Patent Application
20100109846
1. Field of the Invention
The present invention relates to a microstrip antenna configured to conduct radio communication with a communication target and an apparatus for reading a radio frequency identification (RFID) tag information configured to read information from RFID tag using this microstrip antenna.
2. Description of the Related Art
An antenna used in radio communication with a communication target having a configuration provided with a function to switch a polarization phase has been known. Typically, a transmitter or a receiver and a branch point is connected to each other through a transmission line, and a plurality of radio frequency transmission lines are branched from the branch point. Also, each of the radio frequency transmission lines reaches a plurality of power supply points of a radiation patch from respective different directions. When a switching element disposed in an intermediate part of each radio frequency transmission line is operated and only one intermediate part is shortened, the polarization phase is switched between horizontal polarization and vertical polarization.
Recently, as a method for reducing a size of a planar antenna such as a patch antenna, a planar inverted F antenna is known. This planar inverted F antenna is provided with a structure in which one apex of a half-wavelength resonant rectangular microstrip antenna is grounded, and a power supply point is disposed on a side including the apex or close to the side including the apex. As a result, a patch area to a microstrip antenna with the same frequency is reduced to approximately ¼.
Also, if information transmission and reception is conducted with an RFID tag, it is known that communication sensitivity can be increased by switching a polarization phase. However, if the planar inverted F antenna is used, switching of the polarization phase is difficult. On the other hand, if the prior art reference is applied to a usual planar patch antenna, switching of the polarization phase can be performed. However, in this case, it is difficult to reduce the size as in the planar inverted F antenna.
An object of the present invention is to provide a microstrip antenna capable of switching of a polarization phase in communication and size reduction and an apparatus for reading RFID tag information provided with this microstrip antenna.
In order to achieve the above object, according to the invention, there is provided a microstrip antenna comprising a substrate constructed of a dielectric body; a microstrip antenna pattern configured to supply power, formed on one side of the substrate; and a base plate disposed on the other side of the substrate, the microstrip antenna pattern having one power supply point to be connected to a power supply line and two grounding points to be connected to the base plate.
An embodiment of the present invention will be described below referring to the attached drawings.
In
A reader 1, which is an apparatus for reading RFID tag information of the embodiment, is a handheld type. The reader 1 has a main body control part 2 contained in a substantially solid rectangular housing. On this housing of the main body control part 2, a reader antenna unit 3 capable of switching a polarization phase between a longitudinal direction and a lateral direction is disposed at one end portion in the longitudinal direction, and an operation part 7 and a display part 8 are disposed on one of plane parts of the housing. A specific configuration will be described later in detail.
The reader 1 reads information relating to a corresponding article B from the RFID tag T attached to each of the article B via radio communication. A user also functioning as an administrator of the article B manages a storage situation of each article B by using the reader 1. A communicable area 20, that is, a range indicated by a broken line in the figure of the reader 1 is an area spread from the reader antenna unit 3 as a base point. In the RFID tags present in this communicable area 20, only the RFID tag T whose direction of the polarization phase of the tag antenna 151 matches the direction of the polarization phase of the reader antenna unit 3 can conduct favorable radio communication with the reader 1.
In the illustrated example, the directions of the polarization phases of the tag antennas 151 of a large number of RFID tags T are not uniform but either of the longitudinal direction or the lateral direction. In this case, in order to read information from all the RFID tags T present in the communicable area 20, the reader 1 switches the polarization phase of the reader antenna unit 3 between the longitudinal direction and the lateral direction so as to read the information in each direction via radio communication.
In
The reader antenna unit 3 has a microstrip antenna 11 as an apparatus antenna generally formed substantially in a planar shape and a switching circuit 12 that switches a polarization phase direction of the microstrip antenna 11. The microstrip antenna 11 is a so-called patch antenna and its specific configuration will be described later in detail. The switching circuit 12 functions as a switching device and controller.
The CPU 4 executes signal processing according to a program stored in the ROM in advance using a temporary storage function of the RAM so as to perform various control of the entire reader 1.
The RFID tag T has an RFID tag circuit element To provided with the tag antenna 151 and an IC circuit part 150. The RFID tag circuit element To is disposed on a base material, not particularly shown, and the RFID tag T can be attached to the article B. This RFID tag circuit element To will be described later in detail. The tag antenna 151 generally includes a substantially linear dipole antenna, and a longitudinal direction of the dipole antenna is a direction forming the polarization phase.
In
The RF communication control part 9 makes an access to information of the IC circuit part 150 of the RFID tag circuit element To, that is, RFID tag information including a tag ID through the reader antenna unit 3. The RF communication control part includes a transmitting portion 212, a receiving portion 213, and a transmit-receive splitter 214.
