of radio waves radiated from the antenna 8 are arranged so as to protrude from the vicinity of both end edges of the antenna 8.
Hereinafter, a RFID antenna system in the UHF band (800 MHz to 960 MHz) will be described as a specific example.

The antenna 8 has a radio wave radiation surface 28 parallel to the direction of the passage portion 13 (passage direction 12 of the identification target 10) in plan view, and a feeding point 18 of a predetermined (eg, 1300 mm) from the floor surface. It is held by a holding member (not shown) so as to be in the height position.
The casing 11 of the antenna 8 has a rectangular flat plate shape having a long side of 700 mm to 730 mm, a short side of 300 mm to 330 mm, and a thickness of 35 mm to 45 mm, and has an aluminum case and a PVC cover. A circular range drawn with diagonal lines around the feeding point 18 is a radio wave radiation range, and the feeding point 18 is located slightly below the center point of this range.

  Further, a radio wave absorbing floor member 15 that prevents reflection of radiated radio waves from the floor surface may be installed on the (lower) floor surface of the passage portion 13. Further, a radio wave absorbing ceiling member 16 that prevents leakage of radio waves may be installed on the (upper) ceiling side of the passage portion 13.

1, 2, 4, and 5, the apparatus of the present invention has a pair of vertical radio wave absorption side walls 2 and 3 that control the directivity of radio waves radiated from the RFID antenna 8. Is provided in a protruding shape from the vicinity of both end edges to the passage portion 13 side. That is, the radio wave absorption side walls 2 and 3 narrow the half-value angle θ of the beam region 6 in plan view of the radio wave.
Specifically, the radio wave absorption side walls 2 and 3 have a rectangular flat plate shape, and are provided in a state in which the RFID antenna 8 is sandwiched between the front and rear center positions and orthogonal to the RFID antenna 8 (perpendicular to the passage direction 12). It is done. The front end edges 20 and 30 of the side walls 2 and 3 protrude to the same position toward the passage portion 13 side.
In the present invention, the direction orthogonal to the passage portion 13 from the RFID antenna 8 is defined as the front.

Further, when the feeding point 18 of the RFID antenna 8 is provided at the center in the front-rear direction, that is, the lateral dimension W ₁ between the left side wall 2 and the feeding point 18, and the right side wall 3 and the feeding point 18. When the horizontal dimension W ₂ is set to the same size (see FIGS. 2 to 5), W ₁ <W ₂ (see FIGS. 12 to 15), or W ₂ <W ₁ May be set. The RFID antenna 8 of the embodiment performs transmission / reception of radio waves at a single feeding point 18, but an antenna provided at a position where the transmission point and the reception point are separated may be used. Corresponds to the feeding point 18 of the present invention (not shown).

Θ is a half-value angle of the radio wave radiated from the RFID antenna 8 when the pair of side walls 2 and 3 are not provided, and 6 is a beam region forming the half-value angle θ. It is due to the radiation characteristics of the antenna that the beam region 6 is inclined slightly to the left of the normal to the radio wave radiation surface 28 of the RFID antenna 8.
Θc is a control second half angle angle in which the half-value angle θ of the radio wave radiated from the RFID antenna 8 is narrowed (zone-controlled) by the side walls 2 and 3, and is set to be 20 ° to 40 °. The A zone-controlled control area 9 is indicated by dots.
When the control second half value angle θc exceeds 40 °, the radio wave spreads too much, and there is a possibility that even information on a tag in a range not to be read may be read.
On the other hand, by setting a lower limit to the control second half value angle θc, it is possible to maintain a certain amount of radio wave spread, and to ensure a long reading area (distance, passage time) of the RFID tag that passes through the gate, thereby ensuring reading omissions. Can be prevented. From this point of view, the control second half value angle θc is more preferably set to 20 ° or more.
The half-value angle is an angle formed at a point where the power is half the value (a value obtained by subtracting 3 dB from the maximum power) with reference to the point at which the radio wave is emitted most strongly, and represents the sharpness of the beam.

