Radio frequency identification tag antenna for attaching to metal

An antenna includes a polyhedral dielectric material, a feed loop, a polygonal radiating patch, and shorting plates. The feed loop is electrically connected with a radio frequency identification (RFID) tag chip for supplying power to the RFID tag chip. The polygonal radiating patch is magnetically coupled with the feed loop for radiating the electromagnetic waves. Each of the shorting plates disconnects the radiating patch and a ground surface and controls the magnetic coupling of the radiating patch and the feed loop. Accordingly, an RFID tag that can be attached to a metal material is provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2007-0097822 filed in the Korean Intellectual Property Office on Sep. 28, 2007, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a radio frequency identification tag. More particularly, it relates to a radio frequency identification tag for attaching to metal.

The present invention was supported by the IT R&D program of MIC/IITA [2006-S-023-02, Development of Advanced RFID System Technology].

(b) Description of the Related Art

A radio frequency identification (RFID) tag is used in various fields such as distribution and material handling industries together with an RFID reader.

When an object to which the RFID tag is attached accesses a read zone of the RFID reader, the RFID reader transmits an interrogation signal to the RFID tag by modulating an RF signal by using a specific carrier frequency and the RFID tag responds to the interrogation of the RFID reader.

That is, the RFID reader transmits an interrogation signal to the RFID tag by modulating a continuous electromagnetic wave having a specific frequency, and the RFID tag transmits back the electromagnetic wave transmitted from the RFID reader after performing back-scattering modulation in order to transmit its own information stored in the RFID tag's internal memory. The back-scattering modulation is a method for transmitting tag information by modulating the amplitude and/or the phase of a scattered electromagnetic wave when the RFID tag transmits the electromagnetic wave that is initially transmitted from the RFID reader back to the RFID reader by scattering the electromagnetic wave.

Since a passive RFID tag does not include a separate operation power source, it rectifies the electromagnetic wave transmitted from the RFID reader and uses the rectified electromagnetic wave as its own power source to acquire operation power. The intensity of the electromagnetic wave transmitted from the RFID reader should be larger than a specific threshold value for normal operation. However, since the transmission power of the reader is limited by local regulations of each country, it is not possible to unconditionally raise the level of transmission power.

Therefore, the RFID tag should efficiently receive the electromagnetic wave transmitted from the RFID reader to extend the read zone without raising the transmission power level of the reader. A method for raising the receiving efficiency of the RFID tag is to perform complex conjugate matching of an antenna and a radio frequency (RF) front-end of the RFID tag chip so as to maximize the intensity of the signal received by the RFID tag.

A conventional radio frequency identification tag will be described in detail with reference toFIG. 1.

FIG. 1is a configuration of a conventional RFID tag.

As shown inFIG. 1, the RFID tag includes an RFID tag chip10and an RFID tag antenna20.

The RFID tag chip10stores information on an object to which the RFID tag is attached, and modulates the amplitude and/or the phase of an electromagnetic wave transmitted from an RFID reader for transmitting the information of the object. The RFID tag chip10modulates the amplitude and/or the phase of the wave by controlling the amount of power through input impedance, and includes an RF front-end that has input impedance.

The RFID tag antenna20scatters the electromagnetic wave that is modulated by the RFID tag chip10. The RFID tag antenna20includes a dielectric material21, a feed loop23, radiating patches25, and shorting plates27.

The dielectric material21is rectangular-shaped with a relatively low dielectric constant, and a bottom surface of the dielectric material21is a ground surface that contacts the object.

The feed loop23is formed in an upper surface of the dielectric material21, and electrically connected to the RFID tag chip10so as to supply power thereto.

Each of the radiating patches25is formed in the upper surface of the dielectric material21, and excites a current having an out-of-phase characteristic by using a current flowing through the feed loop23and radiates the excited current.

Each of the shorting plates27is formed in a part of a side surface of the dielectric material21and connects the radiating patches25and the ground surface. That is, the shorting plates27disconnect the radiating patches25and the ground surface.

Generally, in an RFID system including an RFID tag and an RFID reader, transmission power of the RFID reader is limited by local regulations of each country. Therefore, in order to extend a read zone of the RFID reader, the RFID tag antenna should have high efficiency, the RFID tag should resonate at a corresponding frequency, and the RFID tag antenna and the RF front-end of the RFID tag chip should be complex-conjugate matched.

