WO2004100309A2

WO2004100309A2 - Apparatus for and method of providing an antenna integral balun

Info

Publication number: WO2004100309A2
Application number: PCT/US2004/013195
Authority: WO
Inventors: Ronald A. Marino
Original assignee: Meadwestvaco Corporation
Priority date: 2003-05-01
Filing date: 2004-04-29
Publication date: 2004-11-18
Also published as: WO2004100309A3; EP1665459A2; EP1665459A4

Abstract

An antenna is provided having an integral balun (202) that together with a transmission medium forms a radiating structure for the antenna. In accordance with an exemplary embodiment of the invention, an antenna in the form of a resonant-loop antenna is provided for use in an RFID reader system. A balun is provided with the resonant-loop antenna in the form of a gap or separation in a portion of a transmission medium (201A, 201B) (e.g., coaxial cable, microstrip, etc.).

Description

APPARATUS FOR AND METHOD OF PROVIDING AN ANTENNA INTEGRAL BALUN

CROSS REFERENCE TO OTHER APPLICATIONS

[0001] This patent application claims the benefit of U.S. provisional patent application number 60/466,721, filed May 1, 2003, the entirety of which is hereby incorporated by reference.

BACKGROUND

[0002] Radio frequency identification (RFID) systems typically use one or more reader antennae to send radio frequency (RF) signals to items tagged with RFID tags. The use of such RFID tags to identify an item or person is well known in the art. In response to the RF signals from a reader antenna, the RFID tags, when excited, produce a disturbance in the magnetic field (or electric field) that is detected by the reader antenna. Typically, such tags are passive tags that are excited or resonate in response to the RF signal from a reader antenna when the tags are within the detection range of the reader antenna. One example of such an RFID system, including details of suitable RF antennae, is described in U.S. Patent No. 6,094,173, which is incorporated by reference herein in its entirety.

[0003] The detection range of the RFID systems is typically limited by signal strength over short ranges, for example, frequently less than about one foot for 13.56 MHz systems. Therefore, portable reader units may be moved past a group of tagged items in order to detect all the tagged items since the tagged items are typically stored in a space significantly greater than the detection range of a stationary or fixed single reader antenna. Alternately, a large reader antenna with sufficient power and range to detect a larger number of tagged items may be used.

[0004] Currently, some RFID applications use resonant-loop reader antennas to identify and monitor objects. Optimal performance of resonant-loop reader antennas can be achieved by incorporation of a device known as a "balun." A "balun" is a device that joins a balanced line (i.e., one that has two conductors with equal currents in opposite directions, such as a twisted pair cable) to an unbalanced line (i.e., one that has just one conductor and a ground, such as a coaxial cable). In other words, a balun operates much like a transformer, in that it is used to convert an unbalanced line to a balanced line, and vice versa. Applications that incorporate a balun are more likely to have improved read characteristics due to a reduction of noise as an outcome of a balanced system. Additionally, antenna designs that do not incorporate a DC connection to the reader may help to overcome grounding problems that may exist in a circuit.

[0005] FIG. 1 illustrates a conventional resonant-loop antenna system 120 having a looped conductor 100, which connects to a toroid-type balun

101, which is part of a tuning circuit 102. The tuning circuit 102 connects to transmission cable 103 that may connect to a device 104 (not shown) such as a transceiver, for example, an RFID reader. The transmission cable 103 is typically characterized by its impedance, which in a simplified form, is approximately the square root of inductance L divided by capacitance C of the transmission cable.

For coaxial cables, the impedance is commonly 50 or 75 ohms. Generally, the transmission cable 103, antenna loop 100, and tuning circuit 102 are connected together in a manner that most efficiently utilizes the RF power at a desired frequency, which for a given RFID system using a loop antenna, such as antenna 120, is typically a "high" frequency such as 13.56 MHz. Another common "low" frequency that is often used for RFID systems is 125 kHz. "Ultrahigh" (UHF) frequencies such as 900 MHz or 2.45 GHz within the RF range are also used with different antenna designs.

