Coupling of RFID straps to antennae using a combination of magnetic and electric fields

An RFID device includes an antenna and an RFID strap. The RFID strap is coupled to the antenna by a combination of magnetic and electric fields. The RFID strap includes an RFID chip and a strap conductor coupling the RFID chip to the antenna. The strap conductor has a loop section positioned generally adjacent to the antenna to magnetically couple the RFID strap to the antenna. The strap conductor also has an extension section overlapping and crossing the antenna to electrically couple the RFID strap to the antenna. By adjusting the size of the RFID strap, the configuration of the RFID strap, the degree of overlap between the extension section and the antenna, and/or the angular orientation of the RFID strap with respect to the antenna, the impedance transformation between the RFID chip and the antenna may be varied to better match the RFID chip and the antenna.

BACKGROUND

Field of the Disclosure

The present subject matter relates to radio frequency identification (“RFID”) devices. More particularly, the present subject matter relates to RFID devices in which an RFID strap is coupled to an associated antenna using a combination of magnetic and electric fields.

Description of Related Art

RFID devices are widely used to associate an object with an identification code. Such devices incorporate an RFID strap that is coupled to an antenna. The RFID strap includes an RFID chip that is programmed with and/or configured to be programmed to include a variety of information, such as an identity of the item to which the RFID device is associated (e.g., a piece of merchandise in a retail setting). The antenna allows the RFID device to communicate with an RFID reader, receiving signals from and/or transmitting signals to the RFID reader.

The RFID strap and antenna may be coupled together in a variety of ways. For example, according to one conventional design, the RFID strap and antenna of an RFID device are coupled via a conductive connection. In another conventional design, the RFID strap and antenna of an RFID device are coupled via an electric field capacitive connection. In yet another conventional design, the RFID strap and antenna of an RFID device are coupled via a magnetic induction field.

Each of the known approaches to coupling the RFID strap and antenna has its own advantages, but it would be advantageous to provide MD devices incorporating the benefits of multiple coupling technologies.

SUMMARY

In one aspect, an RFID device includes an antenna and an RFID strap. The RFID strap is coupled to the antenna by a combination of magnetic and electric fields.

In another aspect, a method is provided for coupling an RFID strap of an RFID device to an antenna of the RFID device. The method includes coupling the RFID strap to the antenna by a magnetic field and by an electric field.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

FIG. 1shows an exemplary RFID strap, generally designated at10, (which may be referred to as a mixed mode strap) of an RFID device according to an aspect of the present disclosure, which allows for coupling between the RFID strap10and an antenna of the RFID device by a combination of magnetic and electric fields. The illustrated RFID strap10includes an RFID chip12(which may be provided according to either a conventional design or a novel design) and a strap conductor, generally designated at14, that couples the RFID chip12to the antenna of the associated RFID device.

The strap conductor14is formed of an electrically conductive material and defines two portions or sections—a loop section16and an extension section18. As will be described in greater detail, the loop section16and the extension section18may be variously configured without departing from the scope of the present disclosure, but in the embodiment ofFIG. 1, the loop section16is generally circular, while the extension section18is generally rectangular.

The loop section16is configured to be positioned generally adjacent to the associate antenna, with the extension section18contacting the antenna, shown inFIGS. 2A, 3A, and 4A-6Bas overlapping and crossing the antenna, which will be discussed in greater detail herein. As used herein, the term “overlapping” is not limited to a configuration in which the extension section18is physically above the antenna (e.g., with the antenna sandwiched between the extension section18and a substrate on which the antenna is mounted) at the locations where the two cross, but also includes a configuration in which the extension section18is positioned physically below the antenna (for example with the extension section18sandwiched between the antenna and a substrate on which the strap conductor14is mounted) at the locations where the two cross. Inasmuch as the extension section18may cross the antenna at multiple locations, it is specifically contemplated that the extension section18may be positioned physically above the antenna at all locations, at none of the locations, and at at least one location, but not at all of the locations. In any of these possible configurations, the extension section18is considered to overlap and cross the antenna at each location.

