Abstract:
A nanocrystalline core antenna for use in electronic article surveillance (EAS) and radio frequency identification (RFID) systems. The nanocrystalline antenna is constructed from nanocrystalline material and exhibits improved detection range in EAS and RFID systems compared to conventional antenna configurations.

Description:
FIELD OF THE INVENTION 
     The present invention relates to core antennas, and, in particular, to core antennas for electronic article surveillance (EAS) and radio frequency identification (RFID) systems. 
     BACKGROUND 
     EAS and RFID systems are typically utilized to protect and/or track assets. In an EAS system, an interrogation zone may be established at the perimeter, e.g. at an exit area, of a protected area such as a retail store. The interrogation zone is established by an antenna or antennas positioned adjacent to the interrogation zone. 
     EAS markers are attached to each asset to be protected. When an article is properly purchased or otherwise authorized for removal from the protected area, the EAS marker is either removed or deactivated. If the marker is not removed or deactivated and moved into the interrogation zone, the electromagnetic field established by the antenna(s) causes a response from the EAS marker. An antenna acting as a receiver detects the EAS marker&#39;s response indicating an active marker is in the interrogation zone. An associated controller provides an indication of this condition, e.g., an audio alarm, such that appropriate action can be taken to prevent unauthorized removal of the item to which the marker is affixed from the protected area. 
     An RFID system utilizes an RFID marker to track articles for various purposes such as inventory. The RFID marker stores data associated with the article. An RFID reader may scan for RFID markers by transmitting an interrogation signal at a known frequency. RFID markers may respond to the interrogation signal with a response signal containing, for example, data associated with the article or an RFID marker ID. The RFID reader detects the response signal and decodes the data or the RFID tag ID. The RFID reader may be a handheld reader, or a fixed reader by which items carrying an RFID marker pass. A fixed reader may be configured as an antenna located in a pedestal similar to an EAS system. 
     Historically, transmitting, receiving, or transceiver antennas in EAS and RFID systems have been configured as loop-type antennas. Recently, however, magnetic core antenna configurations have been explored for use in such systems. Materials utilized as the core material in core antennas have included ferrite and amorphous magnetic material. 
     Ferrite material may be provided as a powder, which is blended and compressed into a particular shape and then sintered in a very high temperature oven. The compound becomes a fully crystalline structure after sintering. Ferrite materials have a higher magnetic permeability than air, and have a relatively low saturation flux density compared, for example, to most amorphous materials. Also, ferrite materials that operate at higher RF (e.g. 15 MHz) frequencies have relatively low permeability and/or saturation flux density. 
     In contrast to ferrite materials, amorphous magnetic materials lack a distinct crystalline structure. Amorphous magnetic materials e.g., VC6025F available from Vacuumschmelze GmBH Co. (D-6450 Hanua, Germany), have been successfully utilized in lower frequency EAS applications, e.g., 58 kHz. However, such amorphous magnetic materials do not perform well in the RF frequency range as core loss and permeability decrease performance for frequencies higher than a few 100 kHz. 
     Accordingly, there is a need for a core antenna for EAS and RFID applications capable of suitable operation frequencies up to the RF range. In addition, there is a need for improved performance of a core antenna in the lower frequency range for EAS as an alternative to ferrite or amorphous materials. 
     SUMMARY OF THE INVENTION 
     An antenna consistent with the invention for use in an EAS or RFID includes: a core including a nanocrystalline magnetic material, and a coil winding disposed around at least a portion of the core. The antenna may be implemented in an EAS or RFID system for generating an electromagnetic field to interrogate a marker by providing a controller configured to provide an excitation signal to excite the antenna for operation at a given frequency. 
     A method of establishing extended detection range in an EAS or RFID system consistent with the invention includes: providing a nanocrystalline core antenna including a core and at least one coil winding disposed around at least a portion of the core, the core including nanocrystalline magnetic material; and exciting the antenna for operation up to and including RF frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, together with other objects, features and advantages, reference should be made to the following detailed description which should be read in conjunction with the following figures wherein like numerals represent like parts: 
         FIG. 1  is block diagram of an exemplary EAS system consistent with the invention; 
         FIG. 2  is a block diagram of one embodiment of a nanocrystalline magnetic core antenna consistent with the invention; 
         FIG. 3  is one exemplary circuit diagram of a controller for use with the system of  FIG. 1 ; 
         FIG. 4  is a perspective view of an exemplary nanocrystalline core antenna consistent with the invention; 
         FIG. 5  is a partial cross-sectional view of the nanocrystalline core of  FIG. 4  showing the insulated laminations and taken along the line  5 — 5  of  FIG. 4 . 
