Abstract:
A UHF-RFID antenna having a central segmented loop surrounded by passive dipole structures provides shaping of the electric and magnetic fields to reduce the number of false positive reads by a UHF-RFID reader at a point of sale.

Description:
BACKGROUND 
     Current RFID (Radio Frequency Identification) systems are able to replace barcode systems in many applications. RFID tagging of clothes and other items such as groceries is seeing increased interest in the respective industries. RFID tagging of goods allows the goods to be tracked throughout the supply chain. At the end of the supply chain is the point of sales (POS) application. Typically, a barcode based product scanner is used at the POS to identify the sold products. Based on the information from the POS terminal, all data throughout the supply chain is updated (e.g. inventory) as well as the generation of a customer&#39;s bill and deactivation of any security system after customer payment is received. 
     Barcode POS systems typically have a very low detection range which means that a barcode tag is only readable when positioned such that the barcode tag faces the light beam of the scanner. This typically requires the tagged object to be repositioned until the proper alignment is achieved with the scanner or the scanner needs to be repositioned with respect to the barcode (e.g. handheld scanner) until the proper alignment is achieved as shown in  FIGS. 1 a - c   .  FIGS. 1 a - b    show product  115  with barcode  120  in orientations which do not permit scanner  110  to scan barcode  120 .  FIG. 1 c    shows product  115  with barcode  120  oriented such that scanner  110  can scan barcode  120 . 
     Using an RFID system for tagging enables a more efficient way to scan products passing a POS because an RFID tag attached to a product need not be aligned with the antenna.  FIGS. 2 a - c    show some of the alignments permissible in an RFID system with product  215 , RFID reader antenna  210  and RFID tag  220 . RFID tag  220  may be read using randomly chosen alignments between reader antenna  210  and product  215 . Typically RFID systems provide a detection range which results in a larger volume than a barcode system. 
     Prior art UHF-RFID systems typically have a problem with false positive reads, such as shown in  FIG. 3 . The electromagnetic radiation pattern of RFID antenna  310  of the reader (not shown) leads to the detection of products  315  with RFID tags  320 ,  321 ,  322  and  323  arranged near RFID antenna  310  at POS  300  when only RFID tag  320  on RFID antenna  310  is to be detected. Hence, products  315  from different customers at POS  300  could be read at the same time. 
     SUMMARY 
     In accordance with the invention, a UHF-RFID reader antenna is disclosed with a defined radiation pattern that provides a controlled read range to suppress false positive readings of RFID tags. Special passive antenna dipole structures are used to control the RF propagation area resulting in a defined read zone with a reduction of false positive reads. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 a - b    show a product with a barcode in orientations which do not permit the scanner to scan the barcode. 
         FIG. 1 c    shows product with a barcode in an orientation which permits the scanner to scan the barcode. 
         FIGS. 2 a - c    show some of the product orientations permissible in an RFID system. 
         FIG. 3  shows the issue of false positive reads in a UHF-RFID system. 
         FIG. 4 a    shows an embodiment in accordance with the invention. 
         FIG. 4 b    shows an embodiment in accordance with the invention. 
         FIG. 5  shows an embodiment in accordance with the invention. 
         FIG. 6 a    shows an embodiment in accordance with the invention. 
         FIG. 6 b    shows an embodiment in accordance with the invention. 
         FIG. 6 c    shows an embodiment not in accordance with the invention. 
         FIG. 6 d    shows an embodiment in accordance with the invention. 
         FIG. 6 e    compares the electric field of an embodiment in accordance with the invention with an embodiment not in accordance with the invention. 
         FIG. 7  shows the coordinate system used for  FIGS. 8 a   - b.    
         FIG. 8 a    shows the gain as a function of angle in the XY plane for an embodiment in accordance with the invention. 
         FIG. 8 b    shows the gain as a function of angle in the XZ plane for an embodiment in accordance with the invention. 
         FIG. 9  shows an embodiment in accordance with the invention. 