[0006] Toroid-type balun 101 is a discrete component, where the toroid coil has a ferrite core. This type of balun is typically hand wound and is a relatively expensive component. The toroid is also relatively large and the ferrite material induces a finite insertion loss. In addition, the use of the ferrite core material may make it difficult to achieve consistent results in the production of the antenna.

SUMMARY

[0007] An antenna is provided having an integral balun that, together with a transmission medium, forms a radiating structure for the antenna. In accordance with an exemplary embodiment of the invention, an antenna in the form of a resonant-loop antenna is provided for use in an RFID reader system. Provided with the resonant-loop antenna is a balun in the form of a gap or separation in a portion of a transmission medium (e.g., coaxial cable, microstrip, etc.).

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 is an exemplary RFID antenna using a toroid-type balun;

[0009] FIG. 2A is an antenna system with an integral balun and tuning circuit according to an embodiment of the invention; [0010] FIG. 2B illustrates in greater detail the integral balun of the FIG. 2A antenna system; and

[0011] FIGS. 3A and 3B respectively illustrate opposite sides of a printed circuit RFID antenna system incorporating an integral balun according to an embodiment of the invention.

DETAILED DESCRIPTION

[0012] Preferred embodiments and applications of the invention will now be described. Other embodiments may be realized and changes may be made to the disclosed embodiments without departing from the spirit or scope of the invention. Although the preferred embodiments disclosed herein have been particularly described as applied to resonant-loop antennas used in the field of RFID reader systems, it should be readily apparent that the invention may be embodied in any antenna structure for any technology having the same or similar problems.

[0013] An exemplary embodiment of the invention may be implemented in an RFID antenna system 200 as illustrated in FIG. 2A. The exemplary antenna system 200 preferably includes a resonant-loop antenna 215 and a tuning circuit 216. In this embodiment, the loop antenna 215 is made of coaxial cable portions 201 A, 201B respectively having first and second outputs 204, 205 that may be taken from center conductors of the coaxial cable portions.

[0014] In this exemplary embodiment, the tuning circuit 216 is composed of capacitors 207 and 209 that are respectively connected to coaxial cable outputs 204 and 205. A bridging capacitor 208 may be connected between the terminals of capacitors 207 and 209. As shown, capacitor 207 is further connected to RF output terminal 210, and capacitor 209 is connected to ground.

[0015] As illustrated in Fig. 2A, loop antenna 215 is provided with a balun 202. In accordance with a preferred embodiment of the invention, balun 202 appears as a gap between conductive portions of coaxial cable 201 A, 201B. In particular, the coaxial cable portions 201A, 201B are each made up of an outer conductor 220A, 220B and a center conductor 225, as shown in Fig. 2B. With this exemplary construction, the balun may appear as the gap 202 between outer conductors 220A, 220B, with the center conductor 225 remaining in electrical contact between coaxial cable portions 201 A, 201B.

[0016] In accordance with a preferred embodiment of the invention, coaxial cable 201A, 201B can be provided as a single, unitary structure laid out in a loop formation. The loop formation initially contains no gaps in either its outer conductor or its center conductor. A balun can be added to the loop formation preferably by removing a section of the outer conductor of the coaxial cable to create a gap in the outer conductor, while leaving the center conductor intact. As shown in Fig. 2B, balun 202 can be formed from the gap between outer conductors 220A, 220B of coaxial cable portions 201A, 201B, respectively with center conductor 225 remaining intact to provide electrical conductivity between portions 201A and 201B. The radiating structure of this resonant-loop antenna can be the outer conductor of the coaxial cable.

[0017] In accordance with a preferred embodiment of the invention, by integrating the balun into the structure, the antenna structure performs a dual function of a balun and a shielded transmission line path to the balun. The balun allows for an equal magnitude and opposite polarity distribution across the gap in the outer conductor of the coaxial cable. This distribution is the identical distribution that the antenna structure would receive, for example, from a balanced transmission line medium. As a result, the feed does not require the radiating structure to have a return path to ground or to the RFID reader.