The loop section16of the RFID strap10couples the RFID chip12to an associated antenna primarily by a magnetic field, with the coupling strength being determined by the distance and relative angle between the RFID chip12and the location or locations at which the strap conductor14contacts the antenna. The extension section18of the RFID strap10couples the RFID chip12to the antenna primarily by capacitance (i.e., electric field) at the location or locations at which it is in proximity to the antenna. Thus, an RFID strap10having a strap conductor14incorporating a loop section16and an extension section18will be coupled to the associated antenna by a combination of electric and magnetic fields. By such a configuration, an RFID device incorporating such an RFID strap10will have the benefits of both electric field and magnetic field coupling. Significantly, coupling the RFID chip12to the associated antenna by a combination of electric and magnetic fields (the characteristics of which may be varied and controlled when designing the RFID device) allows for improved impedance matching of the RFID chip12and the antenna compared to what is possible with only one coupling approach (e.g., only an electric field or only a magnetic field).

FIG. 2Aillustrates an exemplary RFID device, generally designated at20a, incorporating an RFID strap, generally designated at10a, of the general type shown inFIG. 1. The strap conductor, generally designated at14a, of the RFID strap10aofFIG. 2Ais differently shaped compared to the strap conductor14of the RFID strap10ofFIG. 1; for example, strap conductor14ais being generally rectangular instead of having a generally circular loop section16and a generally rectangular extension section18as in strap conductor14, but is otherwise comparable to the strap conductor14ofFIG. 1. In particular, the strap conductor14aofFIG. 2Ahas a loop section16adirectly connected to the RFID chip12aof the RFID strap10a, with an extension section18aof the strap conductor14abeing separated from the RFID chip12aby the loop section16a.

The loop section16aand the extension section18aeach include a pair of legs22a,24aseparated by a gap GL, GEand extending in a direction between the RFID chip12and the antenna26a(which is a generally vertical direction in the orientation ofFIG. 2A). While the legs22aof the loop section16aand the legs24aof the extension section18aare separated by the same gap (i.e., GL=GE) in the embodiment ofFIG. 2A(giving the strap conductor14aits generally rectangular shape), it is within the scope of the present disclosure for the legs of each section to be separated by differently sized gaps, as will be discussed in greater detail in connection with the embodiment ofFIG. 3A.

The difference between the RFID device20ofFIG. 2Aand a conventional RFID device D may be understood and is illustrated by comparingFIG. 2AwithFIG. 2B. The conventional RFID device D ofFIG. 2Bincludes an antenna A that is coupled to an RFID strap S, which includes an RFID chip C and a strap conductor W. As can be seen, the conventional RFID device D ofFIG. 2Bhas an RFID chip C that is coupled to the associated antenna A by an electric field due to the configuration of the strap conductor W of the RFID strap S, whereas the RFID chip12aofFIG. 2Ais coupled to the associated antenna, generally designated at26a, by a combination of electric and magnetic fields, such being accomplished by the configuration, orientation, and positioning of its associated strap conductor14a.

It should be understood that the RFID device20aofFIG. 2Amay include additional and/or differently configured components without departing from the scope of the present disclosure, which is also true for the other RFID devices described herein. For example,FIG. 2Adoes not illustrate a substrate to which the various components of the RFID device20amay be secured. In another embodiment, the RFID strap10amay be an encapsulated item designed to prevent the ingress of water and provide a robust structure. As for the antenna26a, it may be variously configured without departing from the scope of the present disclosure. For example, in one embodiment, the antenna of an RFID device according to the present disclosure may be a flat structure, comprising a cut foil, a wire, or any other suitable material.

FIG. 3Aillustrates a variation of the RFID device20aofFIG. 2A. In the RFID device20bofFIG. 3A, the strap conductor, generally designated at14b, has a loop section16bdirectly connected to the RFID chip12bof the RFID strap10b, with an extension section18bof the strap conductor14bbeing separated from the RFID chip12bby the loop section16b. The loop section16band the extension section18beach include a pair of legs22b,24bseparated by a gap GL, GEand extending in a direction between the RFID chip12band the antenna26b. In contrast to the embodiment ofFIG. 2A, in which the size of the gaps GLand GEis substantially the same in both the loop section16aand the extension section18a, inFIG. 3A, the size of the gap or gaps GLin loop section16bis different from the size of the gap or gaps GEinFIG. 3Aof the loop section1b. In particular, the size of the gap is greater between the legs22bof the loop section16bthan between the legs24bof the extension section18b. In other embodiments, the size of the gaps may be greater between the legs of the extension section than between the legs of the loop section.

Thus, it will be seen that the principal difference between the RFID device20aofFIG. 2Aand the RFID device20bofFIG. 3Ais the configuration of the extension sections18aand18bof their respective strap conductors14aand14b. In particular, the size of the gap GEbetween the legs24aof the extension section18aofFIG. 2Ais greater than the size of the gap GEbetween the legs24bof the extension section18bofFIG. 3A. As a result, there is a difference in the separation between the locations28aand28bat which the extension section18a,18b, respectively, overlaps and crosses the antenna26a,26b, respectively. Varying the separation between these respective locations28a,28baffects the impedance transformation between the RFID chip12a,12band the antenna26a,26b, respectively, such that the separation between the locations28a,28bmay be selectively varied when designing the RFID device20aor20bto satisfy the needs of the device.