         FIG. 6A  is a perspective view of another exemplary nanocrystalline core antenna consistent with the invention illustrating a resonant primary coil winding and a non-resonant secondary coil winding for transmitter, receiver, or transceiver mode operation. 
         FIG. 6B  is an perspective view of a portion of the antenna of  FIG. 6A  showing the primary and secondary windings in greater detail 
         FIG. 7  is a plot of magnetic flux density versus magnetic field intensity for an exemplary nanocrystalline core antenna consistent with the invention. 
         FIG. 8  is a plot of relative permeability versus magnetic field intensity for an exemplary nanocrystalline core antenna consistent with the invention. 
         FIGS. 9A–9C  are detection performance plots illustrating detection range for an EAS tag in lateral, horizontal, and vertical orientations, respectively, in an exemplary system consistent with the invention. 
     
    
    
     DETAILED DESCRIPTION 
     For simplicity and ease of explanation, the present invention will be described herein in connection with various exemplary embodiments thereof associated with EAS systems. A core antenna consistent with the present invention may, however, be used in connection with an RFID system. It is to be understood, therefore, that the embodiments described herein are presented by way of illustration, not of limitation. 
     Turning to  FIG. 1 , there is illustrated an EAS system  100  including a nanocrystalline core antenna  109  consistent with the invention. The EAS system  100  generally includes a controller  110  and a pedestal  106  for housing the core antenna  109 . The controller  110  is shown separate from the pedestal  106  for clarity but may be included in the pedestal housing. In the exemplary embodiment of  FIG. 1 , the antenna  109  is configured as a transceiver and the associated controller  110  includes proper control and switching to switch from transmitting to receiving functions at predetermined time intervals. Those skilled in the art will recognize that there may be a separate transmitting antenna and receiving antenna located on separate sides of the interrogation zone  104 . 
     An EAS marker  102  is placed, e.g. at a manufacturing facility or retail establishment, on each item or asset to be protected. If the marker is not removed or deactivated prior to entering an interrogation zone  104 , the electromagnetic field established by the antenna will cause a response from the EAS marker  102 . The core antenna  109  acting as a receiver will receive this response, and the controller  110  will detect the EAS marker response indicating that the marker is in the interrogation zone  104 . 
     Turning to  FIG. 2 , a block diagram  200  of one embodiment of a nanocrystalline magnetic core antenna consistent with the invention configured as a transceiver antenna is illustrated. In the illustrated embodiment  200 , a winding is placed around the nanocrystalline magnetic core and coupled to a series resonating capacitor C 2 . The core antenna with this winding is represented by the inductor L 2 , the resistor R 2 , and the series resonating capacitor C 2  in the series RLC circuit  218 . As is known to those skilled in the art, the value of the series resonating capacitor C 2  is selected to resonate or tune the antenna circuit at the desired operating frequency. Another winding, represented by L 1 , may be placed around the core antenna and then coupled to the transmission line or cable (depending on the operating frequency)  212 , which is in turn coupled to a controller  210  having appropriate excitation and detection circuitry to support both transmit and receive functions. The winding L 1  is inductively coupled to the series resonating RLC circuit  218 . 
     The controller  210  may be adapted to operate using pulsed or continuous waveform detection schemes, including swept frequency, frequency hopping, frequency shift keying, amplitude modulation, frequency modulation, and the like depending on the specific design of the system. For instance, the controller  210  may provide a limited duration pulse at a given operating frequency, e.g., 8.2 MHz, to the transmission line cable  212  during transmission. The pulse is transmitted via the transmission line cable  212  to the core antenna load. The transmission line cable may have an impedance, e.g., 50 ohms, that matches the signal generator impedance to prevent reflections. At lower frequencies, e.g. 58 kHz, the transmission line or cable is not important in impedance matching. In addition, the impedance transformer L 1  may match the resonant core load impedance of the series RLC circuit  218  to the transmission cable  212 . 
