Abstract:
Wireless tags with a plurality of non-equivalent current pathways altered to collectively represent encoded information, each of which responds differently to an interrogation signal. The element is subjected to a signal stimulating the current pathways, each of which contributes to an overall element response. The information may be recovered from the salient features of this overall response. These salient features include resonant frequency, amplitude, relative peak position, relative peak amplitude, damping, and Q factor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefits of U.S. Provisional Patent Application No. 60/408,141, filed on Sep. 3, 2002 now abandoned. 

   FIELD OF THE INVENTION 
   The present invention relates to remote sensing, tracking, and identification (ID), and in particular to the production and use of inexpensive ID tags. 
   BACKGROUND OF THE INVENTION 
   Various monitoring technologies are known and used to monitor the location of an article or to provide identification in a wide range of contexts. One such technology, known as “tagging,” is commonly employed, for example, in shoplifting security systems, security-badge access systems and automatic sorting of clothes by commercial laundry services. These conventional tagging systems may use some form of radio-frequency identification (RF-ID). In such systems, RF-ID tags and a tag reader (or base station) are separated by a small distance to facilitate near-field electromagnetic coupling therebetween. Far-field radio tag devices (far-field meaning that the sensing distance is long compared to the wavelength and size of the antenna involved) are also known and used for tagging objects at larger distances. 
   The near-field coupling between the RF-ID tag and the tag reader may be used to supply power to the RF-ID tag (so that the RF-ID tag does not require a local power source) and to communicate information to the tag reader via changes in the value of the tag&#39;s impedance. In particular, the RF-ID tag incorporates an active switch, packaged as a small electronic chip, for encoding the information in the RF-ID tag and communicating this information via an impedance switching pattern. The impedance directly determines the reflected power signal received by the reader, and as a result, the RF-ID tag is not necessarily required to generate any transmitted signal. 
   It is desirable in commercial applications or technology to reduce the cost of RF-ID tags. Though simple in principle, RF-ID tags may require sophisticated manufacturing techniques to produce. A more economical alternative involves marker elements adapted to affect an interrogation signal in a measurable, characteristic way. Many such systems utilize magnetic or magnetomechanical tags. For example, a magnetic wire or strip exhibiting harmonic behavior may be stimulated within an interrogation zone by transmitter antenna coils. The coils generate an alternating magnetic interrogation field, which drives the marker into and out of saturation, thereby disturbing the interrogation field and producing alternating magnetic fields at frequencies that represent harmonics of the interrogation frequency. The harmonics are detected by receiver antenna coils, which may be housed in the same structure as the transmitter coils. Accordingly, the appearance of a tagged article within the zone—which may be defined, for example, near the doors of a retail store or library—is readily detected. 
   Inexpensive, magnetic antitheft systems tend to encode very little, if any, information. Essentially, the tag merely makes its presence known. Although some efforts toward enhancing the information-bearing capacity of magnetic tags have been made—see, e.g., U.S. Pat. Nos. 5,821,859; 4,484,184; and 5,729,201, which disclose tags capable of encoding multiple bits of data—the tags themselves tend to be complex and therefore expensive to produce, and may require special detection arrangements that limit the interrogation range (the &#39;859 patent, for example, requires scanning a pickup over the tag) or involve specialized equipment. 
   SUMMARY OF THE INVENTION 
   In one aspect, the invention relates to a device that has a plurality of non-equivalent interacting current pathways that together represent multi-bit information. The device is responsive to a wireless electromagnetic interrogation signal, whereby each of the pathways responds differently to the signal and contributes to the provision of the multi-bit information. Furthermore, some of the non-equivalent current pathways have been altered to encode the represented multi-bit information. 
   In one embodiment, each of the pathways exhibits a different electromagnetic response having at least one electromagnetic resonance, the responses differing from each other in at least one of resonant frequency, amplitude, quality (“Q”) factor, or damping. In various embodiments, each resonance corresponds to a respective different capacitance parameter of the device, a respective different inductance parameter of the device, or both, and the values of the capacitance and inductance are adjustable. 
   In various embodiments the pathways are altered by severing at least one pathway from the other pathways, shorting at least one pathway to a pathway on an opposing face of the device, changing the length of at least one pathway, changing the overlap between at least one pathway and its corresponding pathway on the opposing face of the device, changing the position of at least one pathway relative to the other pathways, changing the spacing between at least one pathway and its corresponding pathway on the opposing face of the device, and changing the capacitance of at least one pathway by placing a metal element between the at least one pathway and its corresponding pathway on the opposing face of the device. 
