Abstract:
An automatic data collection system tracks medical articles by providing a robust electromagnetic (EM) field within an enclosure in which the articles are stored. Respective data carriers, such as RFID tags, attached to each article respond to the electromagnetic field by transmitting data identified with each article. An RFID scanner receives the transmitted RFID tag identification data and a processor compares the received identification data to a data base. The data base associates the identification data with data concerning the medical article to which the RFID tag is affixed, such as the name of the medicine, the size of the dose, and the expiration date. The processor is also programmed to keep track of the number of articles of a particular type remaining in the enclosure, to note receipt of an article in the enclosure, and to note removal of the article.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 14/691,518, filed Apr. 20, 2015, now U.S. Pat. No. 9,223,934, which is a continuation of U.S. application Ser. No. 14/231,513, filed Mar. 31, 2014, now U.S. Pat. No. 9,013,309, which is a continuation of U.S. application Ser. No. 13/776,613, filed Feb. 25, 2013, now U.S. Pat. No. 8,686,859, which is a continuation of U.S. application Ser. No. 12/631,861, filed Dec. 7, 2009, now U.S. Pat. No. 8,384,545, all of which are incorporated by reference in their entireties. 
     
    
     BACKGROUND 
       [0002]    The invention relates generally to the field of automatically collecting data from articles being tracked, and more particularly, to a system and method of establishing a robust electromagnetic field in an enclosure for use in identifying tagged articles. 
         [0003]    There are a number of ways of identifying and tracking articles including visually, optically (bar coding, for example), magnetically, RFID, weighing, and others. Where an automatic system for tracking is desired, RFID is a candidate since identification data may be obtained wirelessly. RFID tags have decreased in cost, which has made them even more attractive for such an application. 
         [0004]    Radio-frequency identification (“RFID”) is the use of electromagnetic energy (“EM energy”) to stimulate a responsive device (known as an RFID “tag” or transponder) to identify itself and in some cases, provide additionally stored data. RFID tags typically include a semiconductor device having a memory, circuitry, and one or more conductive traces that form an antenna. Typically, RFID tags act as transponders, providing information stored in the semiconductor device memory in response to an RF interrogation signal received from a reader, also referred to as an interrogator. Some RFID tags include security measures, such as passwords and/or encryption. Many RFID tags also permit information to be written or stored in the semiconductor memory via an RF signal. 
         [0005]    RFID tags may be incorporated into or attached to articles to be tracked. In some cases, the tag may be attached to the outside of an article with adhesive, tape, or other means and in other cases, the tag may be inserted within the article, such as being included in the packaging, located within the container of the article, or sewn into a garment. The RFID tags are manufactured with a unique identification number which is typically a simple serial number of a few bytes with a check digit attached. This identification number is incorporated into the tag during manufacture. The user cannot alter this serial/identification number and manufacturers guarantee that each serial number is used only once. This configuration represents the low cost end of the technology in that the RFID tag is read-only and it responds to an interrogation signal only with its identification number. Typically, the tag continuously responds with its identification number. Data transmission to the tag is not possible. These tags are very low cost and are produced in enormous quantities. 
         [0006]    Such read-only RFID tags typically are permanently attached to an article to be tracked and, once attached, the serial number of the tag is associated with its host article in a computer data base. For example, a particular type of medicine may be contained in hundreds or thousands of small vials. Upon manufacture, or receipt of the vials at a health care institution, an RFID tag is attached to each vial. Each vial with its permanently attached RFID tag will be checked into the data base of the health care institution upon receipt. The RFID identification number may be associated in the data base with the type of medicine, size of the dose in the vial, and perhaps other information such as the expiration date of the medicine. Thereafter, when the RFID tag of a vial is interrogated and its identification number read, the data base of the health care institution can match that identification number with its stored data about the vial. The contents of the vial can then be determined as well as any other characteristics that have been stored in the data base. This system requires that the institution maintain a comprehensive data base regarding the articles in inventory rather than incorporating such data into an RFID tag. 
         [0007]    An object of the tag is to associate it with an article throughout the article&#39;s life in a particular facility, such as a manufacturing facility, a transport vehicle, a health care facility, a storage area, or other, so that the article may be located, identified, and tracked, as it is moved. For example, knowing where certain medical articles reside at all times in a health care facility can greatly facilitate locating needed medical supplies when emergencies arise. Similarly, tracking the articles through the facility can assist in generating more efficient dispensing and inventory control systems as well as improving work flow in a facility. Additionally, expiration dates can be monitored and those articles that are older and about to expire can be moved to the front of the line for immediate dispensing. This results in better inventory control and lowered costs. 
         [0008]    Other RFID tags are writable and information about the article to which the RFID tag is attached can be programmed into the individual tag. While this can provide a distinct advantage when a facility&#39;s computer servers are unavailable, such tags cost more, depending on the size of the memory in the tag. Programming each one of the tags with informaton contained in the article to which they are attached involves further expense. 
         [0009]    RFID tags may be applied to containers or articles to be tracked by the manufacturer, the receiving party, or others. In some cases where a manufacturer applies the tags to the product, the manufacturer will also supply a respective data base file that links the identification number of each of the tags to the contents of each respective article. That manufacturer supplied data base can be distributed to the customer in the form of a file that may easily be imported into the customer&#39;s overall data base thereby saving the customer from the expense of creating the data base. 
         [0010]    Many RFID tags used today are passive in that they do not have a battery or other autonomous power supply and instead, must rely on the interrogating energy provided by an RFID reader to provide power to activate the tag. Passive RFID tags require an electromagnetic field of energy of a certain frequency range and certain minimum intensity in order to achieve activation of the tag and transmission of its stored data. Another choice is an active RFID tag; however, such tags require an accompanying battery to provide power to activate the tag, thus increasing the expense of the tag and making them undesirable for use in a large number of applications. 
         [0011]    Depending on the requirements of the RFID tag application, such as the physical size of the articles to be identified, their location, and the ability to reach them easily, tags may need to be read from a short distance or a long distance by an RFID reader. Such distances may vary from a few centimeters to ten or more meters. Additionally, in the U.S. and in other countries, the frequency range within which such tags are permitted to operate is limited. As an example, lower frequency bands, such as 125 KHz and 13.56 MHz, may be used for RFID tags in some applications. At this frequency range, the electromagnetic energy is less affected by liquids and other dielectric materials, but suffers from the limitation of a short interrogating distance. At higher frequency bands where RFID use is permitted, such as 915 MHz and 2.4 GHz, the RFID tags can be interrogated at longer distances, but they de-tune more rapidly as the material to which the tag is attached varies. It has also been found that at these higher frequencies, closely spaced RFID tags will de-tune each other as the spacing between tags is decreased. 
