Abstract:
An antenna configured for a radio frequency identification (RFID) device, the antenna comprising a first conductive element over a substrate, the first conductive element extending between a first end and a second end, and a second conductive element over the substrate, the second conductive element including a first path extending between a third end and a fourth end, a second path extending from the third end to a fifth end, and a third path extending from the third end to a sixth end, wherein the first end of the first conductive element is separated from but near one of the fifth end of the second path and the sixth end of the third path of the second conductive element.

Description:
BACKGROUND OF THE INVENTION 
   The present invention generally relates to radio frequency identification (RFID) and, more particularly, to an antenna configured for an RFID tag. 
   Radio frequency identification (RFID) is an important technology in the identification industry and has various applications. RFID tags or labels are widely used to associate an object with an identification code. For example, RFID tags have been used for access control to buildings, security-locks in vehicles and tracking inventory. Information stored on an RFID tag may identify a person seeking access to a secured building or an inventory item having a unique identification number. RFID tags can retain and transmit enough information to uniquely identify individuals, packages, inventory and the like. Generally, in an RFID system, in order to retrieve the information from an RFID tag, an RFID reader may send an excitation signal to the RFID tag using radio frequency (RF) backscatter technology. The excitation signal energizes the tag, which in turn backscatters the stored information to the reader. The reader then receives and decodes the information from the RFID tag. 
   An RFID tag may generally include a chip for data processing and an antenna for data communication. In the RFID industry, it may be important for an RFID tag to efficiently receive or use the energy received from an RFID reader so as to facilitate a subsequent response to the reader or increase an available radio range over which the tag can communicate with the reader in a wireless manner. The efficiency may be improved by impedance matching between the chip and antenna of an RFID tag. Since the chip generally exhibits relatively high capacitive impedance, the antenna may be designed with relatively high inductive impedance to achieve conjugate match. Such high inductive impedance, however, may adversely narrow down the bandwidth of the RFID tag. Furthermore, the material of a substrate that carries an RFID tag may cause variation in the desired inductive impedance of the tag. Also, the capacitive impedance of the chip may vary due to semiconductor manufacturing processes. It may therefore be desirable to have an RFID tag antenna that is able to form complex conjugation with a corresponding chip. It may also be desirable to increase the bandwidth of an RFID tag while achieving complex conjugation for impedance match between the tag antenna and the chip. 
   BRIEF SUMMARY OF THE INVENTION 
   Examples of the present invention may provide an antenna configured for a radio frequency identification (RFID) device, the antenna comprising a first conductive element over a substrate, the first conductive element extending between a first end and a second end, and a second conductive element over the substrate, the second conductive element including a first path extending between a third end and a fourth end, a second path extending from the third end to a fifth end, and a third path extending from the third end to a sixth end, wherein the first end of the first conductive element is separated from but near one of the fifth end of the second path and the sixth end of the third path of the second conductive element. 
   Examples of the present invention may provide an antenna configured for a radio frequency identification (RFID) device, the antenna comprising a first conductive path over a substrate, the first conductive path including a length of one quarter-wavelength long and extending between a first end and a second end, a second conductive path over the substrate, the second conductive path extending between a third end and a fourth end, and a third conductive path over the substrate, the third conductive path including a length of one quarter-wavelength long and extending between the third end and a fifth end, wherein the first end of the first conductive element is separated from but near the fifth end of the third conductive path. 
   Examples of the present invention may provide an antenna configured for a radio frequency identification (RFID) device, the antenna comprising a first conductive element over a substrate, the first conductive element extending between a first end and a second end, and a second conductive element over the substrate, the second conductive element including a first path extending between a third end and a fourth end, and a second path extending from the third end to a fifth end, wherein the first end of the first conductive element is separated from but near the fifth end of the second path of the second conductive element by a gap, the gap being capable of determining a bandwidth of the antenna. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings examples which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. 
     In the drawings: 
       FIG. 1A  is a schematic diagram of a radio frequency identification (RFID) tag consistent with an example of the present invention; 
       FIG. 1B  is a schematic diagram of an antenna configured for the RFID tag illustrated in  FIG. 1A  consistent with an example of the present invention; 
       FIG. 1C  is a schematic diagram of an antenna configured for the RFID tag illustrated in  FIG. 1A  consistent with another example of the present invention; 
       FIG. 1D  is a schematic diagram of an antenna configured for an RFID tag consistent with another example of the present invention; 
       FIG. 2  shows exemplary plots illustrating the impedance of an antenna configured for an RFID tag at different open-circuit distances; and 
       FIG. 3  shows exemplary plots illustrating the return loss of an antenna configured for an RFID tag at different open-circuit distances. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the present examples of the invention illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like portions. 