   In another aspect, the invention relates to an electromagnetically-responsive structure for encoding multi-bit information, whereby the encoded information is wirelessly readable in the frequency domain. The structure includes first and second current pathways having different electromagnetic responses, whereby at least one of the pathways is altered to encode the represented multi-bit information. The first and second current pathways are electromagnetically coupled to each other in such a manner that the coupling results in at least one attribute of the electromagnetic responses approximating a norm. 
   In one embodiment, at least a portion of the multi-bit information is readable through variations in at least one of the salient features of the electromagnetic responses, the salient features including resonant frequency, amplitude, quality (“Q”) factor, and damping. At least one of the current pathways may comprise an open-loop geometry or a closed-loop geometry. 
   In another aspect, the invention relates to a method for encoding multi-bit information on a wireless tag, whereby the encoded information is wirelessly readable in the frequency domain. This method includes providing first and second current pathways having different electromagnetic responses, whereby at least one of the pathways is altered to encode the represented multi-bit information. Further, the first and second current pathways are electromagnetically coupled to each other, such that at least one attribute of the electromagnetic responses approximates a norm. The salient features of the combined electromagnetic responses of the first and second current pathways encode the multi-bit information. 
   In one embodiment, the first and second current pathways are physically adjoined. In various embodiments, the alterations include electrically shorting a pathway, or varying one or more of pathway length, pathway overlap area, or pathway relative position. 
   In yet another aspect, the invention relates to a method of wirelessly sensing multi-bit information comprising the steps of providing a device responsive to a wireless electromagnetic signal and having a plurality of non-equivalent current pathways, wherein at least some of the plurality of pathways have been altered to encode the multi-bit information and respond differently to the excitation signal. Subjecting the device to a wireless electromagnetic excitation signal permits the recovery of the multi-bit information based on an interaction between the device and the signal. Salient features of the electromagnetic response of the device are extracted and the multi-bit information is recovered from the values of the salient features in the response. 
   In one embodiment, each of the salient features corresponds to a different capacitance feature of the device. In another embodiment, each of the salient features corresponds to a different inductance feature of the device. In various embodiments the pathways are altered by severing at least one pathway from the other pathways, shorting at least one pathway to a pathway on an opposing face of the device, changing the length of at least one pathway, changing the overlap between at least one pathway and its corresponding pathway on the opposing face of the device, changing the position of at least one pathway relative to the other pathways, changing the spacing between at least one pathway and its corresponding pathway on the opposing face of the device, and changing the capacitance of at least one pathway by placing a metal element between the at least one pathway and its corresponding pathway on the opposing face of the device. 
   In yet another aspect, the invention relates to an electromagnetically-responsive structure for encoding multi-bit information, whereby the encoded information is wirelessly readable in the frequency domain. The structure includes a first pathway having a first electromagnetic response and having a pair of opposing, electrically-conductive loops electrically connected through at least one point. The structure also includes a second electrically-conductive pathway characterized by having a second electromagnetic response and being electromagnetically coupled to the first loop. The second pathway also has a pair of opposing, electrically-conductive loops electrically connected through at least one point. The electromagnetic coupling results in at least one attribute of the first and second electromagnetic responses approximating a norm. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is pointed out with particularity in the appended claims. The advantages of the invention may be better understood by referring to the following description taken in conjunction with the accompanying drawings in which: 
       FIGS. 1A and 1B  are schematic diagrams depicting the geometry and current paths of an open-loop embodiment of the invention having two branches; 
       FIG. 2  is a circuit diagram depicting an equivalent circuit of the embodiment illustrated in  FIGS. 1A and 1B ; 
       FIGS. 3A and 3B  are plots of the electromagnetic response of the embodiment illustrated in  FIGS. 1A ,  1 B, and  2 ; 
       FIG. 4  is a schematic diagram depicting an embodiment of the present invention having a circular geometry; 
       FIGS. 5A and 5B  are plan and elevational views of an alternative embodiment of the open-loop geometry illustrated in  FIG. 4 ; 
       FIG. 6  is an exploded perspective diagram depicting the embodiment of  FIGS. 5A and 5B ; 
       FIG. 7  is a flow chart of an embodiment of a method in accordance with the present invention; 
       FIG. 8  is a flow chart of a second embodiment of a method in accordance with the present invention; 
       FIGS. 9A and 9B  are schematic diagrams illustrating two approaches for interrogating a tag in accord with the present invention; 
       FIG. 10A  illustrates the electromagnetic response of an embodiment of the invention; and 
       FIG. 10B  presents a converted version of the graphical response illustrated in  FIG. 10A . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention relates to a class of devices (“tags”) adapted for storing multi-bit information and conveying the stored information in response to wireless interrogation signals. In some embodiments these tags include electrical conductors configured as multiple nonequivalent current pathways, whereby each of the conductors contributes to the overall electromagnetic response of the device and at least some of the conductors have been altered to encode the multi-bit information. Generally, the current pathways are defined by electrical conductors configured as open loops, closed loops, or combinations of open and closed loops. An open loop geometry generally includes, for example, linear embodiments, curvilinear embodiments, and combinations thereof. 
