Patent Publication Number: US-2010123553-A1

Title: Rfid tag antenna with capacitively or inductively coupled tuning component

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application No. 61/116,176, filed Nov. 19, 2008, the disclosure of which is incorporated by reference herein in its entirety.” 
    
    
     TECHNICAL FIELD 
     This disclosure relates to radio frequency identification (RFID) systems for article management and, more specifically, to RFID tags. 
     BACKGROUND 
     Radio-frequency identification (RFID) technology has become widely used in virtually every industry, including transportation, manufacturing, waste management, postal tracking, airline baggage reconciliation, and highway toll management. An RFID system may be used to prevent unauthorized removal of articles from a protected area, such as a library or retail store, or as a mechanism for managing a plurality of articles. 
     An RFID system often includes at least one RFID interrogator, often referred to as a “reader,” to interrogate RFID tags to retrieve information from the RFID tags. Each of the RFID tags usually includes information that uniquely identifies the article to which it is affixed. The RFID tags may also include other information associated with the article. The article may be a book, a manufactured item, a vehicle, an animal or individual, or virtually any other tangible article. To detect a tag, the RFID reader outputs RF signals through an antenna to create an electromagnetic field. The field activates RFID tags within a read range of the RFID reader. In turn, the tags produce a characteristic response. In particular, once activated, the tags communicate using a pre-defined protocol, allowing the RFID reader to receive the identifying information from one or more tags in the field. 
     RFID tags for use in such RFID systems typically include an antenna and an RFID integrated circuit (IC) chip. The antenna may, for example, be made from an electrically conductive trace formed on a substrate. The trace forming the antenna may have bonding pads or other connection points for the IC chip. To improve transfer of the RF signals from the reader to the RFID tag antenna and from the RFID tag antenna to the reader, and thereby increase the read range, the antenna may be designed such that an impedance of the antenna matches an impedance of the IC chip. In other words, RFID tags are designed to provide a conjugate impedance match between the IC chip and the antenna. Designing the antenna to match the impedance of the IC chip may be difficult, in part due to the desire to keep a size of the antenna reasonable and usually as small as possible. 
     SUMMARY 
     This disclosure describes RFID tags designed to provide improved impedance matching capabilities. An RFID tag designed in accordance with the techniques of this disclosure includes a radiating component and a tuning component that are located on different layers of the RFID tag. At least a portion of the radiating component and the tuning component overlap, resulting in capacitive and/or inductive coupling. As such, the tuning component provides a mechanism for coupling an IC chip to the radiating component of the RFID tag. Additionally, the tuning component may be used for tuning the antenna, e.g., matching an impedance of the radiating element and an impedance of the IC chip. As such, the radiating element may be designed to provide better gain, polarization purity, larger radar cross section or other parameter, which may degrade when forming the radiating component to include meanders, arched segments or the like. 
     In one embodiment, a radio frequency identification (RFID) tag includes a radiating component formed on a first layer of a substrate. The radiating component includes a straight dipole segment and a loop segment that is electrically coupled to the straight dipole segment. The RFID tag also includes a tuning component formed on a second layer of the substrate. At least a portion of the tuning component substantially overlaps a portion of the radiating component of the first layer of the substrate to couple to the radiating component. Additionally, the RFID tag includes an integrated circuit (IC) that electrically couples to the tuning component. 
     In another embodiment, an antenna for a radio frequency identification (RFID) tag includes a radiating component formed on a first layer of a substrate. The radiating component includes a straight dipole segment and a loop segment that is electrically coupled to the straight dipole segment. The RFID tag also includes a tuning component formed on a second layer of the substrate. The tuning component electrically couples to the radiating component of the first layer of the substrate. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the embodiments will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a radio frequency identification (RFID) system for managing a plurality of articles. 
         FIGS. 2A-2C  are schematic diagrams illustrating an example multi-layer RFID tag that includes a straight radiating component that capacitively couples to a straight tuning component. 
         FIGS. 3A-3C  are schematic diagrams illustrating an example multi-layer RFID tag that includes a straight radiating component that inductively couples to a tuning loop. 
         FIGS. 4A-4C  are schematic diagrams illustrating an example multi-layer RFID tag that includes a radiating component that includes a straight segment and a loop segment that capacitively couples to a straight tuning component. 
         FIGS. 5A-5C  are schematic diagrams illustrating an example multi-layer RFID tag that includes a radiating component that includes a straight segment and a loop segment that inductively couples to a tuning loop. 
         FIGS. 6A and 6B  are schematic diagrams illustrating an example RFID tag that includes a loop radiating component that capacitively couples to an arc-shaped tuning component. 
         FIGS. 7A and 7B  are graphs showing the impedance of several RFID tags over the 900 to 930 MHz range. 
         FIG. 8  is a graph illustrating radiation characteristics of the various RFID tag designs. 
         FIG. 9  is a graph comparing example fields radiated by the RFID tag of  FIG. 3  and a straight dipole antenna. 
         FIG. 10  is a graph illustrating example fields radiated by the RFID tag of  FIG. 5  and a single-layer modified dipole antenna. 
         FIGS. 11A and 11B  are graphs demonstrating the impedance of the RFID tag of  FIG. 6  and a reference RFID tag that includes a loop antenna. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram illustrating an RFID system  2  for managing a plurality of articles. In the example illustrated in  FIG. 1 , RFID system  2  manages a plurality of articles within an area  4 . For purposes of the present description, area  4  will be assumed to be a library and the articles will be assumed to be books or other articles to be checked out. Although the system will be described with respect to managing books or other articles within area  4  to track locations of the articles within area  4  and/or detect checked-in RFID tags to prevent the unauthorized removal of articles from area  4 , it shall be understood that the techniques of this disclosure are not limited in this respect. For example, RFID system  2  could also be used to determine other kinds of status or type information without departing from the scope of this disclosure. Moreover, the techniques described herein are not dependent upon the particular application in which RFID system  2  is used. RFID system  2  may be used to manage articles within a number of other types of environments. RFID system  2  may, for example, be used to manage articles within a corporation, a law firm, a government agency, a hospital, a bank, a retail store or other facility. 
     Each of the articles within area  4 , such as book  6 , may include an RFID tag (not shown in  FIG. 1 ) attached to the respective article. The RFID tags may be attached to the articles with a pressure sensitive adhesive, tape or any other suitable means of attachment. The placement of RFID tags on the respective articles enables RFID system  2  to associate a description of the article with the respective RFID tag via RF signals. For example, the placement of the RFID tags on the articles enables one or more interrogation devices of RFID system  2  to associate a description or other information related to the article. In the example of  FIG. 1 , the interrogation devices of RFID system  2  include a handheld RFID reader  8 , a desktop reader  10 , a shelf reader  12  and an exit control system  14 . Handheld RFID reader  8 , desktop reader  10 , shelf reader  12  and exit control system  14  (collectively referred to herein as “the interrogation devices”) may interrogate one or more of the RFID tags attached to the articles by generating and transmitting RF interrogation signals to the respective tags via an antenna. An RFID tag includes an antenna that receives the interrogation signal from one of the interrogation devices. If a field strength of the interrogation signal exceeds a read threshold, the RFID tag is energized and responds by radiating an RF response signal, a process sometimes referred to as backscattering. That is, the antenna of the RFID tag enables the tag to absorb energy sufficient to power an IC chip coupled to the antenna. Typically, in response to one or more commands contained in the interrogation signal, the IC chip remodulates the interrogation signal to drive the antenna of the RFID tag to output the response signal to be detected by the respective interrogation device. The response signal may include information about the RFID tag and/or its associated article. In this manner, interrogation devices interrogate the RFID tags to obtain information associated with the articles, such as a description of the articles, a status of the articles, a location of the articles, or the like. 