The transmitting portion 212 is a block configured to generate an interrogation wave to access RFID tag information of the IC circuit part 150 of the RFID tag circuit element To through the reader antenna unit 3. The transmitting portion 212 is provided with a crystal oscillator 215A, a Phase Locked Loop (PLL) 215B, a Voltage Controlled Oscillator (VCO) 215C, a transmission multiplying circuit 216, and a variable transmission amplifier 217.
The crystal oscillator 215A generates a reference frequency. The PLL 215B executes control so that a frequency of a carrier wave outputted by the VCO 215C becomes a predetermined frequency based on a command of the CPU 4 according to a frequency generated by the crystal oscillator 215A. The VCO 215C outputs a carrier wave with a frequency determined based on a control voltage generated by the PLL 215B. As the frequency of the generated carrier wave, a UHF band, a micro wave band or a short-wave band frequency, for example, is used.
The transmission multiplying circuit 216 modulates the carrier wave generated based on the signal supplied from the CPU 4. In this example, the transmission multiplying circuit 216 executes amplitude modulation on the basis of a “TX_ASK” signal. In the case of such amplitude modulation, an amplification rate variable amplifier, for example, may be used instead of the transmission multiplying circuit 216.
The variable transmission amplifier 217 amplifies the modulated wave modulated by the transmission multiplying circuit 216. In this example, the variable transmission amplifier 217 performs amplification with an amplification rate determined by a “TX_PWR” signal from the CPU 4. The output of the transmission amplifier 217 is transmitted to a matching circuit 341 of the reader antenna unit 3 and the microstrip antenna 11 through the transmit-receive splitter 214, radiated from the microstrip antenna 11 as an interrogation wave and supplied to the IC circuit part 150 of the RFID tag circuit element To. The interrogation wave is not limited to the modulated signal, that is, the modulated wave, but the wave might be a simple carrier wave.
Into the receiving portion 213, a response wave received at the microstrip antenna 11 of the reader antenna unit 3 and inputted from the RFID tag circuit element To through the matching circuit 341 and the transmit-receive splitter 214 is inputted. The receiving portion 213 is provided with the transmit-receive splitter 214, an I-phase receiving signal multiplying circuit 218, an I-phase lowpass filter 219, an I-phase receiving signal amplifier 221, an I-phase limiter 220, a phase shifter 227, a Q-phase receiving signal multiplying circuit 222, a Q-phase lowpass filter 223, a Q-phase receiving signal amplifier 225, a Q-phase limiter 224, and a Received Signal Strength Indicator (RSSI) circuit 226 as strength detecting means.
The I-phase receiving signal multiplying circuit 218 multiplies and demodulates the response wave from the RFID tag circuit element To received at the microstrip antenna 11 and inputted through the matching circuit 341 and the transmit-receive splitter 214 and the generated carrier wave.
The I-phase lowpass filter 219 takes out only a signal in a required band from the output of the I-phase receiving signal multiplying circuit 218. The I-phase receiving signal amplifier 221 amplifies an output of the I-phase lowpass filter 219. The I-phase limiter 220 amplifies the output of the I-phase receiving signal amplifier 221 and converts it to a digital signal.
The phase shifter 227 delays a phase of the carrier wave generated as above by 90°. The Q-phase receiving signal multiplying circuit 222 multiplies the response wave from the RFID tag circuit element To received at the reader antenna unit 3 and the carrier wave whose phase is delayed by the phase shifter 227 by 90°. The Q-phase lowpass filter 223 takes out only a signal in a required band from the output of the Q-phase receiving signal multiplying circuit 222. The Q-phase receiving signal amplifier 225 amplifies an output of the Q-phase lowpass filter 223. The Q-phase limiter 224 further amplifies the output of the Q-phase receiving signal amplifier 225 and converts it to a digital signal.
A signal “RXS-I” outputted from the I-phase limiter 220 and a signal “RXS-Q” outputted from the Q-phase limiter 224 are inputted into the CPU 4 and processed. The outputs from the I-phase receiving signal amplifier 221 and the Q-phase receiving signal amplifier 225 are inputted into the RSSI circuit 226 and a signal “RSSI” indicating the intensity of these signals is inputted into the CPU 4. As a result, the reader 1 demodulates the response wave from the RFID tag circuit element To by I-Q quadrature demodulation.