   With reference to  FIG. 1A , a representative tag structure  100  having current pathways configured as an open-loop geometry includes a top portion, such as a top half  105 ′, and a bottom portion, such as a bottom half  105 ″, each portion being in electrical communication with the other through an electrical shunt  110 . The top half  105 ′ is further defined by three electrically-conducting traces: a top segment  115 ′; a first top branch  120 ′; and a second top branch  125 ′. The top segment  115 ′, the first top branch  120 ′, and the second top branch  125 ′ are in electrical communication with each other, defining a top node  130 ′. The bottom half  105 ″ is similarly defined by three electrically-conducting traces: a bottom segment  115 ″; a first bottom branch  120 ″; and a second bottom branch  125 ″. The bottom segment  115 ″, the first bottom branch  120 ″, and the second bottom branch  125 ″ are in electrical communication with each other, defining a bottom node  130 ″. 
   The top segment  115 ′ is configurable as an electrically-conducting trace having a length d 1 ′, the first top branch  120 ′ is configurable as an electrically-conducting trace having a length d 2 ′, and the second top branch  125 ′ is configurable as an electrically-conducting trace having a length d 3 ′. Similarly, the bottom segment  115 ″, the first bottom branch  120 ″, and the second bottom branch  125 ″ are each configurable as electrically-conducting traces having respective lengths d 1 ″, d 2 ″, and d 3 ″. To enhance the resonance response of the tag  100 , the top half  105 ′, and the bottom half  105 ″ are configured with a high degree of symmetry. For example, the lengths may be selected such that d 1 ≈d 1 ″, d 2 ′≈d 2 ″, and d 3 ′≈d 3 ″. In some embodiments, as shown in  FIG. 1A , the top half  105 ′ and bottom half  105 ″ are symmetric about the shunt  110  (i.e., the coordinate origin of a Cartesian coordinate system defining a plane containing the tag  100 , as indicated by the dashed line). As will be discussed in greater detail below, the top half  105 ′ and bottom half  105 ″ are generally folded about their respective ends of the shunt  110 , such that the halves  105 ′,  105 ″, overlap each other, residing in parallel, but separate planes. 
     FIG. 1B  illustrates the different current paths of the above-described tag structure  100 . Interrogation by a wireless electromagnetic field induces an electrical current distribution in the tag structure  100 . The current distribution depends upon factors such as the interrogating field&#39;s frequency (or wavelength), its amplitude, and the spatial orientation of its electric and/or magnetic field components. Under interrogation conditions, resonant currents flow along one or more current pathways of the tag  100 . For example, a first current may be induced along the current pathway  135  formed by the first top branch  120 ′, the top segment  115 ′, the shunt  110 , the bottom segment  115 ″, and the first bottom branch  120 ″. Under stimulation by an appropriate interrogating field a first structural resonance occurs in the first current pathway  135 . Similarly, a second current may be induced along a second current pathway  140  formed by the second top branch  125 ′, the top segment  115 ′, the shunt  110 , the bottom segment  115 ″, and the second bottom branch  125 ″. Under stimulation by another interrogating field having different properties, a second structural resonance occurs in the second current pathway  140 . 
   Once folded, the opposing halves (sides) of the structure create a distributed capacitance along the length of the branches. This distributed capacitance allows the electric field to be distributed across the entire area of the tag. This results in more stable resonances that are less susceptible to de-tuning by external electromagnetic influences. This is a desirable property for commercial applications such as packaging labels for materials such as liquids or metal objects. 