Further, the protruding dimension L of the side walls 2 and 3 is set to 0.5 × λ to 3 × λ (λ indicates the wavelength of the radio wave) in terms of electrical length under the condition that the control second half angle θc satisfies the above range. The distance between the side walls 2 and 3 and the left and right vertical sides of the RFID antenna 8 is set to 0.5 × λ or less (0 is in contact) under the same conditions. When the left vertical side portion and the left side wall 2 of the antenna 8 and the right vertical side portion and the right side wall 3 of the antenna 8 are provided with a gap (see FIGS. 1 to 5), respectively. In some cases (see FIG. 12 to FIG. 14) without gaps (in contact). For example, if the 953MH _Z, L is represented by about 0.48 × lambda when 150 mm.

  The radio wave absorption side walls 2 and 3 are formed by sequentially laminating, for example, polycarbonate, an adhesive material, an Ag film, PET, a void (a member having a gap), PET, an ITO film, an adhesive material, and polycarbonate.

Further, as shown in FIGS. 1, 2, and 4, a flat plate-shaped radio wave absorption facing wall 1 may be provided in an opposing manner on the front side of the RFID antenna 8 with the passage portion 13 interposed therebetween. The radio wave absorption facing wall 1 is erected in parallel with the radio wave radiation surface 28 of the RFID antenna 8. Leakage of radio waves can be prevented by connecting the upper and lower ends of the facing wall 1 to the other ends of the radio wave absorbing ceiling member 16 and the floor member 15 without any gaps.
Further, the length P in the left-right direction of the radio wave absorption facing wall 1 is formed so as to correspond to the beam region 6 of the control second half value angle θc in a state where the pair of side walls 2 and 3 are provided. That is, when the distance between the RFID antenna 8 and the opposing wall 1 is X, the RFID antenna 8 is formed so as to satisfy the formula of P ≧ 2Xtan (θc / 2). And the opposing wall 1 is installed so that the electromagnetic wave in the beam area | region 6 may be received without a leak.

FIGS. 6 to 8 show other embodiments, and the difference from the apparatus of FIGS. 1 to 5 is that the radio wave absorption upper wall 4 that narrows the upper region and the lower region of the beam region 6 radiated from the RFID antenna 8 The lower wall 5 is provided.
In FIG. 6, a horizontal radio wave absorption upper wall 4 is provided above the antenna 8 so as to protrude from the RFID antenna 8 toward the passage portion 13. The upper wall 4 has a front end edge 40 protruding toward the passage portion 13 to the same position as the front end edges 20 and 30 of the side walls 2 and 3. In FIG. 7, a horizontal radio wave absorption lower wall 5 is provided below the antenna 8 so as to protrude from the RFID antenna 8 toward the passage portion 13. The front end edge 50 of the lower wall 5 protrudes toward the passage portion 13 to the same position as the front end edges 20 and 30 of the side walls 2 and 3. In FIG. 8, the radio wave absorption upper wall 4 and lower wall 5 are provided in the same configuration as described in FIGS.
By providing the upper wall 4 and the lower wall 5, it is possible to control the angle at which the vertical component of the radio wave spreads upward and downward, and to further prevent reading of other tags and interference with other similar systems.

  9 to 11 show another embodiment of the side walls 2 and 3. In FIG. 9, the side walls 2, 3 are provided in a letter C shape in plan view in which the distance between them is tapered toward the passage portion 13. In the apparatus of FIG. 10, the side walls 2 and 3 are provided in the shape of a letter C in plan view in which the distance between the side walls 2 and 3 extends in a tapered shape toward the passage portion 13 side. Further, in the apparatus of FIG. 11, the front end edges 20 and 30 of the parallel side walls 2 and 3 are bent slightly inward in the front-rear direction. Also in the embodiment of FIGS. 9 to 11, the upper wall 4 and the lower wall 5 described in FIGS. 6 to 8 may be provided in combination.

