Abstract:
There is disclosed an automobile hang tag for use in parking control and the like with an integral radio transponder, together with interrogator apparatus associated therewith in a system for parking control or other purposes. The detection range for the RFID interrogator and transponder is at least about six meters (20 feet) to enable rapid control monitoring of parking facilities from a moving vehicle through the back window or front windshield of parked vehicles. Reliable and trouble-free monitoring of the hang tags in a customary rear view mirror location is facilitated by employing directional antennas in the RFID hang tags, increasing the antenna gain while suppressing false signals and noise and improving the range-to-power ratio for the system. The RFID hang tags may be battery powered but are preferably unpowered RFID transponders. Suitable directional antennas employed may include Yagi antennas, log periodic antennas, stacked dipole antennas, spiral or slot antennas or other monodirectional or bidirectional antennas, preferably those suitable for selected frequencies within the 1,000 megahertz to 10,000 megahertz range.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Not Applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   This invention relates to the field of radio frequency identification transponders (RFID tags), and particular applications thereof involving vehicle identification and parking control systems. 
   RFID tags receive RF electromagnetic radiation from an interrogating transmitter and return information recorded in the tag to a receiver and computer which is usually associated with the interrogator transmitter. 
   RFID tags can and have been used in many ways for locating or identifying tagged objects including animals, people, vehicles, or other objects either stationary or mobile. Usually the RFID tag returns distinctive information from the particular tag which may be variable or invariable in nature. 
   As shown in U.S. Pat. No. 3,098,971 to R. M. Richardson, it has long been proposed that the power necessary to transmit the return signal from the transponder be provided by the much stronger signal that is received by the transponder. This eliminates the necessity for a battery or other power source for the tag. Such an RFID tag is referred to as a passive tag, and it is the preferred form for use in the present invention. The present invention is not limited to such passive tags, however, and could be implemented with battery powered tags. 
   In an RFID tag of the passive type an interrogator signal picked up by the antenna of the tag induces an alternating current in an antenna circuit which may be rectified by an RF diode, and this rectified current can be used in a power supply for the electronic components of a microcircuit. A digital memory element of the microcircuit stores identification information and/or other data. A lower frequency signal generated in the microcircuit is caused to modulate the return signal transmitted from the RFID antenna thereby communicating information coded in the lower frequency signal modulation may be implemented either by altering the antenna loading or by other suitable form of modulation. Thus the RFID tag may be interrogated by a signal which both communicates with the RFID tag and supplies the power for its circuit so that the RFID tag can respond with an information carrying signal from its transmit antenna, all without requiring a battery or other power source for the RFID tag. In some systems information in signals received from the interrogator may be stored in a digital memory of the transponder as well. 
   Further refinements in RFID tag technology were made as shown in U.S. Pat. No. 4,075,632 to Baldwin et al. wherein tags were proposed with logic and read/write memories and transistors for modulating the return signal were also powered by the energy received by the transponder. Such refinements are also shown in U.S. Pat. No. 4,786,907 to Akoelle. 
   While improvements in semiconductor technology to provide microcircuits which are smaller and have lower power requirements have increased the capability of RFID tags and the systems which employ them, there are limitations which have still not been entirely overcome, particularly in the passive type of RFID tags. Use of such tags where the distance between the interrogator and the transponder is more than a few feet or about one meter presents difficulties. The amount of power transmitted by the interrogator is subject to regulations as well as practical limitations so that the effect of the well known square-of-the-distance power reduction factor militates against reliable use at longer ranges. There is an associated problem exacerbated by the low magnitude of the return signal power in that frequently more than one RFID tag is in the area being interrogated, presenting the likelihood that interference between return signals from different tags will adversely affect the reliability of a system. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention relates to systems for parking control or other similar purposes utilizing automobile hang tags incorporating RDIF transponders with directional antennas facilitating remote interrogation of such transponders with increased range and reliability. When employed in a parking control application the detection range for the RFID interrogator and transponder will be at least about 6 meters (about 20 feet) and enables rapid control in monitoring of parking facilities from a moving vehicle employing a transmission path through the back window or the front windshield of parked vehicles. In a system according to the invention customary high gain directional antennas will be employed for the mobile interrogator unit, but the usual omni directional or low gain antennas of RDIF tags are replaced by directional high gain antennas for receiving the transmission from the interrogator and transmitting the reply signal to the interrogator. This is particularly useful in the vehicle parking control application, for in such systems the tags may conveniently be placed in the vehicle in a specified orientation, and the vehicles themselves will be arranged in prearranged parking stalls. 