         FIG. 10  shows an embodiment in accordance with the invention. 
         FIG. 11 a    compares the electric field of an embodiment in accordance with the invention with an embodiment not in accordance with the invention. 
         FIG. 11 b    compares the electric field of an embodiment in accordance with the invention with an embodiment not in accordance with the invention. 
         FIG. 11 c    compares the electric field of an embodiment in accordance with the invention with an embodiment not in accordance with the invention. 
         FIG. 11 d    compares the electric field of an embodiment in accordance with the invention with an embodiment not in accordance with the invention. 
         FIG. 12  shows an alternative embodiment for the segmented loop in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4 a    shows RFID antenna  400  in an embodiment in accordance with the invention. Segmented loop  410  is surrounded by passive dipole structures  420   a  and  420   b  which confine the RF field emitted by segmented loop  410 . Loop segmentation allows an electrically large antenna to behave like an electrically small antenna. The segmented sections provide for very small phase delays between adjacent sections and the currents along segments  515  (see  FIG. 5 ) remain constant in magnitude which results in a strong and uniform magnetic field. Selecting a segment length to be on the order of ⅛ wavelength allows for a compromise between structure complexity and current uniformity in the loop segments. 
     RFID antenna  400  can be made in accordance with the invention by placing conductive material  430  (e.g. copper) on dielectric substrate  440  as shown in  FIG. 4 b   . The thickness of conductive material  430  typically needs to be selected to fit the application. Typically 1.5 mm thickness FR4 material (fiberglass reinforced epoxy laminate) is selected for dielectric substrate  440  and is typically paired with 0.035 mm thickness copper for conductive material  430 . Suitable FR4 material typically has a dielectric constant ∈ r  of approximately 4.3. Dielectric substrate  440  influences the resonance length of RFID antenna  400 . The physical size of an antenna placed on dielectric substrate  440  is scaled down by a scaling factor for the same resonance frequency compared to an antenna having the same resonance frequency surrounded by air as long as dielectric substrate  440  has a higher dielectric constant than air. The scaling factor is proportional to 1/√∈ r . 
     RFID antenna  400  comprises conductor traces, lumped elements (resistors, capacitors, connector(s), balun(s)) and dielectric substrate  440 . RFID antenna  400  has a structure similar to the structure of one layer PCB boards and this typically allows for easy production. 
     RFID antenna  400  can be viewed as comprising two main parts. Segmented loop  410  which operates as the radiating antenna and passive dipole structures  420   a  and  420   b  which shape the radiated field by reflecting and absorbing the radiated energy outside the defined read zone.  FIG. 5  shows segmented loop  410  where segments  515  of segmented loop  410  are separated from each other by gaps  520  and coupled to each other using capacitors  525 . Segmented loop  410  is designed such that the diameter and resonance frequency is appropriate for the desired application. 
     Segmented loop  410  can be scaled arbitrarily where the diameter of segmented loop  410  and the values of capacitors  525  affect the resonance frequency of segmented loop  410 . Segments  515  of segmented loop  410  are typically on the order of one-eighth of the resonant wavelength in length as noted above. If the circumference of segmented loop  410  would require longer segments  515 , additional segmentation is typically introduced to keep segment length constant. 
       FIG. 6 a    shows passive dipole structures  420   a  and  420   b  in an embodiment in accordance with the invention which suppresses the electromagnetic field outside of the desired read zone. The desired read zone is defined mainly by the radiated power of segmented loop  410  (see  FIG. 5 ) and the performance of the passive RFID tag (not shown) which is scanned using antenna  400 . Typically, the read zone is defined for a particular application and then with a knowledge of all the components of the RFID system, a reader antenna such as antenna  400  can be designed having the desired read zone. 
     Passive dipole structures  420   a  and  420   b  are comprised of a total of 4 linear segments  620  and 4 curved segments  610 , respectively. Each pair of linear segments  620  and curved segments  610  is coupled to each other using resistors  650  as shown in  FIG. 6 a   . The length and width of passive dipole structures  420   a  and  420   b  are selected to match the resonance frequency of segmented loop  410 . 
     Passive dipole structures  420   a  and  420   b  function as reflectors and energy absorbers. The distance from segmented loop  410  to passive dipole structures  420   a  and  420   b  has to be appropriately selected to assure proper performance.  FIG. 6 b    shows distances  675  and  680 . Distance  680  typically needs to be selected such that the end of curved segment  610  aligns in the y-direction with the end of linear segment  620  or curved segment  610  overlaps with straight segment  620  (e.g., see  FIG. 6 a   ). 
     Note that in an embodiment in accordance with the invention, curved segment  610  may overlap on the outside of straight segment  620  as shown in  FIG. 6 d    for antenna  666 . 
       FIG. 6 c    shows antenna  600  where distance  680  is not properly adjusted resulting in the elimination of the field suppressing effect but all other dimensions are the same as for antenna  400 . 
       FIG. 6 e    compares the electric field  400   a  of antenna  400  with the electric field  600   a  of antenna  600  along the direction of respective linear segments  620  showing the elimination of the desired field suppressing effect for antenna  600  in an embodiment in accordance with the invention. Electric field  600   a  is plotted from the point x=−100 mm, y=50 mm, z=10 mm to the point x=100 mm, y=50 mm, z=10 mm where x=0, y=0 and z=0 defines the center of segmented loop  410 . Note that if segmented loop  410  is increased in circumference for antenna  400 , typically resulting in a larger read zone, passive dipole structures  420   a  and  420   b  are scaled accordingly to preserve the field suppressing effect and lowering the resonance frequency of segmented loop  410  and passive dipole structures  420   a  and  420   b  but typically not to the same degree. 
     According to the Yagi-Uda configuration, the distance between segmented loop  410  and passive dipole structures  420   a  and  420   b  (see  FIG. 4 a   ) determines the reflective behavior of passive dipole structures  420   a  and  420   b  (see for example: “Antenna Theory and Design”, 2 nd  edition, Stutzman, W. L.; Thiele, G. A.; Wiley 1998 incorporated by reference in its entirety). Note that typical “rules of thumb” for the Yagi-Uda configuration cannot typically be used because there are five coupled antenna structures, four passive dipole structures  420   a  and  420   b  and segmented loop  410  along with dielectric substrate  440  so that numerical simulations are typically needed to find the appropriate geometry. Because the resonance frequency of passive dipole structures  420   a  and  420   b  matches the resonance frequency of segmented loop  410 , passive dipole structures  420   a  and  420   b  couple efficiently to segmented loop  410  to reflect and also partially absorb energy from the radiative field emitted by segmented loop  410 . To prevent passive dipole structures  420   a  and  420   b  from re-radiating, resistors  650  are placed in the middle of each of the passive dipole structures  420   a  and  420   b  (see  FIG. 6 a   ). Resistors  650  function to dissipate the energy absorbed by passive dipole structures  420   a  and  420   b.    
     Typically, RFID antenna  400  is connected to the RFID reader using a cable having a standard SMA (SubMiniature version A) connector, followed by an unbalanced to balanced converter or balun (not shown) to suppress radiating fields in the cable. The balun used is typically a current balun with very high common mode impedance. 
       FIG. 7  shows the coordinate system  700  used for plots  801  and  802  in  FIGS. 8 a  and 8 b   , respectively. 
     Plot  801  in  FIG. 8 a    compares gain pattern  810  for segmented loop  410  without passive dipole structures  420   a  and  420   b  with gain pattern  820  for segmented loop  410  with passive dipole structures  420   a  and  420   b  in the XY plane (see  FIG. 7 ). Plot  801  goes from PHI=−90 degrees to PHI=+90 degrees. Plot  802  in  FIG. 8 b    compares gain pattern  830  for segmented loop  410  without passive dipole structures  420   a  and  420   b  with gain pattern  840  in the XZ plane (see  FIG. 7 ). Plot  802  goes from THETA=0 degrees to THETA=+180 degrees. Note that matching circuit  931  includes the balun (not shown) and the SMA connector (not shown) at gap  930  which serves as the feed-in point introduces asymmetries which are suppressed to some extent by the balun. However, the effect of the balun and the feed-in point is not modeled in  FIGS. 8 a   - b.    
     From  FIGS. 8 a - b    it is apparent that without passive dipole structures  420   a  and  420   b , the largest gains are obtained in the x-direction and y-direction which is the plane of RFID antenna  310  in  FIG. 3  where reduced sensitivity is desired to reduce false positive reads at POS  300 . Passive dipole structures  420   a  and  420   b  reshape gain patterns  810  and  830  into gain patterns  820  and  840 , respectively to enhance sensitivity in the z-direction as shown in  FIG. 8 b    while reducing sensitivity in the x-direction and the y-direction as seen in  FIGS. 8 a - b   . In accordance with the invention, the combination of segmented loop  410  and passive dipole structures  420   a  and  420   b  creates a well-defined read zone for antenna  400  with a higher gain in the z-direction and a suppressed gain in the x-direction and the y-direction. 
       FIG. 9  shows an embodiment in accordance with the invention. Linear segments  980  and  981  of passive dipole structures  420   a  are electrically coupled to each other across gaps  910  by 50Ω resistors  950  which act as terminators. Curved segments  901  and  902  of passive dipole structures  420   b  are electrically coupled to each other across gaps  911  by 50Ω resistors  950  which act as terminators. Gaps  520  separate some of the segments  515  of segmented loop  410  and gaps  520  are bridged by 1.3 pF capacitors  525  which couple the respective segments  515  together to achieve a resonance frequency of about 915 MHz. Note that capacitors  525  resonate out the inductance of segments  515 , keeping the impedance of segmented loop  410  manageable. By varying the value of capacitors  525 , the resonance frequency can be adjusted to frequency values within the UHF RFID band. Gap  925  is bridged by both 1.3 pF capacitor  525  and 91Ω resistor  951  in parallel to achieve more robust matching between the 50Ω system (not shown) comprising the reader and cable and segmented loop  410 . 91Ω resistor  951  functions to sufficiently decrease the Q of segmented loop  410 . Gap  930  corresponds to the feed-in slot for excitation of segmented loop  410 . Matching circuit  931  includes a balun between the cable from the reader and the feed-in slot (gap  930 ). 
       FIG. 10  shows the dimensions for an embodiment in accordance of the invention. The dimensions are determined for the appropriate resonance frequency using computer simulations of the electromagnetic field. Typical computer simulation packages that are used are HFSS (commercial finite element method solver) and CST (Computer Simulation Technology; time domain solver was used). Diameter  1000  of segmented loop  410  is about 5.0 cm. Separation  1090  between curved segment  610  and segmented loop  410  is about 5.6 cm. Separation  1050  between linear segments  620  is about 9.0 cm. Distance  1060  is the length of dielectric substrate  440  which is about 16.5 cm. Separation  1080  between segmented loop  410  and linear segment  620  is about 2.0 cm. Dimension  1010  of curved segments  610  is about 8.0 cm and dimension  1025  of curved segments is about 3.0 cm. Width  1026  of curved segments  515  is about 0.2 cm, width  1005  of curved segments  610  is about 0.2 cm and width  1015  of linear segments  620  is about 0.1 cm. Each linear segment  620  is about 6.6 cm in length and each curved segment  515  is about 1.9 cm in length. All gaps  520 ,  925 ,  930 ,  910 ,  911  are about 0.05 cm across. The size of the gaps  520 ,  925 ,  930 ,  910 ,  911  can be modified depending on the package and footprint of capacitors  525  and resistors  950  that are used. 