However, the conventional RFID tag shown inFIG. 1is not provided with a method for controlling impedance matching of RFID tag chips that have various impedance characteristics.

Further, there are difficulties in miniaturizing the RFID tag antenna and reducing cost.

SUMMARY OF THE INVENTION

The present invention has been made in an effort to realize a small antenna for a radio frequency identification (RFID) tag and provides a RFID tag that can be attached to a metal material.

To achieve the above-described objects, according to one exemplary embodiment of the present invention, an antenna is attached to an object and transmits electromagnetic waves that are modulated by a RFID tag chip, and includes a polyhedral dielectric material, a feed loop, a polygonal radiating patch, and shorting plates. The polyhedral dielectric material includes a first side that is adjacent to the object, a second side that is parallel with the first side, and a third side that connects the first side and the second side. The feed loop has a vertical length and a horizontal length, and is formed in a part of an area in the second side. The area is adjacent to the third side. The feed loop is electrically connected with the RFID tag chip for supplying power to the RFID tag chip. The polygonal radiating patch is formed in a part of the second side and is magnetically coupled with the feed loop for radiating the electromagnetic waves. Each of the shorting plates is formed in a part of the third side, and disconnects the radiating patch and a ground surface, and controls the magnetic coupling of the radiating patch and the feed loop.

Impedance of the antenna is conjugate-matched with impedance of the RFID tag chip.

A relative dielectric constant of the dielectric material is greater than 20.

The dielectric material is made of a ceramic material and has a hexahedral shape.

The feed loop has a concave polygon shape including two sides that are parallel with the third side, and the radiating patch is a concave polygon having more sides than the feed loop.

A reactance component of the impedance of the antenna corresponds to the horizontal length of the feed loop.

A resistance component of the impedance of the antenna corresponds to an area of the shorting plate.

According to another exemplary embodiment of the present invention, an antenna is attached to an object and transmits electromagnetic waves modulated by an RFID tag chip, and includes a hexahedral dielectric material, a feed loop, a polygonal radiating patch, and shorting plates. The hexahedral dielectric material is attached to a bottom surface of the object. The feed loop is formed in a part of an upper surface of the dielectric material, adjacent to a first side among a plurality of sides of the dielectric material, and is electrically connected with the RFID tag chip for supplying power to the RFID tag chip. The polygonal radiating patch is formed in a part of the upper surface of the dielectric material and is magnetically coupled with the feed loop for radiating the electromagnetic waves. Each of the shorting plates is formed in a part of the first side, and disconnects the radiating patch and the bottom surface of the dielectric material.

A relative dielectric constant of the dielectric material is greater than 20.

A sign of a reactance component of impedance of the antenna and a sign of a reaction component of impedance of the RFID tag chip are opposite to each other.

According to the embodiments of the present invention, a small RFID tag antenna can be realized, an RFID tag that can be attached to a metal material can be realized, and an RFID tag antenna that can be efficiently matched with a RFID tag chip by controlling impedance of the RFID tag antenna can be realized.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Throughout this specification and the claims which follow, unless explicitly described to the contrary, the word “comprising” and variations such as “comprises” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements.

A radio frequency identification tag antenna according to an exemplary embodiment of the present invention will now be described with reference to drawings.

An equivalent circuit of a radio frequency identification (RFID) antenna and a radio frequency (RF) front-end according to an exemplary embodiment of the present invention will now be described with reference toFIG. 2.

FIG. 2is an equivalent circuit diagram of an RFID tag antenna and an RF front-end.

As shown inFIG. 2, the equivalent circuit includes a voltage source, impedance of the RFID tag antenna, and impedance of the RF front end. Herein, the impedance Zaof the voltage source and the RFID tag antenna is an equivalent circuit of the RFID tag antenna, and the impedance Zcof the RF front-end is an equivalent circuit of the RF front-end.

The impedance Zaof RFID tag antenna has a real part Raand an imaginary part Xa, and the impedance Zcof the RF front-end has a real part Rcand an imaginary part Xc.

