WO1998005088A1

WO1998005088A1 - Magnetic field antenna and method for field cancellation

Info

Publication number: WO1998005088A1
Application number: PCT/US1997/013240
Authority: WO
Inventors: Mark Allen Schamberger; Stephen Leigh Kuffner; Richard Stanley Rachwalski
Original assignee: Motorola Inc.
Priority date: 1996-07-29
Filing date: 1997-07-25
Publication date: 1998-02-05
Also published as: AU3741497A

Abstract

The present invention provides a magnetic field antenna (1200) and method for utilizing a central loop (1202), carrying a current produced by a signal source or induction by an external field and a plurality of at least three non-collinear peripheral loops (1204, 1206, 1208, 1210), distributed along a perimeter of the central loop (1202), arranged for substantially cancelling a moment of the central loop (1202), and where transmitting, minimizing radiation, and where receiving, for minimizing a current induced by a substantially uniform field.

Description

MAGNETIC FIELD ANTENNA AND METHOD FOR FIELD CANCELLATION

Field of the Invention

The present invention relates generally to RF identification systems and, in particular, to inductively-coupled magnetic loop antenna systems.

Background of the Invention

RF identification (RF ID) systems are known to comprise a plurality of RF ID tags or cards that interact with one or m o re reader/writer terminals or devices. A reader/writer terminal couples both power and data signals to the RF tag circuit, but only when the tag is in the vicinity of the terminal. Once the RF tag circuit couples enough energy to power up the main circuit, the RF tag will conduct a transaction with the reader/writer terminal. Transaction types v a ry from simple identification to financial applications. In order for an RF

ID system to operate properly, it is critical that the terminal and t ag have a reliable and efficient channel of communication. It is v e ry important to understand the coupling mechanisms between the t ag antenna(s) and the terminal antenna(s).

Several different approaches to antenna design have b e e n previously investigated. In general, an RF ID antenna system can b e classified as either electro-static or magneto-static, depending on t h e dominant method of coupling. An electro-static system relies o n capacitive coupling of the power and data signals between the tag a nd terminal, while a magneto-static system relies on inductive coupling . Electro-static and magneto-static systems arc also referred to as "close- coupled" and "remote-coupled" systems, respectively. Electro-static systems typically require that the tag make or nearly make phy sical , but not electrical, contact with the terminal. Magneto-static systems a r e not nearly as limited. The precise coupling region, within which a transaction between tag and terminal can occur, is governed by several factors, including the power available from the terminal, the power required by the tag circuit, and the level of interference present in t h e env i ron ment.

A magneto-static system is relatively easy to understand, since it is essentially a system of mutually coupled inductors. The key to a n efficient system is designing a system of antennas capable of operati ng within a specified coupling region, but which are inoperable outside o f this region. Thus it is necessary to design an antenna with a limited operating range. This is important since there could be several terminals at a transaction center which gives rise to concern about interference. There are also regulatory specifications which limit t h e maximum allowable level of radiation. Again, a coil which minim izes excess radiation would be quite feasible. One last important aspect of a limited coupling region is security, particularly for fi nanci al transactions. It would be highly beneficial to limit the spatial extent through which sensitive information is broadcast.

Thus, there is a need for a magnetic field antenna with a set o f loops and method for substantially canceling a moment of a c en t ral loop, and where transmitting, minimizing radiation, and w h e re receiving, for minimizing a current induced by a substantially un i form field.

Brief Description of the Drawings

FIG. 1 is a schematic representation of one embodiment of a moment canceling antenna arranged in accordance with the p re s ent invent i on . FIG. 2 is a schematic representation of another embodiment of a moment canceling antenna including a collocated antenna wh i c h provides maximum isolation in accordance with the present invention.

FIG. 3 is a diagram of the method of substantial mom en t cancellation for a single antenna system in accordance with th e present invention.

FIG. 4 is a diagram of the method of substantial mom ent cancellation and maximum isolation for a dual antenna system i n accordance with the present invention.

FIG. 5 is a schematic representation of a specific embodiment of a moment canceling antenna arranged in accordance with the pre s en t invention .