As for the difference between the RFID device20bofFIG. 3Aand a conventional RFID device, it may be understood by comparingFIGS. 3A and 3B. The conventional RFID device D′ ofFIG. 3Bincludes an antenna A′ that is coupled to an RFID strap S′, which includes an RFID chip C′ and a strap conductor W′. As can be seen, the conventional RFID device D′ ofFIG. 3Bhas an RFID chip C′ that is coupled to the associated antenna A′ by an electric field due to the configuration of the strap conductor W′ of the RFID strap S′, whereas the RFID chip12bofFIG. 3Ais coupled to the associated antenna26bby a combination of electric and magnetic fields due to the configuration, orientation, and positioning of its associated strap conductor14baccording to the present disclosure.

As described above, the distance between the RFID chip and the associated antenna affects the coupling therebetween for RFID devices according to the present disclosure. This may be understood with reference toFIGS. 4A and 4B, which illustrate RFID devices, respectively designated as20cand20c′, that are substantially identical (including the configuration of the RFID chip12c, the size and shape of the strap conductor14c, and the configuration of the antenna26c), except for the relative position of the RFID strap10cwith respect to the antenna26c. Compared to the embodiment ofFIG. 4A, in the embodiment ofFIG. 4B, the RFID chip12cis closer to the antenna26c. Stated differently, the portion of the strap conductor14con the same side of the antenna26cas the RFID chip12c(i.e., the loop section16c) is smaller inFIG. 4Bthan inFIG. 4A. This difference in the relative positions of the RFID strap10cand the antenna26caffects the resonant frequency of the loop section16cof the strap conductor14c.

In particular, the two locations28cat which the extension section18coverlaps and crosses the antenna26ccauses a section of the strap conductor14cto be bypassed by the antenna conductor, depending on the value of the capacitors, which will change the effective perimeter30,30′ of the loop section16cand, hence, its tuned frequency. The relative positioning of the two locations28c′ along the strap conductor14cofFIG. 4Bis not the same as the relative positioning of the two locations28along the strap conductor14cofFIG. 2A. Thus, the tuned frequencies of the RFID devices20cand20c′ ofFIGS. 4A and 4Bwill be different, with the frequency of the RFID device20c′ ofFIG. 4Bbeing higher than the frequency of the RFID device20cofFIG. 4Adue to its smaller loop section perimeter30′ ofFIG. 4Bwhen compared with the corresponding loop section perimeter30ofFIG. 4A. This variable frequency allows for a standard RFID strap to be adapted to different antennae and device requirements by adjusting the relative positions of the RFID strap and antenna.

While it may be advantageous to adjust the position of the RFID strap with respect to the antenna, if the change in position is the result of manufacturing tolerances, then the change in loop frequency may be undesirable.FIG. 5illustrates an RFID device, generally designated at20d, that is configured to compensate for the relative position of the strap conductor, generally designated at14d, of the RFID strap10dwith respect to the antenna26d. In particular, the RFID device20dofFIG. 5includes a strap conductor14dhaving a loop section16ddirectly connected to the RFID chip12d, with an extension section18dof the strap conductor14dbeing separated from the RFID chip12dby the loop section16d. The loop section16dand the extension section18deach include a pair of legs22d,24dseparated by a gap GL, GEand extending in a direction between the RFID chip12dand the antenna26d. In the embodiment ofFIG. 5, the size of the gaps GLand GEis the same in both the loop section16dand the extension section18d, but the gap size may differ without departing from the scope of the present disclosure.

The width of at least one of the legs22d,24d(and, in some embodiments, of each leg22d,24d) of the strap conductor14dofFIG. 5varies, rather than being uniform. In particular, the width of each leg22d,24din the embodiment illustrated inFIG. 5, tapers from a maximum width at the end32of the RFID strap10dadjacent to the RFID chip12dto a minimum width at the opposite end34. By such a configuration, the width of each leg22d,24dis greater adjacent to the RFID chip12dthan adjacent to the antenna26d. In other embodiments, the width of one or both legs22d,24dmay taper from a minimum width at the end32of the RFID strap10dadjacent to the RFID chip12dto a maximum width at the opposite end34. Additionally, whileFIG. 5illustrates legs22dand24dthat are substantially identical mirror images, it is within the scope of the present disclosure for the legs22dand24dto be differently configured from each other and/or as illustrated inFIG. 5.