     Desktop reader  10  may, for example, couple to a computing device  18  for interrogating articles to collect circulation information. A user (e.g., a librarian) may place an article, e.g., book  6 , on or near desktop reader  10  to check-out book  6  to a customer or to check-in book  6  from a customer. Desktop reader  10  interrogates the RFID tag of book  6  and provides the information received in the response signal from the RFID tag of book  6  to computing device  18 . The information may, for example, include an identification of book  6  (e.g., title, author, or book ID number), a date on which book  6  was checked-in or checked-out, and a name of the customer to whom the book was checked-out. In some cases, the customer may have an RFID tag (e.g., badge or card) associated with the customer that is scanned in conjunction with, prior to or subsequent to the articles which the customer is checking out. 
     As another example, the librarian may use handheld reader  8  to interrogate articles at remote locations within the library, e.g., on the shelves, to obtain location information associated with the articles. In particular, the librarian may walk around the library and interrogate the books on the shelves with handheld reader  8  to determine what books are on the shelves. 
     The shelves may also include an RFID tag that may be interrogated to indicate which shelves particular books are on. In some cases, handheld reader  8  may also be used to collect circulation information. In other words, the librarian may use handheld reader  8  to check-in and check-out books to customers. 
     Shelf reader  12  may also interrogate the books located on the shelves to generate location information. In particular, shelf reader  12  may include antennas along the bottom of the shelf or on the sides of the shelf that interrogate the books on the shelves of shelf reader  12  to determine the identity of the books located on the shelves. The interrogation of books on shelf reader  12  may, for example, be performed on a weekly, daily or hourly basis. 
     The interrogation devices may interface with an article management system  16  to communicate the information collected by the interrogations to article management system  16 . In this manner, article management system  16  functions as a centralized database of information for each article in the facility. The interrogation devices may interface with article management system  16  via one or more of a wired interface, a wireless interface, or over one or more wired or wireless networks. As an example, computing device  18  and/or shelf reader  12  may interface with article management system  16  via a wired or wireless network (e.g., a local area network (LAN)). As another example, handheld reader  8  may interface with article management system  16  via a wired interface, e.g., a USB cable, or via a wireless interface, such as an infrared (IR) interface or Bluetooth™ interface. 
     Article management system  16  may also be networked or otherwise coupled to one or more computing devices at various locations to provide users, such as the librarian or customers, the ability to access data relative to the articles. For example, the users may request the location and status of a particular article, such as a book. Article management system  16  may retrieve the article information from a database, and report to the user the last location at which the article was located or the status information as to whether the article has been checked-out. In this manner, RFID system  2  may be used for purpose of collection, cataloging and circulating information for the articles in area  4 . 
     In some embodiments, an interrogation device, such as exit control system  14 , may not interrogate the RFID tags to collect information, but instead to detect unauthorized removal of the articles from area  4 . Exit control system  14  may include lattices  19 A and  19 B (collectively, “lattices  19 ”) which define an interrogation zone or corridor located near an exit of area  4 . Lattices  19  include one or more antennas for interrogating the RFID tags as they pass through the corridor to determine whether removal of the article to which the RFID tag is attached is authorized. If removal of the article is not authorized, e.g., the book was not checked-out properly, exit control system  14  initiates an appropriate security action, such as sounding an audible alarm, locking an exit gate or the like. 
     RFID system  2  may, in some instances, be configured to operate in an ultra high frequency (UHF) band of the RF spectrum, e.g., between 300 MHz and 3 GHz. In one exemplary embodiment, RFID system  2  may be configured to operate in the UHF band from approximately 900 MHz to 930 MHz. RFID system  2  may, however, be configured to operate within other portions of the UHF band, such as around 868 MHz (i.e., the European UHF band) or 955 MHz (i.e., the Japanese UHF band). Operation within the UHF band of the RF spectrum may provide several advantages including, increased read range and speed, lower tag cost, smaller tag sizes and the like. 
     As mentioned above, the RFID tags for use in such applications include an antenna and an IC chip. To improve transfer of RF energy between the interrogator and the RFID tag, an impedance of the antenna should be substantially tuned to an impedance of the IC chip. In other words, RFID tags are designed to provide a conjugate impedance match between the IC chip and the antenna. Conjugately matching the impedances of the antenna and the IC chip, sometimes referred to as “matching” or “tuning”, results in improved read performance, e.g., read range. 
     To keep IC chip size and cost down, no attempt is typically made to alter the impedance of the IC chip to make it compatible with the impedance of the antenna. As such, the antenna is typically designed such that the impedance of the antenna substantially matches the impedance of the IC chip. Designing the antenna to match the impedance of the IC may be difficult in part due to the desire to keep a size of the antenna small, thereby keeping the size of the overall RFID tag small. To adjust the impedance of the antenna for tuning, a radiating component of the antenna (e.g., the conductive traces forming the antenna) may be designed to include features such as meanders, arched segments, tuning loops and the like. 
     Forming the antenna to include such features may tune an impedance of the antenna close to the desired impedance and keep the size of the antenna to within reason. However, forming the antenna to include such features may result in degradation of other antenna parameters. For example, designing the radiating component of the antenna to include meanders, arched segments and tuning loops may result in degradation of gain, radiation pattern shape, efficiency and polarization purity. Moreover, designing the antenna to include such features may result in a lack of implementation flexibility. For example, impedance of IC chips from different vendors, and even from the same vendor, may vary significantly. As such, designing the radiating component of the antenna to include meanders, arched segments and tuning loops may limit the flexibility of using the antenna with different IC chips. 
     Additionally, designing the radiating component of the antenna to include meanders, arched segments and tuning loops may limit the flexibility in terms of antenna design. 
     An RFID tag designed in accordance with the techniques of this disclosure provides impedance matching capabilities while overcoming some or all of the drawbacks described above. In particular, an RFID tag may be designed to include an antenna that is formed from a radiating component and a tuning component. The radiating component and the tuning component may be located on different layers of a multi-layer RFID tag and couple to one another via a proximal coupling. The proximal coupling may, for example, be a capacitive and/or inductive coupling. The tuning component may provide at least some of the tuning capabilities to substantially match an impedance of the antenna to an impedance of the IC chip. As such, the radiating component may be designed to provide better gain, radiation pattern shape, efficiency, polarization purity, larger radar cross section or other parameter that may degrade when the radiating component is designed to include to meanders, arched segments or the like. Additionally, the RFID tags designed in accordance with the techniques of this disclosure provide improved implementation flexibility. For example, the same antenna may be used with IC chips having different impedances by adjusting the tuning component. 