         [0012]    There are a number of common situations where the RFID tags may be located inside enclosures. Some of these enclosures may have entirely or partially metal or metallized surfaces. Examples of enclosures include metal enclosures (e.g., shipping containers), partial metal enclosures (e.g., vehicles such as airplanes, buses, trains, and ships that have a housing made from a combination of metal and other materials), and non-metal enclosures (e.g., warehouses and buildings made of wood). Examples of objects with RFID tags that may be located in these enclosures include loose articles, packaged articles, parcels inside warehouses, inventory items inside buildings, various goods inside retail stores, and various portable items (e.g., passenger identification cards and tickets, baggage, cargo, individual life-saving equipment such as life jackets and masks) inside vehicles, etc. 
         [0013]    The read range (i.e., the range of the interrogation and/or response signals) of RFID tags is limited. For example, some types of passive RFID tags have a maximum range of about twelve meters, which may be attained only in ideal free space conditions with favorable antenna orientation. In a real situation, the observed tag range is often six meters or less. Therefore, some of the enclosures described above may have dimensions that far exceed the read range of an individual RFID tag. Unless the RFID reader can be placed in close proximity to a target RFID tag in such an enclosure, the tag will not be activated and read. Additionally, metal surfaces of the enclosures present a serious obstacle for the RF signals that need to be exchanged between RFID readers and RFID tags, making RFID tags located behind those metal surfaces difficult or impossible to detect. 
         [0014]    In addition to the above, the detection range of the RFID systems is typically limited by signal strength to short ranges, frequently less than about thirty centimeters for 13.56 MHz systems. Therefore, portable reader units may need to be moved past a group of tagged items in order to detect all the tagged items, particularly where the tagged items are stored in a space significantly greater than the detection range of a stationary or fixed single reader antenna. Alternately, a large reader antenna with sufficient power and range to detect a larger number of tagged items may be used. However, such an antenna may be unwieldy and may increase the range of the radiated power beyond allowable limits. Furthermore, these reader antennae are often located in stores or other locations where space is at a premium and it is expensive and inconvenient to use such large reader antennae. In another possible solution, multiple small antennae may be used but such a configuration may be awkward to set up when space is at a premium and when wiring is preferred or required to be hidden. 
         [0015]    In the case of medical supplies and devices, it is desirable to develop accurate tracking, inventory control systems, and dispensing systems so that RFID tagged devices and articles may be located quickly should the need arise, and may be identified for other purposes, such as expiration dates. In the case of medical supply or dispensing cabinets used in a health care facility, a large number of medical devices and articles are located closely together, such as in a plurality of drawers. Cabinets such as these are typically made of metal, which can make the use of an external RFID system for identification of the stored articles difficult. In some cases, such cabinets are locked due to the presence of narcotics or other medical articles or apparatus within them that are subject to a high theft rate. Thus, manual identification of the cabinet contents is difficult due to the need to control access. 
         [0016]    Providing an internal RFID system in such a cabinet can pose challenges. Where internal articles can have random placement within the cabinet, the RFID system must be such that there are no “dead zones” that the RFID system is unable to reach. In general, dead zones are areas in which the level of coupling between an RFID reader antenna and an RFID tag is not adequate for the system to perform a successful read of the tag. The existence of such dead zones may be caused by orientations in which the tag and the reader antennae are in orthogonal planes. Thus, articles placed in dead zones may not be detected thereby resulting in inaccurate tracking of tagged articles. 
         [0017]    Often in the medical field, there is a need to read a large number of tags attached to articles in such an enclosure, and as mentioned above, such enclosures have limited access due to security reasons. The physical dimension of the enclosure may need to vary to accommodate a large number of articles or articles of different sizes and shapes. In order to obtain an accurate identification and count of such closely-located medical articles or devices, a robust electromagnetic energy field must be provided at the appropriate frequency within the enclosure to surround all such stored articles and devices to be sure that their tags are all are activated and read. Such medical devices may have the RFID tags attached to the outside of their containers and may be stored in various orientations with the RFID tag (and associated antenna) pointed upwards, sideways, downward, or at some other angle in a random pattern. 
         [0018]    Generating such a robust EM energy field is not an easy task. Where the enclosure has a size that is resonant at the frequency of operation, it can be easier to generate a robust EM field since a resonant standing wave may be generated within the enclosure. However, in the RFID field the usable frequencies of operation are strictly controlled and are limited. It has been found that enclosures are desired for the storage of certain articles that do not have a resonant frequency that matches one of the allowed RFID frequencies. Thus, a robust EM field must be established in another way. 
         [0019]    Additionally, where EM energy is introduced to such an enclosure for reading the RFID tags within, efficient energy transfer is of importance. Under static conditions, the input or injection of EM energy into an enclosure can be maximized with a simple impedance matching circuit positioned between the conductor delivering the energy and the enclosure. As is well known to those of skill in the art, such impedance matching circuits or devices maximize the power transfer to the enclosure while minimizing the reflections of power from the enclosure. Where the enclosure impedance changes due to the introduction or removal of articles to or from the enclosure, a static impedance matching circuit may not provide optimum energy transfer into the enclosure. If the energy transfer and resulting RF field intensity within the enclosure were to fall below a threshold level, some or many of the tags on articles within the enclosure would not be activated to identify themselves, leaving an ineffective inventory system. 
         [0020]    It is a goal of many health care facilities to keep the use of EM energy to a minimum, or at least contained. The use of high-power readers to locate and extract data from RFID tags is generally undesirable in health care facilities, although it may be acceptable in warehouses that are sparsely populated with workers, or in aircraft cargo holds. Radiating a broad beam of EM energy at a large area, where that EM energy may stray into adjacent, more sensitive areas, is undesirable. Efficiency in operating a reader to obtain the needed identification information from tags is an objective. In many cases where RFID tags are read, hand-held readers are used. Such readers transmit a relatively wide beam of energy to reach all RFID tags in a particular location. While the end result of activating each tag and reading it may be accomplished, the transmission of the energy is not controlled except by the aim of the user. Additionally, this is a manual system that will require the services of one or more individuals, which can also be undesirable in facilities where staff is limited 
         [0021]    Hence, those of skill in the art have recognized a need for an RFID tag reader system in which the efficient use of energy is made to activate and read all RFID tags in an enclosed area. A further need for establishing a robust EM field in enclosures to activate and read tags disposed at random orientations has also been recognized. A further need has been recognized for an automated system to identify articles stored in a metal cabinet without the need to gain access to the cabinet. A further need has been recognized for a dynamic matching circuit that will automatically adjust the matching circuit operation in dependence on the changes to the enclosure impedance. The present invention fulfills these needs and others. 