     FIG. 2  depicts an equivalent electrical circuit for the embodiment of  FIGS. 1A and 1B . A parallel, open loop resonator is formed by the structure defining the first and second current pathways  135 ,  140  illustrated in  FIG. 1B . Resistor R 1 ′ represents an approximation of the ohmic (“I 2 R”) loss, and the inductor L 1 ′represents an approximation of the series inductance each associated with both the top segment  115 ′ and the first top branch  120 ′. Resistor R 1 ″ represents an approximation of the ohmic loss, and the inductor L 1 ″ represents an approximation of the series inductance, each associated with both the bottom segment  115 ″ and the first bottom branch  120 ″. The capacitor C 1  represents an approximation of the capacitance between the combined top segment  115 ′-first top branch  120 ′ and the combined bottom segment  115 -first bottom branch  120 ″ and is a function of the overlapping area and the dielectric constant of any intervening material. 
   Similarly, resistor R 2 ″ represents an approximation of the ohmic loss, and the inductor L 2 ″ represents an approximation of the series inductance each associated with both the bottom segment  115 ″ and the second bottom branch  125 ″. The capacitor C 2  represents an approximation of the capacitance between the combined top segment  115 ′-second top branch  125 ′ and the combined bottom segment  115 ″-second bottom branch  125 ″ and is also a function of the overlapping area and the dielectric constant of any intervening material. 
   Capacitor C 12 ′ represents an approximation of an interbranch capacitance between the first top branch  120 ′ and the second top branch  125 ′. Likewise, capacitor C 12 ″ represents an approximation of the interbranch capacitance between the first bottom branch  120 ″ and the second bottom branch  125 ″. Capacitors C 12 ′ and C 12 ″ respectively correspond to the interbranch capacitors  145 ′ and  145 ″ illustrated in  FIG. 1B . A current source I S  represents approximate currents induced by an interrogating field. The resulting electrical circuit includes two different resonant, or “tank” circuits. The first tank circuit is defined by the circuit path including the elements L 1 ′, R 1 ′, C 1 , R 1 ″, and L 1 ″. The second tank circuit is defined by the circuit path including the elements L 2 ′, R 2 ′, C 2 , R 2 ″, and L 2 ″. Each tank circuit exhibits a resonant response relating to the values of its electrical components. 
   A representative electrical response of a tag, such as the tag structure  100  of  FIGS. 1A ,  1 B, and  2 , is illustrated in  FIGS. 3A and 3B . A first electrical response curve  150 ′ depicts the value of an electrical response parameter versus frequency. As illustrated, the vertical coordinate axis represents a range of amplitude for the electrical response parameter. The horizontal axis represents a range of frequencies over which the electrical response parameter is observed. The first electrical response curve  150 ′ shows an electrical response of the tag structure  100  resulting from stimulation by a continuous, swept-frequency electrical source. It should be understood, however, that different electrical responses are possible. For example, stimulation by a series of discrete frequencies or stimulation by a short electromagnetic pulse that can simultaneously excite many frequencies. 
   The electrical response parameter may be for example, the impedance of the tag as measured by its reflected or absorbed power. Alternatively, the electrical response parameter can also be the voltages induced in a reader coil resulting from a pulse excitation of the tag. These approaches are illustrated schematically in  FIGS. 14A and 14B . 
   The electrical response curve  150 ′ includes one or more resonant features  155 ′,  155 ″ (generally  155 ). Referring to  FIG. 3A , the resonant feature  155  may include a relative maximum, or peak value. Alternatively or in addition, the resonant feature  155  may include a relative minimum, or valley value. Generally, each resonant feature  155  is defined by feature aspects including a resonant frequency (e.g., F 1 , F 2 , etc.), a relative maximum (or minimum) amplitude (e.g., E 1 , E 2 , etc.), and a shape factor (e.g., Q 1 , Q 2 , etc.) relating to the “sharpness” of the resonance response. The shape factor may be defined, for example, by a quality factor “Q” relating to values of the equivalent electrical circuit parameters (R, L, C) at resonant frequency. One such quality factor is defined in equation 1: 
                 Q   ≡       f   r       Δ   ⁢           ⁢     f   r                 (     Equation   ⁢           ⁢   1     )               
where f r  is a particular individual resonant frequency and Δf r  is the width of the associated peak.