       FIG. 3  is a more detailed block diagram of an exemplary controller  310  configured for operation using a pulse detection scheme. The controller  310  may include a transmitter drive circuit  318 , which includes signal generator  311  and transmitter amplifier  312 . The signal generator  311  supplies an input signal to the transmitter  312  at a desired frequency such as RF frequency levels. The term “RF” as used herein refers to a range of frequencies between 9 KHz and 300 MHz. 
     The transmitter  312  drives the nanocrystalline magnetic core antenna represented by inductor LA, resistor RC, and resonating capacitor CR. The transmitter drive circuit  318  thus provides a burst to the core antenna at a given frequency for a short period of time to produce a sufficient electromagnetic field at a sufficient distance from the core antenna in an associated interrogation zone. A marker in the interrogation zone excited by this electromagnetic field produces a sufficient response signal for detection when the core antenna is connected to the receiver circuit portion of the controller  310 . 
     After a short delay following the transmission burst, the nanocrystalline magnetic core antenna is coupled to the receiver circuit  322  when the switch controller  324  instructs the switch S 1  to open. The switch controller  324  effectively switches the core antenna into and out of transmit and receive modes. During the transmitter pulse, the receiver circuit  322  is isolated from the antenna load at node  330  through the decoupling network formed by capacitor CDEC and resistor RDEC and the input protection circuit  334 . After the transmission pulse, there is sufficient delay to allow the energy from the transmitter circuit  318  to fully dissipate. The switch controller  324  then disconnects the transmitter amplifier  312  from the antenna by opening switch S 1 . The alternating transmit and receive modes continue in such a pulse mode. 
     A perspective view of a nanocrystalline magnetic core antenna  400  consistent with the invention is illustrated in  FIG. 4 . The core antenna  400  may be utilized as the transceiver antenna of the system of  FIGS. 1 and 2 , a transmitter antenna, or a receiver antenna. The nanocrystalline magnetic core antenna  400  includes a core assembly  404  with a coil winding  406  thereon. The coil winding  406  may be coupled to a transmission line and controller as previously detailed. Those skilled in the art will recognize that the dimension of a core antenna consistent with the invention may vary depending on application and performance requirements. In exemplary embodiments, the core may have a length in a range from 20 to 80 cm, and may have a cross-sectional area from 0.02 to 1 cm 2 . 
       FIG. 5  is a partial cross sectional view of the core assembly  404  taken along the line  5 — 5  of  FIG. 4 . In the illustrated exemplary embodiment, the core assembly  404  generally includes stacked ribbons  508  of nanocrystalline material laminated together with a suitable insulation coating  510 . The insulation coating  510  electrically isolates each ribbon  508  from adjacent ribbons to reduce eddy current losses. 
     As will be recognized by those skilled in the art, nanocrystalline material begins in an amorphous state achieved through rapid solidification techniques. After casting, while the material is still very ductile, a suitable coating such as SiO 2  may be applied to the material. This coating remains effective after annealing and prevents eddy currents in the laminate core. The material may be cut to a desired shape and bulk annealed to form the nanocrystalline state. The resulting nanocrystalline material exhibits excellent high frequency behavior, and is characterized by constituent grain sizes in the nanometer range. The term “nanocrystalline material” as used herein refers to material including grains having a maximum dimension less than or equal to 40 nm. Some materials have a maximum dimension in a range from about 10 nm to 40 nm. 
     Exemplary nanocrystalline materials useful in a nanocrystalline core antenna consistent with the invention include alloys such as FeCuNbSiB, FeZrNbCu, and FeCoZrBCu. These alloys are commercially available under the names FINEMET, NANOPERM, and HITPERM, respectively. The insulation material  510  may be any suitable material that can withstand the annealing conditions, since it is preferable to coat the material before annealing. Epoxy may be used for bonding the lamination stack after the material is annealed. This also provides mechanical rigidity to the core assembly, thus preventing mechanical deformation or fracture. Alternatively, the nanocrystalline stack may be placed in a rigid plastic housing. 