[0018] In accordance with a preferred embodiment of the invention, the length of coaxial cable portion 201A taken from its output 204 to the balun 202 may be optimized to enhance the tuning performance and Q of the RFID antenna system. Preferably, when optimizing the length, the physical distance should be taken preferably as a function of the impedance locus of the antenna loop. An optimized length of coaxial cable portion 201A will position the impedance locus for optimal tuning. Those skilled in the art will understand that the matching components (e.g., lumped element capacitors and inductors, the susceptive properties of open or short circuited sections of transmission lines, etc.) must be placed at a junction along the transmission line where they will optimally match the circuit of interest.

[0019] An example of such a component is a "single stub" tuner. The load impedance, which may not perfectly match the characteristic impedance of the operating system, is rotated about a Smith chart to a point where the real part of the admittance is the inverse of the characteristic impedance of the system. A Smith chart, which is well-known in the art, provides a graphical procedure for solving transmission line problems, such as impedance and placement position along mismatched transmission lines. The stub length is adjusted so the susceptance cancels out the susceptance of the line at the junction where the stub is placed. An example of this is referenced in Liao, Samuel, Microwave Devices and Circuits. Prentice-Hall, Inc., Englewood Cliffs, New Jersey, 1980, which is hereby incorporated by reference in its entirety.

[0020] In accordance with an embodiment of the invention, the length of coaxial cable portion 201B taken from the balun 202 to output 205 may undergo the same optimization. If the second output 205 is left in an open-circuit condition, the resultant impedance of this open circuited stub is governed by the equation:

[0021] If the second output 205 is connected to the outer jacket of the coaxial cable, the impedance is governed by the equation:

where:

Zo=Characteristic impedance of coaxial line; β=2iT/λ; and L=line length.

[0022] The resultant Q of the circuit will be significantly lower and the tuning circuit will not require a resistor to lower the Q of the antenna system. An optional resistive component (shown in dotted line as resistor 203 in FIG. 2A) may be used but this may make the antenna a less efficient radiator by absorbing power that would otherwise be transmitted by the antenna. The resistor 203 can also be shunted across the outputs 204 and 205.

[0023] In implementing one or more embodiments of the invention, any antenna material or other known component(s) may be used to form an antenna structure. As shown in FIGS. 3A and 3B, for example, a resonant-loop antenna 300 can be formed from a microstrip conductor, where the back plane or wider side of the microstrip acts as the radiating structure. FIG. 3A illustrates a first side of the antenna, where a conductive trace 301 is formed on an insulating surface 302. The conductive trace 301 in FIG. 3A is preferably not grounded. A balun may be added to a selected portion of the trace 301. In a preferred embodiment of the invention, a gap in the metallization of trace 301 may be created to form balun 310. A connector 303 can couple the antenna 300 to any additional electronics or antennas that may be used.

[0024] FIG. 3B shows a second side of antenna 300, where a conductive trace 304 is formed in an insulating surface 302, and where the conductive trace of the first side of the antenna (shown in FIG. 3A) is shown as dotted lines in FIG. 3B. In this embodiment, conductive trace 304 follows the periphery of the microstrip surface and further connects to tuning circuit 305, which is also coupled to connector 303.

[0025] While preferred embodiments of the invention have been described and illustrated, it should be apparent that many modifications to the embodiments and implementations of the invention can be made without departing from the spirit or scope of the invention. For example, while a co- located tuning circuit 216 has been specifically illustrated herein, it should be apparent that any known tuning circuit (whether locally or remotely located) may be utilized. Although not required, an optional ground line 206 may be incorporated in the circuit for applications requiring a DC path to the RFID reader. In addition the output 205 may be left open circuited depending upon the tuning mechanism desired.