       FIGS. 2A-2C  are schematic diagrams illustrating an example RFID tag  20  that includes a radiating component  22  that capacitively couples to a tuning component  24 .  FIG. 2A  is an exploded view of RFID tag  20 ,  FIG. 2B  is a top view of RFID tag  20  and  FIG. 2C  is a cross section view of RFID tag  20  from A to A′. As illustrated in the exploded view of RFID tag  20  of  FIG. 2A , RFID tag  20  includes a first layer  28 A that includes tuning component  24  and a second layer  28 B that includes radiating component  22 . In one embodiment, radiating component  22  may be formed on a first side of a substrate  29  and tuning component  24  may be formed on a second, e.g., opposite, side of substrate  29 . In another embodiment, radiating component  22  and tuning component  24  may formed on separate substrates. Substrate  29  may comprise any dielectric material, and, in one example, may be a thin, plastic substrate. Radiating component  22  and tuning component  24  may, in some instances, be formed using various fabrication techniques. Radiating component  22  and tuning component  24  may, for example, be printed onto substrate  29 . Alternatively, a conductive layer, such as copper, aluminum, or other conductive material, may be deposited on substrate  29 , e.g., via chemical vapor deposition, sputtering, or any other depositing technique, and radiating component  22  and tuning component  24  may be shaped via etching, photolithography, masking, or similar technique. 
     In the example RFID tag  20  illustrated in  FIGS. 2A-2C , radiating component  22  is a straight dipole element that has a length L RAD  and a width W RAD . Tuning component  24  is a straight tuning element that has a length L TUN  and a width W TUN . Radiating component  22  and tuning component  24  are arranged such that radiating component  22  and tuning component  24  are coupled via a proximal coupling. For example, radiating component  22  and tuning component  24  may be arranged such that there is substantial overlap between a portion of radiating component  22  and tuning component  24 . In the example top view illustrated in  FIG. 2B , there is a substantial overlap between a portion of the length and width of radiating component  22  of the first layer and the length and width of tuning component  24  of the second layer. In other words, when viewed from the top, the portion of the length and width of radiating component  22  is directly above the length and width of tuning component  24 . 
     The overlap between tuning component  24  and radiating component  22  provides capacitive coupling between tuning component  24  and radiating component  22  for transferring RF energy, e.g., RF signals, between radiating component  22  and an IC chip  26  that is electrically coupled to the tuning component  24 . As will be described in further detail below, the capacitive coupling may also be used as the tuning element. IC chip  26  may be electrically coupled to tuning component  24  via one or more feedpoints, e.g., bonding pads or other means for interconnection. IC chip  26  may be bonded to the feedpoints using flip chip bonding, wire bonding or the any other attachment mechanism. 
     The length L RAD  of radiating component  22  may, for example, be greater than approximately 100 mm (about 4 inches), and more preferably between approximately 130 mm and 180 mm (between about 5 and 7 inches), and even more preferably approximately 165 mm (slightly over 6.5 inches). The width W RAD  of the radiating component  22  may be less than approximately 4 mm (about 0.15 inches), and more preferably approximately 1 mm (about 0.04 inches). The length L TUN  of tuning component  24  may be between approximately 10 mm and 50 mm (between about 0.4 and 2.0 inches), and more preferably between approximately 20 mm and 40 mm (between about 0.79 and 1.57 inches). The width W TUN  of the tuning component  24  may be less than approximately 4 mm (about 0.15 inches), and more preferably approximately 1 mm (about 0.04 inches). In one embodiment, one or more conductive traces that form radiating component  22  and/or tuning component  24  may have a minimum trace width of a selected manufacturing process, e.g., approximately 1 mm. Although in the example illustrated in  FIGS. 2A-2C  radiating component  22  and tuning component  24  have substantially the same widths, the width W TUN  of tuning component  24  may be wider or narrower than the width W RAD  of radiating component  22 . 
     The long, narrow aspect of radiating component  22  may allow RFID tag  20  to be concealed, i.e., rendered covert, on or within the article while still allowing RFID tag  20  to be interrogated even when partially covered by some object. For example, RFID tag  20  may be placed within a gutter of a book or on an inside portion of a spine of the book to conceal RFID tag  20  from an observer. RFID tag  20  may, however, still be interrogated when a hand of a person holding the book is partially covering RFID tag  20 . 
     As described above, arranging radiating component  22  and tuning component  24  such that there is substantial overlap between a portion of radiating component  22  and tuning component  24  results in capacitive coupling between radiating component  22  and tuning component  24 . In this manner, tuning component  24  functions as a mechanism for interconnecting radiating component  22  with IC chip  26 . In one example, a conductive trace forming tuning component  24  may act as a first capacitive plate and the portion of a conductive trace of the radiating component  22  that overlaps the tuning component may act as a second conductive plate. An electric field exists between the overlapping conductive traces to provide the capacitive coupling between tuning component  24  and radiating component  22 . In general, the more overlapping surface area between radiating component  22  and tuning component  24 , the larger the tuning capacitance. The amount of overlap may be controlled, for example, by adjusting a length and/or width of tuning component  24  or positioning of tuning component  24  with respect to the radiating component  22 . 
     Additionally, the distance between the overlapping portions of radiating component  22  and tuning component  24 , e.g., the thickness of substrate  29 , may further be used to control the tuning capacitance. Although the predominant coupling between radiating component  22  and tuning component  24  of RFID antenna  20  is capacitive, the coupling may include at least some inductive coupling as well. 
     The length L TUN  and the width W TUN  of tuning component  24  may also be adjusted to provide improved impedance matching between an impedance of radiating component  22  and IC chip  26 . Matching an impedance of the antenna to the impedance of IC chip  26  improves transfer of RF energy between the interrogator and the RFID tag. Generally, IC chip  26  has a complex impedance with a resistance (i.e., real part of the impedance) and a negative reactance (i.e., imaginary part of the impedance). The reactance is typically a large negative value due to the input circuitry of the IC. Thus, to achieve conjugate matching, tuning component  24  may be designed to provide the antenna with an equivalent resistance and equal and opposite positive reactance. In particular, the length L TUN  and the width W TUN  of tuning component  24  may be designed to provide impedance matching. For example, as the length L TUN  of tuning component  24  or the width W TUN  of tuning component  24  is increased, the reactance becomes more positive. Additionally, the amount of capacitive impedance provided by tuning component  24  may be adjusted by controlling a distance between the overlapping portions of radiating component  22  and tuning component  24 , e.g., the thickness of substrate  29 . In other words, the thickness of substrate  29  may also be used for tuning Although radiating component  22  and tuning component  24  overlap in the example RFID antenna  20  of  FIG. 2 , the techniques described herein are not limited to such an embodiment. In some instances, radiating component  22  and tuning component  24  may be offset such that the components are not substantially overlapping. In this case, radiating component  22  and tuning component  24  are still located proximal to one another to provide proximal coupling (e.g., via inductive or capacitive coupling) for transferring RF energy and providing tuning capabilities, but do not substantially overlap. In other words, there may be no overlap or only partial overlap between radiating component  22  and tuning component  24 . 
     In accordance with one aspect of this disclosure, tuning component  24  and radiating component  22  are of different lengths so that any field radiated by tuning component  24  does not play a major role in the transmission and/or reception of radiation by RFID tag  20 . Thus, the dominant source of radiation is still the straight dipole radiating component. For example, RFID tag  20  may be designed such that the field radiated by tuning element  24 , if any, is less than 5 percent of the entire field radiated by RFID tag  20 . By designing the tuning component  24  of RFID tag  20  to be less than one-quarter of the length of radiating component  22 , and more preferably less than one-eighth of the length of radiating component  22 , tuning component  24  may be designed to radiate a field within the limits provided above. The example RFID tag  20  illustrated in  FIGS. 2A-2C  is representative of one RFID tag configuration in accordance with this disclosure. The illustrated embodiment should not be limiting of the techniques as broadly described in this disclosure. For example, although tuning component  24  is positioned to overlap a center portion of radiating component  22 , tuning component  24  may be offset from the center portion of radiating component  22 . Moreover, tuning component  24  and radiating component  22  may be formed in different shapes, some of which are illustrated in  FIGS. 3-6 . Additionally, tuning component  24  may be constructed from multiple elements in addition to or instead of conductive traces. For example, tuning component may be made up of conductive traces and tuning capacitors.  FIGS. 3A-3C  are schematic diagrams illustrating an example RFID tag  30  that includes a radiating component  32  that inductively couples to a tuning component  34 .  FIG. 3A  is an exploded view of RFID tag  30 ,  FIG. 3B  is a top view of RFID tag  30  and  FIG. 3C  is a cross section view of RFID tag  30  from B to B′. As illustrated in the exploded view of RFID tag  30  of  FIG. 3A , RFID tag  30  includes a first layer  38 A that includes tuning component  34  and a second layer  38 B that includes radiating component  32 . Radiating component  32  and tuning component  34  may be formed on opposite sides of a single substrate  29  or on separate substrates. Radiating component  32  and tuning component  34  may be formed using various fabrication techniques. 
     In the example RFID tag  30  illustrated in  FIGS. 3A-3C , radiating component  32  is a straight dipole element that has a length L RAD  and a width W RAD  and tuning component  34  is a tuning loop that has a length L TUN  and a width W TUN . The tuning loop illustrated in  FIGS. 3A-3C  is formed in the shape of a rectangle. The tuning loop may, however, take on different shapes. For example, the tuning loop may be formed in the shape of a half-circle, a half-oval, triangle, trapezoid or other symmetric or asymmetric shape. 
     Radiating component  32  and tuning component  34  are arranged such that there is substantial overlap between a portion of radiating component  32  and a portion of tuning component  34 . When radiating component  32  and tuning component  34  are formed using conductive traces, at least a portion of the conductive traces (or traces) forming tuning component  34  substantially overlap with at least a portion of the conductive trace (or traces) forming radiating component  32 . In the example top view illustrated in  FIG. 3B , there is a substantial overlap between a portion of the length and width of radiating component  32  of second layer  38 B and a length and width of one side of the tuning loop of tuning component  34  of first layer  38 A. In other words, the portion of radiating component  32  of second layer  38 B is located directly below the one side of the tuning loop that forms tuning component  34  on the first layer  38 A. In the example illustrated in  FIGS. 3A-3C , the side of the tuning loop of tuning component  34  that overlaps radiating component  32  is symmetrically located with respect to a center of radiating component  32 . In other embodiments, however, the side of the tuning loop that overlaps radiating component  32  may be asymmetrically located with respect to the center of radiating component  32 . 
     The overlap between the portion of radiating component  32  and the one side of the tuning loop of tuning component  34  provides inductive coupling. In particular, RF energy is transferred between the overlapping portions of tuning component  34  and radiating component  32  via a shared magnetic field. For example, as current flows through radiating component  32 , a current is induced in the tuning loop of tuning component  34 , thereby transferring RF energy from radiating component  32  to tuning component  34 . In the embodiment illustrated in  FIGS. 3A-3C , inductive coupling dominates because tuning component  34  is a closed loop through which current can easily flow. Although the coupling between the overlapping portions of radiating component  32  and tuning component  34  is predominately inductive coupling, the coupling may include at least some capacitive coupling as well. 
     The length L RAD  of radiating component  32  may, for example, be greater than approximately 100 mm (about 4 inches), and more preferably between approximately 130 mm and 180 mm (between about 5 and 7 inches), and even more preferably approximately 165 mm (slightly over 6.5 inches). The width W RAD  of the radiating component  32  may be less than approximately 4 mm (about 0.15 inches), and more preferably approximately 1 mm (about 0.04 inches). 
     The length L TUN  of tuning component  34  may be between approximately 10 mm and 50 mm (between about 0.4 and 2.0 inches), and more preferably between approximately 20 mm and 40 mm (between about 0.79 and 1.57 inches). The width W TUN  of the tuning component  34  may be less than approximately 6 mm (about 0.25 inches), and more preferably less than approximately 4 mm (about 0.15 inches). In one embodiment, width W TUN  of tuning component  34  may be less than or equal to approximately four times a width of conductive traces that form the tuning loop. In such an embodiment, the width of conductive traces forming the sides of the tuning loop are equal to 1X, and a space between an inside edge of the conductive trace forming the side of the tuning loop overlapping radiating component  32  and an inside edge of the conductive trace forming an opposite side of the tuning loop may be equal to approximately 2X, where X is equal to the conductive trace width. Thus, the width W TUN  of tuning component  34  may have a width that is approximately four times the width of the conductive traces forming the tuning loop. In another embodiment, the space between the inside edge of the conductive trace forming the side of the tuning loop overlapping radiating component  32  and the inside edge of the conductive trace forming an opposite side of the tuning loop may be equal to approximately 1X, resulting in a width that is approximately three times the width of the conductive traces. In some instances, the conductive traces that form tuning component  34  may have a minimum trace width of a selected manufacturing process, e.g., approximately 1 mm. 
     Again, the long, narrow aspect of radiating component  32  may allow RFID tag  30  to be concealed, i.e., rendered covert, on or within the article while still allowing RFID tag  30  to be interrogated even when partially covered by some object. For example, RFID tag  30  may be placed within a gutter of a book or on an inside portion of a spine of the book to conceal RFID tag  30  from an observer. RFID tag  30  may, however, still be interrogated when a hand of a person holding the book is partially covering RFID tag  30 . 
     In addition to providing the coupling with radiating component  32 , tuning component  34  may also provide impedance matching. In particular, the length L TUN  and the width W TUN  of tuning component  34 , i.e., the tuning loop, may be adjusted to match an impedance of radiating component  32  and IC chip  26 . For example, as the length L TUN  or width W TUN  of tuning component  34  is increased, the reactance becomes more positive. Additionally, the amount of inductive coupling between tuning component  34  and radiating component  32  may be adjusted by controlling a distance between the overlapping portions of radiating component  32  and tuning component  34 , e.g., the thickness of substrate  29 . In this manner, the thickness of substrate  29  may also be used for impedance matching (or tuning). 
     Matching an impedance of the antenna to the impedance of IC chip  26  improves transfer of RF energy between the interrogator and the RFID tag. 
     Although radiating component  32  and tuning component  34  overlap in the example RFID antenna  30  of  FIG. 3 , the techniques described herein are not limited to such an embodiment. In some instances, radiating component  32  and tuning component  34  may be offset such that the components are not substantially overlapping. In this case, radiating component  32  and tuning component  34  are still located proximal to one another to provide proximal coupling (e.g., via inductive or capacitive coupling) for transferring RF energy and providing tuning capabilities, but do not substantially overlap. In other words, there may be no overlap or only partial overlap between radiating component  32  and tuning component  34 . 
     In accordance with one aspect of this disclosure, the dimensions of tuning component  34  are selected so that any field transmitted or received by tuning component  34  does not play a major role in the transmission and/or reception of radiation by RFID tag  30 . Thus, the dominant source of radiation is still the straight dipole radiating element. For example, RFID tag  30  may be designed such that the field radiated by tuning component  34 , if any, is less than 5 percent of the entire field radiated by RFID tag  30 . By designing a circumference (or perimeter of tuning component  34  of RFID tag  30  to be less than one-quarter of the length of radiating component  32 , and more preferably less than one-eighth of the length of radiating component  32 , tuning component  34  may be designed to radiate a field within the limits provided above. 
     IC chip  26  may be electrically coupled to tuning component  34  via one or more feedpoints, e.g., bonding pads or other means for interconnection. IC chip  26  may be bonded to the feedpoints using flip chip bonding, wire bonding or the any other attachment mechanism. 
     As illustrated in  FIGS. 3A and 3B , IC chip  26  couples to the tuning component  34  on the side of the tuning loop opposite from the side of the tuning loop that inductively couples to radiating component  32 . However, IC chip  26  may couple to tuning component  34  on any side of the tuning loop, including the side that inductively couples to the radiating component  32 . 
