AN ATTENUATION DEVICE FOR AN ANTENNA OF AN INTERROGATOR FIELD OF INVENTION

The present invention relates to the field of radio frequency identification (RFID). In one form, the invention relates to an interrogator antenna for interrogating an RFID transponder,

The invention has been developed primarily for interrogating multiple passive transponders which are attached to objects to be identified by those respective transponders and will be described hereinafter with reference to that application. A typical application is the identification of RFID transponders or other FRID devices, such as those embedded in plastic tokens or cards that are stacked on each other.

It will be convenient to hereinafter describe the invention in relation to identification of RFID transponders, however it should be appreciated that the present invention is not limited to that use only. BACKGROUND ART

The discussion throughout this specification comes about due to the realisation of the inventors and/or the identification of certain prior art problems by the inventors. . In prior art systems, interrogators use antenna coils to generate an oscillating magnetic field which interrogates or excites an RFID transponder. These interrogator antennae have generally been open frame coils. Where strong magnetic fields are required (roughly greater than 1A/m) and the coils are larger than about 0.3m x. 0.3m these coils tend to emit high levels of stray magnetic fields which can contravene occupational health and safety (OH&S) regulations and/or electromagnetic emission (EM) regulations. In answer to these concerns it is known to enclose the interrogating coils within a tubular metal housing generally constructed from four walls. These walls tend to collectively define both an entrance port and an exit port to allow objects, and the attached RFID transponders, to be moved in and out of the interrogator field. The housing is preferentially made of a high conductivity metal such as copper or aluminium. Although poorer conductivity metals have also been used they are less desirable due to their higher losses and the greater wall thickness required to adequately

screen the fields. Leakage of the fields out of the entrance and exit ports must also be suppressed and special suppression structures are available.

^• One example is an interrogator known as a Tunnel Reader Programmer (TRP). An example of a TRP for interrogating transponders on pallets or conveyors which meets all OH&S and EM regulations in Australia is disclosed in US Patent No. 5,258,766 and international application PCT/AU95/00436,

While a TRP has excellent shielding properties a major drawback is that it is only suitable for applications where the RFID transponders are moved in and out of the TRP, usually on a conveyor or similar. TRP are inherently unsuitable for applications requiring the interrogator to operate on a flat surface such as a table or wall. For these applications flat planar antenna coils are required however these coils can suffer from high stray field emissions levels. Due to the reciprocal nature of the magnetic field they are also susceptible to interference from external sources. Since the radiation of stray fields and the reception of external interference are reciprocal reducing either one inherently reduces the other.

There have been several methods employed to reduce the stray magnetic fields of a planar coils. Figure 1 illustrates a number of prior art methods employed. The stray fields can be reduced by winding the coil as a figure of eight so that the magnetic field generated by each half of the coil is oppositely directed to the other half. In this way the fields from each half of the coil cancel at a distance. The method of using counter wound coils to generate self cancelling fields at a distance can be extended to three (or more) coils provided the magnetic moment of the coils are designed to cancel each other.

One problem with this method is that counter wound coils have inherently higher inductance and resistance than simply wound planar coils. Another disadvantage of both simply wound and counter wound coils is that there are large fringing fields around the coil which may result in transponders being interrogated when near the coil but not in the defined interrogation zone. For example this would be a problem with an under table reader (for example of a gaming table) where transponders are placed on the table for identification and inspection purposes. Tags near the table, but not on the defined reading surface, may inadvertently be interrogated. A further disadvantage is that where two or

more interrogator coils are adjacent, the fringing fields will interact and may prevent the correct operation of the interrogators.

Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.

An object of the present invention is to provide an improved shield for an antenna associated with an interrogator. A further object of the present invention is to alleviate at least one disadvantage associated with the prior art. SUMMARY OF INVENTION

The present invention provides, in an inventive aspect, an interrogator for and/or method of interrogating an RFID transponder, the interrogator comprising at least one coil for generating a magnetic interrogating field, and a conductive loop that is placed in proximity to at least one coil in at least one plane, the coil being positioned substantially within the conductive loop and the conductive loop being an electrical short circuit.