 
   The multi-bit information encoded within the tag structure  100  is thus observable and retrievable through the electrical response represented in curve  150 . Namely, respective portions of the multi-bit information may be encoded using the values of the resonant features  155  (i.e., the resonant frequency, the amplitude, and/or the Q value).  FIG. 3A  illustrates a resonant tag  100  whose response encodes two bits of information. In this figure, each bit of the multi-bit information is represented by a different resonant feature  155 ′,  155 ″ associated with a resonant frequency. The resonant frequency generally refers to that frequency at which the relative maximum (or minimum) value of the resonant feature  155  occurs. Thus, each bit of information may be distinguished according to its associated resonant frequency, each bit of information being associated with a respective frequency (i.e., F 1 , F 2 ), or with a respective frequency range (i.e., Δf 1 , Δf 2 ). 
   A value for each bit of information may thus be encoded in one or more of the respective amplitude, frequency, and/or the Q values of each resonant feature  155 . In the exemplary embodiment encoding binary information illustrated in  FIGS. 3A and 3B , an amplitude value occurring within a range about a first norm (e.g., within an amplitude range extending from a value of about Δ 2  below the norm, to a value of about Δ 1  above the norm) represents a binary “1.” Accordingly, the electrical response curve  150 ′ is representative of the multi-bit binary informational value of “11 2 .” 
   Amplitude ranges about a norm can be defined by absolute amplitude values, or as a percentage, e.g., 10%, of the norm value. The amplitude ranges may also be asymmetric about the norm (for example, Δ 1  above the norm being 20% of the norm and Δ 2  below the norm being 10% of the norm). 
   Similarly, an amplitude value occurring within a range about a second norm represents a binary “0.” In some embodiments, as illustrated in  FIG. 3B , the absence of a resonant amplitude in a range about a first norm represents a binary “0.” That is, the amplitude of the electrical response curve is below the amplitude value of “NORM−Δ 2 ” over the entire frequency range associated with a bit of the multi bit information. Accordingly, the electrical response curve  150 ″ is representative of the multi-bit binary informational value of “10 2 .” 
   For embodiments having more than two norm values, multiple bits of information may be encoded at each resonant frequency. For example, at each resonant frequency, an embodiment may exhibit any of three allowed amplitude states (“0,” “1,” and “2,”) thereby tripling the information encodable by frequency alone. Alternatively or additionally, a resonant peak  155  having an associated Q value occurring within a first range of Q values may represent a binary “0”; whereas, a resonant peak  155  having an associated Q value occurring within a second range of Q values would represent a binary “1.” The analog data concerning the resonant frequencies may be converted to digital values using other techniques known to the art. 
   The multi-bit information stored in a particular tag may be modified in several ways through modification of the metal layer of the tag, the dielectric layer of the tag, or both. The metal and dielectric layers may be modified during the time of manufacture or during post-manufacture processing. In accord with the present invention, a typical embodiment of a tag includes at least one convenient location on the tag for implementing changes to the metal layer, the dielectric layer, or both. 
   Several types of modifications to the metal layer permit changes to the resonant frequencies of an individual tag design. Each resonance peak may be individually deactivated by severing its branch&#39;s connection to the rest of the structure or shorting opposing branches across the dielectric thickness. Increasing the length of a given branch adds additional turns without overlapping with the metal layer of the opposing face of the tag, increasing the inductance of that branch and altering its resonant frequency. Varying the amount of overlap between each branch and its corresponding branch on the opposing face of the tag by changing the trace length or the trace width affects the capacitance of the branch and, accordingly, its resonant frequency. The mutual inductance among the different branches of the tag may be varied by shifting the relative position of the individual metal traces, producing changes in the relative position of the individual resonance peaks. As in any multi-resonant structure, there is typically a coupling between the individual resonances that limits the extent to which each resonance may be tuned without affecting the values of the other resonant frequencies. 
   Likewise, the dielectric layer of the tag may be altered to modify the resonant frequencies of an individual tag design. The thickness of the dielectric, either globally or in localized areas, may be varied to change the resonant frequency of the tag. Holes in the dielectric layer create shorts between metal traces on opposite faces of the tag, eliminating resonance peaks. Inserting metal between the opposite faces of the tag, such as through the introduction of a dielectric having an embedded patterned metal layer, also reduces the capacitance of the tag, affecting its resonant frequencies. 