[0026] While embodiments directed to a resonant-loop antenna for an RFID system have been disclosed, the invention may easily be deployed or embodied in any antenna-related environment or application in accordance with the teachings herein.

[0027] The depiction of antenna structures 215 and 301/304 as substantially contiguous transmission lines (e.g., coaxial cable 201 (Fig. 2)) formed from a unitary structure is for illustration purposes only. Any number of segments or portions may be used to form the antenna structures, including balun 202, used when implementing the invention.

[0028] It is to be understood therefore that the invention is not limited to the particular embodiments disclosed (or apparent from the disclosure) herein, but only limited by the claims appended hereto.

Claims

CLAIMS[0029] What is claimed as new and desired to be protected by Letters Patent of the United States is:

1. An antenna comprising:

a transmission line medium; and

a balun integrated with said transmission line medium such that said transmission line medium and said balun operate as a radiating structure of the antenna.

2. The antenna as recited in claim 1, wherein the antenna is a resonant-loop antenna.

3. The antenna as recited in claim 1, wherein the antenna comprises a coaxial cable having a center conductor and an outer conductor.

4. The antenna as recited in claim 3, wherein the balun is formed from a gap in the outer conductor of the coaxial cable.

5. An RFID reader antenna, comprising:

a resonant-loop antenna, formed from a length of unbalanced line having at least one conductor; and

a balun, formed by a gap in the at least one conductor of the unbalanced line.

6. The RFID reader antenna according to claim 5, wherein the length of unbalanced line is such that at least one of the following are enhanced: antenna tuning and Q.

7. The RFID reader antenna according to claim 5, wherein the unbalanced line is a coaxial cable having primary and secondary conductors.

8. The RFID reader antenna according to claim 7, wherein the primary conductor is a center conductor of the coaxial cable.

9. The RFID reader antenna according to claim 7, wherein the secondary conductor is an outer conductor of the coaxial cable, and wherein the coaxial cable is a single, unitary structure.

10. The RFID reader antenna according to claim 5, wherein the primary conductor of the unbalanced line is coupled to a tuning circuit.

11. An RFID reader antenna, comprising:

a first microstrip conductor, said first microstrip conductor having a first path around an area on a first surf ce; and

a second microstrip conductor, said second microstrip conductor having a second path around an area of a second surface opposite said first surface, said second conductor having a separation of a predetermined length in said second path.

12. The RFID antenna of claim 11, wherein the separation in said second path forms a balun.

13. The RFID antenna of claim 11, wherein said first microstrip conductor is coupled to a tuning circuit.

14. The RFID antenna of claim 14, wherein said second microstrip conductor path is wider than the first microstrip conductor path.

15. The RFID antenna of claim 14, wherein said second microstrip conductor is coupled to ground.

16. The RFID reader antenna according to claim 11, wherein the length of the first and second microstrip conductors are such that at least one of the following are enhanced: antenna tuning and Q.

17. A method of providing an antenna, the method comprising:

arranging a transmission line in a loop to form a radiating structure, the transmission line having a primary and a secondary conductor; and providing a balun as an integral part of the radiating structure wherein the transmission line and balun form an antenna.

18. The method according to claim 17, wherein providing a balun comprises forming a gap in the secondary conductor of the transmission line.

19. The method according to claim 18, wherein the transmission line is a coaxial cable, and wherein the primary conductor is a center conductor and the secondary conductor is an outer conductor of the coaxial cable.

20. The method according to claim 19, wherein forming a gap comprises removing a section of the outer conductor of the coaxial cable to form the balun.

21. The method according to claim 18, wherein the transmission line is a microstrip transmission medium.

22. A method of providing an antenna on a structure having opposing first and second surfaces, the method comprising:

arranging a first microstrip conductor in a loop around an area of the first surface; arranging a second microstrip conductor in a loop around an area of the second surface to form a radiating structure; and

providing a balun as an integral part of the radiating structure.

23. The method according to claim 22, wherein providing a balun comprises forming a gap in the second microstrip conductor.