   Thus according to the invention the combined directivity and associated gain for both the interrogator antenna and the transponder antenna are utilized to increase the range and sensitivity for response signals from hang tags in a predetermined preferred direction, while at the same time reducing the likelihood of interference from other hang tags in nearby vehicles. It is important that this system serves to increase the normal range-to-power ratio permitting use of lower power interrogator signals well within regulatory requirements. Suitable directional antennas employed may include Yagi antennas, log periodic antennas, stacked dipole antennas, spiral or slot antennas or other known monodirectional or bidirectional antenna forms. It is preferred that any radio frequencies employed be within the 1,000 megahertz to 10,000 megahertz range; a basic design approach for such antennas may include scaling of television or other communication antennas to smaller size and higher frequency. 
   Other known interference reduction techniques may be employed in the controllers software of the system, and, if desired, further interference reduction may be achieved by using radar techniques to maintain a range window corresponding to response signal return time. By this approach transponder return signals from more than a specified distance, 8 meters or 25 feet for example, may be suppressed to eliminate interference from such sources. 
   Certain specific designs of directional antennas are of interest and in particular those derived utilizing the antenna design techniques of the prior art listed below and incorporated by reference herein. Also listed below are vehicular RFID related patents. 
   U.S. Pat. No. 4,782,345 issued to Landt on Nov. 1, 1988; U.S. Pat. No. 4,786,907 issued to Koelle on Nov. 22, 1988; U.S. Pat. No. 4,816,839 issued to Landt on Mar. 28, 1989; U.S. Pat. No. 5,525,991 issued to Nagura et al. on Jun. 11, 1996; U.S. Pat. No. 5,661,473 issued to Paschal on Aug. 26, 1997; U.S. Pat. No. 5,771,021 issued to Veghte et al. on Jun. 23, 1998; U.S. Pat. No. 5,777,561 issued to Chieu et al. on Jul. 7, 1998; U.S. Pat. No. 5,912,632 issued to Dieska et al. on Jun. 15, 1999; U.S. Pat. No. 6,118,379 issued to Kodukula et al. on Sep. 12, 2000; U.S. Pat. No. 6,121,880 issued to Scott et al. on Sep. 19, 2000; U.S. Pat. No. 6,215,402 issued to Rao Kodukula et al. on Apr. 10, 2001; U.S. Pat. No. 6,236,315 issued to Helms et al. on May 22, 2001; U.S. Pat. No. 6,278,413 issued to Hugh et al. on Aug. 21, 2001; U.S. Pat. No. 6,307,524 issued to Britain on Oct. 23, 2001; U.S. Pat. No. 6,320,509 issued to Brady et al. on Nov. 20, 2001; and U.S. Pat. No. 6,353,443 issued to Ying on Mar. 5, 2002. 
   It is an object of the present invention to provide an automobile hang tag of convenient size and shape including an RFID transponder enabling the information recorded in the hang tag to be accessed by radio frequency identification (RFID) apparatus as well as visually or optically. 
   It is another object of the present invention to provide an RFID tag to be temporarily affixed in the interior of an automobile with a transponder circuit and directional antenna enabling it to be reliably interrogated from a distance of at least one car length (about 20 feet or 6 meters). 