Next, the radio wave of the RFID antenna 8 is actually measured and the result is considered.
First, the results of measuring the radio wave environment (electric field distribution) around the RFID antenna 8 by variously changing the conditions such as the installation position in the apparatus of FIGS. 1 and 2 are shown in FIGS. Further, FIG. 16 shows the result of measurement with the pair of side walls 2 and 3 omitted (conventional example), and FIG. 17 shows the measurement result of another conventional apparatus. A frame indicated by 23 is a measurement range in a plan view measured by a reception antenna (not shown) of the periphery of the RFID antenna 8. The numbers given along the measurement range 23 are positions (coordinates) from the reference point O at the center in the front-rear direction of the facing wall 1 to the RFID antenna 8 side and the front side. Measurement is performed at a number of locations while the receiving antenna is moved within the measurement range 23.

Both of the radio wave absorption side walls 2 and 3 have a vertical dimension of 2000 mm, a horizontal dimension of 1000 mm, and a thickness dimension of 80 mm (including a gap of 73 mm) (FIG. 1 shows only a part of the side walls 2 and 3). . The distance X between the RFID antenna 8 and the radio wave absorption facing wall 1 was set to 3000 mm. The feeding point 18 of the RFID antenna 8 was set at a height of 1300 mm from the floor surface. The feeding point 18 is provided on the left side of the left and right center.
The measurement frequency of the RFID antenna 8 is 953 MHz. The measurement receiving antenna installed in the measurement range 23 was a vertically polarized dipole antenna, and the measurement height position was set to 1300 mm.

FIG. 12, FIG. 13, and FIG. 14 show the electric field distributions when the protrusion dimension L of the device is set to 300 mm, 500 mm, and 750 mm. FIG. 16 shows an electric field distribution when measurement is performed without providing the front wall 2 and the rear wall 3. A white area indicated by 24 indicates an unreadable area where the information of the RFID tag 7 cannot be read because the electric field intensity is less than a certain value. The shaded area 22 indicates a readable area where the electric field strength is a predetermined value or more and the information of the RFID tag 7 can be read, and the shaded area can be read more strongly.
The readable area 22 in FIG. 16 is wide, whereas in FIGS. 12 to 14, the readable area 22 narrowed by the zone control by the front wall 2 and the rear wall 3 was measured. Moreover, it turns out that it is narrowed more largely as the protrusion dimension L becomes large. Although the measurement of the range behind the measurement range 23 is omitted, the readable area is also zone-controlled in the omitted range.

  FIG. 15 shows the electric field distribution when L is set to 500 mm and the distance between the pair of side walls 2 and 3 is set to be about twice that of the apparatus shown in FIGS. Since the readable area 22 is too wide, tag information in an undesired place is read, and the zone control is not appropriate.

FIG. 17 shows the electric field distribution when the pair of side walls 2 and 3 are not provided in the apparatus of FIG. 1 and the radio wave reflection opposing wall 14 is provided instead of the radio wave absorption opposing wall 1. According to FIG. 17, the direct wave from the RFID antenna 8 and the reflected wave from the radio wave reflection facing wall 14 are combined, and a plurality of null points 17 are generated in the readable area 22 at particularly weakening points. There is a problem that the tag becomes unresponsive.
Although not shown in the drawings, conventionally, measures have been taken to cover the RFID antenna with a radio wave shielding material, but it has been difficult to predict the arrival path of the reflected wave by the shielding material.

  FIG. 18 shows the displacement of the electric field intensity measured at a fixed distance from the RFID antenna 8 (feeding point 18) in the measurements of FIGS. 12, 13, 14, and 16. The horizontal axis shows the angle in the left-right direction measured with reference to the normal line standing on the RFID antenna 8 passing through the feeding point 18, the plus side shows the angle to the right side, and the minus side shows the angle to the left side. The vertical axis indicates the electric field strength. The half-value angle of the radio wave having a field strength 3 dB lower than the strongest point of the field strength is, as is apparent from the graph of FIG. 18, when the pair of side walls 2 and 3 are not provided (corresponding to FIG. 16). As the half-value angle θ is the largest and the protrusion dimension L is increased (in the order of FIGS. 12, 13, and 14), the control second-half value angle θc is decreased.