The RFID tag antenna transmits the maximum power to the RF front-end of a RFID tag chip by conjugate matching the impedance Zaof the RFID tag antenna and the impedance Zcof the RF front-end as shown in Equation 1.
Ra=Rc
Xa=−Xc[Equation 1]

A typical impedance value of the RF front-end is 50Ω, but in the exemplary embodiment of the present invention, the impedance of the RF front-end has a complex value. That is, the impedance Zcof the RF front-end has a relatively small resistance component Rcand a relatively large capacitive reactance component Xc. Therefore, the impedance Xaof the antenna should have a small resistance component Raand a large inductive reactance component Xa, and should simultaneously resonate at a corresponding frequency.

With reference toFIG. 3orFIG. 4, an RFID tag including a patch-structured RFID tag antenna according to the exemplary embodiment of the present invention will be described.

FIG. 3is a configuration of an RFID tag according to an exemplary embodiment of the present invention.

As shown inFIG. 3, the RFID tag according to the exemplary embodiment of the present invention includes an RFID tag chip10and an RFID tag antenna100.

The RFID tag chip10includes information on an object to which an RFID tag is attached, and modulates the amplitude and/or the phase of electromagnetic waves transmitted from the RFID reader so as to transmit the object information. The RFID tag chip10controls the amount of power by using an input impedance so as to modulate the amplitude and/or the phase of the electromagnetic waves, and includes an RF front-end having the input impedance.

The RFID tag antenna100includes a ceramic dielectric material110, a feed loop130, a radiating patch150, and shorting plates170aand170b.

The hexahedral ceramic dielectric material110has a relative dielectric constant that is above 20, and a bottom surface of the dielectric material110is a ground surface that contacts the object. According to the exemplary embodiment of the present invention, the RFID tag antenna100can be realized by a small antenna by using the ceramic dielectric material110.

The feed loop130is formed in a part of an upper surface of the dielectric material110and has a vertical length L1and a horizontal length L2. The feed loop130is electrically connected to the RFID tag chip10and supplies power thereto through a current. The feed loop130controls the reactance component Xaof the impedance of the RFID tag antenna100by changing the vertical length L1and the horizontal length L2. In addition, the feed loop130is a concave hexahedral shape with ‘L’-shaped inside inFIG. 3, and the concave hexahedral shape of the RFID tag antenna100can be variously designed by controlling the vertical length L1and the horizontal length L2.

The radiating patch150is formed in a part of an upper surface of the dielectric material110, together with the feed loop130, and is designed to have a concave polygon shape for convenience in magnetic coupling with the feed loop130. The radiating patch150radiates an electromagnetic wave that is modulated by the RFID tag chip10. In design of the RFID tag antenna100, a resonance frequency of the RFID tag antenna100can be controlled by changing a resonance length of the radiating patch150. In addition, the radiating patch150has an octagon shape inFIG. 3, and the octagon-shaped radiating patch150can be designed to have various concave polygon shapes according to a structure of the feed loop130.

The shorting plate170ais partially formed inside the dielectric material110, and the shorting plate170bis partially formed in a side surface that is close to the horizontal length L2of the feed loop130among four side surfaces of the dielectric material110. The shorting plates170aand170bconnect the radiating patch150and the ground surface to disconnect the radiating patch150and the ground surface. Unlike the structure of the shorting plates170aand170binFIG. 1, the shorting plates170aand170binFIG. 3are formed in a surface that is close to the feed loop130according to the exemplary embodiment of the present invention, and therefore the resistance component Raof the impedance of the RFID tag antenna100can be controlled by changing each length S1and S2of the shorting plate170aand170b, that is, each area of the shorting plate170aand170b.

Herein, the radiating patch150and the feed loop130are magnetically coupled, and the magnetic coupling can serve as an impedance transformer in the RFID tag antenna100.

FIG. 4is a configuration of an RFID tag according to another exemplary embodiment of the present invention.

As shown inFIG. 4, the RFID tag according to the present exemplary embodiment of the present invention includes an RFID tag chip10and an RFID tag antenna200.

The RFID tag chip10includes information on an object to which an RFID tag is attached, and modulates the amplitude and/or the phase of electromagnetic waves transmitted from an RFID reader so as to transmit the object information. The RFID tag chip10controls the amount of power by using input impedance so as to modulate the amplitude and/or the phase of the wave, and includes an RF front-end having the input impedance.