       SUMMARY OF THE INVENTION 
       [0022]    Briefly and in general terms, the present invention is directed to a system for reading a data carrier that is attached to an article, the data carrier being responsive to electromagnetic energy (EM) of a frequency f 1  in response to which the data carrier provides unique identification data, the system comprising a metallic enclosure having electrically conductive walls, the enclosure have a natural frequency of resonance f 2 , a probe mounted through one of the walls of the enclosure, the probe configured to inject electromagnetic energy into the enclosure, wherein the position of the probe in relation to the walls of the enclosure is selected to optimize power transfer into the enclosure, a storage device located within the enclosure configured to receive data carriers with respective articles to be stored, and a receiving antenna disposed within the enclosure and configured to receive the unique identification data provided by the data carriers. 
         [0023]    In a further aspect in accordance with the invention, the frequency f 1  is different from the natural frequency of resonance of the enclosure f 2 , and the position of the probe is selected so that reflected phase of EM from the walls is equal at the probe position, i.e, they are in phase at the probe position. 
         [0024]    In yet more detailed aspect, the system further comprises an impedance matching circuit configured to more closely match impedance of the probe to impedance of the enclosure. The impedance matching circuit comprises an active impedance matching circuit. 
         [0025]    In other aspects in accordance with the invention, the storage device comprises a drawer adapted to move into and out of the enclosure. A portion of the drawer forms a portion of the electrically conductive enclosure when the drawer is in a predetermined position in relation to the enclosure. The drawer is non-electrically conductive except for a portion of the drawer that forms a portion of the electrically conductive enclosure. Further, the drawer is slidable into and out of the enclosure through an opening in the enclosure, the drawer having a front panel that is electrically conductive and that contacts the electrically conductive enclosure when the drawer is slid to a predetermined position within the enclosure. 
         [0026]    In another more detailed aspect, the receiving antenna is located on an opposite side of the storage device from the probe. 
         [0027]    In yet further aspects, the system further comprises a plurality of probes, each probe being mounted through a wall of the enclosure, each of the probes configured to inject electromagnetic (EM) energy into the enclosure at its respective position, wherein the position of each probe in relation to the walls of the enclosure is selected to optimize power transfer into the enclosure to provide coverage of a portion of the enclosure with an electromagnetic field, the position of the probes further selected such that the probes may be sequentially activated to inject their respective EM energy into the enclosure to provide a composite EM field. 
         [0028]    In yet other detailed aspect, there is provided a system for tracking an article having a data carrier attached thereto, the data carrier being responsive to electromagnetic energy of a frequency f 1  in response to which the data carrier broadcasts unique identification data, the system comprising a metallic enclosure having electrically conductive walls, the enclosure have a natural frequency of resonance of f 2 , wherein f 2  is different from f 1 , a probe mounted through one of the walls of the enclosure, the probe configured to inject electromagnetic energy into the enclosure, wherein the position of the probe in relation to the walls of the enclosure is selected to optimize power transfer into the enclosure, a drawer movably disposed within the enclosure, the drawer configured to store articles having respective data carriers, and a receiving antenna disposed within the enclosure and configured to receive the unique identification data provided by data carriers. 
         [0029]    In yet another aspect in accordance with the invention, there is provided a system for establishing a robust electromagnetic field within an enclosure, the system comprising a metallic enclosure having electrically conductive walls, the enclosure have a natural frequency of resonance f 2 , a probe mounted through one of the walls of the enclosure, the probe configured to inject electromagnetic energy (EM) into the enclosure, the EM having a frequency of f 1 , wherein f 1  and f 2  are different, wherein the position of the probe in relation to the walls of the enclosure is selected to optimize power transfer into the enclosure, a storage device located within the enclosure configured to receive data carriers with respective articles to be stored, and a receiving antenna disposed within the enclosure and configured to receive the unique identification data provided by the data carriers. 
         [0030]    The features and advantages of the invention will be more readily understood from the following detailed description that should be read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a schematic diagram of a drawer that may be positioned within a medical dispensing cabinet, showing the storage of a plurality of medical articles randomly positioned in the drawer, each of those articles having an integral RFID tag oriented randomly; 
           [0032]      FIG. 2  is a perspective view of a medication dispensing cabinet having five drawers, one of which is similar to the schematic view of  FIG. 1 , the cabinet also having an integral computer for controlling access to the cabinet and performing inventory tracking by periodically reading any RFID tags placed on articles stored within the cabinet, and for reporting the identified articles to a remote computer; 
           [0033]      FIG. 3  is a block and flow diagram showing an embodiment in which an RFID reader transmits activating EM energy into a drawer containing RFID tags with a single transmitting antenna, receives the data output from the activated RFID tags with a single receiving antenna, a computer controlling the transmission of activating energy and receiving the data from the activated RFID tags for processing; 
           [0034]      FIG. 4  is a block and flow diagram similar to  FIG. 3  showing an embodiment in which an RFID reader transmits activating EM energy into a drawer containing RFID tags with two transmitting antennae, receives the data output from the activated RFID tags with three receiving antennae, and as in  FIG. 3 , a computer controlling the transmission of activating energy and receiving the data from the activated RFID tags for processing; 
           [0035]      FIG. 5  shows an enclosure with a single probe and a connector, the probe being configured to inject EM energy into the enclosure and excite a TE mode; 
           [0036]      FIG. 6  shows an enclosure with a single probe and a connector, the probe being configured to inject EM energy into the enclosure and excite a TM mode; 
           [0037]      FIG. 7  shows a plot of coupled power in an enclosure as a function of frequency for a resonant enclosure where F n  is the natural resonance frequency of the enclosure; 
           [0038]      FIG. 8  shows a plot of coupled power (ordinate axis) in an enclosure as a function of frequency (abscissa axis), where f f  is a forced resonance frequency, or otherwise referred to as a frequency that is not equal to the resonant frequency of the enclosure, and f n  is the natural resonant frequency of the enclosure, showing the establishment of a robust field of coupled power in the enclosure at the f f  frequency; 
           [0039]      FIG. 9  shows an enclosure with two probes each with a connector for injecting EM energy into the enclosure, one probe being a TM probe and the other being a TE probe; 
           [0040]      FIG. 10  shows a probe, a connector, and an attenuator that is used to improve the impedance match between the probe and the enclosure; 
           [0041]      FIG. 11  shows a probe, a connector, and a passive matching circuit that is used to improve the impedance match between the probe and enclosure; 
           [0042]      FIG. 12  shows an active matching circuit connected between a probe located in an enclosure and a transceiver, the active matching circuit comprising a tunable capacitor, a dual-directional coupler, multiple power sensors, and a comparator used to provide a closed-loop, variable matching circuit to improve the impedance match between the probe and the enclosure; 