The present invention provides, in another inventive aspect, in an interrogator for interrogating an RFID transponder, the interrogator having at least one coil for generating a magnetic interrogating field, a conductive loop being formed of a relatively low reluctance material and being configured as an electrical short circuit and being adapted to be placed substantially around at least one of the at least one coil and in at least one plane. The present invention provides, in a further inventive aspect, a method of attenuating a magnetic field emanating from an interrogator adapted to interrogate an RFID transponder, the method comprising the steps of generating a magnetic interrogating field with a coil; and providing a low impedance conductive loop proximate the coil, the loop being formed in a manner having an electrical short circuit.

The present invention provides, in yet another inventive aspect, an interrogator for interrogating an RFID transponder, the interrogator comprising at least one coil for generating a magnetic interrogating field, and a conductive loop

that totally surrounds the coil, where the coil is positioned inside the conductive loop.

The present invention provides, in another inventive aspect, a method for interrogating an RFID transponder, the method comprising the steps of generating a magnetic interrogating field with a coil; and providing a low impedance conductive loop at the periphery of the coil for confining the generated magnetic field within the conductive loop.

The attenuation device of the present invention may be a shield.

Other aspects and preferred aspects are .disclosed in the specification and/or defined in the appended claims, forming a part of the description of the invention.

In essence, in the present invention it has been found that an interrogator constructed in accordance with the preferred embodiments of the invention serve to confine the coils magnetic fieid within a conductive loop that surrounds the coii(s) reducing the fringing fields. When surrounded by a conductive loop the field produced by an interrogator's coil induces eddy currents in the conductive loop. These eddy currents produce their own field which in effect subtracts from the interrogator coil's field such that the total net field passing through the conductive loop is (nearly) zero and hence the fifed outside the conductive loop must also be (nearly) zero. This alleviates a problem normally associated with planar coils where strong fringing fields are present at large distances from the coil

The present invention has been found to result in a number of advantages, such as: • The confined field will only read transponders ciose to the interrogator coil

• The strength of any externally radiated field from the interrogator coil is attenuated

• Coupling of external interference to the interrogator coil is reduced

• Coils with conductive loops can be operated close to each other without interfering with each others operation

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood

that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

Further disclosure, objects, advantages and aspects of the present application may be better understood by those skilled in the relevant art by reference to the following description of preferred embodiments . taken in conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the. present invention, and in which:

Figure 1 illustrates an example of prior art methods of reducing stray magnetic fields;

Figures 2(a) and 2(b) illustrate a three dimensional sketch and schematic cross sectional view of a coil and the generated magnetic field; Figures 3(a) to 3(b) illustrate a three dimensional sketch and schematic cross sectional view of the invention;

Figure 4 illustrates a three dimensional sketch of the invention where there is more than one coil;

Figure 5 iilustrates, in cross section, the magnetic fields associated with Figure 4 when only one coil is generating a magnetic field;

Figure 6 illustrates an electrical model for .the coii and conductive loop combination;

Figure 7 illustrates a set of equations that that represents how the conductive loop operates; Figure 8 illustrates two coils in close proximity, where the conductive loops serve to reduce the coupling between the coils; and

Figure 9 illustrates the conductive loop being grounded to screen against electric fields. DETAILED DESCRIPTION For the prior art methods shown in Figure 1 , the magnetic field can be calculated by vector addition of the magnetic field from each loop portion. Close to the coil to a distance less than or about the linear dimension of the coii, the field is dominated by the closet loop portion of the coil. At greater distances the

fields from the other portion(s) of the coil add to the field and eventually at a distance of about 10 times the linear dimension of the coil, the fields will substantially cancel (effectively). This is provided that the total magnetic moment of the coils loops is zero. Clearly close to and adjacent the coil, the field is strong and transponders may be interrogated unintentionally.

Figures 2(a) and 2(b) show a three dimensional sketch and schematic cross sectional view of a coil and the generated magnetic field. As it can be seen, the magnetic field extends well beyond the coil boundaries.

Figures 3(a) and 3(b) shows a three dimensional sketch and schematic cross sectional view of an embodiment according to an aspect of the present invention where a conductive loop has been placed around the coil, Preferably, the loop should have a very low resistance at the operating frequency. The resistance should be less than 1 ohm and more preferably less than 100 milliohms and even more preferably 10 milliohms or less and be relatively coplanar with the coil, although proximity to the coil is acceptable. Preferably, the loop is made of aluminium or copper since these materials have a low resistance, however other suitable conductive and relatively low resistance materials may be used. The loop effectively operates as a shorted loop through which the net magnetic flux is substantially zero because it is a short circuited loop. If the coil is inside the loop, or the loop substantially surrounds the coil at least in plane, this means that the field generated by the loop is effectively trapped inside the loop since the flux passing through the coil must equal the flux passing between the coil and the loop as shown in Figure 3(b). The arrangement shown in Figure 3 is advantageous where two or more coils must operate in close proximity and stray magnetic coupling between the coils must be substantially minimised.