     The example RFID tag  30  illustrated in  FIGS. 3A-3C  is representative of one RFID tag configuration in accordance with this disclosure. The illustrated embodiment should not be limiting of the techniques as broadly described in this disclosure. For example, although tuning component  34  is positioned to overlap a center portion of radiating component  32 , tuning component  34  may be offset from the center portion of radiating component  32 . Moreover, tuning component  34  and radiating component  32  may be formed in different shapes, some of which are illustrated in FIGS.  2  and  4 - 6 . Additionally, tuning component  34  may be constructed from multiple elements in addition to or instead of conductive traces. For example, tuning component may be made up of conductive traces and tuning capacitors.  FIGS. 4A-4C  are schematic diagrams illustrating an example RFID tag  40  that includes a radiating component  42  that capacitively couples to a tuning component  44 .  FIG. 4A  is an exploded view of RFID tag  40 ,  FIG. 4B  is a top view of RFID tag  40  and  FIG. 4C  is a cross section view of RFID tag  40  from C to C′. As illustrated in the exploded view of RFID tag  40  of  FIG. 4A , RFID tag  40  includes a first layer  48 A that includes tuning component  44  and a second layer  48 B that includes radiating component  42 . Radiating component  42  and tuning component  44  may be formed on opposite sides of a single substrate  29  or on separate substrates. Radiating component  42  and tuning component  44  may be formed using various fabrication techniques. 
     In the example RFID tag  40  illustrated in  FIGS. 4A-4C , radiating component  42  includes a straight antenna segment  46  coupled to a conductive loop segment  47 . In other words, radiating component  42  may be viewed as a straight dipole antenna with loop segment  47  added. In one embodiment, straight segment  46  and loop segment  47  may be electrically conductive traces disposed on substrate  29 . For example, straight antenna segment  46  may be formed from a first electrically conductive trace and loop segment  47  may be formed of a second electrically conductive trace and coupled to the first conductive trace forming straight antenna segment  47 . 
     Loop segment  47  of radiating component  42  illustrated in  FIGS. 4A-4C  is formed in the shape of a rectangle. Loop segment  47  of radiating component  42  may, however, take on different shapes. For example, loop segment  47  may be formed in the shape of a half-circle, a half-oval, triangle, trapezoid or other symmetric or asymmetric shape. Additionally, loop segment  47  is symmetrically located with respect to the straight segment  46 . In other words, straight segment  46  extends an equal distance in both directions beyond loop segment  47 . In other embodiments, however, loop segment  47  may be asymmetrically located with respect to the straight segment  46 . 
     Radiating component  42  has a length L RAD  and a width W RAD . The length L RAD  of radiating component  42  may, for example, be greater than approximately 100 mm (about 4 inches), and more preferably between approximately 140 mm and 180 mm (between about 5 and 7 inches), and even more preferably approximately 165 mm (slightly over 6.5 inches). The width W RAD  of the radiating component  42  may be less than approximately 6 mm (about 0.25 inches), and more preferably less than approximately 4 mm (about 0.15 inches). 
     In one embodiment, width W RAD  of radiating component  42  may be less than or equal to approximately four times a width of conductive traces that form loop segment  47 . In such an embodiment, the width of conductive traces forming the sides of the tuning loop are equal to 1X, and a space between an inside edge of the conductive trace forming loop segment  47  and an inside edge of the conductive trace forming straight segment  46  may be equal to approximately 2X, where X is equal to the conductive trace width. Thus, the width W RAD  of radiating component  42  may have a width that is approximately four times the width of the conductive traces forming the tuning loop. In another embodiment, the space between the inside edge of the conductive trace forming loop segment  47  and the inside edge of the conductive trace forming straight segment  46  may be equal to approximately 1X, resulting in a width W RAD  that is approximately three times the width of the conductive traces. In some instances, the conductive traces that form tuning component  44  may have a minimum trace width of a selected manufacturing process, e.g., approximately 1 mm 
     Tuning component  44  is a straight tuning element that has a length L TUN  and a width W TUN . The length L TUN  of tuning component  44  may be between approximately 10 mm and 50 mm (between about 0.4 and 2.0 inches), and more preferably between approximately 20 mm and 40 mm (between about 0.79 and 1.57 inches). The width W TUN  of the tuning component  44  may be less than approximately 4 mm (about 0.15 inches), and more preferably approximately 1 mm (about 0.04 inches). In one embodiment, tuning component  44  is formed from a conductive trace that has the same width as radiating component  42 . Radiating component  42  and tuning component  44  are arranged such that there is substantial overlap between a portion of radiating component  42  and at least a portion of tuning component  44 . In the example top view illustrated in  FIG. 4B , there is a substantial overlap between a portion of loop segment  47  of radiation component  42  and a length and width of tuning component  44 . In the example illustrated in  FIGS. 4A-4C , tuning component  44  is symmetrically located with respect to center of the portion of loop segment  47 . In other embodiments, however, tuning component  44  may be asymmetrically located with respect to the center of the portion of the loop segment  47 , but still proximal to at least a portion of loop segment  47 . 
     The overlap between the portion of loop segment  47  and tuning component  44  results in capacitive coupling between tuning component  44  and radiating component  42 . In this manner, tuning component  44  transfers RF energy between radiating component  42  with IC chip  26 . In one example, a conductive trace forming tuning component  44  may act as a first capacitive plate and the portion of loop segment  47  that overlaps tuning component  44  may act as a second conductive plate. An electric field exists between the overlapping conductive traces to provide the capacitive coupling between tuning component  44  and radiating component  42 . In general, the more overlapping surface area between radiating component  42  and tuning component  44 , the larger the tuning capacitance. The amount of overlap may be controlled, for example, by adjusting a length and/or width of tuning component  44  or the positioning of tuning element  44  with respect to the radiating element  42 . Although the predominant coupling between radiating component  22  and tuning component  24  of RFID antenna  20  is capacitive, the coupling may include at least some inductive coupling as well. In addition to providing the coupling with radiating component  42 , tuning component  44  may also provide impedance matching. In particular, the length L TUN  and the width W TUN  of tuning component  44  may be adjusted to match an impedance of radiating component  42  and IC chip  26 . For example, as the length L TUN  and/or width W TUN  of tuning component  44  is increased, the reactance becomes more positive. Additionally, the distance between the overlapping portions of radiating component  42  and tuning component  44 , e.g., the thickness of substrate  29 , may further be used to control the tuning capacitance. Matching an impedance of the antenna to the impedance of IC chip  26  improves transfer of RF energy between the interrogator and the RFID tag. Although the predominant coupling between radiating component  42  and tuning component  44  of RFID tag  40  is capacitive, the coupling may include at least some inductive coupling as well. 
     The antenna may further be tuned to match the impedance of IC chip  26  by modifying dimensions of loop segment  47 . For example, a length or width of the loop segment  47  may be adjusted to match the impedance of the antenna to the impedance IC chip  26 . 
     Additionally, a number of aspects of loop segment  47  may also be modified to improve the operation of RFID tag  40 . For example, a length of the loop segment may be adjusted to affect the sensitivity of RFID tag  40 . A longer length L LOOP  may increase the sensitivity of RFID tag  40  to signal interference, loss caused by the presence of dielectric material (e.g., pages and other binding materials) and changes in dipole length. Alternatively, or additionally, the shape of loop segment  47  may also be adjusted to affect sensitivity of RFID tag  42 . 
     Although radiating component  42  and tuning component  44  overlap in the example RFID antenna  40  of  FIG. 4 , the techniques described herein are not limited to such an embodiment. In some instances, radiating component  42  and tuning component  44  may be offset such that the components are not substantially overlapping. In this case, radiating component  42  and tuning component  44  are still located proximal to one another to provide proximal coupling (e.g., via inductive or capacitive coupling) for transferring RF energy and providing tuning capabilities, but do not substantially overlap. In other words, there may be no overlap or only partial overlap between radiating component  42  and tuning component  44 . 
     In accordance with one aspect of this disclosure, the dimensions of tuning component  44  are selected so that any field transmitted or received by tuning component  44  does not play a major role in the transmission and/or reception of radiation by RFID tag  40 . Thus, the dominant source of radiation is still the straight dipole radiating element. For example, RFID tag  40  may be designed such that the field radiated by tuning element  44 , if any, is less than 5 percent of the entire field radiated by RFID tag  40 . By designing the tuning component  44  of RFID tag  40  to be less than one-quarter of the length of radiating component  42 , and more preferably less than one-eighth of the length of radiating component  42 , tuning component  44  may be designed to radiate a field within the limits provided above. 
     The example RFID tag  40  illustrated in  FIGS. 4A-4C  is representative of one RFID tag configuration in accordance with this disclosure. The illustrated embodiment should not be limiting of the techniques as broadly described in this disclosure. For example, although tuning component  44  is positioned to overlap a center portion of radiating component  42 , tuning component  44  may be offset from the center portion of radiating component  42 . Moreover, tuning component  44  and radiating component  42  may be formed in different shapes, some of which are illustrated in  FIGS. 2 ,  3 ,  5  and  6 . Additionally, tuning component  44  may be constructed from multiple elements in addition to or instead of conductive traces. For example, tuning component may be made up of conductive traces and tuning capacitors.  FIGS. 5A-5C  are schematic diagrams illustrating an example RFID tag  50  that includes a radiating component  52  that inductively couples to a tuning component  54 .  FIG. 5A  is an exploded view of RFID tag  50 ,  FIG. 5B  is a top view of RFID tag  50  and  FIG. 5C  is a cross section view of RFID tag  50  from D to D′. As illustrated in the exploded view of RFID tag  50  of  FIG. 5A , RFID tag  50  includes a first layer  58 A that includes tuning component  54  and a second layer  58 B that includes radiating component  52 . Radiating component  52  and tuning component  54  may be formed on opposite sides of a single substrate  29  or on separate substrates. Radiating component  52  and tuning component  54  may be formed using various fabrication techniques. 
     In the example RFID tag  50  illustrated in  FIGS. 5A-5C , radiating component  52  that has a length L RAD  and a width W RAD . Radiating component includes a straight antenna segment  56  coupled to a conductive loop segment  57 . In other words, radiating component  52  may be viewed as a straight dipole antenna with loop segment  57  added. In one embodiment, straight segment  56  and loop segment  57  may be electrically conductive traces disposed on substrate  29 . Tuning component  54  is a tuning loop that has a length L TUN  and a width W TUN . 
     Loop segment  57  of radiating component  52  and the tuning loop of tuning component  54  are formed in the shape of a rectangle in the illustrated example. Loop segment  57  and tuning component  54  may, however, take on different shapes. For example, loop segment  57  may be formed in the shape of a half-circle, a half-oval, triangle, trapezoid or other symmetric or asymmetric shape. Loop segment  57  and the tuning loop may be of the same shape or different shapes. 
     The length L RAD  of radiating component  52  may, for example, be greater than approximately 100 mm (about 5 inches), and more preferably between approximately 150 mm and 180 mm (between about 5 and 7 inches), and even more preferably approximately 165 mm (slightly over 6.5 inches). The length L TUN  of tuning component  54  may be between approximately 10 mm and 50 mm (between about 0.5 and 2.0 inches), and more preferably between approximately 20 mm and 40 mm (between about 0.79 and 1.57 inches). 
     The width W RAD  of the radiating component  52  and the width W TUN  of tuning component  54  may be less than approximately 6 mm (about 0.25 inches), and more preferably less than approximately 5 mm (about 0.15 inches). As described above, the width W RAD  of radiating component  52  and the width W TUN  of tuning component  54  may, in some instances, be less than or equal to approximately four times a width of conductive traces that form loop segment  57  and the tuning loop, respectively. The conductive traces that form tuning component  54  may have a minimum trace width of a selected manufacturing process, e.g., approximately 1 mm. Although illustrated in  FIGS. 5A-5C  as being approximately the same width, radiating component  52  and tuning component  54  may have different widths. 
     Radiating component  52  and tuning component  54  are arranged such that there is substantial overlap between a portion of radiating component  52  and at least a portion of tuning component  54 . In the example top view illustrated in  FIG. 5B , there is a substantial overlap between loop segment  57  of radiation component  52  and the tuning loop of tuning component  54 . Alternatively, only a portion of loop segment  57  may overlap the tuning loop of tuning component  54 . 
     The overlap between loop segment  57  and the tuning loop of tuning component  54  results in inductive coupling between radiating component  52  and tuning component  54 . In particular, RF energy is transferred between the overlapping portions of tuning component  54  and radiating component  52  via a shared magnetic field. For example, as current flows through loop segment  57  of radiating component  52 , a current is induced in the tuning loop of tuning component  54 , thereby transferring RF energy from radiating component  52  to tuning component  54 . In the embodiment illustrated in  FIGS. 5A-5C , inductive coupling dominates because tuning component  54  is a closed loop through which current can easily flow. Although the coupling between the overlapping portions of radiating component  54  and tuning component  54  is predominately inductive coupling, the coupling may include at least some capacitive coupling as well. 
     In addition to providing the coupling with radiating component  52 , tuning component  54  may also provide impedance matching. In particular, the length L TUN  and the width W TUN  of tuning component  54 , i.e., the tuning loop, may be adjusted to match an impedance of radiating component  52  and IC chip  26 . For example, as the length L TUN  and/or width W TUN  of tuning component  54  is increased, the reactance becomes more positive. Additionally, the distance between the overlapping portions of radiating component  52  and tuning component  54 , e.g., the thickness of substrate  29 , may further be used to control the tuning capacitance. Matching an impedance of the antenna to the impedance of IC chip  26  improves transfer of RF energy between the interrogator and the RFID tag. 
     The antenna may further be tuned to match the impedance of IC chip  26  by modifying dimensions of loop segment  57  of radiating component  52 . For example, a length or width of the loop segment  57  may be adjusted to match the impedance of the antenna to the impedance of IC chip  26 . Additionally, a number of aspects of loop segment  57  may also be modified to improve the operation of RFID tag  50 . For example, a length of the loop segment may be adjusted to affect the sensitivity of RFID tag  50 . A longer length L LOOP  may increase the sensitivity of RFID tag  50  to signal interference, loss caused by the presence of dielectric material (e.g., pages and other binding materials) and changes in dipole length. Alternatively, or additionally, the shape of loop segment  57  may also be adjusted to affect sensitivity of RFID tag  52 . 
     Although radiating component  52  and tuning component  54  overlap in the example RFID antenna  50  of  FIG. 5 , the techniques described herein are not limited to such an embodiment. In some instances, radiating component  52  and tuning component  54  may be offset such that the components are not substantially overlapping. In this case, radiating component  52  and tuning component  54  are still located proximal to one another to provide proximal coupling (e.g., via inductive or capacitive coupling) for transferring RF energy and providing tuning capabilities, but do not substantially overlap. In other words, there may be no overlap or only partial overlap between radiating component  52  and tuning component  54 . 
     In accordance with one aspect of this disclosure, the dimensions of tuning component  54  are selected so that any field transmitted or received by tuning component  54  does not play a major role in the transmission and/or reception of radiation by RFID tag  50 . Thus, the dominant source of radiation is still the straight dipole radiating element. For example, RFID tag  50  may be designed such that the field radiated by tuning element  54 , if any, is less than 5 percent of the entire field radiated by RFID tag  50 . For example, by designing a circumference or perimeter of tuning component  54  of RFID tag  50  to be less than one-quarter of the length of radiating component  52 , and more preferably less than one-eighth of the length of radiating component  52 , tuning component  54  may be designed to radiate a field within the limits provided above. 
     The example RFID tag  50  illustrated in  FIGS. 5A-5C  is representative of one RFID tag configuration in accordance with this disclosure. The illustrated embodiment should not be limiting of the techniques as broadly described in this disclosure. For example, although tuning component  54  is positioned to overlap a center portion of radiating component  52 , tuning component  54  may be offset from the center portion of radiating component  52 . Moreover, tuning component  54  and radiating component  52  may be formed in different shapes, some of which are illustrated in  FIGS. 2-4  and  6 . Additionally, tuning component  54  may be constructed from multiple elements in addition to or instead of conductive traces. For example, tuning component may be made up of conductive traces and tuning capacitors.  FIGS. 6A and 6B  are schematic diagrams illustrating an example RFID tag  60  that includes a radiating component  62  that capacitively couples to a tuning component  64 .  FIG. 6A  is an exploded view of RFID tag  60  and  FIG. 6B  is a top view of RFID tag  60 . As illustrated in the exploded view of RFID tag  60  of  FIG. 6A , RFID tag  60  includes a first layer  68 A that includes tuning component  64  and a second layer  68 B that includes radiating component  62 . Radiating component  62  and tuning component  64  may be formed on opposite sides of a single substrate or on separate substrates using various fabrication techniques. 
     In the example RFID tag  60  illustrated in  FIGS. 6A and 6B , radiating component  62  is a loop antenna. The loop antenna illustrated in  FIGS. 6A and 6B  includes a single loop that is shaped like a circle. In other embodiments, however, the loop antenna may have more than one loop. Additionally, the loop antenna may take on different shapes, e.g., an oval shape, a rectangular shape, a square shape, a trapezoid shape or other symmetric or asymmetric shape. Radiating component  62  includes a length L RAD  and a width W RAD . In the example illustrated in  FIGS. 6A and 6B , the length L RAD  of radiating component  62  is the circumference of the circle-shaped loop. The circle-shaped loop of radiating component  62  may have a circumference that is approximately half of a wavelength. In one example, the circle-shaped loop of radiating component  62  may have a radius of approximately 22 mm (about 0.87 inches). Thus, the length L RAD  of radiating component  62  is approximately 138 mm (about 5.43 inches). The width W RAD  of radiating component  62  may be a thickness of the conductive trace or other conductive element that forms the loop, which may be less than approximately 4 mm (about 0.15 inches), and more preferably approximately 1 mm (about 0.04 inches). 
     Tuning component  64  is an arc segment that has a length L TUN  and a width W TUN . The arc segment that forms tuning component  64  may be a portion of a loop of the same radius as the loop antenna forming radiating component  62 . In one example, the arc segment may be approximately one-eighth of the portion of a loop of the same radius. In this example, the length L TUN  of the tuning component  64  is approximately 17.25 mm (about 0.68 inches). IC chip  26  is electrically coupled to tuning component  62 . 
     Radiating component  62  and tuning component  64  are arranged such that there is substantial overlap between a portion of radiating component  62  and tuning component  64 . In the example top view illustrated in  FIG. 6B , there is a substantial overlap between radiating component  62  and tuning component  64  along a portion of the circumference of radiating component  62 . The substantial overlap between tuning component  64  and radiating component  62  provides capacitive coupling between tuning component  64  and radiating component  62  for transferring RF energy, e.g., RF signals, between radiating component  62  and an IC chip  26  that is electrically coupled to the tuning component  64 . Although the predominant coupling between radiating component  62  and tuning component  64  of RFID antenna  60  is capacitive, the coupling may include at least some inductive coupling as well. Tuning component  64  may also provide improved impedance matching between an impedance of radiating component  62  and IC chip  26 . Tuning component  64  may provide a resistance and reactance to match the impedance of the antenna to the impedance of IC chip  26 . In particular, the length L TUN  and the width W TUN  of tuning component  64  may be designed to provide impedance matching. For example, as the length L TUN  and/or the width W TUN  of tuning component  64  is increased, the reactance becomes more positive. 
     Additionally, the distance between the overlapping portions of radiating component  62  and tuning component  64 , e.g., the thickness of substrate  29 , may further be used to control the tuning capacitance. 
     Although radiating component  62  and tuning component  64  overlap in the example RFID antenna  60  of  FIG. 6 , the techniques described herein are not so limited. In some instances, radiating component  62  and tuning component  64  may be offset such that the components are not substantially overlapping. In this case, radiating component  62  and tuning component  64  are still located proximal to one another to provide proximal coupling (e.g., via inductive or capacitive coupling) for transferring RF energy and providing tuning capabilities, but do not substantially overlap. In other words, there may be no overlap or only partial overlap between radiating component  62  and tuning component  64 . 
     In accordance with one aspect of this disclosure, tuning component  64  is significantly smaller than radiating component  62  so that any field radiated by tuning component  64  does not play a major role in the transmission and/or reception of radiation by RFID tag  60 . Thus, the dominant source of radiation is still the loop antenna. For example, RFID tag  60  may be designed such that the field radiated by tuning element  64 , if any, is less than 5 percent of the entire field radiated by RFID tag  60 . By designing the tuning component  64  of RFID tag  60  to be less than one-quarter of the length of radiating component  62 , and more preferably less than one-eighth of the length of radiating component  62 , tuning component  64  may be designed to radiate a field within the limits provided above. 
     The example RFID tag  60  illustrated in  FIGS. 6A-6C  is representative of one RFID tag configuration in accordance with this disclosure. The illustrated embodiment should not be limiting of the techniques as broadly described in this disclosure. For example, although tuning component  64  is positioned to overlap a center portion of radiating component  62 , tuning component  64  may be offset from the center portion of radiating component  62 . Moreover, tuning component  64  and radiating component  62  may be formed in different shapes, some of which are illustrated in  FIGS. 2-5 . Additionally, tuning component  64  may be constructed from multiple elements in addition to or instead of conductive traces. For example, tuning component may be made up of conductive traces and tuning capacitors.  FIGS. 7A and 7B  are graphs showing the impedance of RFID tag  30  of  FIG. 3 , RFID tag  40  of  FIG. 4 , RFID tag  50  of  FIG. 5  and a reference RFID tag over the 900 to 930 MHz range. The reference RFID tag was constructed on a single side of the substrate and included a straight dipole segment and a loop segment, similar to radiating components  42  and  52  of  FIGS. 4 and 5 , respectively. 
     Resistance curve  70 A corresponds with RFID tag  30 , resistance curve  71 A corresponds with RFID tag  40 , resistance curve  72 A corresponds with RFID tag  50  and resistance curve  73 A corresponds with the reference RFID tag. The RFID tags tested had a length L RAD  of 165 mm, a trace width of 1 mm, a length L TUN  of 26 mm, and a spacing between an inside edge of the conductive trace forming the sides of the loop of 2 mm. Reactance curve  70 B corresponds with RFID tag  30 , reactance curve  71 B corresponds with RFID tag  40 , reactance curve  72 B corresponds with RFID tag  50  and reactance curve  73 B corresponds with the reference RFID tag. As illustrated in the graphs of  FIG. 7A , the real part of the impedance, i.e., the resistance, of RFID tags  30 ,  40 ,  50  showed little change from the real part of the impedance of the reference RFID tag over the UHF RFID band of interest (900-930 MHz). As illustrated in the graphs of  FIG. 7B , the imaginary part of the impedance, i.e., the reactance, of RFID tags  30  and  50  showed little change from the imaginary part of the impedance of the reference RFID tag over the UHF RFID band of interest (900-930 MHz). However, the imaginary part of the impedance of RFID tag  40  showed an increase in capacitance that causes the imaginary component of the impedance to be reduced over the UHF RFID band. The impedance of RFID tag  40  may further be adjusted by adjusting the overlapped region and/or the length of the tuning loop of radiating component  42 . As such, tuning component  44  may be useful in matching the impedance of the antenna with the impedance of IC chip  26 . 
     Table 1 illustrates empirical results of the various RFID tag designs. Table 1 represents changes in impedance as the length of the tuning component (i.e., L TUN ) was adjusted. Again, the reference tag design was a single layer modified dipole antenna that included a straight dipole segment and a loop segment, similar to radiating components  42  and  52  of  FIGS. 4 and 5 , respectively. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Length of tuning 
                 Impedance 
               