           [0043]      FIG. 13  provides a side cross-sectional view of the cabinet of  FIG. 2  at the location of a drawer with the drawer removed for clarity, showing the placement of two probe antennae in a “ceiling mount” configuration for establishing a robust EM field in the drawer when it is in place in the cabinet in the closed position; 
           [0044]      FIG. 14  is a perspective view of the metallic enclosure showing the probe configuration of  FIG. 13  again showing the two probe antennae for establishing a robust EM field in a drawer to be inserted; 
           [0045]      FIG. 15  is a cutaway perspective side view of the metallic enclosure or frame in which are mounted the dual probe antennae of  FIGS. 13 and 14  with the drawer removed for clarity; 
           [0046]      FIG. 16  is a frontal perspective view of the view of  FIG. 14  with a cutaway plastic drawer in place in the metallic enclosure and further showing the dual ceiling mount probe antennae protected by an electromagnetically inert protective cover, and further showing cooling system components mounted at the back of the cabinet near the drawer&#39;s back, the drawing also showing a partial view of a drawer slide mechanism for ease in sliding the drawer between open and closed positions in the cabinet, the drawer front and rear panels having been cutaway in this view; 
           [0047]      FIG. 17  is a frontal perspective view at the opposite angle from that of  FIG. 16  with the plastic drawer completely removed showing the dual ceiling mount probe antennae protected by the EM inert protective cover mounted to the metallic enclosure, and further showing the cooling system components of  FIG. 16  mounted at the back of the cabinet as a spring loading feature to automatically push the drawer to the open position when the drawer&#39;s latch is released, the figure also showing a mounting rail for receiving the slide of the drawer; 
           [0048]      FIG. 18  is a schematic view with measurements in inches of the placement of two TE 01  mode probes in the top surface of the enclosure shown in  FIGS. 13-15 ; 
           [0049]      FIG. 19  is a schematic view of the size and placement within the drawer of  FIG. 16  of two microstrip or “patch” antennae and their microstrip conductors disposed between respective antennae and the back of the drawer at which they will be connected to SMA connectors in one embodiment, for interconnection with other components; 
           [0050]      FIG. 20  is diagram of field strength in an embodiment of an enclosure with a probe placed in the enclosure at a position in accordance with the diagram of  FIG. 19 ; 
           [0051]      FIG. 21  is a lower scale drawing of the field intensity diagram of  FIG. 20  showing a clearer view of the field intensity nearer the front and back walls of the enclosure; and 
           [0052]      FIGS. 22A and 22B  together present a block electrical and signal diagram for a multiple-drawer medical cabinet, such as that shown in  FIG. 2 , showing the individual multiplexer switches, the single RFID scanner, and power control. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0053]    Referring now in more detail to the exemplary drawings for purposes of illustrating embodiments of the invention, wherein like reference numerals designate corresponding or like elements among the several views, there is shown in  FIG. 1  a schematic representation of a partial enclosure  20  in which a plurality of medical articles  22  are stored, each with a respective RFID tag  24  that has a unique identification number. The partial enclosure may comprise a drawer having a front  26 , a left side  28 , a right side  30 , a rear  32 , and a bottom  34 . These articles are randomly distributed in the drawer with the RFID tags facing in various and random directions. 
         [0054]    As used in regard to the embodiments herein, “reader” and “interrogator” refer to a device that may read or write/read. The data capture device is always referred to as a reader or an interrogator regardless of whether it can only read or is also capable of writing. A reader typically contains a radio frequency module (a transmitter and a receiver, sometimes referred to as a “transceiver”), a control unit and a coupling element (such as an antenna or antennae) to the RFID tag. Additionally, many readers include an interface for forwarding data elsewhere, such as an RS-232 interface. The reader, when transmitting, has an interrogation zone within which an RFID tag will be activated. When within the interrogation zone, the RFID tag will draw its power from the electrical/magnetic field created in the interrogation zone by the reader. In a sequential RFID system (SEQ), the interrogation field is switched off at regular intervals. The RFID tag is programmed to recognize these “off” gaps and they are used by the tag to send data, such as the tag&#39;s unique identification number. In some systems, the tag&#39;s data record contains a unique serial number that is incorporated when the tag is manufactured and which cannot be changed. This number may be associated in a data base with a particular article when the tag is attached to that article. Thus, determining the location of the tag will then result in determining the location of the article to which it is attached. In other systems, the RFID tag may contain more information about the article to which it is attached, such as the name or identification of the article, its expiration date, it dose, the patient name, and other information. The RFID tag may also be writable so that it can be updated. 
         [0055]    As used in regard to the embodiments herein, “tag” is meant to refer to an RFID transponder. Such tags typically have a coupling element, such as an antenna, and an electronic microchip. The microchip includes data storage, also referred to as memory. 
         [0056]      FIG. 2  presents a representative medical dispensing cabinet  40  comprising a plurality of movable drawers  42 . In this embodiment, there are five drawers that slide outwardly from the cabinet so that access is provided to the contents of the drawers.  FIG. 1  is a schematic diagram of a representative drawer that may be positioned within the cabinet of  FIG. 2  for sliding outward to provide access to the drawer&#39;s contents and for sliding inward into the cabinet to secure the drawer&#39;s contents. The cabinet also comprises an integral computer  44  that may be used to control access to the drawers and to generate data concerning access and contents, and to communicate with other systems. In this embodiment, the computer generates data concerning the number and type of articles in the drawers, the names of the patients for whom they have been prescribed, the prescribed medications and their prescribed administration dates and times, as well as other information. In a simpler system, the computer may simply receive unique identification numbers from stored articles and pass those identification numbers to an inventory control computer that has access to a data base for matching the identification numbers to article descriptions. 
         [0057]    Such a cabinet may be located at a nursing station on a particular floor of a health care institution and may contain the prescriptions for the patients of that floor. As prescriptions are prepared for the patients of that floor, they are delivered and placed into the cabinet  40 . They are logged into the integral computer  44 , which may notify the pharmacy of their receipt. A drawer may also contain non-prescription medical supplies or articles for dispensing to the patients as determined by the nursing staff. At the appropriate time, a nurse would access the drawer in which the medical articles are stored through the use of the computer  44 , remove a particular patient&#39;s prescriptions and any needed non-prescription articles, and then close the drawer so that it is secured. In order to access the cabinet, the nurse may need to provide various information and may need a secure access code. The drawers  42  may be locked or unlocked as conditions require. 
         [0058]    The computer  44  in some cases may be in communication with other facilities of the institution. For example, the computer  44  may notify the pharmacy of the health care institution that a patient&#39;s prescription has been removed from the cabinet for administration at a particular day and time. The computer may also notify the finance department of the health care institution of the removal of prescriptions and other medical articles for administration to a particular patient. This medication may then be applied to the patient&#39;s account. Further, the computer  44  may communicate to administration for the purpose of updating a patient&#39;s Medication Administration Record (MAR), or e-MAR. The medication cabinet  40  computer  44  may be wirelessly connected to other computers of the health care institution or may have a wired connection. The cabinet may be mounted on wheels and may be moved about as needed or may be stationary and unable to move. 