FIG. 6 is a schematic representation of a specific embodiment of a moment canceling antenna, with balanced flux, including a collocated antenna which provides maximum isolation in accordance with t h e present invention.

FIG. 7 is a schematic representation of another specific embodiment of a moment canceling antenna, with a slight flu x imbalance, including a collocated antenna which provides m ax im um isolation in accordance with the present invention.

FIG. 8 is a schematic representation of a particular embodiment which contains a square central loop, square peripheral loops located internal to the central loop at its vertices, and a square collocated loop located concentric to the central loop with its vertices bound by t h e interior of the peripheral loops.

FIG. 9 is a schematic representation of a particular embodi ment which contains a square central loop, square peripheral loops located internal to the central loop at its vertices, and a square collocated loop located concentric to the central loop with its vertices exterior to t h e central loop.

FIG. 10 is a schematic representation of a particular embodiment which contains a square central loop, square peripheral loops located external to the central loop connected by extensions from the vertices along the central loops diagonals, and a square collocated loop located concentric to the central loop with its vertices contained on t h e extensions .

FIG. 1 1 is a schematic representation of a particular embodi ment which contains a square central loop, square peripheral loops located external to the central loop at its vertices, and a polygonal collocated loop located concentric to the central loop which follows the peri m eter of the combination of central and peripheral loops.

FIG. 12 is a schematic representation of a particular em bodiment which contains a square central loop and square peripheral loops located externally at its vertices which are magnetically coupled to t h e central loop and contain distributed capacitive elements.

FIG. 13 is a schematic representation of a coupling pattern for a square loop which indicates a normalized mutual inductance.

FIG. 14 is a schematic representation of a coupling patte rn showing the magnitude for the mutual inductance for the loop geometry of FIG. 5.

FIG. 15 is a schematic representation of a coupling p atte rn showing both the magnitude and phase for the mutual inductance fo r the loop geometry of FIG. 5.

FIG. 16 is a schmatic representation of isolation data for the two- inductor system of FIG. 6. Detailed Description of a Preferred Embodiment

One embodiment of the magnetic field antenna is illustrated i n

FIG. 1, which includes a central conducting loop and a plurality of a t least three non-collinear peripheral conducting loops distributed al on g a perimeter of the central loop. The peripheral loops are arranged suc h that there is substantial cancellation of the moment of the central loop. The antenna current is either generated by a signal source or i nduced by an external field. This current represents either a power signal, a data signal, or a combination of power and data signals. W h en transmitting, the magnetic fields, particularly the intermediate and far- field components, produced by this antenna are minimized. W h e n receiving, this antenna minimizes current induced by a substanti al ly uniform field, as could be caused by background interference. In th i s context, a substantially uniform field refers to one that is uniform ov e r the volume of the antenna. The conductors used to construct t h e antenna could include wires, printed circuit board traces, or conductive ink patterns.

Another embodiment of the magnetic field antenna is illustrated in FIG. 2, which includes the antenna from FIG. 1 as well as a second collocated loop antenna. The function of the second antenna is to transmit/receive data and/or provide power to an external RF receiver/transm itter.

The loops used in the magnetic field antennas of FIG. 1 and FIG. 2 are in the shape of N-sided polygons or closed three-d imensional contours. This includes the central and peripheral loops of the ante nn a in FIG. 1 as well as the collocated loop in FIG. 2. For the magnetic field antenna of FIG. 1, the peripheral loops are attached to the central loop by a direct electrical connection, a three-dimensional extension, o r indirectly via magnetic coupling. A direct connection is established b y breaking the central loop and inserting a peripheral loop at the bre ak point. An extension is established similar to a direct connection, except that an additional pair of conductors is used to offset the position of a peripheral loop relative to the break point in the central loop. If a peripheral loop is not electrically connected to the central loop by a direct connection or an extension, then the only connection is v i a magnetic field coupling. The positions of the breakpoints, and t h e length and orientation of the extensions are arbitrary. Also, the central and peripheral loops of FIG. 1 and the collocated loop of FIG. 2 e a c h contain a predetermined number of turns, wherein each loop may h av e a different number of turns. Additionally, the central and pe ri ph e ral loops of FIG. 1 and the collocated loop of FIG. 2 each contain a predetermined distribution of series/parallel c i rcu i t elements/networks, including inductors, capacitors, resistors, sho rt circuits, open circuits, and active devices. This predeterm ined distribution of circuit elements/networks may be different for e ac h loop.