   In more detail, each of the current pathways  135 ,  140  illustrated in  FIG. 1B  typically corresponds to at least one bit of the multi-bit information encoded within the tag structure  100 . The resonant features  155  associated with each bit of the multi-bit information stored by the tag structure  100  are related to the physical properties of the current pathways  135 ,  140 . The physical properties include the physical dimensions of the current pathways  135 ,  140 —such as an overall length of the tag structural elements defining the current pathway  135 ,  140 , their related widths, or their related thicknesses. The length of a particular current pathway  135 ,  140  relates generally to the resonant frequency F 1 , F 2 . Thus, the resonant frequency may be controlled by the length of the current pathway  135 ,  140 . 
   As previously described in relation to  FIG. 2 , the tag structural elements defining a respective current pathway  135 ,  140  have an associated distributed inductance value related to the electrical conductor, represented by lumped inductors L 1 ′ and L 2 ′. Similarly the tag structural elements defining respective current pathways have an associated distributed capacitance value, generally related to the overlap of the respective structural elements. This capacitance value may be represented by equivalent lumped capacitors C 1  and C 2 . This represents yet another way to modify the resonant spectrum of the tag, hence the ability of a single tag to encode multi-bit information. 
   Additionally, resonant features  155  associated with each bit of the multi-bit information stored by the tag structure  100  may be modified by varying the separation distance between the opposing folded halves  105 ′,  105 ″. The amount of physical separation distance controls the capacitance between the halves  105 ′,  105 ″, thereby affecting the resonant frequency value. Similarly, the physical properties (e.g., dielectric constant or electrical permittivity, ∈) of a material disposed between the respective halves  105 ′,  105 ″ may be used to tune the resonant frequency, again by altering an associated capacitance. 
   Inter-branch capacitance between different branches of the tag is created by the overlap area between branches on opposing folded halves  105 ′,  105 ″ of the tag. The inter-branch capacitance may be used to control the relative amplitude value of resonant features  150 , thereby adjusting the respective amplitudes to approach within a boundary about the norm. A tag structure  100  that encodes informational values using norms is advantageous as it simplifies the detection of the encoded information. For example, an interrogated tag that produces an amplitude response substantially bounded about a norm simplifies the dynamic range requirements of a tag receiver by limiting the dynamic range to a known value. Limiting expected amplitude variations about a norm also simplifies detection circuitry in a tag reader because the detector&#39;s decision circuitry (i.e., circuitry distinguishing between a binary 0 or 1) may be designed to operate using well-defined, bounded amplitude ranges. 
   A tag producing an amplitude response whereby informational values are substantially bounded by respective norms may more readily accommodate tags using amplitude to store more than one bit of information in a given frequency range. A suitably designed tag, when interrogated, produces an amplitude response having amplitude values substantially contained within respective norms. For example, a ternary system can have at least two different amplitude norms. As the tag&#39;s amplitude response is constrained about the norms, the different norm values can be selected relatively close to each other (e.g., both amplitude values being within the same order of magnitude). 
   Variations in the material of the current pathways  135 ,  140  may be used to, among other things, vary the respective Q value. For example, providing a current pathway  135  made of a first material having a given resistivity and/or electrical conductivity results in a first structural electrical loss value (e.g., resistors R 1 ′ and R 2 ′ of  FIG. 2 ). As the Q value varies with the value of structural loss, increasing the value of R 1 ′ and R 2 ′ tends to reduce the associated Q value. Similarly, reducing the structural loss by reducing the values of R 1 ′ and R 2 ′ tends to increase the associated Q value. 
     FIG. 4  illustrates an open loop tag structure  200  having a top half  205 ′, shown in black, and a bottom half  205 ″, shown in crosshatch, with current paths corresponding to those of  FIG. 1B . The top half  205 ′ and bottom half  205 ″ are interconnected by a shunt  210  at the point where the two halves of the tag are folded. The top half  205 ′ includes a first segment  215 ′ in electrical communication at one end with the shunt  210 . The top half  205 ′ also includes a first top branch  220 ′ in electrical communication at one end to another end of the first segment  215 ′, and a second top branch  225 ′, also electrically connected at one end to the top branch  220 ′. The first segment  215 ′, the first top branch  220 ′, and the second top branch  225 ′ are interconnected, thereby forming a node  230 ′. 