         [0059]    Systems that use RFID tags often employ an RFID reader in communication with one or more host computing systems that act as depositories to store, process, and share data collected by the RFID reader. Turning now to  FIGS. 3 and 4 , a system and method  50  for tracking articles are shown in which a drawer  20  of the cabinet  40  of  FIG. 2  is monitored to obtain data from RFID tags disposed with articles in that drawer. As mentioned above, a robust field of EM energy needs to be established in the storage site so that the RFID tags mounted to the various stored articles will be activated, regardless of their orientation. 
         [0060]    In  FIGS. 3 and 4 , the tracking system  50  is shown for identifying articles in an enclosure and comprises a transmitter  52  of EM energy as part of an RFID reader. The transmitter  52  has a particular frequency, such as 915 MHz, for transmitting EM energy into a drawer  20  by means of a transmitting antenna  54 . The transmitter  52  is configured to transmit the necessary RFID EM energy and any necessary timing pulses and data into the enclosure  20  in which the RFID tags are disposed. In this case, the enclosure is a drawer  20 . The computer  44  of an RFID reader  51  controls the EM transmitter  52  to cycle between a transmit period and a non-transmit, or off, period. During the transmit period, the transmitted EM energy at or above a threshold intensity level surrounds the RFID tags in the drawer thereby activating them. The transmitter  52  is then switched to the off period during which the RFID tags respond with their respective stored data. 
         [0061]    The embodiment of  FIG. 3  comprises a single transmitting probe antenna  54  and a single receiving antenna  56  oriented in such a manner so as to optimally read the data transmitted by the activated RFID tags located inside the drawer  20 . The single receiving antenna  56  is communicatively coupled to the computer  44  of the reader  50  located on the outside of the drawer  20  or on the inner bottom of the drawer. Other mounting locations are possible. Coaxial cables  58  or other suitable signal links can be used to couple the receiving antenna  56  to the computer  44 . A wireless link may be used in a different embodiment. Although not shown in the figures, those skilled in the art will recognize that various additional circuits and devices are used to separate the digital data from the RF energy, for use by the computer. Such circuits and devices have not been shown in  FIGS. 3 and 4  to avoid unneeded complexity in the drawing. 
         [0062]    The embodiment of  FIG. 4  is similar to the embodiment of  FIG. 3  but instead uses two transmitting probe antennae  60  and  62  and three receiving antennae  64 ,  66 , and  68 . The configuration and the number of transmitting probe antennae and receiving antennae to be used for a system may vary based at least in part on the size of the enclosure  20 , the frequency of operation, the relationship between the operation frequency and the natural resonance frequency of the enclosure, and the expected number of RFID tags to be placed in it, so that all of the RFID tags inside the enclosure can be reliably activated and read. The location and number of RFID reader components can be dependent on the particular application. For example, fewer components may be required for enclosures having a relatively small size, while additional components, such as shown in  FIG. 4 , may be needed for larger enclosures. Although shown in block form in  FIGS. 3 and 4 , it should be recognized that each receiving antenna  56 ,  64 ,  66 , and  68  of the system  50  may comprise a sub-array in a different embodiment. 
         [0063]    The transmit antennae ( 54 ,  60 , and  62 ) and the receive antennae ( 56 ,  64 ,  66 , and  68 ) may take different forms. In one embodiment as is discussed in more detail below, a plurality of “patch” or microstrip antennae were used as the reader receiving antennae and were located at positions adjacent various portions of the bottom of the drawer while the transmit antennae were wire probes located at positions adjacent portions of the top of the drawer. It should be noted that in the embodiments of  FIGS. 3 and 4 , the RFID reader  50  may be permanently mounted in the same cabinet at a strategic position in relation to the drawer  20 . 
         [0064]    One solution for reliably interrogating densely packed or randomly oriented RFID tags in an enclosure is to treat the enclosure as a resonant cavity. Establishing a resonance within the cavity enclosure can result in a robust electromagnetic field capable of activating all RFID tags in the enclosure. This can be performed by building an enclosure out of electrically conductive walls and exciting the metallic enclosure, or cavity, using a probe or probes to excite transverse electric (TE) or transverse magnetic (TM) fields in the cavity at the natural frequency of resonance of the cavity. This technique will work if the cavity dimensions can be specifically chosen to set up the resonance at the frequency of operation or if the frequency of operation can be chosen for the specific enclosure size. Since there are limited frequency bands available for use in RFID applications, varying the RFID frequency is not an option for many applications. Conversely, requiring a specific set of physical dimensions for the enclosure so that the natural resonant frequency of the enclosure will equal the available RFID tag activating frequency will restrict the use of this technique for applications where the enclosure needs to be of a specific size. This latter approach is not practical in view of the many different sizes, shapes, and quantities of medical articles that must be stored. 
         [0065]    Referring now to  FIG. 5 , a rectangular enclosure  80  is provided that may be formed as part of a medical cabinet, such as the cabinet shown in  FIG. 2 . It may be embodied as a frame disposed about a non-metallic drawer in such a cabinet. The enclosure  80  is formed of metallic or metallized walls  82 , floor  83 , and ceiling  84  surfaces, all of which are electrically conductive. All of the walls  82 , floor  83 , and ceiling  84  may also be referred to herein as “walls” of the enclosure.  FIG. 5  also shows the use of an energy coupling or probe  86  located at he top surface  84  of the enclosure  80 . In this embodiment, the probe takes the form of a capacitor probe  88  in that the probe  88  has a first portion  94  that proceeds axially through a hole  90  in the ceiling  84  of the enclosure. The purpose of the coupling is to efficiently transfer the energy from the source  52  (see  FIGS. 3 and 4 ) to the interior  96  of the enclosure  80 . The size and the position of the probe are selected for effective coupling and the probe is placed in a region of maximum field intensity. In  FIG. 5 , a TE 01  mode is established through the use of capacitive coupling. The length and distance of the bent portion  94  of the probe  88  affects the potential difference between the probe and the enclosure  80 . 
         [0066]    Similarly,  FIG. 6  presents an inductive coupling  110  of the external energy to an enclosure  112 . The coupling takes the form of a loop probe  114  mounted through a side wall  116  of the enclosure. The purpose of this probe is to establish a TM 01  mode in the enclosure. 
         [0067]    The rectangular enclosures  80  and  112  shown in  FIGS. 5 and 6  each have a natural frequency of resonance f n , shown in  FIG. 7  and indicated on the abscissa axis  118  of the graph by f n . This is the frequency at which the coupled power in the enclosure is the highest, as shown on the ordinate axis  119  of the graph. If the injected energy to the enclosure does not match the f n  frequency, the coupled power will not benefit from the resonance phenomenon of the enclosure. In cases where the frequency of operation cannot be changed, and is other than f n , and the size of the enclosure cannot be changed to obtain an f n  that is equal to the operating frequency, another power coupling apparatus and method must be used. In accordance with aspects of the invention, an apparatus and method are provided to result in a forced resonance f f  within the enclosure to obtain a standing wave within the enclosure with constructive interference. Such a standing wave will establish a robust energy field within the enclosure strong enough to activate all RFID tags residing therein. 
         [0068]    When an EM wave that is resonant with the enclosure enters, it bounces back and forth within the enclosure with low loss. As more wave energy enters the enclosure, it combines with and reinforces the standing wave, increasing its intensity (constructive interference). Resonation occurs at a specific frequency because the dimensions of the cavity are an integral multiple of the wavelength at the resonance frequency. In the present case where the injected energy is not at the natural resonance frequency f n  of the enclosure, a solution in accordance with aspects of the invention is to set up a “forced resonance” in an enclosure. This forced resonance is different from the natural resonance of the enclosure in that the physical dimensions of the enclosure are not equal to an integral multiple of the wavelength of the excitation energy, as is the case with a resonant cavity. A forced resonance can be achieved by determining a probe position, along with the probe length to allow for energy to be injected into the cavity such that constructive interference results and a standing wave is established. The energy injected into the enclosure in this case will set up an oscillatory field region within the cavity, but will be different from a standing wave that would be present at the natural resonance frequency f n  of a resonant cavity. The EM field excited from this forced resonance will be different than the field structure found at the natural resonance of a resonant cavity, but with proper probe placement of a probe, a robust EM field can nevertheless be established in an enclosure for RFID tag interrogation. Such is shown in  FIG. 8  where it will be noted that the curve for the forced resonance f f  coupled power is close to that of the natural resonance f n . 
         [0069]    Turning now to  FIG. 9 , an enclosure  120  having two energy injection probes is provided. The first probe  86  is capacitively coupled to the enclosure  120  in accordance with  FIG. 5  to establish a TE 01  mode. The second probe  114  is inductively coupled to the enclosure  120  in accordance with  FIG. 6  to establish a TM 01  mode. These two probes are both coupled to the enclosure to inject energy at a frequency f f  that is other than the natural resonance frequency f n  of the enclosure. The placement of these probes in relation to the ceiling  126  and walls  128  of the enclosure will result in a forced resonance within the enclosure  120  that optimally couples the energy to the enclosure and establishes a robust EM field within the enclosure for reading RFID tags that may be located therein. The placement of these probes in relation to the walls of the enclosure, in accordance with aspects of the invention, result in the forced resonance curve f f  shown in  FIG. 8 . 
         [0070]    Referring briefly to  FIG. 10 , an impedance matching circuit  121  is shown that functions to match the impedance of a source of energy  122  to the enclosure  120 . The impedance matching circuit is located between the coaxial cable  122  that feeds activating energy to the enclosure  120  and the capacitively coupled probe  88  through a hole in the metallic ceiling  126  of the enclosure. While the hole is not shown in the drawing of  FIG. 10 , the insulator  123  that electrically insulates the probe from the metallic ceiling is shown. In this case, the matching circuit  121  consists of only a resistive attenuator  124  used to reduce reflections of energy by the enclosure  120 . However, as will be appreciated by those of skill in the art, capacitive and inductive components are likely to exist in the enclosure and in the coupling  88 .  FIG. 11  on the other hand presents an impedance matching circuit  124  having passive reactive components for use in matching the impedance of the coaxial cable/energy source  122  and the enclosure  120 . In this exemplary impedance matching circuit  124 , an inductive component  125  and a capacitive component  127  are connected in series, although other configurations, including the addition of a resistive component and other connection configurations, are possible. 
         [0071]    Passive components such as resistors, inductors, and capacitors shown in  FIGS. 10 and 11  can be used to form matching circuits to match the impedances of the energy source and the enclosure. This will aid in coupling power into the enclosure. However, the passive matching circuit will improve the impedance match for a specific enclosure loading, such as an empty enclosure, partially loaded, or fully loaded enclosure. But as the enclosure contents are varied, the impedance match may not be optimized due to the variation in contents in the enclosure causing the impedance properties of the enclosure to change. 
         [0072]    This non-optimal impedance match caused by variation in enclosure loading can be overcome by the use of an active impedance matching circuit which utilizes a closed loop sensing circuit to monitor forward and reflected power. Referring now to  FIG. 12 , an active matching circuit  130  is provided that comprises one or several fixed value passive components such as inductors  132 , capacitors  134 , or resistors (not shown). In addition, one or several variable reactance devices, such as a tunable capacitor  134 , are incorporated into the circuit; these tunable devices making this an active impedance matching circuit. The tunable capacitor  134  can take the form of a varactor diode, switched capacitor assembly, MEMS capacitor, or BST (Barium Strontium Titanate) capacitor. A control voltage is applied to the tunable capacitor  134  and varied to vary the capacitance provide by the device. The tunable capacitor  134  provides the capability to actively change the impedance match between the probe  140  and the enclosure  142 . 
         [0073]    To complete the active matching circuit, a dual directional coupler  144  along with two power sensors  146  can be incorporated. The dual directional coupler  144  and the power sensors  146  provide the ability to sense forward and reflected power between the RFID transceiver  148  and the active matching circuit  130  and enclosure  142 . Continuous monitoring of the ratio of forward and reflected power by a comparator  150  provides a metric to use to adjust the tunable capacitor  134  to keep the probe  140  impedance matched to the enclosure  142 . An ability to continuously monitor and improve the impedance match as the contents of the enclosure are varied is provided with the active matching circuit  130 . 
         [0074]    Referring now to the side cross-sectional view of  FIG. 13 , two ceiling-mounted  160  probe antennae  162  and  164  are shown mounted within an enclosure, which may also be referred to herein as a cavity  166 , which in this embodiment, operates as a Faraday cage. As shown, the Faraday cage  166  comprises walls (one of which is shown)  168 , a back  170 , a floor  172 , a ceiling  160 , and a front  161  (only the position of the front wall is shown). All surfaces forming the cavity are electrically conductive, are electrically connected with one another, and are structurally formed to be able to conduct the frequency of energy f f  injected by the two probes  162  and  164 . In this embodiment, the cavity  166  is constructed as a metal frame  167  that may form a part of a medical supply cabinet similar to that shown in  FIG. 2 . Into that metal frame may be mounted a slidable drawer. The slidable drawer in this embodiment is formed of electrically inert material, that is, it is not electrically conductive, except for the front. When the drawer is slid into the cabinet to a closed configuration, the electrically conductive front panel of the drawer comes into electrical contact with another part or parts of the metallic frame  167  thereby forming the front wall  161  of the Faraday cage  167 . 
         [0075]    The amount of penetration or retention into the cavity by the central conductor  180  of each probe is selected so as to achieve optimum coupling. The length of the bent portion  94  of the probe is selected to result in better impedance matching. The position of the probe in relation to the walls of the cavity is selected to create a standing wave in the cavity. In this embodiment, the probe antennae  162  and  164  have been located at a particular distance D 1  and D 3  from respective front  161  and back  170  walls. These probe antennae, in accordance with one aspect of the invention, are only activated sequentially after the other probe has become inactivated. It has been found that this configuration results in a standing wave where the injected energy waves are in phase so that constructive interference results. 