       FIGS. 6A and 6B  are perspective views of another exemplary nanocrystalline magnetic core antenna  600  consistent with the invention. As shown, the core antenna  600  includes a nanocrystalline core assembly  602  with a primary resonant coil winding  604  and a secondary non-resonant coil winding  606 . A capacitor  608 , shown particularly in  FIG. 6B , is coupled to the primary winding  604  for tuning the resonant frequency of the primary winding. 
     Providing multiple windings  604 ,  606  on a single core  602  allows use of the core to transmit at one frequency and receive at another frequency as long as sufficient frequency separation is provided. Using two windings operating at separate frequencies, such as 58 kHz and 13.56 MHz, also allows use of a single antenna as a transmitter and/or receiver at either frequency so that the antenna assembly can be plugged into a system operating at either frequency without special tuning. Additionally, multiple windings may be used such that the transmitter winding is tuned to 13.56 MHz and the receiver winding is tuned to 6.78 MHz (half-frequency) to facilitate operation using a frequency division scheme. 
     Turning to  FIG. 7 , there is provided a BH plot  700  for an exemplary nanocrystalline magnetic core antenna consistent with the invention constructed as shown in  FIG. 4  using a FINEMET core. The exemplary nanocrystalline magnetic core antenna was 60 cm long by 0.5 cm wide, by 0.5 cm high and operated at 1 KHz. In general, the plot includes a linear region at fields below saturation (H fields between about +/−170 A/m) and a flat region at saturation (H fields above and below about +/−250 A/m). The slope of the linear region determines the permeability. In general, a higher permeability results in a more sensitive antenna when configured to act as a receiver antenna. 
       FIG. 8  is a plot  800  of relative permeability versus H-field in Aim at a frequency of 1 kHz for the same exemplary 60 cm×0.5 cm×0.5 cm nanocrystalline magnetic core antenna exhibiting the BH plot of  FIG. 7 . As indicated, the relative permeability is about 5000 or higher at H fields between 0 and about 100 A/m. The permeability decreases relatively linearly until saturation at about 250 A/m where it begins to drop off even further. Of course, as the antenna operating frequency increases, permeability decreases. Nonetheless, high permeability is maintained compared to conventional core antenna configurations. For example, the same exemplary 60 cm×0.5 cm×0.5 cm nanocrystalline magnetic core antenna exhibiting the BH plot of  FIG. 7  and permeability characteristic of  FIG. 8  and operated at frequencies from 8.2 to 13.56 MHz exhibits a minimum relative permeability of 300. Due to the relatively high permeability and saturation level of nanocrystalline material, as indicated, for example, in  FIGS. 7 and 8 , a nanocrystalline core antenna used as a receiver antenna exhibits increased detection performance compared to conventional core antenna configurations. 
       FIGS. 9A–9C  are detection performance plots  900 ,  902 ,  904  illustrating detection range for an EAS tag in lateral, horizontal, and vertical orientations, respectively, associated with an axially arranged pair of nanocrystalline magnetic core antennas consistent with the invention. The two nanocrystalline magnetic core antennas were 60 cm long×0.5 cm wide×0.5 cm thick and provided in a 58 kHz detection configuration. The dimensions of the plots in each of  FIGS. 9A–9C  correspond to the height and width dimensions of the tested area. The shaded area of each figure shows detection of an EAS tag. Non-shaded areas are areas in which an EAS tag is not detected. As shown, the exemplary antenna configuration exhibits a detection range between about 0 cm and 90 cm over a large height range from about 0 cm to 150 cm. In addition, the detection rate, also referred to as the pick rate, for the lateral orientation was 93.1%. The pick rate for the horizontal orientation was 79.3%, and the pick rate for the vertical orientation was 95.6%. The exhibited detection range and pick rates compare favorably with those of amorphous core antennas. 
     There is thus provided a nanocrystalline core antenna for use in EAS and RFID systems. The nanocrystalline antenna is constructed from nanocrystalline material and exhibits excellent performance characteristics at RF frequencies. The performance of the antenna results in improved detection range in EAS and RFID systems compared to conventional antenna configurations. 
     The embodiments that have been described herein, however, are but some of the several which utilize this invention and are set forth here by way of illustration but not of limitation. It is obvious that many other embodiments, which will be readily apparent to those skilled in the art, may be made without departing materially from the spirit and scope of the invention as defined in the appended claims.