Abstract:
An antenna for a Radio-Frequency IDentification (RFID) system is disclosed that comprises a resonant structure, an RFID load element, and a floating coupling element. One of the two terminals of the RFID load element is connected directly to the resonant structure, and the other terminal is connected to the floating coupling element. The floating coupling element is electrically isolated from the resonant structure; its presence provides an improved impedance match to the RFID load element.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This case claims priority of the following provisional applications: 
         [0002]    (1) U.S. provisional application No. 61/207,467; and 
         [0003]    (2) U.S. provisional application No. 61/273,814. 
         [0004]    This case is a Continuation-in-Part and claims priority of co-pending U.S. Ser. No. 12/621,451 titled “Multiple-Cavity Antenna” and filed on Nov. 18, 2009. 
         [0005]    This case claims priority to PCT case titled “Multiple-Cavity Antenna”, which was submitted to the USPTO/Mail Stop PCT on Feb. 15, 2010 via FedEx Airbill 8682 2381 6624. 
     
    
     FIELD OF THE INVENTION 
       [0006]    The present invention relates to antenna design for radio communication in general, and, more particularly, to antenna design for Radio-Frequency IDentification (RFID) systems. 
       BACKGROUND OF THE INVENTION 
       [0007]    Radio communication systems have existed for over a century. During this period of time, antenna designers have generated a wide variety of antenna designs with the goal of achieving good performance in a variety of operating conditions. 
         [0008]    Generally, the goal of the antenna designer when designing, for example, a receiving antenna, is to maximize power transfer between an electromagnetic signal incident on the antenna, and the resulting electrical signal generated by the antenna. The higher the power transfer, the higher the received signal-to-noise ratio, which usually results in better receiver performance. 
         [0009]    Also, traditionally, radio receivers have comprised electronic circuitry and a separate receiving antenna interconnected to one another through a suitable cable connection. In such systems, antenna designers must consider the distorting influence of the cable connection and the electronic circuitry on the electromagnetic behavior of the antenna. 
         [0010]    More recently, with the advent of small radio systems based on integrated circuit technology, it has become possible to make so-called Radio-Frequency IDentification (RFID) systems, wherein an entire radio receiver is housed in a package much smaller than the receiving antenna. In such systems, the almost-complete elimination of the distorting influence of the cable connection and the electronic circuitry enables novel antenna designs. 
         [0011]    So-called passive RFID receivers can be much smaller than the receiving antenna in part because they do not require a power supply. Power to operate the receiver is derived from the received radio signal itself. The signal generated by the receiving antenna is rectified by one or more diodes to yield a direct-current (DC) voltage that is used to power the receiver. 
         [0012]    Ideal diodes are perfect conductors when a forward voltage is applied and are perfect insulators when a reverse voltage is applied. Real diodes only approximate this behavior. In particular, real diodes require a minimum forward voltage before becoming good conductors. Accordingly, the signal generated by the receiving antenna, must have a voltage higher than the minimum required by the diodes, before a DC voltage becomes available to power the RFID receiver. 
         [0013]    So, in contrast with traditional antenna design, the goal for the design of passive-RFID-receiver antennas is to maximize not the received-signal power, but rather the received-signal voltage. 
         [0014]    It is well known in the art that antennas are reciprocal devices, meaning that an antenna that is used as a transmitting antenna can also be used as a receiving antenna, and vice versa. Furthermore, there is a one-to-one correspondence between the behavior of an antenna used as a receiving antenna and the behavior of the same antenna used as a transmitting antenna. This property of antennas is known in the art as “reciprocity.” 
         [0015]    An antenna used as a transmitting antenna accepts an electrical signal applied at an input port and produces a transmitted electromagnetic signal that propagates through three-dimensional space. It is well known in the art how to represent such a transmitted electromagnetic signal as a vector in a vector space, for example, as a superposition of spherical harmonics. The behavior of a transmitting antenna at a given frequency can be fully characterized by reporting, for example, the spherical-harmonic components of the transmitted electromagnetic signal that it generates in response to a test electrical signal at that frequency that is applied to the antenna&#39;s input port. 
