Security tag with three dimensional antenna array made from flat stock

A three-dimensional dipole antenna system for an RFID tag that optimizes detection for a given available volume in which to situate the RFID tag.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to security tags and, more particularly, to an ultra-high frequency (UHF) dipole antenna system for a radio frequency identification (RFID) tag that optimizes detection for a given available volume in which to situate the RFID tag.

2. Description of Related Art

Low cost tags manufactured by continuous feed processes are normally formed from tag stock and are therefore two-dimensional. The performance of two-dimensional tags is generally a strong function of the orientation of the tag's antenna relative to the antenna of the tag interrogator and reader.

One approach used for reducing the sensitivity of tags to their orientation with respect to the interrogator/reader include increasing the effective area of the tag antenna so that greater energy is extracted from the incident electromagnetic field. Another approach, used with dipole antennas, is to orient two or more antennas at angles to each other within the plane of the tag stock. However, both of the aforementioned approaches results in a larger tag, adding manufacturing expense and reducing marketability.

To accommodate the use of two dipole antennae in RFID tags, one company, Matrics, Inc. of Rockville, Md., has developed an RFID system IC (e.g., on Matrics Tag X1020) that provides for a plurality of RF inputs along with a ground terminal.

However, especially where UHF frequencies (e.g., 850 MHz–950 MHz) and microwave frequencies (e.g., 2.3 Ghz–2.6 Ghz) are used in communicating with RFID tags, there remains a need for a UHF(or microwave) dipole antenna system that optimizes detection for a given volume in which the RFID tag is positioned.

BRIEF SUMMARY OF THE INVENTION

An antenna configuration for use in a security tag (e.g., an RFID security tag) that optimizes the receipt of a signal issued from an interrogator or reader. The antenna configuration comprises: a first dipole and a second dipole arranged in a non-parallel nor collinear configuration to form a plane (e.g., a web material) comprising the first and second dipoles; and a third dipole being positioned out of the plane.

A method of fabricating a three-dimensional antenna for a security tag (e.g., an RFID security tag) for optimizing the receipt of a signal issued from an interrogator or reader. The method comprises the steps of: (a) providing a web material (e.g., substrate, flat stock, paper, plastic, etc.); (b) forming a first dipole and a second dipole on the web material and wherein the first dipole and the second dipole are formed to be non-parallel nor collinear with respect to each other; (c) forming a third dipole on the web material; (d) cutting the web material to free a portion of the third dipole from the web material; and (e) displacing the free portion out of the web material.

DETAILED DESCRIPTION OF THE INVENTION

There is shown at20inFIG. 1an RFID security tag comprising a three-dimensional antenna. The RFID security tag20comprises three dipole antennae coupled to an RFID integrated circuit (IC)22. A dipole antenna in the x-axis (seeFIG. 1Afor axes orientation) comprises antenna stubs X1and X2. A dipole antenna in the y-axis comprises antenna stubs Y1and Y2. Finally, a third dipole antenna in the z-axis comprises antenna stubs Z1and Z2. This RFID security tag20can be packaged in an enclosure, e.g., a ball-shaped enclosure, a cubic box-shaped enclosure, etc. The RFID security tag20is ideal for placement in shipping pallets, for example, or incorporation into packing or packaging materials. The presence of a dipole in all three dimensions optimizes detection, by the RFID tag20, of a signal issued from an interrogator or reader (not shown) for a given volume in which the tag20is present, especially for signals in the UHF frequency range (e.g., 850 MHz–950 MHz) and in the microwave range (e.g., 2.3 Ghz–2.6 Ghz). Thus, the three-dimensional antenna forms an improvement over a two-dimensional antenna and operates better than a straight or wavy single dipole antenna.

A very economical method to produce the z-axis dipole is to use security tag flat stock processes for creating all of the dipoles. In particular, as shown inFIGS. 2A–2B, all three dipoles are fabricated on a flat sheet of web material (FIG. 2A) and electrically coupled to the RFID IC22. At a subsequent stage, the web material connecting one of the dipoles is cut (see lines C inFIG. 2A), allowing the free end (FE) of each dipole stub to be folded out of the x-y plane (FIG. 2B) and perpendicular to the other two dipoles (seeFIG. 2Cfor axes orientation).