               
                   
                 Tag design 
                 component (mm) 
                 (Ohms) 
               
               
                   
                   
               
             
            
               
                   
                 Reference 
                 32 
                 52 + j158 
               
               
                   
                 RFID tag 20 
                 28 
                 4.3 − j60   
               
               
                   
                   
                 57 
                 164 + j97  
               
               
                   
                 RFID tag 30 
                 26 
                 34 + j132 
               
               
                   
                   
                 32 
                 47 + j158 
               
               
                   
                   
                 38 
                 75 + j191 
               
               
                   
                 RFID tag 40 
                 26 
                 36 + j8  
               
               
                   
                   
                 32 
                 52 + j48  
               
               
                   
                   
                 38 
                 82 + j70  
               
               
                   
                 RFID tag 50 
                 26 
                 29 + j135 
               
               
                   
                   
                 32 
                 39 + j170 
               
               
                   
                   
                 38 
                 34 + j228 
               
               
                   
                   
               
            
           
         
       
     
     As illustrated in the table, the impedance of the tuning component with a loop segment length of 32 mm is 52+j158. For RFID tag  20  of  FIG. 2 , the capacitive coupling between radiating component  22  and tuning component  24  increases as the length of the overlapping region increases, e.g., as the length L TUN  of tuning component  24  increases. In particular, when the straight segment that forms tuning element  24  increases from 28 mm to 57 mm, the impedance changes from 4.3−j60 to 164+j97. In this manner, the tuning element may provide additional elements for tuning without increasing a footprint of RFID tag  20 . For RFID tag  30  of  FIG. 3 , the inductive coupling between radiating component  32  and tuning component  34  increases as the length of the overlapping region increases, e.g., as the length L TUN  of tuning component  34  increases. Likewise, for RFID tag  50  of  FIG. 5 , the inductive coupling between radiating component  52  and tuning component  54  increases as the length of the overlapping region increases. As such, tuning component  35 ,  54  of RFID tag  30 ,  50  may provide additional elements for tuning the imaginary component to a higher value. 
     For RFID tag  40  of  FIG. 4 , the capacitive coupling between radiating component  42  and tuning component  44  increases as the length of the overlapping region increases, e.g., as the length L TUN  of tuning component  44  increases. Increasing the region of overlap will cause the overlap area to act as one unit piece of metal and thus the increase should asymptotically approach the impedance of the reference case. This can be seen by the increase in the imaginary component as the over lap increases. In this manner, tuning component  44  of RFID tag  40  may provide an additional element for tuning the imaginary component to a higher value. 
       FIG. 8  is a graph illustrating gains of various RFID tag designs to illustrate radiation characteristics of the various RFID tag designs.  FIG. 8  shows radiation characteristics (e.g., gains) of four RFID tag designs; RFID tag  30  of  FIG. 3 , RFID tag  40  of  FIG. 4 , RFID tag  50  of  FIG. 5  and a reference RFID tag. The reference RFID tag was a single layer modified dipole antenna that included a straight dipole segment and a loop segment, similar to radiating components  42  and  52  of  FIGS. 4 and 5 , respectively. The two peaks illustrated in  FIG. 8  are characteristic of a dipole type antenna. 
     As illustrated in  FIG. 8 , the radiation characteristics for each of the RFID tag designs are substantially the same, as the lines of the separate RFID tag designs are nearly indistinguishable. In other words, although there are four separate lines each representing one of the RFID tag designs, the radiation characteristics of each RFID tag design are so similar that the four lines appear as substantially one line. Thus, the radiation characteristics of the RFID tags  30 ,  40  and  50  that have a radiating component on one layer and a tuning component on a second layer continue to have radiation characteristics are substantially the same as the single-sided modified dipole reference antenna. As such, the tag designs of RFID tag  30 ,  40  and  50  are advantageous because not only do they have the same radiation characteristics of the reference antenna, but include tuning components that may provide additional inductance and/or capacitance for tuning purposes to further improve performance.  FIG. 9  is a graph illustrating example fields radiated by RFID tag  30  of  FIG. 3  and a straight dipole antenna. In particular, the graph of  FIG. 9  shows two example fields; a first field radiated by RFID tag  30  of  FIG. 3  and a second field radiated by a straight dipole antenna. As described in detail in  FIG. 3 , RFID tag  30  includes a radiating component  32  that is a straight dipole element and a tuning component  34  that is a tuning loop. As shown in the graph of  FIG. 9 , the resulting fields radiated by RFID tag  30  of  FIG. 3  is substantially the same magnitude as the field radiated by the reference straight dipole antenna, indicating that the field radiated by tuning component  34  does not play a major role in transmission and/or reception of radiation by RFID tag  30 . In fact, the magnitude of the field radiated by RFID tag  30  and the straight dipole antenna are so similar that they appear as a single line. In other words, although there are two separate lines illustrated in  FIG. 9 , the lines are so similar that they appear as a single line. 
     The graph of  FIG. 9  was obtained by performed modeling in which an excitation voltage was placed on tuning component  34  until a current with a magnitude of one amp was flowing on tuning component  34 . The current flowing on radiating component  32  was determined. Next, the electric field produced by the entire structure of RFID tag  30  with one amp current flowing on tuning component  34  was determined at a fixed far-field distance. Then, tuning component  34  was removed and a source was placed at the center of the straight dipole antenna of the reference RFID tag. A magnitude of the voltage source was adjusted to produce the same current as was induced by tuning component  34 . The resulting electric field produced by the straight dipole antenna was determined at a fixed far-field distance. Again, the results illustrated in  FIG. 9  indicated that tuning component  34  does not play a major role in transmission and/or reception of radiation by RFID tag  30 . Therefore, tuning component  34  simply provides a mechanism to connect IC chip  26  to the radiating component  32  without affecting radiating properties of RFID tag  30   
       FIG. 10  is a graph illustrating example fields radiated by RFID tag  50  of  FIG. 5  and a reference modified dipole antenna. As described in detail in  FIG. 5 , RFID tag  50  includes a radiating component  52  that includes a straight dipole segment  56  and a loop segment  57 , and a tuning component  54  that is a tuning loop. The reference antenna was a modified dipole antenna similar that is substantially the same as radiating component  52 , but with no tuning component  54 . As shown in the graph of  FIG. 10 , the resulting fields radiated by RFID tag  50  of  FIG. 50  is substantially the same magnitude as the field radiated by the reference modified dipole antenna, thus indicating that the field radiated by tuning component  54  does not play a major role in transmission and/or reception of radiation by RFID tag  50 . The largest difference, which is only 2-3 V/m, occurs at the peaks at L TUN  lengths of 30 and 50 mm. 
     The graph of  FIG. 10  was obtained by modeling performed as described above with respect to  FIG. 9 . Again, the results illustrated in  FIG. 10  indicated that tuning component  54  does not play a major role in transmission and/or reception of radiation by RFID tag  50 . 
     Therefore, tuning component  54  simply provides a mechanism to connect IC chip  26  to the radiating component  52  without affecting radiating properties of RFID tag  50 . 
       FIGS. 11A and 11B  are graphs demonstrating the impedance of RFID tag  60  of  FIG. 6  and a reference RFID tag. As described in detail above, RFID tag  60  had a radiating component  62  that is a circle-shaped conductive loop and tuning component  64  is an arc segment of a portion of a circle-shaped loop of the same radius. The reference RFID tag had the same geometry as the radiating component  62  of RFID tag  60 , i.e., circle-shaped loop, but with no tuning component  64  on a second layer. The amount of overlap corresponds with a length of the arc segment forming tuning component  64 . RFID tag  60  and the reference RFID tag were modeled using CST Microwave Studios. These loop antennas have a radius of 22 mm. Resistance curve  110 A corresponds with arc segment forming tuning component  64  having a length of 26 mm, resistance curve  111 A corresponds with arc segment forming tuning component  64  having a length of 32 mm, resistance curve  112 A corresponds with arc segment forming tuning component  64  having a length of 38 mm and resistance curve  113 A corresponds with the reference RFID tag with no tuning component  64 . As illustrated in the graphs, the impedance can be tuned using the capacitively coupled overlap between tuning component  64  and radiating component  62 . As the length of the overlap increased, the impedance comes closer to approximating the reference design. 
     Various embodiments have been described. The embodiments described are described for purposes of limitation and, therefore, should not be limiting. Other designs may be encompassed within the scope of this disclosure. For example, the radiating component may be a multi-layer radiating component. In other words, portions of the radiating component may be formed on different layers of the RFID tag and be coupled using vias or using capacitive/inductive coupling. In this case, the tuning element may be located on a different layer of the RFID tag than the portions of the radiating component, and be arranged to overlap with at least a portion of the radiating component of the other layers. These and other embodiments are within the scope of the following claims.