         [0016]    Such a characterization can be used to derive, unambiguously, the behavior of the same antenna when it is used as a receiving antenna. In this case, the input port becomes an output port that generates an output electrical signal in response to an incident electromagnetic signal propagating through three-dimensional space. The incident electromagnetic signal can be specified by, for example, by specifying its spherical-harmonic components. The resulting electrical signal can then be derived through a scalar product with the spherical-harmonic components of the transmitted electromagnetic signal at the same frequency, as is well known in the art. 
         [0017]    A consequence of reciprocity is that an antenna can be fully characterized in terms of its properties as either a transmitting antenna or as a receiving antenna. A full characterization of an antenna when used in one mode (transmitting or receiving) uniquely and unambiguously defines the properties of the antenna when used in the other mode. 
         [0018]    For example, in order to understand or measure the radiation pattern of an antenna it is frequently easier to feed an electric signal into the antenna and then observe the electromagnetic field generated by the antenna. This task can be performed experimentally or computationally. The radiation pattern of the antenna that is obtained through this method also applies when the antenna is used as a receiving antenna. Hereinafter, antennas will be interchangeably referred to as receiving or transmitting, and their properties will be discussed as they apply to either transmission or reception, as convenient to achieve clarity. It will be clear to those skilled in the art how to apply what is said about an antenna used in one mode (receiving or transmitting) to the same antenna used in the other mode. 
         [0019]      FIG. 1  depicts monopole antenna  100  in accordance with the prior art. Monopole antenna  100  comprises monopole  110 , ground plane  120  and co-axial cable connection  130 . Monopole antenna  100  is a very common type of antenna and is representative of how many antennas operate. When an electrical signal is applied to co-axial cable connection  130 , an electric field appears between monopole  110  and ground plane  120 . If the electrical signal has a frequency at or near the so-called “resonant” frequency of the antenna, a large fraction of the power of the electrical signal is converted into an electromagnetic signal that is radiated by the antenna. If the electrical signal has a frequency that is substantially different from the resonant frequency of the antenna, a relatively small fraction of the signal&#39;s power is radiated; most of the power is reflected back into the co-axial cable connection. 
         [0020]    In principle, it is possible to make an antenna that radiates efficiently at many frequencies, without exhibiting a band of resonance. In practice, it is difficult to make such antennas, and resonant structures (hereinafter also referred to as “resonators”) are commonly used to make antennas that radiate efficiently. 
         [0021]      FIG. 2  depicts resonant structure  200 , which is an example of a type of resonant structure commonly used to make antennas in the prior art. Resonant structure  200  comprises a length of wire  240  bent in the shape of the letter U, with an input-output port  220  comprising connection points  230 - 1  and  230 - 2 . As depicted in  FIG. 2 , the two connection points are attached to the two ends of the wire. 
         [0022]    The frequency of resonance of resonant structure  200  depends on its length. The structure can be modeled as a twin-lead transmission line  210  with a short at one end (i.e., the end opposite input-output port  220 ). The structure is resonant at a frequency for which the length of the transmission line is about one quarter of a wavelength. The range of frequencies near the resonant frequency over which the resonant structure exhibits acceptably good performance is known as the “band of resonance.” 
         [0023]    Resonant structure  200  exhibits resonance in a manner similar to monopole antenna  100 . Near the resonant frequency, the electromagnetic fields generated by the voltages and currents on wire  240  become stronger, and a larger fraction of the power of an electrical signal applied to input-output port  220  is radiated as an electromagnetic signal. Accordingly, resonant structures that exhibit this behavior are referred to as “electromagnetically-resonant.” 
         [0024]      FIG. 3  depicts folded-dipole antenna  300 , which is an example of a common type of antenna in the prior art. Folded-dipole antenna  300  can be modeled as being composed of two instances of resonant structure  200  connected in series. When used as a transmitting antenna, an electrical signal is applied through balanced transmission line  320 . 
         [0025]    Although folded-dipole antenna  300  can be modeled as being composed of two instances of resonant structure  200  connected in series, the signal that it generates when used as a receiving antenna is not the sum of the signals that each instance of resonant structure  200  would generate if used by itself because of the mutual coupling between the two instances of resonant structure  200 . 
         [0026]      FIG. 4  depicts antenna-with-load-element  400 , which is an example of a type of antenna in the prior art for RFID systems known as RFID tags. Antenna-with-load-element  400  comprises: conductive sheets  410 - 1 , and  410 - 2 , electrical connection  420 , connection points  440 - 1  and  440 - 2 , and load element  430 , interrelated as shown. 
         [0027]    Conductive sheets  410 - 1  and  410 - 2 , together with electrical connection  420 , form resonant structure  450 . Load element  430  receives the signal generated by resonant structure  450  through connection points  440 - 1  and  440 - 2 . When used to implement an RFID tag, load element  430  is small relatively to the size of conductive sheets  410 - 1  and  410 - 2 . 
         [0028]    To implement an RFID tag, load element  430  acts as both a receiver and a transmitter. In particular, in a passive RFID tag, transmission is accomplished through a technique known as “modulated backscatter” wherein load element  430  controls the impedance that it presents to the received signal. Modulated backscatter is based on the fact that, in any radio receiver, a portion of the electromagnetic signal incident on the receiving antenna is reflected. The amplitude and phase of the reflected signal depend on the impedance connected to the antenna port, so that load element  430  modulates the reflected signal by controlling its own impedance. 
         [0029]    The impedance of an RFID load element depends on the design and implementation of the device and, typically, it is non-linear, meaning that is varies as a function of the amplitude of the applied signal. As a consequence, the goal of maximizing received-signal voltage is difficult to achieve. There is a need for methods to couple an antenna to an RFID load element that achieve the desired impedance match. 
       SUMMARY OF THE INVENTION 
       [0030]    Embodiments of the present invention comprise a resonant structure, an RFID load element, and a floating coupling element. One of the two terminals of the RFID load element is connected directly to the resonant structure, and the other terminal is connected to the floating coupling element. The resonant structure can be realized, for example, as a resonant cavity, as is well known in the art. The floating coupling element is electrically isolated from the resonant structure, and its size, shape and position, relative to the resonant structure, are adjusted so as to achieve the desired impedance match. In particular, the electromagnetic field that forms in the volume of space between the floating coupling element and the resonant structure, and around the floating coupling element, provides the desired coupling between the load element and the resonant structure. The volume of space between the floating coupling element and the resonant structure can be regarded a second cavity, the resonant structure being the first cavity. The size, shape and other physical and electrical parameters of this second cavity determine the shape and behavior of the electromagnetic field and the coupling that it provides. The advantage provided by the floating coupling element derives from the availability of these parameters, that the antenna designer can adjust to achieve a particular coupling. The impedance match between the load element and the resonant structure depends on the coupling, and the flexibility provided by the floating coupling element makes it easier to achieve a good impedance match. Hereinafter we will refer to the floating coupling element as “floating element” with the understanding that its purpose is to provide coupling between the load element and the resonant structure. 
         [0031]    In the prior art, both terminals of the load element are usually directly connected to one or more resonant structures. In some prior-art implementations, one or both terminals of the load element are not connected. Embodiments of the present invention achieve a better impedance match through the use of the floating coupling element. The ability to vary the shape, size and position of the floating coupling element provide antenna designers with additional parameters that they can adjust, through simulation or prototyping, to achieve the desired impedance match. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  depicts a monopole antenna in the prior art. 
           [0033]      FIG. 2  depicts a resonant structure in the prior art. 
           [0034]      FIG. 3  depicts a folded-dipole antenna in the prior art. 
           [0035]      FIG. 4  depicts an example of a type of antenna in the prior art for RFID tags. 
           [0036]      FIG. 5  depicts a single-cavity antenna with floating element in accordance with a first illustrative embodiment of the present invention. 
           [0037]      FIG. 6  depicts a single-cavity antenna with floating element and dielectric in accordance with a second illustrative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]      FIG. 5  depicts single-cavity antenna with floating element  500  in accordance with a first illustrative embodiment of the present invention. Single-cavity antenna with floating element  500  comprises: conductive sheets  510 - 1 , and  510 - 2 , electrical connection  520 , connection points  540 - 1  and  540 - 2 , load element  530 , and floating element  570 , positioned and interrelated as shown. In particular, floating element  570  is a flat piece of conductive material parallel to, and at distance  560  from, conductive sheet  510 - 1 ; conductive sheets  510 - 1  and  510 - 2 , together with electrical connection  520 , form resonant structure  550 ; and load element  530  is electrically connected between resonant structure  550  and floating element  570  through connection points  540 - 1  and  540 - 2 . 
         [0039]    Conductive sheets  510 - 1  and  510 - 2 , and electrical connection  520  are identical to conductive sheets  410 - 1  and  410 - 2 , and electrical connection  420  of  FIG. 4 . They form resonant structure  550 , which is identical to resonant structure  450 . But load element  530  can be different from load element  430  because it does not need to have the same impedance. 
         [0040]    The purpose of floating element  570  is to couple connection point  540 - 2  to resonant structure  550  without the need for a direct electrical connection. Floating element  570  is electrically isolated from conductive sheet  510 - 1 . Coupling between floating element  570  and conductive sheet  510 - 1  occurs through electro-magnetic fields that develop between floating element  570  and conductive sheet  510 - 1  when the antenna is used to receive a radio signal. 
         [0041]    The size of floating element  570 , and its distance from conductive sheet  510 - 1 , are not negligible, compared to the size parameters of resonant structure  550 . Examples of such size parameters are: the lengths and widths of conductive sheets  510 - 1  and  510 - 2 , the distance between the two sheets, the relative position of one sheet with respect to the other. Because of its non-negligible size and distance from sheet  510 - 1 , the impedance that is coupled to load element  530  is different from the impedance that is coupled to load element  430  in the prior art. The precise value of the impedance can be adjusted by varying the size and shape of floating element  570 , and by varying its position relative to conductive sheet  510 - 1 . The exact values that achieve a particular impedance that is desirable in a particular implementation can be derived through techniques well known in the art such as simulation or prototyping. 
         [0042]    Although the shape of floating element  570  is depicted as a rectangle in  FIG. 5 , it will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention wherein the shape is different. In particular, the shape of floating element  570  affects the impedance coupled to load element  630 , and it is one of the parameters that can be varied for the purpose of achieving a desired impedance. For example, and without limitation, the shape of floating element  570 , can be a regular or irregular polygon, a circle or ellipse, a serpentine shape, a multi-pointed star. 
         [0043]    Although floating element  570  is depicted as flat, it will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention wherein floating element  570  is not flat. For example, and without limitation: floating element  570  can be a piece of conductive material with non-negligible thickness, and its thickness can be an additional parameter that can be adjusted to achieve a desired impedance; floating element  570  can be shaped as a dome, or as a more complex three-dimensional structure; floating element  570  can be realized as one or more wires arranged in a three-dimensional shape, wherein the diameter of the wires can be an additional parameter that can be adjusted to achieve a desired impedance. 
         [0044]    Although floating element  570  is depicted as parallel to conductive sheet  510 - 1 , it will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention wherein floating element  570  is not parallel to conductive sheet  510 - 1 . In particular, the exact angle and orientation of floating element  570  relative to conductive sheet  510 - 1  affect the impedance coupled to load element  530 , and are additional parameters that can be varied for the purpose of achieving a desired impedance. 
         [0045]    Although floating element  570  is depicted as floating unsupported in mid air relative to sheet  510 - 1 , it will be clear to those skilled in the art how to support floating element  570 . For example, and without limitation, non-conductive supporting devices such as plastic or teflon screws, or spacers; or glue can be used to support floating element  570 . Alternatively, it is possible to make load element  530  with sufficient mechanical strength and rigidity such that the connection to load element  530  through connection point  540 - 2  is sufficient to support floating element  570  in the desired position. One alternative method to support floating element  570  is presented below as part of a second illustrative embodiment of the present invention. Other methods to support floating element  570  will be clear to those skilled in the art. 
         [0046]    Although floating element  570  is depicted as being at a distance  560 , from conductive sheet  510 - 1 , that is less than the distance between sheet  510 - 1  and sheet  510 - 2 , it will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention wherein floating element  570  is at a different distance. For example, and without limitation, floating element  570  can be in the same plane as sheet  510 - 2 , so that distance  560  is the same as the distance between sheet  510 - 1  and sheet  510 - 2 ; or distance  560  can be larger than the distance between sheet  510 - 1  and sheet  510 - 2 . 
         [0047]      FIG. 6  depicts single-cavity antenna with floating element and dielectric  600  in accordance with a second illustrative embodiment of the present invention. Single-cavity antenna with floating element and dielectric  600  comprises: conductive sheets  610 - 1 , and  610 - 2 , electrical connection  620 , connection points  640 - 1  and  640 - 2 , load element  630 , floating element  670 , positioned and interrelated as shown. 
         [0048]    Conductive sheets  610 - 1  and  610 - 2 , and electrical connection  620  are identical to conductive sheets  510 - 1  and  510 - 2 , and electrical connection  520  of  FIG. 5 . They form resonant structure  650 , which is identical to resonant structure  550 . Load element  630 , connection points  640 - 1  and  640 - 2 , and floating element  670  are identical, respectively, to load element  530 , connection points  540 - 1  and  540 - 2 , and floating element  570 . The salient difference between this second embodiment and the first embodiment depicted in  FIG. 5  is the presence of dielectric  670  between floating element  670  and conductive sheet  610 - 1 . 
         [0049]    Dielectric  610  is made of dielectric material whose dielectric properties provide additional parameters that can be varied for the purpose of achieving a desired impedance. Also, dielectric  610  can be made sufficiently strong to provide mechanical support for floating element  670 . 
         [0050]    Although dielectric  610  is depicted as having the shape of a parallelepiped whose size and shape match the size and shape of floating element  670 , it will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention wherein dielectric  610  has other sizes and shapes. For example, and without limitation:
       i. dielectric  610  can occupy only part of the space between floating element  670  and conductive sheet  610 - 1 ;   ii. dielectric  610  can extend beyond the outline of floating element  670  over portions of or over the entirety of the perimeter of floating element  670 ;   iii. dielectric  610  can comprise different regions made from different dielectric materials;   iv. dielectric  610  can be part of a printed-circuit board, resonant structure  650  can be realized as a patch antenna, and floating element  670  can be realized as a patch of conductive material; or   v. a combination of i, ii, iii, or iv.       
 
         [0056]    Although  FIG. 5  and  FIG. 6  depict embodiments of the present invention comprising a single-cavity resonant structure, it will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention that comprise multiple resonant structures or multiple resonant cavities. Also, although the resonant cavities depicted in  FIG. 5  and  FIG. 6  do not comprise a dielectric, it will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention with resonant cavities that comprise a dielectric. For example, and without limitation, dielectric  670  can be realized as a single block of dielectric material that extends beyond the outline of floating element  670  and into the space between conductive sheets  610 - 1  and  610 - 2 . 
         [0057]    Although this disclosure sets forth embodiments of the present invention as applicable for implementing RFID systems, it will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention that are applicable to other types of radio-communication systems. For example, and without limitation, a radio receiver or transmitter characterized by a high input or output impedance can advantageously utilize an antenna with a floating coupling element in accordance with an embodiment of the present invention. 
         [0058]    It is to be understood that this disclosure teaches just one or more examples of one or more illustrative embodiments, and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure, and that the scope of the present invention is to be determined by the following claims.