A method for providing substantial moment cancellation for a magnetic field antenna is illustrated in FIG. 3. This method contains t h e embodiment of FIG. 1. A current is provided to the central loop of th i s antenna. Simultaneously, this same current is provided to the plural ity of at least three non-collinear peripheral loops distributed along a perimeter of the central loop , as shown in FIG. 1. This current is e i t h e r generated by a signal source or induced by an external field, and i t represents either a power signal, a data signal, or a combination o f power and data signals. For a substantially balanced-moment antenna, i t is necessary to design the central and peripheral loops such that t h e central moment is substantially canceled by the net peripheral moment. The moment is calculated as the product of the number of turns in a loop, the loop current, and the loop area. This assumes that the turns a r e essentially overlapping, as could be done with insulated w i re conductors. On the other hand, if the conductors are printed circuit board traces, it is necessary to use spiral loops to achieve more turn s . For this topology, the moment is calculated as the sum of the moments for each turn in the spiral loop. Under these conditions, the m agnetic field antenna of FIG. 1 provides a substantially canceled net m agneti c moment. This results in minimal radiation when transmitting, an d minimal induced current from substantially uniform fields w h e n recei ving .

A method for providing substantial moment cancellation a n d maximum isolation between antennas for a magnetic field antenna i s illustrated in FIG. 4. This method contains the embodiment of FIG. 2. The primary antenna is that of FIG. 1, which is the combination of t h e central and plurality of at least three non-collinear peripheral loops distributed along a perimeter of the central loop. The secondary antenna is the collocated loop. The primary antenna operates prec i sely as described in the previous method. A current, different from that i n the primary antenna, is provided to the secondary antenna. This current is either generated by a signal source or induced by an external field, and it represents either a power signal, a data signal, or a combination of power and data signals. The primary antenna i s designed for substantial moment cancellation as described in t h e previous method. The secondary antenna is designed with a specific shape to provide maximum isolation with respect to the p ri m a ry antenna. The shape is a function of the primary antenna topology . Specific dimensions arc readily calculated via numerical simulation o f the magneto-static or quasi-magneto-static fields.

A specific magnetic field antenna is illustrated in FIG. 5. The central and peripheral loops are single-turn squares. The fo u r peripheral squares are attached to the central square and its vertices . Also, the peripheral squares are half-scaled versions of the c ent ral square. Clearly the net peripheral moment has an equal magnitude to the central moment. Due to the opposite direction of current flow in t h e peripheral coils relative to the central coil, the two moments are 180° out of phase. This condition produces optimal moment cancellation.

A specific magnetic field antenna system is illustrated in FIG. 6. I t includes the antenna from FIG. 5, as well as a collocated square loop which is coplanar and concentric with the other antenna. The dimensions of the collocated square loop are optimized via numeri cal simulation so that there is no coupling to the other antenna, thu s providing maximum isolati.on. For this configuration, there is a n optimum dimension of the collocated square loop such that its vertices lie within the perimeter of the peripheral square loops of the o th e r antenn a .

Another specific magnetic field antenna system is illustrated i n FIG. 7. It includes an antenna similar to that in FIG. 5, except th at peripheral loops are modified such that there is a slight mom ent imbalance. There is also a collocated square loop which is coplanar a n d concentric with the other antenna. The dimensions of the collocated square loop are optimized via numerical simulation so that there is n o coupling to the other antenna, thus providing maximum isolation. For this configuration, there is an optimum dimension of the collocated square loop such that its vertices lie outside the perimeter of the oth e r antenna, thus requiring that it circumscribes this other antenna.

FIG. 8 is a schematic representation of an example c onfi gu ration in accordance with the present invention. The central, peripheral, a n d collocated loops are all square in shape. The peripheral loops a r e directly connected to the central loop, located in its interior. The collocated loop partially overlaps the peripheral loops to in c reas e isolation. FIG. 9 is a similar geometry except that the collocated loop circumscribes the combination of central and peripheral loops to maximize isolation. Note that the peripheral loops are connected a s illustrated in FIG. 5, such that their moments oppose the c entral moment.