         [0076]      FIG. 14  is a front perspective view of the probe configuration of  FIG. 13  again showing the two probe antennae  162  and  164  located in a Faraday-type enclosure  166  for establishing a robust EM field in an article storage drawer to be inserted. It should be noted again that the Faraday cavity  166  is constructed as a metallic frame  167 . In this figure, the cavity is incomplete in that the front surface of the “cage” is missing. In one embodiment, this front surface is provided by an electrically conductive front panel of a slidable drawer. When the drawer is slid into the cabinet, the front panel will make electrical contact with the other portions of the metallic frame  167  thereby completing the Faraday cage  166 , although other portions of the drawer are plastic or are otherwise non-electrically conductive. In the embodiment discussed and shown herein, the two probe antennae  162  and  164  are both located along a centerline between the side walls  166  and  168  of the frame  166 . The enclosure in one embodiment was 19.2 inches wide with the probe antennae spaced 9.6 inches from each side wall. This centered location between the two side walls was for convenience in the case of one embodiment. The probes may be placed elsewhere in another embodiment. In this embodiment, the spacing of the probes  162  and  164  from each other is of little significance since they are sequentially activated. Although not shown, two receiving antennae will also be placed into the Faraday cage  166  to receive response signals from the activated RFID tags residing within the cavity  166 . 
         [0077]    It will also be noted from reference to the figures that the probes each have a bent portion used for capacitive coupling with the ceiling  160  of the cavity, as is shown in  FIG. 13 . The front probe  162  is bent forward while the back probe  164  is bent rearward A purpose for this configuration was to obtain more spatial diversity and obtain better coverage by the EM field established in the drawer. Other arrangements may be possible to achieve a robust field within the cavity  166 . Additionally two probes were used in the particular enclosure  166  so that better EM field coverage of the enclosure  166  would result. 
         [0078]      FIG. 15  is a cutaway perspective side view of the dual probe antennae  162  and  164  of  FIGS. 13 and 14 , also with the drawer removed for clarity. The front probe  162  is spaced from the left side wall by ½λ of the operating frequency F f  as shown. It will be noted that the probes each have a bent portion used for capacitive coupling with the ceiling  160  of the enclosure  166  as shown in  FIG. 13 . The front probe  162  is bent forward for coupling with the more forward portion of the enclosure while the back probe  164  is bent rearward for coupling with the more rearward portion of the enclosure  166  to obtain more spatial diversity and obtain better coverage by the EM field in the drawer. Other arrangements may be possible to achieve a robust field and further spatial diversity and coverage within the enclosure. 
         [0079]      FIG. 16  is a frontal upward-looking perspective view of the frame  167  forming a Faraday cage  166  showing a portion of a drawer  180  that has been slidably mounted within the frame  167 . The front metallic panel of the drawer has been removed so that its sliding operation can be more clearly seen. It will also be noted that the dual ceiling mount probe antennae  162  and  164  have been covered and protected by an electromagnetically inert protective cover  182 . The drawer is formed of a non-metallic material, such as a plastic or other electromagnetic inert material having a low RF constant. The back  184  of the drawer has also been cut away so that a cooling system comprising coils  186  and a fan  188  located in the back of the frame  167  can be seen. In this case, the drawer  180  is slidably mounted to the Faraday cage frame with metallic sliding hardware  190 . The sliding hardware of the drawer is so near the side of the frame  167  of the enclosure  166  and may be in electrical contact with the metallic slide hardware of the side walls  168  of the enclosure that these metallic rails will have only a small effect on the EM field established within the enclosure. 
         [0080]      FIG. 17  is an upward looking, frontal perspective view at the opposite angle from that of  FIG. 16 ; however, the drawer has been removed. The frame  167  in this embodiment includes a mounting rail  192  for receiving the slide of the drawer  180 . In this embodiment, the mounting rail is formed of a metallic material; however, it is firmly attached to a side  168  of the Faraday cage and thus is in electrical continuity with the cage. The figure also shows a spring mechanism  194  used to assist in sliding the drawer outward so that access to the articles stored in the drawer may be gained. The spring is configured to automatically push the drawer outward when the drawer&#39;s latch is released. 
         [0081]      FIG. 18  is a schematic view showing measurements of the placement of two TE 01  mode capacitive coupling probes  162  and  164  in the ceiling  160  of the frame  167  shown in  FIGS. 13-15 . In this embodiment, the frequency of operation with the RFID tags is 915 MHz, which therefore has a wavelength of 0.32764 meters or 1.07494 feet. One-half wavelength is therefore 0.16382 meters or 6.4495 inches. The length of the capacitive coupling bent portion  200  of each of the probes is 5.08 cm or 2.00 in. The length of the axial extension  202  of the probes into the enclosure is 3.81 cm or 1.50 in., as measured from the insulator  204  into the enclosure  166 . The probe configuration and placement in the embodiment was based on an operation frequency of 915 MHz. In one embodiment, the enclosure  166  had a depth of 16.1 inches (40.89 cm), a width of 19.2 inches (48.77 cm) and a height of 3 inches (7.62 cm). It was found that the optimum probe placements for this size and shape (rectangular) enclosure and for the 915 MHz operating frequency were: the front probe was spaced from the front wall by 5.0 inches (12.7 cm) and the rear probe was spaced from the back wall by 5.0 inches (12.7 cm). As discuss above, the probes in this embodiment would only be activated sequentially. 
         [0082]      FIG. 19  is a schematic view of the size and placement within the enclosure  166  of  FIG. 16  of two microstrip or “patch” antennae  210  and  212  and their microstrip conductors  214  and  216  disposed between the respective antennae and the back of the enclosure at which they will be connected to SMA connectors (not shown) in one embodiment. Feed lines  58  ( FIG. 3 ) may be connected to those SMA connectors and routed to the computer  44  for use in communicating the RFID signals for further processing. The measurements of the spacing of some of the microstrip components are provided in inches. The spacing of 9.7 in. is equivalent to 24.64 cm. The width of the microstrip line of 0.67 in. is equivalent to 17.0 mm. The spacing of 1.4 in. is equivalent to 3.56 cm. Other configurations and types of receiving antennae may be used, as well as different numbers of such antennae. In the present embodiment, the receiving antennae are mounted on insulation at the bottom inside surface of the metallic enclosure frame  167  so that the receiving patch antennae are not in contact with the metal surfaces of the Faraday cage. 
         [0083]    Referring now to  FIG. 20 , the field intensity or field strength in the enclosure discussed above is shown with the ordinate axis shown in volts/meter and the abscissa axis shown in meters. It will be seen from the diagram that the maximum field intensity occurs at about 5.0 inches (0.127 m) which results from the probe positioned at 5.0 inches (12.7 cm) from the front wall and at a 915 MHz operating frequency. Referring now to  FIG. 21 , the scale has been reduced although the large rise in field intensity can be seen at 5.0 inches. It can also be more clearly seen that the field intensity falls off at the right wall but remains strong very close to the left wall. Therefore in an embodiment, a second probe was used that was placed 5.0 inches (12.7 cm) from the right wall thereby resulting in a mirror image field intensity to that shown in  FIG. 21 . The two probes  162  and  164  are activated sequentially and are not both activated simultaneously. It will be noted that better EM field coverage of the enclosure  166  is obtained with the two probes and that RFID tags on articles positioned close to the front wall  161  will be activated by the front probe  162  and that RFID tags on articles positioned close to the rear wall  170  will be activated by the rear probe  164  (see  FIG. 13 ). 
         [0084]    Although not intending to be bound by theory, in deriving the probe location for TE modes in a square or rectangular non-resonant cavity, the following equation can be useful: 
         [0000]    
       
