Abstract:
An RFID antenna comprising an elongated structure existing along an axis that is long compared to the signal wavelength and including twin ribbon-like feed lines of electrically conductive material, the feed lines being in a common plane and being uniformly laterally spaced from one another, and a plurality of radiating perturbations associated with the feed lines at a plurality of locations spaced along the feed lines, at each location each feed line has its own individual perturbation or portion of a perturbation.

Description:
This application claims the priority of U.S. Provisional Application No. 61/191,687, filed Sep. 11, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention pertains to radio frequency identification (RFID) systems and, in particular, to an improved antenna for such applications. 
     PRIOR ART 
     RFID technology is expected to greatly improve control over the manufacture, transportation, distribution, inventory, and sale of goods. A goal, apparently not yet realized on a widespread scale, is the identification of goods down to a unit basis at a given site. To accomplish this goal, each item will carry a unique tag that, when it receives radiation from an RFID antenna, will send back a modulated unique signal verifying its presence to the antenna. The antenna, in turn, receives this transmitted signal and communicates with a reader that registers reception of this signal and, therefore, the presence and identity of the subject item. 
     Typically by its nature, an RFID tag identifying a subject item is polarized so that its response to a radio signal will depend on its alignment with the polarization of the signal radiated by the RFID antenna. Items can be expected to be randomly positioned in the space being surveyed by the RFID system and, therefore, the system should be capable of reading these items. Signal fading due to interference, absorption, reflection and the like can adversely affect the ability of an RFID antenna to reliably read an RFID tag. These conditions make it desirable to be able to transmit as much electromagnetic signal power as government regulations allow. 
     An RFID antenna should be relatively inexpensive to produce, practical to handle and ship, and be simple to install. Additionally, the antenna should be unobtrusive when installed and, ideally, easily concealed. 
     SUMMARY OF THE INVENTION 
     The invention provides a novel RFID antenna structure particularly suited for reading RFID tags at the item level. The antenna is capable of reading such tags in a near zone as they exist in storage, display or as they pass through a control zone such as a door or other portal, whether or not in bulk and/or in random orientation. The antenna of the invention produces radio frequency electric field beams of diverse polarization and direction. This diversity ensures that at least some beam component with a polarization matching that of each RFID tag will illuminate such a tag to ensure that a signal can be generated by the tag and thereby be detected. 
     In a preferred embodiment, the antenna is an elongated structure producing a near-field radiation that is used to monitor a cylindrical or semi-cylindrical zone. The axis of the antenna is located at or adjacent to the axis of the cylindrical zone to be monitored. By way of example, the antenna can be arranged vertically. In this configuration, the antenna is capable of monitoring nearby shelves, pallets, display cabinets, or doorways, for example. 
     In the disclosed embodiments, the antenna comprises twin-feed lines extending along an elongated axis and perturbations or radiators spaced along the length of the antenna. The feed lines can comprise a pair of spaced, preferably flat, coplanar conductors, and the radiators can extend as branches or stubs laterally from the feed lines. 
     In the preferred embodiments, the stubs are skewed with respect to the antenna axis. The skew or angularity of the stubs relative to the axis develops a favorable polarization pattern. The feed line conductors, ideally, are disposed along a serpentine path, centered about the axis that reduces interference with radiation patterns from the stubs by orienting the stubs normal or nearly normal to the feed lines. 
     The preferred antenna arrangement is characterized by diversity of both electric field polarization and beam direction, and at the same time a relatively uniform signal strength coming from each radiator. This beam diversity enables the antenna to be driven and radiate at a high power level, without violating Federal Communication Commission (FCC) rules, to ensure RFID tag illumination and, therefore, reliable tag reading. The beam diversity of direction and polarization obtained by the preferred antenna construction, additionally, enhances performance by ensuring that an RFID tag in the antenna operating range with any orientation will be illuminated with an aligned polarized beam. Beam diversity is further increased by using multiple antennas to cover the same zone. 
     The skewed polarization and beam separation characteristic of the preferred antenna enables an identical antenna or antennas to be flipped on its axis and/or inverted relative to a first antenna to further increase the beam diversity in both polarization and direction. 