  From the above measurement results, the radio waves radiated from the RFID antenna 8 can be effectively zone-controlled, so that tag information at undesired locations is not read. In addition, radio waves and interference from other similar systems can be prevented, and tag information can be read accurately. Also, since no Null point occurs, reading of tag information is prevented from being missed. In addition, although the measurement result about RFID of UHF band was shown, even if it measures in other frequencies, the same zone control is performed.

Further, when a radio wave reflector is used as in the prior art, the transmitting antenna constitutes a highly directional antenna such as a parabolic or horn antenna, and the antenna gain increases. According to the present invention, since the gain of the transmitting antenna is not affected, the EIRP (effective radiation power: product of antenna gain and transmission output) value regulated as the radio wave strength in the UHF band is not changed. There is an advantage that can be controlled.
In FIG. 1, the antenna 8 can be arranged on the left and right side walls of the passage portion 13 (not shown). When one of the antennas 8 and 8 is turned on, the other is turned off, and the radio waves are alternately Make a call. Alternatively, it is also preferable to arrange the antenna 8 on the ceiling (upper) of the passage 13 so that the radio wave is transmitted downward.

  As described above, in the transmission antenna according to the present invention, the pair of vertical radio wave absorption side walls 2 and 3 that control the directivity of the radio wave radiated from the antenna 8 are disposed so as to protrude from the vicinity of both ends of the antenna 8. Therefore, the radiated radio wave can be controlled (zone control) with a simple configuration, and reading of tag information in an undesired place can be prevented. In addition, interference with radio waves radiated from another adjacent antenna (same system) can be prevented. Therefore, it is possible to reliably and accurately measure only the desired identification target 10. Further, the antenna 8 can be said to be technically rational without increasing the antenna gain and with increased directivity.

  Further, the half-value angle θ in plan view of the radio wave radiated from the antenna 8 is narrowed by the pair of radio wave absorption side walls 2 and 3, and the narrowed control latter half-value angle θc is set to 40 ° or less. It is possible to prevent reading even the tag information in the range. In addition, it is desirable that the control second half value angle θc is 20 ° or more, and by doing so, the range in which the radio wave reaches is not too narrow, so reading omission can be reliably prevented.

1 is a perspective view showing an embodiment of a transmission antenna according to the present invention. It is a top view. It is a front view which shows an antenna. It is a top view for description. It is a principal part front view. It is a principal part front view which shows another Example. It is a principal part front view which shows another Example. It is a principal part front view which shows another Example. It is a principal part top view which shows another Example. It is a principal part top view which shows another Example. It is a principal part top view which shows another Example. It is an explanatory plan view showing an electric field distribution. It is an explanatory plan view showing an electric field distribution. It is an explanatory plan view showing an electric field distribution. It is an explanatory plan view showing an electric field distribution. It is an explanatory plan view showing an electric field distribution. It is an explanatory plan view showing an electric field distribution. It is a graph which shows the measurement result of electric field strength.

Explanation of symbols

2 Radio wave absorption side wall 3 Radio wave absorption side wall 8 Antenna θ Half value angle θc Control second half value angle

Claims (2)

  A pair of vertical radio wave absorption side walls (2) and (3) for controlling the directivity of radio waves radiated from the antenna (8) are provided so as to protrude from the vicinity of both ends of the antenna (8). Transmit antenna.   The half-value angle (θ) in plan view of the radio wave radiated from the antenna (8) is narrowed by the pair of radio wave absorption side walls (2) and (3), and the narrowed control second-half value angle (θc) is 40 °. The transmission antenna according to claim 1, wherein: