1. Field of the Invention
This invention relates to the field of Radio Frequency Identification (RFID) tags and labels, and in particular to tags and labels that include a surface insensitive antenna structure.
2. Description of the Related Art
Current inventory and manufacturing methods rely on the ability to track and identify items of concern, such as inventory items, packages, individual parts, etc., and to communicate information concerning such items of concern in a wireless manner for processing and use. One known method of tracking and providing information on such items of concern is to attach to each such item a wireless communication device, such as an active or passive transponder, incorporated into an identification tag or label that responds to radio frequency interrogation and commands. The tag may store or represent information concerning the item to which it is attached, such as a unique identifying number, item status such as opened or unopened, location, and the like. The tag may be attached to an individual item or to packaging containing multiple items.
The present invention deals with the problems that arise in attempting to design and manufacture an RFID tag that has general applicability and can efficiently operate when the packaging and containers on which such a tag may be attached vary widely. For example, some items of interest are shipped in bulk in a single container made of cardboard or plastic, heavier items may be shipped in wooden boxes, and liquids and viscous materials may be shipped in metal containers. Specifically, the present invention is directed toward meeting the problems that arise in attempting to design and manufacture an antenna structure that will operate efficiently and properly over a wide range of such packaging and containers.
In general, an antenna connected to an RFID tag is designed for operation on a specific or narrow range of substrates on which it may be attached or upon which it may be otherwise coupled to. Other substrates will cause the radiation efficiency of the antenna to deteriorate from the designed optimal mounting substrates. Thus, the antenna, and consequently the tag, will no longer radiate properly as designed. This loss of antenna efficiency may be due to a number of variable packaging factors. One is that each substrate has its own dielectric characteristics that typically affect the impedance matching between the wireless communication device and its antenna. Impedance matching ensures the most efficient energy transfer between an antenna and the wireless communication device.
The substrates on which such antennas are attached therefore are important in designing the antenna and the subsequent operating efficiency of the RFID tag itself. Common substrates vary from the non-conductive, such as cardboard, to the conductive, such as foil. In fact, even within a single type of substrate the dielectric characteristics may vary, such as in cardboard packaging, the thickness of the cardboard substrate can differ from package to package, even the moisture content of the cardboard due to weather changes may cause a change in its dielectric characteristics. The radiation efficiency and operation of the antenna can be affected by these highly variable factors even if the antenna was designed for attachment to a cardboard substrate. Therefore, a need exists to provide an antenna for use with a wireless communication device in an RFID tag whose impedance and therefore radiation efficiency is substantially insensitive to the substrate on which it may be attached.
Some prior art systems attempt to overcome the problem of the varying dielectric characteristics of the packaging on which the tag is applied by mounting and encapsulating the wireless communication device and antenna system on a known substrate and then attaching the encapsulated system onto the subject packaging so that the RFID tag “floats” independently of the packaging on which it is mounted, i.e., it is separated from the package by some distance, such as label/ticket that is attached with a plastic fastener. The problem with this encapsulated RFID tag is that it is cumbersome, expensive, difficult to mount and protrudes from the surface of the packaging on which it is mounted, leaving it susceptible to damage and prone to fall off during handling of the packaging, leaving the item untagged.
To overcome these noted problems, RFID tag embodiments have been developed that are directly attached by adhesion to or printing on the surface of the packaging thereby resulting in a lower tag profile that is less prone to damage or removal during handling of the packaging. However, direct surface mounting of these antennas and tags cause their efficiency to suffer from the varying dielectric characteristics of the surfaces on which they are attached. Thus, these systems require that different tags be used on different packaging resulting in added cost, complexity and manufacturing inconvenience. Since it is preferable to tag each inventory item separately, the need for so many different tags multiples the problems greatly.
Other known prior art RFID tags have attempted to overcome the problem of the varying dielectric characteristics of the packaging and provide a generally, “one size fits all” tag, by designing tag antenna systems that are insensitive to the surfaces on which they are mounted.
One known such RFID tag structure that is insensitive to its mounting surface, is that described in U.S. Pat. No. 6,501,435 to King et al., titled “Wireless Communication Device and Method.” This tag structure compensates for the varying substrate dielectric characteristics on which the RFID tabs are attached by utilizing an antenna structure where the radiating tabs are asymmetric with regard to their shape and size. The tab(s) may be attached to the surface of the package or dielectric material opposite the tab(s).