     In the preferred embodiment, the beam diversity is obtained in a counter-intuitive manner by scanning the beams of signal components polarized in the vertical or axial direction of the antenna while the signal components polarized in directions perpendicular to the antenna axis radiate in beams nearly perpendicular to the antenna axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevational view, in a mid-plane, of a preferred embodiment of an antenna of the invention; 
         FIG. 2  is a fragmentary enlarged view of the antenna of  FIG. 1  showing near zone electric fields; 
         FIG. 3  is a fragmentary cross-section of the antenna taken at the plane  3 - 3  in  FIG. 1 ; 
         FIG. 4  is a schematic diagram of horizontally and vertically polarized beams radiated from the antenna; 
         FIG. 5  is an illustration of the feed or input end of the antenna; 
         FIG. 6  illustrates the use of adjacent identical antennas with different orientations; 
         FIG. 7  illustrates an arrangement useful for covering a semi-cylindrical zone on one side of the antenna; 
         FIG. 8  is an alternative antenna construction; 
         FIG. 9  is a second alternative antenna construction; 
         FIG. 10  is a third alternative antenna construction; and 
         FIG. 11  shows use of two of the antennas of the type shown in  FIG. 10 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  illustrates a preferred form of an RFID antenna  10 . The antenna is elongated along a longitudinal axis  11 . The antenna  10  includes a pair of coplanar twin ribbon-like conductors or strips  12  having a gap or space  13  therebetween. The conductors  12 , also referred to herein as feed lines, are made of copper or aluminum, for example, and can be relatively thin self-supporting foil or can be printed, deposited, or otherwise fabricated on a thin carrier film  14  of suitable dielectric material such as Mylar®, or etched from a printed circuit board. 
     Preferably at uniformly spaced locations along the length of the antenna  10  are pairs of stubs (i.e. dipoles) or branch radiators  16 , each stub of a pair being in electrical continuity with an associated one of the conductors or feed lines  12 . The stubs  16  are conveniently formed conductors such as the same material used for the feed lines  12 , are coplanar with the feed lines, and are integrally formed with these lines so as to ensure electrical continuity with these lines. 
     In one antenna design intended for use to monitor space within a room, the antenna has a nominal length of about 7′ and the antenna is used with its axis  11  upright or vertical. The conductors  12  are each about ½″ wide and the space or gap  13  between them is about ⅛″. The stubs  16  conductor width is used to adjust the radiator&#39;s bandwidth. For typical applications the stubs are somewhat narrower than the feed lines and their lengths can be varied from about 2″ at a feed end of the antenna  10  to about 3″ at the terminal end. In a 7′ antenna length seven pairs or dipoles of stubs  16  are used with a spacing of about 12″ measured along the axis  11  of the antenna. The distance from a feed or feed matching section  17  described below, to the first pair of stubs  16  is about 4″ measured along the center of the gap  13  and the distance from the last pair of stubs  16  can be about 2″ from a short  18  between the conductors  12  forming the termination of the antenna. Alternatively, the termination can be an open circuit or an impedance load. Note that the impedance termination can also create radiation, which can be used to excite RFID tags. 
       FIG. 3  is a cross-sectional view of the antenna  10  illustrating a sandwich-like construction. The conductors  12  and the stubs  16  are printed, laminated, or otherwise disposed on the carrier film  14  between two low density dielectric boards or panels  21 . Alternatively, the conductors  12  and stubs  16 , if sufficiently self-supporting, can be laminated directly to one of the boards  21  so as to eliminate the film  14 . As another alternative, the conductors  12  and stubs  16  can be printed directly on a board  21 . The boards  21  can be extruded low-density, (1.5 lbs/ft 3 ) polystyrene foam for instance. Protective heavy plastic film  22 , for example 0.040″ thick, is held firmly or bonded on the exterior surfaces of the foam boards  21 . The boards  21 , conductive strips  12 , stubs  16 , any film  14 , and film  22  can be solidly held and/or bonded by suitable adhesives together to produce a relatively rigid antenna package, if desired. The presence of the boards  21  ensures that surrounding structures, materials or goods are not so close to the antenna  10  when it is installed as to significantly adversely affect the performance of the antenna. 
     The stubs or radiators  16 , have an orientation that is skewed at an angle to the axis  11  of the antenna. Ideally, the stubs  16  lie at an angle of about 45° with respect to the axis  11 . The two stubs or branches  16  forming a dipole at each location along the length of the antenna  10  are preferably in alignment such that both lie along a common line. 
       FIG. 5  shows a manner of feeding the antenna  10  from a coax cable  26 . A feed matching section  17 , in the form of a quarter wavelength impedance transformer, includes two conductive strips  28  on a suitable thin non-conductive substrate such as the Mylar® sheet  14  on which the antenna feed lines  12  are carried. The strips  28  are electrically connected to the feed line conductors  12  and are separated by a narrow gap  29  of about 1 mm. A center conductor  31  of the coax cable  26  is electrically connected to one of the strips  28  such as by a mechanical connector in the form of a metal clamp  32  with integral barbs that, after piercing the respective strip, are crimped tightly against the underside of the film  14  carrying the strip or if the strip is self-supporting, against the opposite side of the strip. An outer conductor  33  of the coax cable  26  is similarly electrically connected to the other strip  28  by an associated metal clamp or connector  34 . The metal clamps or connectors  32 ,  34 , may be soldered between their respective conductors  31 ,  33  and feed strips  28 , to assure a reliable electrical connection between these elements. Because of the stepped nature of the quarter wavelength impedance transformer, it tends to radiate a small signal level as well. Even this small radiation can be useful for RFID applications as discussed here. 