     More generally, separations  1080  and  1090  are the distances from segmented loop  410  to dipole structures  420   a  and  420   b , respectively. Separations  1080  and  1090  together with the resonance length of dipole structures  420   a  and  420   b  determine distances  675  and  680  (see  FIG. 6 b   ). Hence, distances  675  and  680  are determined by diameter  1000  of segmented loop  410 , the resonance length of dipole structures  420   a  and  420   b  and separations  1080  and  1090 , respectively. It is important that curved segment  610  overlaps with straight segment  620 ; the amount of overlap is determined by diameter  1000  of segmented loop  410 , the resonance length of dipole structures  420   a  and  420   b  and separations  1080  and  1090 , respectively. When the geometries of segmented loop  410  and dipole structures  420   a  and  420   b  do not allow for an overlap due to, for example, scaling, the limits of a functioning antenna  400  in accordance with the invention are reached and actions are required to ensure there is an overlap. For example, dielectric substrate  440  may be replaced with a dielectric substrate having a lower dielectric constant to allow for an increase in the length of dipole structures  420   a  and  420   b  to create an overlap. 
     Curved dipole segments  610  are curved at a specific angle and comprise arc segments of a circle whose diameter typically needs to be about 60 percent to 70 percent larger than diameter  1000  of segmented loop  410 . This requirement together with separations  1080  and  1090 , diameter  1000  of segmented loop  410  and the length of dipole structures  420   a  and  420   b  ensures that separation  675  is within the proper range. 
       FIGS. 11 a - d    show the electric field  1120  along the direction of passive dipole structures  420  and the electric field  1130  at for the same locations with passive dipole structures  420  removed for an embodiment in accordance with the invention. 
       FIGS. 11 a  and 11 b    show electric field  1120  along the direction of top passive dipole structures  620  (x=−100 mm, y=50 mm, z=10 mm to x=100 mm, y=50 mm, z=10 mm where x=0, y=0 and z=0 is the center of segmented loop  410 ) and bottom passive dipole structures  620  (x=−100 mm, y=−50 mm, z=10 mm to x=100 mm, y=−50 mm, z=10 mm where x=0, y=0 and z=0 is the center of segmented loop  410 ), respectively. For comparison, electric field  1130  with all passive dipole structures  620  and  610  removed is shown. 
       FIG. 11 c    shows electric field  1125  along the direction of passive dipole structure  610  on the left side of  FIG. 9  (x=−100 mm, y=−50 mm, z=10 mm to x=−100 mm, y=50 mm, z=10 mm where x=0, y=0 and z=0 is the center of segmented loop  410 ) which has matching circuit  931  including a balun. For comparison, electric field  1140  with all passive dipole structures  610  and  620  removed is shown. 
       FIG. 11 d    shows electric field  1126  along the direction of passive dipole structure  610  on the right side of  FIG. 9  (x=100 mm, y=−50 mm, z=10 mm to x=100 mm, y=50 mm, z=10 mm where x=0, y=0 and z=0 is the center of segmented loop  410 . For comparison, electric field  1140  with all passive dipole structures  610  and  620  removed is shown. Note the difference in the electric fields  1125  and  1126  as well as electric fields  1140  and  1150  due to the location of the feed-in point (part of matching circuit  931 ) on the left side of segmented loop  410  and 91Ω resistor  951  in  FIG. 9 . 
       FIG. 12  shows segmented loop  1200  as an alternative to segmented loop  410  in accordance with the invention. Segmented loop is ellipsoidal in shape and generates a field that extends further to the left and right than the field for segmented loop  410  assuming the minor elliptical axis of segmented loop  1200  is about the radius of segmented loop  410 . Note that low order polygonal segmented loops such as rectangular or square segmented loops are typically to be avoided as sharp corners disrupt an in-phase and constant in magnitude current. Because a current flux occurs at the edges of a conductive path, there is typically a higher current density at the inner angle of a sharp corner compared to the outer angle of the sharp corner as the current chooses the shortest possible path. This typically leads to unwanted radiation. 
     While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.