FIG. 10 is a schematic representation of an example con fi guration in accordance with the present invention. Again, the central , peripheral, and collocated loops are all square in shape. The peri ph e ral loops are exterior to the central loop and connected at the its vertices via extensions, which are in this case planar. The collocated loop i s concentric with the central loop. Its size could be adjusted to change the self-inductance as well as its mutual inductance to the combination of central and peripheral coils. Note that the peripheral loops are connected as illustrated in FIG.5, such that their moments oppose the central moment.

FIG. 11 is a schematic representation of an example configuration in accordance with the present invention. The central and peripheral loops are square in shape. The peripheral loops are exterior to the central loop and are connected directly at the vertices of the central loop. The collocated loop is a polygonal shape which follows the perimeter of the combination of the central and peripheral loops. This configuration also yields high isolation. Note that the peripheral loops are connected as illustrated in FIG.5, such that their moments oppose the central moment.

FIG. 12 is a schematic representation of an example configuration in accordance with the present invention. The central (1202) and peripheral loops (1204, 1206, 1208, 1210) are all square in shape. The peripheral loops are centered along the exterior diagonals of the center loop. There is no direct electrical connection present, rather they are connected via magnetic field coupling. In addition, there are capacitors (1212) distributed along each edge of each of the peripheral loops. The capacitive loading can be used to alter the loop impedance, thus changing the magnitude of the induced currents in the peripheral loops.

For FIGs. 13-15 a small square loop inductor is used as a probe for performing the mutual inductance calculations via magneto-static simulation.

FIG. 13, numeral 1300, is a schematic representation of a coupling pattern for a square loop which indicates a normalized mutual inductance. The single square loop geometry produces a central peak (1302) in the coupling pattern which is confined within the perimeter of the loop.

FIG. 14, numeral 1400, is a schematic representation of a coupling pattern showing the magnitude for the mutual inductance for the loop geometry of FIG. 5. There is central peak (1402) that corresponds to t h e central peak (1302) of FIG. 13 that is due to the central square loop. I n addition, there are four smaller outlying peaks (1404, 1406, 1408, 1410) produced by the peripheral loops. The radiation corresponding to t h e outlying peaks (1504, 1506, 1508, 1510) substantially cancels t h e radiation corresponding to the central peak (1502). FIG. 15, n um e ra l 1500, is a schematic representation of a coupling pattern showing bo th the magnitude and phase for the mutual inductance for the loop geometry of FIG. 5. FIG. 15 is identical to FIG. 14 except that the outlyi ng peaks are inverted , which is indicative of the 180° phase shift between the central and peripheral loop currents.

FIG. 16, numeral 1600, is a schematic representation of isolation data for the two- inductor system of FIG. 6. Mutual inductance is sho wn on the y-axis, and the x-axis is an edge length of a square, co-located concentric loop. The first peak (1602) occurs when the co-located loop (a second inductor) overlaps the central loop of the combination o f central and peripheral loops of the first inductor. The second pe a k (1604) occurs when the co-located loop partially overlaps the outermost edges of the peripheral loops. The coupling null (1606) occurs for a square co-located loop whose vertices lie within the peripheral loops, thus occurring between the first and second peaks.

The present invention may be embodied in other specific form s without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only a s illustrative and not restrictive. The scope of the invention is, therefore , indicated by the appended claims rather than by the fo rego ing description. All changes which come within the meaning and range o f equivalency of the claims are to be embraced within their scope.

Claims

We claim:

1. A magnetic field antenna having a conductor comprising:

A) a central loop, carrying a current produced by one of: A l ) a signal source; and

A2) induction by an external field; an d

B) a plurality of at least three non-collinear peripheral loops, distributed along a perimeter of the central loop, arranged fo r substantially canceling a moment of the central loop, and where transmitting, minimizing radiation, and where receiving, for minimizing a current induced by a substantially uniform field.