Due to the varying width of the legs22dand24dof the strap conductor14dthat are illustrated inFIG. 5, the degree of overlap at the locations28dwhere the strap conductor14dcrosses the antenna26ddepends upon the relative positions of the RFID strap10dand the antenna26d. For example, if the strap conductor14dis positioned with the RFID chip12drelatively close to the antenna26d, then there will be a greater degree of overlap due to the relatively large width of the legs22dand24dcloser to the RFID chip12d. Conversely, if the strap conductor14dis positioned with the RFID chip12dspaced farther from the antenna26d, then there will be a lesser degree of overlap due to the relatively small width of the legs22dand24dfarther from the RFID chip12d.

The capacitance between the legs22dand24dand the antenna26dis proportional to the coupling area, meaning that the shape of the legs22dand24dcan assist in stabilizing the frequency in the event of deviations from the intended position of the RFID strap10dwith respect to the antenna26d. In particular, as the RFID strap10dis moved to position the RFID chip12dcloser to the antenna26d, inductance decreases (due to the decreased length of the perimeter30dof the loop section16d), while capacitance increases (due to the greater degree of overlap between the extension section18dof the strap conductor14dand the antenna26dat the intersection locations28d). Depending on the values of inductance, capacitance, and RFID chip capacitance, the product of inductance and capacitance (which controls the resonant frequency) may be compensated for in order to be relatively stable regardless of the proximity of the RFID chip12dto the antenna26d, thereby compensating for positional tolerance in applying the RFID strap10dto the antenna26d.

FIGS. 6A and 6Billustrate embodiments of RFID devices20eand20e′ according to the present disclosure incorporating a strap conductor10ewith a complex shape. The RFID devices20eand20e′ ofFIGS. 6A and 6Bare comparable to the RFID devices ofFIGS. 2A, 3A, and 4A-5, with an antenna26eand an RFID strap10eincluding an RFID chip12ecoupled to the antenna26eby a strap conductor14ecomprised of a loop section16eand an extension section18e. The principal difference is that the strap conductor14eofFIGS. 6A and 6Bhas a generally elliptical shape. As a result, adjusting the relative positions of the RFID strap10eand antenna26ein the illustrated “x” direction not only changes the length of the perimeter30e,30e′ of the loop section16e(as in the other embodiments), but also changes the distance between the locations28eat which the extension section18eoverlaps and crosses the antenna26edue to the variable-width gap GEof the extension section18e. Other complex shapes besides an ellipse may also be employed without departing from the scope of the present disclosure.

FIGS. 6A and 6Balso illustrate a manner in which the impedance transformation between the RFID chip12eand the antenna26emay be adjusted by varying the angular orientation of the RFID strap10ewith respect to the antenna26e. The strap conductor14emay be understood as having a center line36extending in a direction between the RFID chip12eand the antenna26e, which is shown inFIG. 6Aas a vertical line extending in the “x” direction. In the orientation ofFIG. 6A, the center line36is substantially perpendicular to the antenna26eat the location where the center line36crosses the antenna26e. This orientation of the RFID strap10ewith respect to the antenna26emay be considered as representing an angular orientation of zero.FIG. 6Bshows the RFID strap10ewith a non-zero angular orientation, with the RFID strap10ebeing rotated some angle Θ about the center of the strap conductor14e, such that the center line36crosses the antenna26eat a non-perpendicular angle Θ and at a different position (illustrated as a horizontal offset in the “y” direction inFIG. 6B). Other angular orientations may also be employed without departing from the scope of the present disclosure, and it should also be understood that varying the angular orientation of the RFID strap is not limited to the embodiment ofFIGS. 6A and 6B, but may be employed with any of the RFID devices according to the present disclosure.

By adjusting the angular orientation of the RFID strap10e, the space between the locations28eat which the extension section18eof the strap conductor14ecrosses the antenna26emay be varied. Adjusting the angular orientation also varies the length of the perimeter30e,30e′ of the loop section16e, while varying the position of the RFID chip12ewith respect to the antenna26ein both the “x” and “y” directions (FIG. 6B). Changing any one of these factors (along with the size and/or shape of the strap conductor) will vary the impedance transformation between the RFID chip12eand the antenna26e, allowing for a better match between the RFID chip12eand the antenna26e.