     FIG. 1A  is a schematic diagram of a radio frequency identification (RFID) tag  10  consistent with an example of the present invention. Referring to  FIG. 1A , the RFID tag  10  may include a chip  11  and an antenna  12 . The chip  11  may be coupled or secured to a substrate  13  and is electrically connected to the antenna  12  on or over the substrate  13 . The chip  11  may include suitable electrical components such as, for example, resistors, capacitors, inductors, batteries, memory devices and processors for providing suitable interaction with an RFID reader through the antenna  12 . In general, the chip  11  may exhibit a relatively high capacitive impedance (Z C ), which may be provided by chip manufactures and can be expressed as follows.
   Z   C   =R   C   −jX   C    
   Where R C , the real number of Z C , represents a resistance of the chip  11 , and X C , the imaginary number of Z C , represents a capacitive reactance of the chip  11 . 
   The substrate  13  may form the basis for a personal identification badge, a label, a package container and the like. Suitable materials for the substrate  13  may include but are not limited to hard materials such as glass, epoxy, ceramic, Teflon and FR4, or organic materials such as paper, synthetic paper, plastic and polyimide. The resonance frequency of the antenna  12  may vary as the material, electrical characteristics and thickness of the substrate  13  vary. 
   The antenna  12  may include inductive materials such as, for example, copper, copper alloy, aluminum and inductive ink. An antenna pattern of the inductive material may be formed on or over the substrate  13  through etching, deposition or printing processes or other processes. In general, the antenna  12  may exhibit a relatively high inductive impedance (Z L ), which can be expressed as follows.
 
 Z   L   =R   L   +jX   L  
 
   Where R L , the real number of Z L , represents a radiation resistance of the antenna  12 , and X L , the imaginary number of Z L , represents an inductive reactance of the antenna  12 . In designing the antenna  12 , it may be desirable to form complex conjugation for Z C  and Z L  while improving the bandwidth of the antenna  12 . 
   Referring back to  FIG. 1A , the antenna  12  may include two or more sub sets, such as a first antenna element  12 - 1  and a second antenna element  12 - 2 . The first antenna element  12 - 1  may include a first conductive path (referred to as “the first path CD” hereinafter) extending between nodes “C” and “D, a second conductive path (referred to as “the second path CAG” hereinafter) extending from node “C” to node “A” and then to node “G”, and a third conductive path (referred to as “the third path CBH” hereinafter) extending from node “C” to node “B” and then to node “H”. The first path CD may have a length W 4 , which is configured to achieve a desired inductance reactance value, i.e., X L . In one example of the present invention, the value of X L  increases as the length W 4  increases. Furthermore, at least a portion of the second path CAG, for example, the path CA, and at least a portion of the third path CBH, for example, the path CB, may form a path ACB having a length H 1 , which is configured to achieve a desired radiation resistance value, i.e., R L . In one example of the present invention, the value of R L  increases as the length H 1  increases. 
   Each of the second path CAG, the third path CBH and the second antenna element  12 - 2  is a quarter-wavelength transmission path, whose length is one quarter wavelength long, or an odd multiple of one quarter wavelength long. In one example, the RFID tag  10  may accept one or more of various frequencies, such as at least one of three frequency bands. An example of those three frequency bands may include a microwave band at or near 2.45 gigahertz (GHz)), an ultra high frequency (UHF) band in the range of 860 megahertz (MHz) to 960 MHz, and a high frequency (HF) band at or near 13.65 MHz. In other examples, the RFID tag  10  may accept another or other combination of frequency bands depending on its applications. The antenna  12  may be configured to obtain sufficient antenna gain to transceive electric waves in a desired waveband. Using a frequency of 915 MHz in the UHF band as an example, each of the second path CAG, the third path CBH and the second antenna element  12 - 2  may have a length of approximately 32 centimeters (=3×10 8  m/915 M). 
   The second antenna element  12 - 2  may include a first end “E” and a second end “F”, which may function to serve respectively as a shorting point and a feeding point of the RFID antenna  12 . The first end “E” of the second antenna element  12 - 2  may be electrically connected to a pin or pad (not shown) of the chip  11 , while one end “D” of the first path CD may be electrically connected to another pin or pad (not shown) of the chip  11 . Furthermore, the second end “F” of the second antenna element  12 - 2  may be separated from but near one end “G” of the second path CAG. The distance between the ends F and G is d 1 , which may affect the coupling of electrical fields and in turn the bandwidth of the antenna  12 . In one example of the present invention, the amount of electrical coupling decreases as the distance d 1  increases. A desired bandwidth may be obtained by changing the amount of electrical coupling. The first antenna element  12 - 1  may be characterized as being “open-circuit” coupled to the second antenna element  12 - 2 . Specifically, the second antenna element  12 - 2  is “open-circuit” coupled to the second path CAG at the end “G”. In another example, the second antenna element  12 - 2  may be open-circuit coupled to the third path CBH at the end “H”. 