   It is still another object of the present invention to provide a vehicle control system with RFID hang tags for mounting in the interior of vehicles for identification thereof and/or data related thereto, together with interrogator apparatus mounted in a vehicle enabling rapid identification among a plurality of vehicles such as those within a parking area to enable control and monitoring of parking facilities from a moving vehicle or for other purposes. 
   It is yet another object of the present invention to provide a vehicle identification system including automobile hang tags with individual RFID transponders and a vehicle mounted RFID interrogator such that the transmission and reception characteristics of the transponder and interrogator facilitate communication of coded information from the transponder through the rear window (or the front windshield) of vehicles while the interrogator is being transported along a line of parking stalls in a parking facility. 
   It is a further object of the present invention to provide RFID hang tags for such a parking control system which are not battery powered (i.e., passive tags) having the necessary range of operation through exploitation of antenna gain and directivity. 
   A still further object of the present invention is to provide RFID tags with antennas having bidirectional or monodirectional radiation patterns in the horizontal plane. 
   Other objects and advantages of the invention will be apparent from consideration of the following description in conjunction with the appended drawings described below. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  is a partially schematic diagram of a parking facility with vehicles fitted with RFID auto hang tag responders and a monitoring vehicle with interrogator apparatus according to the invention; 
       FIG. 2  is a block diagram of one form of prior art conventional RFID tag circuit useful in implementing the present invention; 
       FIG. 3  is a block diagram of another form of prior art conventional RFID tag circuit useful in implementing the present invention; 
       FIG. 4A  is a schematic block diagram of an RFID transponder circuit and antenna for use in RFID hang tag transponders according to the invention; 
       FIG. 4B  is a schematic block diagram of an interrogator circuit used to interrogate and record information from RFID hang tag transponders as shown in the above Figures; 
       FIG. 5  is a partially schematic diagram of a first embodiment of RFID hang tag transponder according to the invention also illustrating visual indicia on the hang tag (which is omitted in following figures for simplicity and clarity); 
       FIG. 6  is a partially schematic diagram of a second embodiment of RFID hang tag transponder according to the invention; 
       FIG. 7  is a partially schematic diagram of a third embodiment of RFID hang tag transponder according to the invention; 
       FIG. 8  is a partially schematic diagram of a fourth embodiment of RFID hang tag transponder according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows partially schematically a parking facility and an exemplary form of parking control system incorporating features of the present invention. The parking facility  3  could be of any form but is illustrated having a double row of parking stalls separated by barriers  5 . 
   Vehicles such as automobile  11  are parked front to front on either side of the barriers  5 . The vehicles may be of any style and automobile  11  has a common configuration including a vehicle hood  31 , front windshield  33 , side windows  37  and  39 , metal roof  41 , back window  43 , and rear portion  45 . According to the invention a distinctive automobile hang tag  35  is situated near the rear view mirror of automobile  11 ; such hang tag has a long range, preferably directional propagation, RFID tag incorporated therein. Each of the other vehicles shown, automobiles  13 ,  15 ,  17 ,  19 ,  21 ,  23 ,  27  and  29  will be understood to be generally similar to automobile  11  for the purpose of explaining  FIG. 1 . It should be understood, however, that other styles of vehicles such as vans, small trucks and the like can be accommodated in the system with little or no adjustment. 
   According to this embodiment of the invention an interrogator  55  for communicating with the RFID transponder of hang tags  35  is preferably mounted in or on a security or control vehicle  50 . Vehicle  50  has a front hood  51 , front windshield  53 , left side window  57 , and right side window  59 . The interrogator  55  may conveniently be mounted at a left side window  57  and/or right side window  59 . The transmitter of the interrogator  55  preferably has a directivity pattern provided by its antenna which helps to eliminate interference from multiple response signals of adjacent automobiles such as  21  and  23  as shown in  FIG. 1 . The optimum directivity may be determined by consideration of the parameters shown in  FIG. 1 . These parameters are: 
   T-average center to center distance of rear view mirrors holding parking permit hang tag with RFID. 
   D—depth of field of RF interrogator. 
   V—variation in length of vehicles. 
   A—aperture (cone) of the signal from RF interrogator. 
   B—aperture (cone) of the signal from RFID transponder. 
   For clarity the B-aperture for the RFID transponder is shown relative to the transponder  35  of vehicle  27  as pattern  47 . It will be understood however that all the RFID transponders  35  will have similar directivity as indicated by pattern  47  and its B-aperture. 
   As shown in  FIG. 1  the desirable directivity parameters cause the maximum for A to be such that at the end of zone D, representing depth of field, that the width of aperture cone  56  is no greater than T. This provides the desired effect of transmitting the interrogating signal essentially to one transponder at a time and effectively eliminates the likelihood of interference from simultaneous response from two adjacent vehicles. 
   According to the invention a similar employment of directivity in the transponder antenna enhances this effect and the reliability of the system as shown by the pattern  47  and the B aperture thereof. In the case of the hang tag transponders  35  the directivity pattern  47  increases the sensitivity for transmissions from interrogator  55  when aligned with vehicle  27  and at the same time renders the transponder  35  of vehicle  27  relatively insensitive when the interrogator vehicle  50  is in a position to interrogate some other vehicle such as vehicle  29 . Usually the pattern for transmitting a response from RDIF transponders  35  will have similar directivity and, to some degree, this may also reduce likelihood of interfering signals being received at the interrogator  55 . While there will be unwanted reflected radiation in this environment, it too will tend to be suppressed by the directivity considerations discussed above. 
   The diagrams of  FIG. 1  indicating interrogator and transponder directivity are schematic and it will be appreciated that actual RF antenna transmission and reception patterns are not precisely directional in the manner depicted in  FIG. 1 . It is reasonable to consider that the angle of directivity such as A or B represents the half power point of the pattern. Of course the pattern does not sharply terminate at a prescribed distance. Also, in some embodiments the pattern will be bidirectional rather than as shown in  FIG. 1 . In situations such as shown in  FIG. 1  where there are double rows of vehicles so that a transmission from an interrogator vehicle  50  which is aligned with a vehicle in one of the rows will also be aligned with a more distant vehicle in a second row, the directivity of the interrogator signal clearly will not aid in preventing signal interference of two such aligned vehicles. However, the response of the more distant transponder will be lessened by the operation of the square of the distance rule for attenuation of distant signals. With passive transponders this attenuation by distance may approach a fourth power effect as is encountered in radar technology. If problems would be created by interference from substantially more distant RFID transponders, it may be eliminated by employing radar technology to suppress signals beyond a predetermined range by a range gate technique based on transmission return times. 
   Transponder antennas, such as directional antenna  140 , preferably have horizontal polarization (as provided by horizontally disposed dipoles, for example). Other radiation polarization such as vertical polarization or circular polarization may be employed for the transponder antennas (and the interrogator antennas) as circumstances may indicate. Except for  FIG. 8 , the transponder antennas shown schematically have horizontal polarization, so that polarization would preferably be employed for the interrogator antenna. Discrimination against extraneous signals may be achieved by selection of horizontal polarization, and horizontal dipoles provide better directivity in the horizontal plane for suppressing response from untargeted RFID tag transponders. 
   The radiation pattern for RFID transponders  35  is indicated in  FIG. 1  as being monodirectional, although the pattern could alternatively be bidirectional. The monodirectional pattern has the advantage that automobiles in a more distant row will usually have their RFID tags facing in an opposite direction thereby significantly reducing any possibility of interference from such tags. The advantage of bidirectional radiation patterns for the RFID tags is that they would be responsive through rear window or front window transmission paths without changing their orientation. Antennas with monodirectional and bidirectional radiation patterns will each be described. 
   In referring to bidirectional and monodirectional antennas it will be understood that there is some directivity associated with even the most omni-directional antennas. In referring to bidirectional or monodirectional antennas in this discussion it will be understood that these terms are meant to describe antennas with significantly greater directivity than an ordinary simple dipole antenna without intended directional characteristics. 
   As shown in  FIG. 2  prior art RFID tags of small dimensions using microcircuits powered only by energy received from the interrogator are known. Conventional forms thereof as shown in  FIG. 2  include an antenna  32 A connected to a ground  58 A and antenna transmission line  50 A. A tag rectification power supply  34 A receives energy from antenna  32 A and converts some of received radio frequency energy to DC to be conveyed through lines  52 ,  54  and  56  to a tag current source and other elements requiring direct current energization. 
   The transponder function is implemented by a tag receiver section  36 A which feeds signals to a tag clock section  40 A (in this embodiment) and to a tag logic section  42 A. The tag clock section  40 A also feeds a signal  102 A to the tag logic section  42 A which transmits information signals to and from a tag memory section  44 A. The operation of such RFID transponders is described in detail in the references listed in the background section above. 
     FIG. 3  shows a schematic block diagram of a prior art RFID tag of common form suitable for battery powered tags. The transponder circuit  18 B including antenna  40 B may be powered by an optional battery  48 B (or may be adapted to use a portion of received energy to power the circuit as illustrated in  FIG. 2 ). Radio frequency energy from antenna  40 B is fed to detector  42 B (and in the case of passive RFID tags would also supply energy for a rectification power supply). The detector signal from  42 B is provided to control unit  38 B having all the necessary logic and control functions programmed in a digital microcircuit. Control unit  38 B has access to and from data memory  46 B. Battery  48 B would be utilized if the transponder was to be battery powered rather than passive. The control unit  38 B directs information signals received from detector  42 B and from data memory  46 B to control modulator  44 B which in turn sends the information modulated response signal to antenna  40 B for transmission back to the system interrogator apparatus. The transponder apparatus shown in  FIG. 3  may have the capability of recording data received through antenna  40 B and detector  42 B into data memory  46 B if that function is desired. On the other hand, data memory  46 B may be a read-only memory pre-programmed to simply identify the RFID transponder tag  18 B and thus the vehicle or other object with which it is associated. 
     FIG. 4  is a schematic block diagram of exemplary electronic circuitry for the RFID transponder incorporated in automobile hang tags according to the present invention. Electronic circuitry is preferably implemented with microcircuit technology employing one or more integrated circuits. A transponder microcircuit  118  is connected to receive microwave signals from and send microwave signals to a directional antenna  140 , to be more fully described below. 
   Signals received from antenna  140  are processed in a detector  142  and communicated to a control unit  138 . Control unit  138  has micro circuitry programmed to carry out the control operations for the transponder in accordance with known computer digital logic technique and standards. The primary function of control unit  138  is to recognize an interrogation signal received through directional antenna  140  and to access a data memory  146  for data to respond to the interrogation signal. Data memory  146  is connected to control unit  138  to provide this capability. In more sophisticated RFID implementations the control unit  138  may have the capability of receiving data signals beyond the simple interrogation command and to store such data by writing it into the data memory  146 . In this more sophisticated implementation the data stored in data memory  146  would be accessible for transmission in a response to a subsequent interrogation. These additional functions are optional, however, and the basic implementation of the RFID transponder requires a read-only memory for the identification function of the RFID microcircuit. 
   An optional battery  148  is shown in  FIG. 4A  as included in microcircuit  118 . As previously described, the preferable form of RFID tag is a passive one without a battery for reasons of simplicity and economy, but where circumstances prevent transmission of a signal with sufficient power to the transponder microcircuit to provide the required electrical energy, a battery  148  may be included to overcome that problem. 
   It is well known that the RFID circuits as shown in  FIG. 4A  can be made smaller, more compact and more efficient at higher frequencies and shorter wavelengths. Although frequencies from 100 megahertz to 900 megahertz could be employed for RFID apparatus according to the invention it is preferable that frequencies of about 900 megahertz and above be employed. Of these preferable frequencies it is probably best to employ frequencies of about 2,300 megahertz and above to allow compact size for the physical structure of the tag while providing strong directivity and high gain for the directional antenna. 
   The modulation frequency for microcircuit  118  encoding control information and data is subject to much variation as known in the art and may be selected from the range of 1 kilohertz to 1 megahertz. Amplitude modulation or other forms such as frequency or pulse modulation may be employed. The operation of the directional antenna  140  which is a primary feature of the invention favors higher operation frequencies and the concomitant shorter wavelengths, but otherwise no particular characteristics for known RFID transponder circuitry are required by the invention. 
   The interrogator circuit  218  shown in  FIG. 4B  is of any generally conventional form known in the art except for the need for a directive antenna such as antenna  240 . 
   In  FIG. 4B  a usual arrangement where antenna  240  is employed both as a transmitter  242  antenna and a receiver  244  antenna is illustrated. It will be understood however that, particularly in the interrogator  55 , and to a lesser extent in transponders  35 , one might choose to have separate antennas for receiving and transmitting. 
   Interrogator circuit  218  includes a control circuit  238  preferably implemented with digital micro circuitry and programmed to control operations of transmitter  242  and receiver  244  and to communicate with memory  246  and computer  247 , all of which are preferably implemented with integrated circuit technology in one or more circuits. Memory  246  is preferably a read/write memory and computer  247  includes a central processor unit and usual associated displays for locally communicating information to the operator. 
   In most instances the computer  247  will have a link to a base computer which will serve to collect, compile and store information and data received from one or more interrogators over a period of time. Interrogator circuit  218  is provided with a power supply  248  preferably including a storage battery rechargeable from a vehicle direct current system or from fixed alternating current receptacles. 
   The interrogator circuit  218  and the antenna  240  may be of well known conventional form provided only that the antenna  240  is capable of providing the directivity needed to carry out preferred forms of the invention. Since there are no severe limitations on the size or shape of the antenna  240 , which will typically be mounted in or on a vehicle  50 , desired antenna directivity can be achieved by common known techniques in the antenna art without difficulty. 
   More detail of an exemplary preferred form of auto hang tag with RFID transponder  35  is shown in  FIG. 5 . Hang tag  35  may be of a shape commonly used for such hang tags without RFID capability and may include an engagement opening  101  for affixing the tag in the vehicle at the rear view mirror with the tag oriented with the normal to its plane surface approximately horizontal and directed longitudinally fore and aft of the vehicle. This will normally provide a line of sight from one side of a tag through the vehicle front windshield and a line of sight from the opposite face of the tag through the vehicle rear window. If this situation does not prevail with a particular vehicle, accommodation can be made to locate the tag to provide a line of sight through a vehicle window so that microwave frequency radiation to and from the tag is not blocked or reflected. Usually heating wires in rear windows will not present a problem, but such can be overcome by using a different window. 
   As indicated in  FIG. 5 , hang tag  35  includes a transponder circuit  118  as previously described. While an optional battery  148  is shown it preferable that the transponder circuit be powered with received radio frequency energy so that battery  148  is not necessary. 
   As indicated in  158  the tag  35  will customarily include visual indicia that is found on auto hang tags without RFID capability. The showing of indicia  158  is exemplary only and indicia may occupy a larger area on the tag and be present on both the front and back surfaces thereof. 
   A particular form of directional antenna generally referred to as a stacked folded dipole antenna is illustrated in schematic form to be employed in the hang tag  35  of  FIG. 5 . By employing two or more vertically stacked horizontal dipoles, antenna  140  has increased directivity and increased gain as compared with a simple dipole antenna. A transmission line  152  of appropriate form connects folded dipole  151  to the transponder circuit  118  and an additional transmission line  153  also connects a second folded dipole  155  in parallel to the transponder circuit  118 . Transmission line  153  may extend to additional folded dipoles not shown in  FIG. 5 . 