Figure 4 illustrates a three dimensional sketch of the invention where there is more than one coil and Figure 5 illustrates, in cross section, the magnetic fields associated with Figure 4 when only one coil is generating a magnetic. In Figure 4 three coils are shown, although obviously, the present invention is applicable to any number of coils, and/or in any orientation. It is also applicable to more complex coil shapes such as the shapes shown in Figure 1. When one of these coils is active, the flux passing through the coil must equal the flux passing between the coi! and the loop as shown in Figure 5. The three coil arrangement

shown in Figure 4 has been found to be advantageous for reading transponders over a large area such as a table top, for example a gaming table.

It is evident that since the stray magnetic field outside the coil and conductive loop combination has been reduced in the embodiments discussed above then likewise by reciprocity the susceptibility to external magnetic interference will also likewise be reduced. Hence the coil conductive loop combination is considered substantially resistant to external magnetic interference.

Grounding the loop can also serve to screen the coil from external electric interference in the same manner as a faraday screen, as illustrated in figure 9.

Figure 6 shows an electrical model for the coil and conductive loop combination. The coil is L1 driven by current H . The conductive loop inductance and resistance are L2 and r2 respectively. The mutual coupling between L1 and

L2 is m. Current 11 induces a voltage V2 in L2 and causes current I2 to fiow in L2.

Figure 7 illustrates a set of equations that represent the operation of the conductive loop.

From Faraday's Law the net flux φ1 caused by current 11 passing through L2 is given by: V2 = s.φ1 equation 1 where s is the complex frequency. The current I2 that flows in the conductive loop is given by: I2 = V2 / (r2 + sL2) equation 2 and the counter flux φ2 generated by I2 passing through the conductive loop is given by: φ2 = L2.I2 equation 3

Hence the total net flux passing through the conductive loop φT due to current 11 is: φT = φ1 - φ2 equation 4 Solving for φT gives: φT = φ1 . (r2 / s.L2) equation 5

Since r2 « s.L2 . The total net flux equals the total flux present outside the conductive loop as a fraction of the flux generated by the coil and represents the

flux leakage from the conductive loop. • For a 0,5m loop made of 10cm wide copper sheet typical values for r2 and s.L2 are 10 miiliohms and 49 ohms respectively at a frequency of 13.56MHz. Hence the ratio; r2 / s.L2 = 0.0002 equation 6 and hence the leakage from the conductive loop is considered to be very small.

Figure 8 shows two coils in close proximity where the conductive loops serve to reduce the coupling between the coils. Since the leakage field is small, the coils can be brought into . close proximity without compromising their performance. Also for the multi coil combination shown in Figures 4 and 5, a relatively small leakage current means that the return flux from each coil is substantially confined to be inside the conductive loop and hence transponders may be interrogated when substantially placed inside or proximate the conductive loop.

. External eiectric fields may induce voltages on the coil and interfere with the interrogator operation. Figure 9 shows how the conductive loop may be grounded to screen against external eiectric fields.

While this invention has been described in connection with specific embodiments thereof, it wiil be understood that it is capable of further modification(s). This application is intended to cover any variations uses or adaptations of the invention following in genera!, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth.

As the present invention may be embodied in several forms without departing from the spirit of the essential characteristics of the invention, it should be understood that the above described embodiments are not to limit the present invention unless otherwise specified, but rather should be construed broadly within the spirit and scope ,of the invention as defined in the appended claims.

Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the invention and appended claims. Therefore, the specific embodiments are to be understood to be illustrative of the many ways in which the principles of the present invention may be practiced. In the following claims, means-plus-function clauses are intended to cover structures as

performing the defined function and not only structural equivalents, but also equivalent structures. For example, although a nail and a screw may not be structural equivalents in that a naif employs a cylindrical surface to secure wooden parts together^ whereas a screw employs a helical surface to secure wooden parts together, In the environment of fastening wooden parts, a nail and a screw are equivalent structures.

"Comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof." Thus, unless the context clearly requires otherwise, throughout the description and the claims; the words 'comprise', 'comprising', and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".