   Skilled persons in the art will understand that the antenna  12  may be designed with various antenna patterns while achieving the desired electrical characteristics such as the desired impedance of the RFID tag  10 .  FIG. 1B  is a schematic diagram of an antenna  121  configured for the RFID tag  10  illustrated in  FIG. 1A  consistent with an example of the present invention. Referring to  FIG. 1B , the antenna  121  may be formed on or over a paper substrate and may accept a radiation frequency of approximately 915 MHz in one example. And the lengths H 1  and W 4 , which may respectively determine the radiation resistance and inductive reactance of the antenna  121 , may respectively be approximately 44 millimeter (mm) and 25 mm. The open-circuit gap d 1 , which may determine the amount of electrical coupling and in turn the bandwidth of the antenna  121 , may be approximately 0.5 mm. Other parameters of the antenna  121  may also be set according to its applications. For example, a set of parameters may include lengths W 1  of approximately 2 mm, W 2  of approximately 58.5 mm, W 3  of approximately 10 mm, W 5  of approximately 40 mm and H 2  of approximately 1 mm. Furthermore, the gap d 2 , which may depend on the pin gap of the chip  11 , may be approximately 0.25 mm. 
     FIG. 1C  is a schematic diagram of an antenna  122  configured for the RFID tag  10  illustrated in  FIG. 1A  consistent with another example of the present invention. Referring to  FIG. 1C , the antenna  122  may include a first antenna element  21  and a second antenna element  22 . The first antenna element  21  may further include a first path  21 - 1 , a second path  21 - 2  and a third path  21 - 3 . Each of the second path  21 - 2 , the third path  21 - 3  and the second antenna element  22  may be one quarter-wavelength long. The second path  21 - 2  may include a meander or winding structure, such as the one illustrated in  FIG. 1C , which may be one quarter wavelength long. Furthermore, the second antenna element  22  may employ a meander or winding structure, such as the one illustrated in  FIG. 1C , which may be one quarter wavelength long. 
   The above-mentioned parameters for the antenna  121  illustrated in  FIG. 1B  and the antenna  122  illustrated in  FIG. 1C  may be determined based on simulation, such as with the help of a simulation software. In one example, HFSS™ by the Ansoft Corporation (Pittsburgh, United States) may be used. HFSS™ may support three-dimensional (3D) electromagnetic-field simulation for high performance electronic design. For example, the HFSS may support the electromagnetic simulation of high-frequency and high-speed components, and has been widely used for the design of antennas and RF and/or microwave components as well as on-chip embedded passives, printed circuit board (PCB) interconnects and high-frequency integrated-circuit (IC) packages. 
     FIG. 1D  is a schematic diagram of an antenna  30  configured for an RFID tag consistent with another example of the present invention. Referring to  FIG. 1D , the antenna  30  may include a first element  31  and a second element  32 . The first element  31  may further include a first conductive path  31 - 1 , a second conductive path  31 - 2  and a third conductive path  31 - 3 . Each of the first, second and third conductive paths  31 - 1 ,  31 - 2  and  31 - 3  and the second element  31  may include a meander or winding structure. Furthermore, each of the second and third conductive paths  31 - 2  and  31 - 3  and the second element  32  may be one quarter wavelength long. With the help of a simulation software, the parameters associated with the antenna  30  may be determined. 
     FIG. 2  shows exemplary plots illustrating the impedance of an antenna configured for an RFID tag at different open-circuit distances. The plots may be provided by a simulation software product such as the HFSS. The antenna may include a similar antenna pattern and associated parameters to the antenna  121  illustrated in  FIG. 1B . Referring to  FIG. 2 , the capacitive reactance of the chip decreases as the frequency increases, while the resistance of the chip may remain at a constant independent of the frequency. The resistance and inductive reactance of the antenna may vary as the frequency varies at different gaps, i.e., 0.5 mm, 1.0 mm and 1.5 mm. Conjugate matching in impedance between the chip and the antenna at each of the different gaps may therefore be determined. 
     FIG. 3  shows exemplary plots illustrating the return loss of an antenna configured for an RFID tag at different open-circuit distances. The plots may be provided by a simulation software product such as the HFSS. The antenna may include a similar antenna pattern and associated parameters to the antenna  121  illustrated in  FIG. 1B . Referring to  FIG. 3 , when a return loss greater than 10 dB is concerned, the antenna has a relatively wide bandwidth greater than approximately 70 MHz in the front and rear parts of a center frequency 910 MHz. 
   In describing representative examples of the present invention, the specification may have presented the method and/or process of the present invention as a particular sequence of steps. However, to the extent that the method or process does not rely on the particular order of steps set forth herein, the method or process should not be limited to the particular sequence of steps described. As one of ordinary skill in the art would appreciate, other sequences of steps may be possible. Therefore, the particular order of the steps set forth in the specification should not be construed as limitations on the claims. In addition, the claims directed to the method and/or process of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the sequences may be varied and still remain within the spirit and scope of the present invention. 
   It will be appreciated by those skilled in the art that changes could be made to the examples described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular examples disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.