   By employing folded dipoles  151  and  155  the electrical length of each dipole is greater than its physical length and hence their optimum frequency may be lower than for an unfolded dipole; thus the antenna  140  of  FIG. 5  with its folded dipoles will, if necessary, provide optimum quarter-wave (or other) antenna design for lower frequencies while keeping within the practical space restrictions imposed by the hang tag  35 . 
   It may be noted that the antenna  140  of stacked folded dipole configuration shown in  FIG. 5  is bidirectional rather than monodirectional. As previously discussed, this has the advantage that the transponder will respond equally well from two opposite directions so that it could be interrogated successfully through either the rear window or the front windshield of an automobile. It has the disadvantage that hang tags other than the one being targeted by the interrogator might respond in a manner to interfere, but any such problem can be overcome by processing received signals at the interrogator utilizing a range gate or other known techniques. 
   Antennas specifically for use in RFID tags implemented with printed circuit techniques have been developed and an adaptation of such antennas is utilized in the hang tag embodiment illustrated in  FIG. 6 . The antenna  160  incorporated in auto hang tag  6  illustrated in  FIG. 6  is a spiral antenna similar to the antennas explained and discussed in U.S. Pat. No. 6,353,443 referred to above and the references cited therein. 
   Hang tag  6  may be of similar configuration to hang tag  35  and may be provided with an engagement opening  101 . Hang tag  6  as well as the other hang tags illustrated herein may be modified to be dual purpose serving also as a magnetic data card by providing a magnetic strip  168  on an edge of the card located so as not to interfere with the operation of the RFID function of the hang tag. 
   Hang tag  6  has a transponder circuit  118  and may optionally be provided with a battery  148 . Transponder circuit  118  is connected by a transmission line  152  to directional antenna  160 ; antenna  160  comprises a metallic strip  167  having a spiral or other convoluted shape; which fulfills the radiator function of the antenna. Optional matching elements may be provided for magnetic strip  167  such as matching bridge  165  connected between grounding post  161  and feed pin  163 . Using known design considerations the directional antenna  160  and the metallic strip  167  will be configured to provide a desirable directivity pattern, center response frequency and bandwidth for the transponder function of hang tag  6 . 
   Although automobile hang tags are conventionally in the form of a flat generally planar sheet material there is no fundamental or operational restriction to such a shape, and  FIG. 7  shows an auto hang tag  7  with a folded configuration that makes it somewhat 3-dimensional rather than 2-dimensional. Auto hang tag  7  may be provided with an engagement opening  101  for securing on an arm of a rear view mirror. On its upper portion it may also be provided with a transponder circuit similar to those previously described using printed circuit and integrated circuit technology. The antenna portion  170  of the hang tag  7  is of a flat planar configuration but it is disposed at an angle F relative to the upper portion with transponder circuit  118 . Angle F is preferably selected to be about 90 degrees or less so that the hang tag antenna portion  170  will be approximately horizontal, rather than vertical as in the previously described embodiments. 
   A horizontal antenna configuration in  FIG. 7  has the advantage that more highly directive and higher gain antenna structures in general communication use can be adapted for use in the RFID transponder of the hang tag. For example, a log periodic antenna is schematically illustrated in  FIG. 7 . A plurality of simple dipoles  172 ,  173 ,  174 ,  175  and  176  are situated in a horizontal array with their axis perpendicular to the upper portion of auto hang tag  7 . In accordance with well known design considerations for log periodic antennas the dipoles  172 – 176  are of increasing length at greater distances from transponder circuit  118 . The dipoles  172 – 176  are connected by a harness  171  which in usual practice introduces a 180 degree phase shift between successive dipoles. The resulting antenna configuration provides a high gain, highly directive antenna for reception or transmission in the direction of arrow  177  shown in dashed lines. The hang tag of  FIG. 7  (and other embodiments) preferably provides a minimum space of 2×4 inches (5 by 10 centimeters) for the antenna elements. 