2. A method for providing substantial moment cancellation for a magnetic field antenna comprising the steps of : A) providing, along a conductor arranged in a central loop, a current produced by one of:

A l ) a signal source; and A2) induction by an external field; an d B) simultaneously providing the current to a plurality of a t least three non-collinear peripheral loops that are distributed along a perimeter of the central loop and arranged for substantially cance ling a moment of the central loop, and where transmitting, minimizing radiation, and where receiving, for minimizing a current induced by a substantially uniform field.

3. A method for providing substantial moment cancellation an d maximum isolation between antennas for a magnetic field a n te nn a comprising the steps of :

A) providing, along a conductor arranged in a central loop, a first current produced by one of:

A l ) a signal source; and A2) induction by an external field; an d B ) simultaneously providing the first current to a plurality o f at least three non-collinear peripheral loops that are distributed along a perimeter of the central loop and arranged for substantially c ance l ing a moment of the central loop, and where transmitting, minimizing radiation, and where receiving, for minimizing a current induced by a substantially uniform field, and further utilizing a co- located loop antenna that has a second current, wherein the collocated loop antenna has p redete rm ined dimensions with respect to a combination of the central loop and t h e non-collinear peripheral loops and where the first current represen ts at least one of power and data signals, the second current represents a t least one of data and power signals.

4. A magnetic field antenna having a conductor comprising:

A) a central rectangular/square loop, carrying a cu rren t produced by one of:

A l ) a signal source; and

A2) induction by an external field; an d

B) a plurality of four peripheral rectangular/square loops, distributed at each corner of the central rectangular/square loop an d located internally/externally to the central rectangular/square loop, arranged for substantially canceling a moment of the c en tral rectangular/square loop, and where transmitting, minimizing radiation, and where receiving, for minimizing a current induced by a substantially uniform field.

5. A magnetic field antenna having a conductor comprising:

A) a central rectangular/square loop, carrying a c u rren t produced by one of:

A l ) a signal source; and A2) induction by an external field; an d B) a plurality of four peripheral rectangular/square loops, distributed at each corner of the central rectangular/square loop an d located internally/externally to the central rectangular/square loop, arranged for substantially canceling a moment of the central rectangular/square loop, and where transmitting, minimizing radiation, and where receiving, for minimizing a current induced by a substantially uniform field, and further including: C) a rectangular/square collocated loop having a predetermined dimension that provides maximum isolation with respect to a combination of the central rectangul ar/square loop and t h e plurality of four peripheral rectangular/square loops.

6. A radio frequency identification reader/writer terminal h a v i n g a magnetic field antenna with a conductor comprising:

A2) induction by an external field; an d

7. A radio frequency identification/transaction tag having a magnetic field antenna with a conductor comprising: A) a central loop, carrying a current produced by one of:

A l ) a signal source; and

A2) induction by an external field; an d B ) a plurality of at least three non-collinear peripheral loops, distributed along a perimeter of the central loop, arranged fo r substantially canceling a moment of the central loop, and where transmitting, minimizing radiation, and where receiving, for minimizing a current induced by a substantially uniform field.

8. The radio frequency identification/transaction tag of claim 48 wherein the shape of the collocated loop antenna is one of: A) an N-sided polygon, where N is a predetermined intege r; an d

B) a predetermined closed contour in th ree-d i mensi on al space.

9. The radio frequency identification/transaction tag of claim 48 wherein the shape of the central loop is one of:

A) an N-sided polygon, where N is a predetermined i n te ge r wherein the peripheral loops are connected to the central loop a t predetermined points by one of: direct electrical connection; and extensions in three-dimensional space; and magnetic coupling; an d

B) a predetermined closed contour in three-dimensional space wherein the peripheral loops are connected at predete rm ined points to the central loop by one of: direct electrical connection; and extensions in three-dimensional space; and magnetic coupling.

10. The radio frequency identification/transaction tag of claim 48 wherein the shapes of each of the unique peripheral loops are one of:

A) an N-sided polygon, where N is a predetermined integer, that is connected to the central loop at a predetermined point by one of: direct electrical connection; and extensions in three-dimensional space; and magnetic coupling; an d

B) a predetermined closed contour in three-di mensional space connected to the central loop at a predetermined point by one of: direct electrical connection; and extensions in three-dimensional space; and magnetic coupling.