The dipole antennae of the RFID tag20of the present invention can be produced using conventional processes using etching, printing (e.g., copper or silver inks, flexographic printing), die cutting, laser cutting, etc. The web material24may comprise any flat stock or substrate including paper or plastic, etc. In the preferred embodiment, the thickness of the web material24could be in the range of 25 to 90 microns; the antenna stubs X1-Z2or elements122/124(seeFIGS. 3–5and corresponding text), e.g., metal trace, could be in the range of 7 to 60 microns or more. However, it is known to those skilled in the art that the thickness of the web material24and the antenna stubs X1-Z2/elements122/124are not restricted in any way to those ranges and those ranges do not limit the scope of the invention in any way. In fact, it is within the broadest scope of the present invention to include antenna stubs/elements that are embedded in the web material24, including where the antenna stubs/elements are flush with the surface of the web material24. The RFID IC22can be electrically coupled to the antenna stubs using wire bonding, flip chip processes, contact cementing, etc. Coupling the stubs/elements to the RFID IC22can be accomplished using rectifiers and even multiplexers to provide the signals received from the various dipoles to the RFID IC22. Thus, it is within the broadest scope of the present invention to include any process whereby the stubs of all of the dipoles are formed on or in the substrate and then electrically coupled to the RFID IC22. Moreover, it is also within the broadest scope of the present invention to include the security tag manufacturing processes disclosed in U.S. Patent Application Ser. No. 60/547,235 entitled Security Tags, Apparatus and Methods for Making the Same, filed on Feb. 23, 2004 or disclosed in U.S. patent application Ser. No. 10/235,733 entitled Security Tag and Process for Making the Same, filed Sep. 5, 2002, both of whose entire disclosures are incorporated by reference herein and both of which are owned by the same Assignee, namely Checkpoint Systems, Inc., as the present application. The antenna stubs (X1-Z2) may include tuning stubs that can be trimmed and holding bars for impedance matching that can be modified to properly tune (e.g., in-line tuning of the dipoles while they reside on/in the substrate) the three dipoles before the z-axis stubs Z1and Z2are lifted out of the x-y plane.

It should be understood that although the preferred embodiment includes a third dipole (stubs Z1/Z2) that is orthogonally oriented with respect to said first and second dipoles, it is within the broadest scope of the present invention to include a third dipole having stubs that are positioned out of the x-y plane formed by the first and second dipoles but are not necessarily orthogonal to that plane. Thus, the angles θ1and θ2shown inFIG. 2Bmay be between 0° and 90° with respect to a horizontal reference line in the x-y plane. Moreover, it is also within the broadest scope of the invention wherein the angles θ1and θ2are not equal.

Referring toFIG. 3, there is shown (i.e., a plan view) another embodiment120of a two-dimensional antenna array having multiple dipole elements formed on tag stock24for use with electronic article surveillance (EAS) and RFID type tags. In particular, two folded dipole elements are shown in this embodiment120, an outer element122around the perimeter of the cut tag stock24and an inner element124within the area of the outer element122(the RFID IC22is not shown). The inner element124comprises dipole stubs124A and124B. Preferably, the inner124and outer elements122are formed on the non-conductive tag stock substrate24by any of several tag manufacturing process (all of which were previously described above for the tag20and all of which are applicable to embodiment120) that result in an electrically conductive trace which form the antenna stubs. Such processes include, but are limited to, die cutting, conductive ink printing, etching of a conductive foil and additive plating. The substrate is preferably a polymeric material but could be another substantially non-conductive material such as paper.

Referring toFIG. 4, the embodiment120ofFIG. 3is shown folded into a three-dimensional antenna array. The three-dimensional antenna array is formed from the two-dimensional antenna array by cutting the substrate24(which is in the x-y plane; seeFIG. 4A) around the periphery of the inner element124using die cutting or a similar process, and folding the inner element122into an upright position, the plan of which is at an angle to the x-y plane of the outer element122.FIG. 5shows the inner element124of the antenna ofFIG. 4at a substantially perpendicular angle with respect to the outer element122, installed within the back shell126of a hard tag (e.g., a reusable security tag) housing, including a portion of a lock housing10. Unlike the preferred embodiment20, the second embodiment120is formed by having both dipole stubs124A and124B on the same side of the flat stock24.

It should be noted that although the inner element124of the second embodiment120is orthogonally oriented with respect to the outer element122, it is within the broadest scope of the present invention to include an inner element124having stubs124A/124B that are positioned out of the x-y plane formed by the outer element122but are not necessarily orthogonal to that plane. Thus, the angles θAand θBshown inFIG. 4may be between 0° and 90° with respect to a horizontal reference line in the x-y plane. Moreover, it is also within the broadest scope of the invention wherein the angles θAand θBare not equal.

The three-dimensional antenna array as shown inFIGS. 1–5is not limited to the specific implementation of the depicted embodiments. For example, the inner124and outer122elements need not be folded dipoles but could be other antenna configurations such as loops, and the array could be a combination of various antenna element configurations such as loops and dipoles. Further, the elements of the two-dimensional antenna need not be formed within each other but could be adjacent to each other. Also, the number of elements may be more than two and the elements may be oriented at arbitrary angles with respect to each other and still be within the spirit of the invention.

As would be clear to those skilled in the art, by extending the antenna array into a third dimension, the performance of the antenna array is improved relative to the size of the tag stock consumed to form the antenna array. By maintaining the same area as a two-dimensional antenna array, the performance of the antenna array is increased without increasing the cost of the tag. Alternatively, the antenna area may be reduced to achieve the same performance as a two-dimensional antenna array but in a less expensive tag.