   Similarly, the bottom half  205 ″ includes a first segment  215 ″ in electrical communication at one end with the shunt  210 . The bottom half  205 ″ includes a first bottom branch  220 ″ in electrical communication at one end to another end of the first segment  215 ″, and a second bottom branch  225 ″ electrically connected at one end to the same end of the bottom branch  220 ″, thereby forming a node  230 ″ at the intersection of the first segment  215 ″, the first branch  220 ″, and the second branch  220 ″. 
   As illustrated, the top and bottom halves  205 ′,  205 ″ are configured as arcs defining at least a portion of a circle or, more generally, an ellipse. In one embodiment, the top and bottom halves  205 ′,  205 ″ are disposed in opposite orientations as viewed from one side, such that the electrical current in either half flows in the same general direction (e.g., clockwise, or counterclockwise). The respective lengths of the segments  205 ′,  205 ″ (d 1 ), the first branch  210 ′,  210 ″ (d 2 ), and the second branch  215 ′,  215 ″ (d 3 ) are selectable. The relative lengths d 1 , d 2 , d 3 , as well as the diameter of the open loop, control the extent of any overlap between respective elements of the tag structure  200 . Similarly, the relative lengths of d 2 , d 3 , as well as the diameter of the open loop, control the extent to which the first branch  210 ′ and second branch  215 ′ are disposed adjacently to each other, thereby affecting a related inter-branch capacitance value. 
   An angular, open loop tag structure  250  is shown in  FIGS. 5A and 5B . The structure  250  has two current pathways  255 ,  260  corresponding to those illustrated in  FIG. 4 . The current pathways  255 ,  260  may be fashioned from an electrical conductor (e.g., copper, aluminum, nickel, silver, gold) or from combinations of electrical conductors (e.g., nickel-plated copper), e.g., from one or more piecewise linear segments, each having a substantially rectangular cross section, with a width W and a height h. 
   As illustrated, the current pathways  255 ,  260  may be disposed upon a dielectric material  265 . The dielectric material  265  may be fashioned from any suitable dielectric material, with associated dielectric constant, ∈. Some exemplary dielectric materials include Mylar sheet, Duroid, fiberglass, ceramic, silicon, polypropylene, polyethylene (PET), and more generally, polymeric materials. The dielectric material may be fashioned in a planar configuration having a thickness, t. Alternatively, the dielectric material  265  may be fashioned in a tape, such that the current pathways  255 ,  260  are disposed upon one side of the tape. In this manner, the resulting tag structure  250  may be formed by folding the dielectric tape in a controlled manner, such that a top half  270 ′ and a bottom half  270 ″ substantially overlap each other. 
   The current pathways  255 ,  260  may be fixedly applied to the dielectric material  265  in any suitable manner, such as the varied techniques available for forming conductive etches upon a printed circuit board (e.g., chemical etching, photo-resist etching, silk screening, printing, etc.). In one embodiment, the current pathways  255 ,  260  are formed independently from the dielectric material  265 . For example, the material of the current pathways  255 ,  260  may include an adhesive  275  on one side, such as a pressure sensitive adhesive. The adhesive  275  allows the current pathways  255 ,  260  to be bonded to the dielectric material  265  in the manner of tape. The top and bottom halves  270 ′,  270 ″ may be interconnected by a shunt  280 , which may be a conductive material (e.g., a pin, screw, rivet, a solder joint, a plated through hole, or another segment of adhesive strip in which the adhesive is itself conductive). 
   To control the resonance response of the resulting tag structure  250 , the overall lengths, as well as the relative lengths, of the current pathways  255 ,  260  may be preselected. The lengths of each of the current pathways  255 ,  260  may be predetermined and selected during a fabrication process for the tag  150  (i.e., the lengths are established when the conductive pathways are first applied to the conductive material  265 ). Alternatively, the lengths may be determined after fabrication, e.g., by selective removal of material by the user. 
   Some methods of varying the lengths include prefabricating the tag  250  with the current pathways having a maximum length. Thus, the length may be selectably shortened after fabrication by removing one or more portions of the current pathways  255 ,  260  from at least one end of the current pathways  255 ,  260 . For example, the ends of the current pathways  255 ,  260  may be cut away. Additionally, the ends of one or more of the current pathways  255 ,  260  may be selectively short circuited, e.g., from the top half  270 ′ to the bottom half  270 ″ using a shorting device  285 , such as a conductive pin, screw, rivet, or even a dimple that places the top and bottom halves  270 ′,  270 ″ into physical contact with each other.  FIG. 6  illustrates in exploded view the tag  250  of  FIGS. 5A and 5B . 