The matching circuit 341 performs impedance matching when the microstrip antenna 11 is connected to the transmitting portion 212 or the receiving portion 213 through the transmit-receive splitter 214 (See
The switching circuit 12 is a switch circuit using a publicly known FET for radio frequency and a diode, for example. The switching circuit 12 connects either of the two grounding points disposed on the microstrip antenna 11 to the base plate 22 by a control signal from the CPU 4. By means of a connection switching operation by this switching circuit 12, the polarization phase direction of the microstrip antenna 11 is switched to either of two directions crossing each other at a right angle, that is, a vertical direction or a horizontal direction shown by a broken line in
In
The microstrip antenna pattern 21 is generally constructed by a conductive material and is formed in a substantially square thin plate shape. The dielectric substrate 23 is generally constructed by a high dielectric material with high dielectric constant. The dielectric substrate 23 is formed in a substantially square thin plate shape with a dimension larger than that of the microstrip antenna pattern 21. The base plate 22 is generally constructed by a conductive material. The base plate 22 is formed in a thin plate shape covering substantially the whole of the plane on the other side of the dielectric substrate 23. In a part of the dielectric substrate 23 on which wiring is formed, a surface thereof is exposed.
At three spots of the base plate 22 and the dielectric substrate 23, through holes 25s, 25e1, and 25e2 are formed, respectively. The microstrip antenna pattern 21 is connected to the matching circuit 341 (See
Also, the microstrip antenna pattern 21 is connected to switching circuit 12 disposed on a surface of the base plate 22 through the other through holes 25e1 and 25e2. These connection points are grounding points Pe1 and Pe2.
As shown in
Also, the two grounding points Pe1 and Pe2 are disposed on the outer edge portion of the microstrip antenna pattern 21 formed substantially in a square. The grounding points Pe1 and Pe2 are, in this example, disposed at both ends of one side of the microstrip antenna pattern 21 on which the power supply point Ps is disposed and at corner parts sandwiching the power supply point Ps. At this time, a distance from the grounding point Pe1 to the power supply point Ps and the distance from the grounding point Pe2 to the power supply point Ps is substantially equal to each other. Each of the grounding points Pe1 and Pe2 is connected to the switching circuit 12 through a connection line. The switching circuit 12 connects either of the two grounding points Pe1 and Pe2 to the base plate 22 according to a switching control signal from the CPU 4. By means of this connection switching, the direction of the polarization phase in the microstrip antenna 11 is switched. A specific principle of the switching of polarization phase direction will be described later in detail.
In
In the example shown in
In this configuration, a signal of either 1 or 0 is inputted into an input signal line from the CPU 4. If 1 is inputted from the CPU 4, for example, a control signal of 1 is inputted as it is to the SPST switch 31a on the upper side in the figure. As a result, the switch 31a is closed to form a closed circuit, and the grounding point Pe1 of the microstrip antenna 11 is grounded. On the other hand, to the SPST switch 31b on the lower side in the figure, a control signal of 0 inverted through the negative logic circuit N is inputted. As a result, the switch 31b is opened to form an open circuit or an open state, and the grounding point Pe2 of the microstrip antenna 11 is brought into a non-grounded state. Also, in case that 0 is inputted from the CPU 4, each of the switches 31a and 31b performs connection operations opposite to each other. In this way, the connection control is executed so that only either one of the two grounding points Pe1 and Pe2 of the microstrip antenna 11 is grounded.
The switching circuit 12 shown in
The switching circuit 12 shown in
As the switching circuit 12, various types of switching circuit can be applied as appropriate other than the above three circuits.
Here, the most distinctive characteristic of the reader 1 in this embodiment is that in the microstrip antenna 11 disposed on the reader antenna unit 3, switching is made such that either one of the two grounding points Pe1 and Pe2 is connected to the base plate 22. As a result, while size reduction of the antenna is realized, the direction of the polarization phase in radio communication can be switched. The details will be sequentially described below.
In
As described above, the power supply point Ps is disposed at a position close to one side, and a radio frequency signal is supplied to this power supply point Ps. In this case, an electric current flows in a direction from a side closest to power supply point Ps of the microstrip antenna pattern 21A, that is, from the side on the left side in the figure to the opposite side. As a result, charge distribution becomes the maximum at the center position along the direction, and the charge distribution is substantially 0 at the both end positions along the direction. Also, voltage distribution becomes the maximum in an absolute value with opposite signs to each other, that is, in negative and positive at the both end positions along the direction and the voltage is 0 at the center position along the direction (See a graph in
Here, at a position where the voltage distribution becomes 0, since a potential is originally 0, an electromagnetic-wave characteristic of the entire microstrip antenna pattern 21A is not affected if grounding is performed at this point. Moreover, when all such points where the voltage distribution becomes 0 gather, it forms substantially a straight line, but if the collected line of the grounding points is grounded, the electromagnetic-wave characteristic is not affected, either. In two areas of the microstrip antenna pattern 21A divided by the collected line of the grounding points as above, the electric current does not flow in an area where the power supply point Ps is not disposed. As a result, as shown in
Moreover, the grounding points are disposed so that they gather at one point at either one of the both ends of the collected line, that is, a corner part. In this case, due to the reason similar to the above, an area of the microstrip antenna pattern 21B substantially in a rectangular shape can be reduced to a half along the longitudinal direction of the rectangle. With this configuration, the electric current flows in a direction from the corner part where the grounding point Pe1 is disposed to the opposite corner part on a diagonal line. As a result, the microstrip antenna pattern 21 can be formed so that a length of the diagonal line with which a length of the current-flowing path becomes the longest is approximately one fourth of the wavelength in radio communication, that is, λ/4.