   To utilize the directivity of auto hang tag  7  in an automobile one would situate the antenna portion extending in the opposite position of the desired directivity. That is, for reception and transmission through the automobile windshield one would locate the antenna portion  170  extending toward the rear of the vehicle, and, conversely, for rear window reception the antenna portion  170  would extend toward the windshield of the vehicle. In the example illustrated in  FIG. 7  the transponder circuit  118  is of the passive type not requiring a battery due, in part, to the high gain characteristic of the associated antenna. The log periodic antenna of  FIG. 7  may be designed to have a relatively broad band width (and greater energy reception) if desired, but that would not in all cases be desirable. The angle F shown in  FIG. 7  is somewhat less than 90 degrees and approximately 75 degrees to cause the orientation of the antenna ray formed by dipoles  172 – 176  to be oriented very nearly horizontally when the auto hang tag  7  is hanging in place at a vehicle rear view mirror. The directivity of the antenna is thereby aligned with the vehicle rear window or windshield as desired. Alternatively, the angle F could be somewhat adjustable by the user to accommodate different vehicle configurations. 
   Another preferred embodiment illustrated in  FIG. 8  shows an auto hang tag having an L-shaped configuration similar to  FIG. 7 , but with a different form of monodirectional antenna. Hang tag  8  is provided with an engagement opening  101  and a magnetic strip  188  allowing it a dual purpose use. The hang tag  8  has an extension tab  187  to accommodate a desired length of magnetic strip  188  if necessary. The auto hang tag  8  of  FIG. 8  as well as all other embodiments normally will be provided front and/or back with visible indicia or optically readable indicia which is not shown in  FIGS. 6–8  for clarity of illustration. Hang tag  8  has a transponder circuit  118  connected by a transmission line  152  to a simple dipole antenna  183  formed by metallic strips on a flap  182  embodying antenna  180  of auto hang tag  8 . 
   Associated with the dipole  183  is a reflector  181  in the form of a metallic strip on the main vertical portion of hang tag  8 . If desired, metallic strip  181  could alternatively be located on flap  182 . Further metallic strips  184 ,  185  and  186  serve as directors thereby providing a Yagi antenna configuration with a directivity indicated by arrow  189  in dashed lines. This Yagi antenna for the RFID transponder for auto hang tag  8  may be configured for a particular frequency of operation using frequency scaling techniques for the very common Yagi antenna design used in numerous forms of radio frequency communication apparatus. As is well understood, a Yagi antenna as shown in  FIG. 8  will have a highly directional radiation pattern principally in the direction of arrow  189  and will provide an antenna gain many times greater than a simple dipole antenna. 
   The amount of improvement in gain over a simple dipole antenna required for a particular implementation of the invention will depend upon operational requirements such as range and interference rejection. It should be noted that even a simple dipole antenna has some degree of directivity with radiation in a direction endwise discriminated against as compared with the broadside directions. Thus, when the antenna to be associated with the RFID transponder of a hang tag is described or defined as directive or directional it should be understood that the directional quality is not only the inherent directionality incidental to the use of a simple dipole antenna. 
   The use of the RFID system with auto hang tags according to the invention presently deemed to be of primary importance is vehicle control and monitoring in parking facilities. The invention is not limited to such uses however, and may find uses in highway toll collection or traffic control or other future uses beyond those specifically described herein. 
   While numerous variations and modifications for the invention have been described shown or suggested above, it should be understood that other modifications and alternative embodiments will be apparent to those skilled in the art and accordingly the scope of the invention is not to be deemed limited to those variations specifically described shown or suggested, but is rather to be determined by reference to the appended claims.