The RFID tag antenna200includes a ceramic dielectric material210, a feed loop230, a radiating patch250, and shorting plates270aand270b.

The hexahedral ceramic dielectric material210has a relative dielectric constant that is above20, and a bottom surface of the dielectric material210is a ground that contacts the object. According to the present exemplary embodiment of the present invention, the RFID tag antenna200can be realized by a small antenna by using the ceramic dielectric material210.

The feed loop230is formed in a part of an upper surface of the dielectric material210, and has a vertical length L3and a horizontal length L4. The feed loop230is electrically connected to the RFID tag chip10and supplies power thereto through a current. The feed loop230controls a reactance component Xaof impedance of the RFID tag antenna200by adjusting the vertical length L3and horizontal length L4of the feed loop230. In addition, the vertical length L1of the feed loop130inFIG. 3is changed to the vertical length L3of the feed loop230and the horizontal length L2of the feed loop130inFIG. 3is changed to the horizontal length L4such that the feed loop230has a rectangular-shaped interior.

The radiating patch250is formed in a part of an upper surface of the dielectric material210, together with the feed loop230, and is designed to have a concave polygon shape for convenience in magnetic coupling with the feed loop230. The radiating patch250radiates the wave that is modulated by the RFID tag chip10. In design of the RFID tag antenna200, a resonance length of the radiating patch250can be changed in order to control a resonance frequency.

Each of the shorting plates270aand270bis placed in a side surface that is close to the horizontal length L2of the feed loop230among four side surfaces of the dielectric material210, and connects the radiating patch250and a ground surface to disconnect the radiating patch250and a ground surface. According to the exemplary embodiment of the present invention, unlike the structure of the shorting plates inFIG. 1, the shorting plates270aand270bare formed in a surface that is close to the feed loop230, and therefore a resistance component Raof an impedance of the RFID tag antenna200can be controlled by changing each length S3and S4of the shorting plates270aand270b. That is, the resistance component Raof the impedance of the RFID tag antenna can be controlled by changing the size of each area of the shorting plates270aand270b.

In this instance, the radiating patch250and the feed loop230are magnetically coupled, and the magnetic coupling can serve as an impedance transformer in the RFID tag antenna200.

With reference toFIG. 5toFIG. 6, variations of the impedance of the RFID tag antenna according to the exemplary embodiment of the present invention will be described in detail.

FIG. 5shows an impedance variation of the RFID tag antenna accordance with a change of a length variation of the feeding loop according to the exemplary embodiment of the present invention.

The smith chart ofFIG. 5shows an impedance variation of the RFID tag antenna according to a change of the horizontal length L2of the feed loop130in the RFID tag antenna100inFIG. 3.

As shown inFIG. 5, the resistance component Raof the impedance of the RFID tag antenna100is constantly sustained and the inductive reactance component Xaof the impedance of the RFID tag antenna100is increased when the horizontal length L2of the feed loop130is changed from 14.7 mm to 16.7 mm.

FIG. 6shows an impedance variation of the RFID tag antenna in accordance with a change of a length of the shorting plate according to the exemplary embodiment of the present invention.

The Smith chart ofFIG. 6shows the impedance variation of the RFID tag antenna in accordance with a change of the length of the shorting plate170aof the RFID tag antenna100inFIG. 3.

As shown inFIG. 6, the inductive reactance component Xaof the impedance of the RFID tag antenna100is constantly maintained and the resistance component Raof the impedance of the RFID tag antenna100is increased when the length S1of the shorting plate170ais changed from 3 mm to 4 mm.

As shown inFIG. 5andFIG. 6, the RFID tag antenna can be efficiently matched to impedance of an RFID tag chip that has a comparatively large capacitive reactance component Xcto a resistance component Rcaccording to the exemplary embodiment of the present invention.

With reference toFIG. 7, a return loss between the RFID tag antenna and the RFID tag chip according to the exemplary embodiment of the present invention will be described.

FIG. 7shows a return loss of the RFID tag antenna according to the exemplary embodiment of the present invention.

As shown inFIG. 7, an operation bandwidth of the RFID tag antenna is 7 MHz with reference to return loss of 3 dB when a volume of the RFID tag antenna according to the exemplary embodiment of the present invention is 20 mm×24 mm×3 mm and a relative dielectric constant of the ceramic dielectric material is 22.