In this embodiment, based on consideration in the above (A), a configuration as shown in
As a result, by switching such that either one of the two grounding points Pe1 and Pe2 is connected to the base plate 22, the polarization phase direction is switched. As a result, in the microstrip antenna pattern 21 in this embodiment particularly formed in the square shape as illustrated, the two polarization phase directions switched to cross each other at a right angle.
As having been described above, in this embodiment, the grounding points Pe1 and Pe2 are arranged at positions on a reference line where the voltage distribution becomes 0 in the microstrip antenna pattern 21. Also, by arranging the grounding points Pe1 and Pe2 at end portions of the antenna pattern 21, a length direction in the current flow direction of the antenna pattern 21 can be reduced to a half while a normal radio communication function is maintained. Also, since the two grounding points Pe1 and Pe2 are provided for the one power supply point Ps, by selectively switching the grounding points Pe1 and Pe2 in use, the direction of the current flowing on the antenna pattern 21 can be switched. As a result, the direction of the polarization phase can be changed without providing two power supply points. As described above, a small-sized microstrip antenna whose direction of the polarization phase in communication is variable can be realized.
Also, particularly in this embodiment, by providing the grounding points Pe1 and Pe2 on the outer edge portion of the microstrip antenna pattern 21, the electric current can be made to flow from the outer edge portion where the grounding points Pe1 and Pe2 are located toward the outer edge portion on the opposite side. At this time, the two grounding points Pe1 and Pe2 are disposed so as to sandwich the single power supply point Ps. As a result, if the electric current flows from the outer edge portion where the grounding points Pe1 and Pe2 are located toward the outer edge portion on the opposite side, the direction can be made considerably different and to cross each other at an angle close to approximately 90°. As a result, the two polarization phase directions substantially crossing each other at a right angle can be switched for use.
Particularly in this embodiment, the microstrip antenna pattern 21 is formed in a quadrangle shape, especially in a square shape in this example. The two grounding points Pe1 and Pe2 are provided at both end portions on one side of the quadrangle, and the single power supply point Ps is disposed at the intermediate part of the side of the quadrangle. As a result, a small-sized square antenna can be realized, and by selectively switching the two grounding points Pe1 and Pe2, two linear polarizations crossing each other at a right angle can be switched for use.
In use by switching the two linear polarization phases crossing each other at a right angle as above, as shown in
Also, particularly in this embodiment, the power supply point Ps is disposed at a midpoint of the two grounding points Pe1 and Pe2. As a result, whichever of the grounding points is selected, the impedance of the antenna is hardly changed, and the same matching circuit 341 can be used.
Also, particularly in this embodiment, the switching circuit 12 is a switch circuit provided with two 1-input and 1-output switches (See
The present invention is not limited to this embodiment but capable of various variations in a range not departing from its gist and technical idea. Such variations will be sequentially described below.
For example, if a microstrip antenna pattern formed in a circular shape is used, the configuration in which the square shaped microstrip antenna pattern 21 is disposed on the microstrip antenna 11 is described, but the present invention is not limited to that. That is, a microstrip antenna pattern formed in a circular shape, for example, may be used.
A microstrip antenna pattern 21C of an example shown in
Then, as the microstrip antenna pattern 21C of this variation shown in
As shown in
Other than those described above, methods of the embodiment and each variation may be combined as appropriate for use. Though not specifically exemplified, the present invention can be put into practice with various changes made in a range not departing from the gist of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-229861 | Sep 2007 | JP | national |
This is a Continuation-in-part under 35 U.S.C. §120 of International Application No.: PCT/JP2008/064414, filed Aug. 11, 2008, which was not published under PCT article 21(2) in English. The disclosure of PCT/JP2008/064414 is herein incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2008/064414 | Aug 2008 | US |
| Child | 12686725 | US |