     Inspection of  FIG. 1  shows that pairs of stubs or branches  16  alternate from a positive slope (the first, third, fifth, and seventh stub pairs) to a negative slope (the second, fourth, and sixth stub pairs). The feed lines  12  act as a two-wire transmission line, from which it is well known that the current on one feed line is out of phase by 180° to the current in the other feed line. This allows the currents in each pair of the stubs  16  to be in phase and, therefore, produce radiated signals that reinforce one another. The short between the feed lines  12  at the terminal end  18  is about a ¼ wavelength or less from the last pair of stubs  16 . 
     The serpentine path of the feed lines  12  has been found to advantageously limit the influence these lines would otherwise generally have on the directional character and strength of the radiated signals produced by the stubs  16 . The serpentine configuration of the feed lines  12  serves to space the distal or free ends of the stubs  16  from the feed lines and produces the ideal electric field patterns shown in  FIG. 2 . 
     Radiation from a stub  16  is polarized parallel or nearly parallel to the stub. In  FIGS. 1 and 4 , the stubs, i.e. dipoles  16  are arranged at an angle of +45° or −45° to the axis  11 . Radiation of the angled stubs  16  has both horizontal and vertical components in the sense that the axis  11  of the antenna  10  is vertically oriented. The horizontally polarized radiation components of all of the stubs  16  of the antenna  10  are all polarized in the same direction and roughly in-phase such that they create radiation beams  41  that are nearly perpendicular to the antenna axis  11 . In addition, horizontally polarized beams  45  are end fire beams produced as a consequence of the nearly full wavelength spacing between the stubs or radiators  16 . On the other hand, the vertically polarized radiation components of adjacent stubs  16  are in opposite directions and therefore oppose one another. The interaction of these opposing vertically polarized radiation components produces scanned conical beams tilted off the plane perpendicular to the axis  11  by about ±40°, the angle depending in part on the proximity of the stubs  16  to one another. This phenomenon is schematically depicted in  FIG. 4  where horizontally polarized signal components travel in beams  41  nearly perpendicular to the antenna axis  11  and in the end fire direction; whereas, the vertically polarized signal components are radiated in terms of tilted conical beams  42   u  and  42   d . Because of the complex phasing action between all the stubs and termination, these beams will not all be excited to the same radiation level. Thus,  FIG. 4  is an over-simplification and in-use of the antenna the RFID tagged items are illuminated in the near zone of the antenna.  FIG. 4  is depicting the horizontally and vertically polarized radiation beams as seen in the far field of the antenna. 
     From this analysis, it will be understood that the antenna  10  is characterized by a high degree of radiation diversity in the near zone where it operates. The antenna  10  affords both vertically and horizontally polarized signal components, and these signal components are directed in widely divergent beam paths. This diversity reduces the risk of signal fading in areas of the space or zone the antenna  10  is intended to illuminate or survey. Further, the separation of the vertically and horizontally polarized beams  41 ,  42 ,  45  allows the antenna to be efficiently driven with a maximum wattage without violating FCC regulations because the power is not concentrated in a single beam, thus providing an effective and inexpensive antenna unit composed of multiple radiators. References to vertical and horizontal orientation throughout this disclosure are for convenience in the explanation, but it will be understood that the antenna  10  can be used in any orientation and the planes of polarization and beam direction will be similarly reoriented. 
     The 45° degree angle of the stubs  16  to the longitudinal axis  11  is of great benefit because it allows a duplicate antenna to be flipped over 180° about its axis relative to a first antenna and produce radiation polarization in planes that are orthogonal to the polarization planes of the first antenna. This arrangement, which significantly improves the signal polarization and beam diversity, is shown by the side-by-side placement of the antenna  10  and the antenna  10   a  in  FIG. 6 . For even greater radiation diversity, antenna  10   b  can be inverted and for still further diversity, a fourth duplicate antenna  10   c  can be flipped on its axis and inverted adjacent to the antenna  10 . Any combination of two or more of the antenna orientations depicted in  FIG. 6  can be used. For greatest effectiveness, each of the provided antennas  10 ,  10   a ,  10   b , and/or  10   c , where more than one is used, is operated alone in a sequence with the other(s). 