         
           
             N 
             = 
             
               2 
               × 
               
                 
                   
                     L 
                     2 
                   
                   - 
                   
                     L 
                     1 
                   
                 
                 
                   λ 
                   g 
                 
               
             
           
         
       
     
         [0085]    where:
       N=positive non-zero integer, for example 1, 2, 3, etc.   L 1 =distance between probe and back wall   L 2 =distance between probe and front wall   λ g =wavelength in the cavity       
 
         [0090]    L 1  cannot be zero for TE modes, which implies that the probe for TE mode excitation cannot be at the front or back wall. For TM modes, the equation is the same, but N can equal zero as well as other positive integers. The probe position cannot be λ g /2 from the front or back wall. An L 1  and an L 2  are chosen such that N can be a positive integer that satisfies the equation. For example, for the enclosure  166  discussed above: 
         [0091]    L 1 =4.785 inches 
         [0092]    L 2 =11.225 inches 
         [0093]    λ g =12.83 inches 
       Therefore, 
       [0094]    
       
         
           
             N 
             = 
             
               
                 2 
                 × 
                 
                   
                     11.215 
                     - 
                     4.785` 
                   
                   12.83 
                 
               
               = 
               1.0 
             
           
         
       
     
         [0095]    The actual enclosure had the probe located at a slightly different location (5.0 inches) than that indicated by the equation (4.785 inches) which was possibly due to the insertion of a plastic drawer in the cavity, which introduces a change in the phase from the reflected signals. The equation above is set up such that the reflected phase from both front and back walls is equal, i.e., they are “in phase” at the probe location. 
         [0096]    The wavelength in the enclosure, λ g , can be calculated using waveguide equations. Equations for a rectangular cavity are shown below. The cutoff frequency is required for this calculation. The equations will change for a cylindrical cavity or for other shapes. 
         [0097]    The cutoff frequency is at the point where g vanishes. Therefore, the cutoff frequency in Hertz is: 
         [0000]    
       
         
           
             
               
                 ( 
                 
                   f 
                   c 
                 
                 ) 
               
               mn 
             
             = 
             
               
                 1 
                 
                   2 
                    
                   π 
                    
                   
                     μɛ 
                   
                 
               
                
               
                 
                   
                     
                       ( 
                       
                         
                           m 
                            
                           
                               
                           
                            
                           π 
                         
                         a 
                       
                       ) 
                     
                     2 
                   
                   + 
                   
                     
                       ( 
                       
                         
                           n 
                            
                           
                               
                           
                            
                           π 
                         
                         b 
                       
                       ) 
                     
                     2 
                   
                 
               
                
               
                 ( 
                 Hz 
                 ) 
               
             
           
         
       
     
         [0098]    The cutoff wavelength in meters is: 
         [0000]    
       
         
           
             
               
                 ( 
                 
                   λ 
                   c 
                 
                 ) 
               
               mn 
             
             = 
             
               
                 2 
                 
                   
                     
                       
                         ( 
                         
                           
                             m 
                              
                             
                                 
                             
                           
                           a 
                         
                         ) 
                       
                       2 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             n 
                              
                             
                                 
                             
                           
                           b 
                         
                         ) 
                       
                       2 
                     
                   
                 
               
                
               
                 ( 
                 m 
                 ) 
               
             
           
         
       
     
         [0099]    where:
       a=inside width   b=inside height   m=number of ½-wavelength variations of fields in the “a” direction   n=number of ½-wavelength variations of fields in the “b” direction   ξ=permittivity   μ=permeability       
 
         [0106]    The mode with the lowest cutoff frequency is called the dominant mode. Since TE 10  mode is the minimum possible mode that gives nonzero field expressions for rectangular waveguides, it is the dominant mode of a rectangular waveguide with a&gt;b and so the dominant frequency is: 
         [0000]    
       
         
           
             
               
                 ( 
                 
                   f 
                   c 
                 
                 ) 
               
               10 
             
             = 
             
               
                 1 
                 
                   2 
                    
                   a 
                    
                   
                     μɛ 
                   
                 
               
                
               
                 ( 
                 Hz 
                 ) 
               
             
           
         
       
     
         [0107]    The wave impedance is defined as the ratio of the transverse electric and magnetic fields. Therefore, impedance is: 
         [0000]    
       
         
           
             
               Z 
               TE 
             
             = 
             
               
                 
                   E 
                   x 
                 
                 
                   H 
                   y 
                 
               
               = 
               
                 
                   
                     j 
                      
                     
                         
                     
                      
                     w 
                      
                     
                         
                     
                      
                     μ 
                   
                   γ 
                 
                 = 
                 
                   
                     
                       
                         j 
                          
                         
                             
                         
                          
                         w 
                          
                         
                             
                         
                          
                         μ 
                       
                       jβ 
                     
                     ⇒ 
                     
                       Z 
                       TE 
                     
                   
                   = 
                   
                     
                       k 
                        
                       
                           
                       
                        
                       η 
                     
                     β 
                   
                 
               
             
           
         
       
     
         [0108]    The guide wavelength is defined as the distance between two equal phase planes along the waveguide and it is equal to: 
         [0000]    
       
         
           
             
               λ 
               g 
             
             = 
             
               
                 
                   
                     2 
                      
                     π 
                   
                   β 
                 
                 &gt; 
                 
                   
                     2 
                      
                     π 
                   
                   k 
                 
               
               = 
               λ 
             
           
         
       
       
         
           
             
               
                 where 
                  
                 
                     
                 
                  
                 
                   k 
                   c 
                 
               
               = 
               
                 
                   
                     
                       ( 
                       
                         
                           
                             m 
                              
                             
                                 
                             
                              
                             π 
                           
                            
                           
                               
                           
                         
                         a 
                       
                       ) 
                     
                     2 
                   
                   + 
                   
                     
                       ( 
                       
                         
                           
                             n 
                              
                             
                                 
                             
                              
                             π 
                           
                            
                           
                               
                           
                         
                         b 
                       
                       ) 
                     
                     2 
                   
                 
               
             
             ; 
             and 
           
         
       
       
         
           
             β 
             = 
             
               
                 
                   k 
                   2 
                 
                 - 
                 
                   k 
                   c 
                   2 
                 
               
             
           
         
       
     
         [0109]      FIGS. 22A and 22B  together provide a block electrical and signal diagram for a multiple-drawer medical cabinet, such as that shown in  FIG. 2 . In this case, the cabinet has eight drawers  220 , shown in both  FIGS. 22A and 22B . Each drawer includes two top antennae, two bottom antennae and a lock with a lock sensor  222  for securing the drawer. Signals to and from the antennae of each drawer are fed through an RF multiplexer switch  224 . Each RF multiplexer switch  224  in this embodiment handles the routing of RF signals for two drawers. RFID activation field and RFID received signals are fed through the respective RF multiplexer switch  224  to a main RFID scanner  230  (see  FIG. 22B ). The scanner  230  output is directed to a microprocessor  232  (see  FIG. 22B ) for use in communicating relevant information to remote locations, in this case by wired connection  234  and wireless connection  236  (see  FIG. 22B ). Various support systems are also shown in  FIGS. 22A and 22B , such as power connections, power distribution, back up battery (see  FIG. 22B ), interconnection PCBA, USB support (see  FIG. 22A ), cooling (see  FIG. 22B ), and others. 
         [0110]    In accordance with one embodiment, drawers are sequentially monitored. Within each drawer, the antennae are sequentially activated by the associated multiplexer  224 . Other embodiments for the signal and electrical control systems are possible. 
         [0111]    Although RFID tags are used herein as an embodiment, other data carriers that communicate through electromagnetic energy may also be usable. 
         [0112]    Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense, which is as “including, but not limited to.” 
         [0113]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments and elements, but, to the contrary, is intended to cover various modifications, combinations of features, equivalent arrangements, and equivalent elements included within the spirit and scope of the appended claims.