   Referring now to  FIG. 7 , in operation a tag structure encoding multi-bit information is provided (step  400 ). The tag structure is then interrogated (step  410 ). In one embodiment, the interrogation includes subjection to an interrogating electromagnetic signal. Finally, the encoded multi-bit information is retrieved from the tag structure (step  420 ). 
   In more detail, referring now to  FIG. 8 , interrogation of the tag structure may include transmitting a radio frequency (RF) signal in proximity to the tag structure (step  430 ). The interaction of the interrogation signal with the tag structure results in electrical currents being induced within the conductive elements of the tag structure. These currents, in turn, generate electromagnetic fields that represent an electromagnetic (RF) response signal. A suitably configured receiver receives the response signal from the tag structure (step  440 ). The received response signal is processed to identify the one or more resonance features encoding the multi-bit information (step  450 ). Each of the identified resonance responses is further processed to characterize the resonant features (e.g., resonant frequency, amplitude, Q value, etc.) (step  460 ). Finally, the characterized features are interpreted (step  470 ) to recover the encoded multi-bit information 
     FIG. 9A  illustrates one embodiment of an interrogator  500 . The interrogator  500  includes a source (such as a swept-frequency voltage source  505 ), a signal detector (such as a square-law power detector  510 ) and a directional coupler  515 . The directional coupler  515  may include three ports  520 ,  525 ,  530 . The directional coupler  515  generally transfers signals received at the first port  520  to the third port  530 , and transfers signals received at the third port  530  to the second port  525 . The voltage source  515  is in electrical communication with the first port  520 , transmitting an interrogation signal thereto. The third port  530  is in electrical communication with an antenna  535 , which transmits the interrogation signal received from the voltage source  505  to one or more tags of opportunity located within the range of the antenna  535 . 
   The antenna  535  then receives a response signal generated by a tag responsive to the interrogation signal. The antenna  535  forwards the received response signal to the third port  530 . The directional coupler  515  transfers the received response signal to the detector  510  via the second port  525 . The detector  510  receives the response signal and performs a detection function. The detected signal may be forwarded to a computer or other processor for additional processing. 
     FIG. 9B  illustrates an alternative embodiment of an interrogator  550  configured without a directional coupler. The interrogator  550  includes the swept-frequency voltage source  505  and the detector  510 . The interrogator is in electrical communication with a transmit antenna  535 ′ through a first port  555 . The interrogator is also in electrical communication with a receive antenna  535 ″ through a second port  560 . The source  505  transmits the interrogation signal to a tag of opportunity via the transmit antenna  535 ′. The detector  510  receives a response signal from an interrogated tag via the receive antenna  535 ″. 
   In one embodiment, referring now to  FIGS. 10A &amp; 10B , a frequency response signal  570  is received at a detector. In one embodiment, the detector receives a time-domain response signal from the interrogated tag and converts the received signal into the frequency domain. For example, the detector may perform a fast Fourier transform of a detected time-domain signal. In other embodiments, the detector may include a “chirp” transform device (e.g., using a surface acoustic wave transducer) to produce a representative frequency response curve from a received time-domain response curve. In this example, the frequency response signal  570  includes two resonant features  575 ,  580 . The resonant features  575 ,  580  respectively include relative maximum values at F 1  and F 2 . Each of the resonant features  575 ,  580  also respectively includes an associated maximum amplitude, E 1  and E 2 , each approaching a norm and having an associated Q value, Q 1 , and Q 2 . 
   In one embodiment, the detector samples the received signal at a number of frequencies, providing a binary “1” output if the sampled response value is above a detection threshold, such as the norm value minus a predetermined threshold, Δ 2 . Similarly, the detector provides a binary “0” output if the sampled response curve is below the detection threshold for that sample. A single “1” value may indicate a first resonance, the resonant frequency being approximately related to the sample number. Similarly, a cluster of adjacent 1&#39;s may also indicate a single resonant peak  575  having an associated Q value lower than the Q value of the resonant peak  580 . As discussed above, digital data may be extracted from the analog resonant frequency information in accord with the present invention using a variety of techniques known to the art. 
   Having shown the preferred embodiments, one skilled in the art will realize that many variations are possible within the scope and spirit of the claimed invention. It is therefore the intention to limit the invention only by the scope of the claims.