     An RFID tag  46  is preferably permanently attached to the antenna  10  and is unique to the particular antenna to which it is attached. Still further, a non-RF machine readable tag  47 , again unique to the particular antenna, like an optically readable UPC label or a magnetically encoded tag is also preferably attached to the antenna  10 . When the antenna is installed, a technician can scan the non-RF tag  47  and thereby electronically record its location and RFID tag identity at the installation site. At any time thereafter, a reader system can test a particular antenna (with its identity and location previously stored in an electronic memory) by driving it and determining if it senses its own RFID tag. 
       FIG. 7  diagrammatically illustrates an antenna  10  arranged to monitor a semi-cylindrical zone. As shown, a conducting metal plate  51  is spaced some distance (which is normally close to one-quarter wavelength) behind the vertical antenna  10 . Reflection from the conducting plate  51  reinforces the forward radiation while blocking back radiation. It will be appreciated that rather than a single antenna, multiple antennas such as arranged in  FIG. 6  can be used in the installation depicted in  FIG. 7 . 
     In  FIGS. 8-11 , antenna constructions can employ ribbon-like feed lines and radiation areas like those described in connection with  FIGS. 1-3  and can be mounted and protected in the same way.  FIG. 8  is a fragmentary view of a portion of an antenna  60  with parallel feed lines  61  segments and dual stub radiators  62 . The antenna  60  obtains a desired 45° polarization although the abrupt bends in the feed lines  61  may also radiate energy. 
     Referring now to  FIG. 9 , there is shown an embodiment of an antenna  65  wherein coplanar strip feed lines or conductors  66  are arranged to cause radiation from the half wavelength sections  67   a - e . As shown in  FIG. 9 , the rectangular radiators  67   a - e  are wider near a termination end  68  as compared to the feed end  69 . The spacing between the feed lines  66  changes abruptly for roughly a half wavelength section and then changes back to the original spacing. The currents in the feed lines behave similarly to a loop or patch antenna. Currents travel in opposite directions in the two coplanar feed lines  66 . Therefore, the currents I 1 , I 2 , and I 3 , have the directions shown in  FIG. 9  in each feed line or strip  66 . The fields radiated by the currents I 2  flowing in opposite directions in the two parallel lines  66  will tend to cancel. The field of currents I 1  flowing in the two collinear lines or strips  66  will not cancel each other because they are in phase and flowing in the same direction. The same is true for I 3 . The fields of currents I 1  and I 3  do not cancel each other because there is a 180° phase shift due to the half wavelength spacing along the feed line. This gives the antenna  65  a strong polarization component normal to the axis of the feed lines  66 . The antenna  65  does not have the 45° polarization of the earlier disclosed embodiments but represents an antenna design using the basic configuration of coplanar strip feed lines. 
     Referring now to  FIG. 10 , an antenna  75  having dual feed lines  76 , produces radiation from bends in the feed lines. The fields radiated by currents I 1  in the two parallel strips will cancel because they are equal and opposite, as will the currents I 2 . However, the fields radiated by currents I 3  and I 4  will not cancel each other because of the 180° phase shift due to the half wavelength separation along the feed line. The radiation from I 3  and I 4  has the desired 45° polarization. The power radiated by I 3  and I 4  may be controlled by reducing the offset distance to less than a half wavelength. As the currents get closer together their radiated fields will tend to cancel each other. Another way to control the radiation level at a junction is to vary the bend angle. The bend angle shown in  FIG. 10  is 90°. If the angle is reduced, such as the 45° angle shown in  FIG. 8 , the radiation will be reduced relative to that radiated by a 90° bend. 
     Because of the ±45° polarization of the alternating bend embodiment of  FIG. 10 , it is possible to combine this antenna  75  with a second identical antenna flipped 180° about its axis. The second antenna  75  will provide orthogonal polarization and may be mounted relatively close to the first antennas shown in  FIG. 11 . This concept is shown for antenna  75 , but it could be used for antenna  10  or  60  as well. Here, the second antenna is shown directly over the first antenna, and can even be shifted one-half period along the axis. For antennas  10  and  60 , the second antenna could be rotated 180 degrees about its axis to create the orthogonal polarization as well. The two antennas can be separated using a low density dielectric panel or foam, for example, that is thick enough to prevent excessive coupling between the two feed lines. In this manner, two antennas can be easily mounted in the same package with two ports or feeds. 
     While the invention has been shown and described with respect to particular embodiments thereof, this is for the purpose of illustration rather than limitation, and other variations and modifications of the specific embodiments herein shown and described will be apparent to those skilled in the art all within the intended spirit and scope of the invention. Accordingly, the patent is not to be limited in scope and effect to the specific embodiments herein shown and described nor in any other way that is inconsistent with the extent to which the progress in the art has been advanced by the invention.