Abstract:
A Radio-Frequency IDentification (RFID) receiver is disclosed that comprises a plurality of resonant structures arranged to form an antenna. The resonant structures are interconnected in series and are arranged, relative to one another, so as to achieve a received electrical signal with an increased voltage, when the antenna is exposed to an incident electromagnetic signal. This occurs for a majority of all possible incident electromagnetic signals and, therefore, an RFID receiver based on such an antenna provides, in a majority of cases, an improved performance.

Description:
FIELD OF THE INVENTION 
     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 
     Radio communication systems have existed for over a century. During this period of time, antenna designers have generated a wide variety of designs with the goal of achieving good performance in a variety of operating conditions. 
     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. 
     Also, traditionally, radio receivers have comprised an electronic assembly 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 assembly on the electromagnetic behavior of the antenna. 
     More recently, with the advent of small radio systems based in 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 absence of the distorting influence of the cable connection and the electronic assembly enables novel antenna designs. 
     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. 
     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 larger than the minimum required by the diodes, before a DC voltage becomes available to power the RFID receiver. 
     So, in contrast with traditional antenna design, the goal for RFID-receiver antennas is to maximize not the received-signal power, but rather the received-signal voltage. 
     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.” 
     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. 
     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. 
     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. 
     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. 
       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 form 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. 
     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. 
       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. 
     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 one quarter of a wavelength. The range of frequencies over which the resonant structure exhibits good resonance is known as the “band of resonance.” 
     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.” 
       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 . 
     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 . 
       FIG. 4  depicts antenna with load element  400 , which is an example of a type of antenna in the prior art for 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. 
     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 . 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention comprise a plurality of resonant structures arranged to form an antenna. An antenna in accordance with the present invention comprises multiple resonant structures interconnected in series and arranged, relative to one another, so as to achieve a received electrical signal with an increased voltage. In particular, when exposed to an incident electromagnetic signal, an antenna in accordance with the present invention generates a received electrical signal whose voltage amplitude (hereinafter “amplitude”) is larger than the amplitude of the electrical signals generated by the individual resonant structures comprised by the antenna. This occurs for a majority of all possible incident electromagnetic signals and, therefore, an RFID receiver based on such an antenna provides, in a majority of cases, an improved performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a monopole antenna in the prior art. 
         FIG. 2  depicts a resonant structure in the prior art. 
         FIG. 3  depicts a folded-dipole antenna in the prior art. 
         FIG. 4  depicts an example of a type of antenna in the prior art for RFID tags. 
         FIG. 5  depicts a dual-resonator antenna with a load element in accordance with a first illustrative embodiment of the present invention. 
         FIG. 6  depicts a dual-resonator antenna with a dielectric in accordance with a second illustrative embodiment of the present invention. 
         FIG. 7  depicts a multiple-resonator antenna in accordance with a third illustrative embodiment of the present invention. 
         FIG. 8  depicts a multiple-resonator antenna with a load element in accordance with the third illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 5  depicts dual-resonator antenna with load element  500  in accordance with a first illustrative embodiment of the present invention. Dual-resonator antenna with load element  500  comprises: conductive sheets  510 - 1 ,  510 - 2 , and  510 - 3 , electrical connections  520 - 1  and  520 - 2 , connection points  540 - 1  and  540 - 2 , and load element  530 , interrelated as shown. Conductive sheet  510 - 1  comprises hole  550  through which passes load element  530 . 
     Conductive sheet  510 - 2  is substantially parallel to conductive sheet  510 - 2 . These two sheets, together with electrical connection  520 - 1 , form a first resonant structure similar to resonant structure  450 . Conductive sheet  510 - 3  is substantially parallel to conductive sheet  510 - 2 . These two sheets, together with electrical connection  520 - 1 , form a second resonant structure similar to resonant structure  450 . Conductive sheets  510 - 2  and  510 - 3  are on opposite sides of conductive sheet  510 - 1 , so that the first and second resonant structures share conductive sheet  510 - 1 . Because conductive sheet  510 - 1  is shared between the two resonant structures, it provides an electrical connection between the two structures whereby the two resonant structures are connected in series. 
     Connection point  540 - 1  is on the first resonant structure and connection point  540 - 2  is on the second resonant structure. When the antenna is used as a receiving antenna, the voltage between the two connection points (hereinafter the “output voltage”) results from the two voltages generated by the two resonant structures (hereinafter the “resonant voltages”) in response to an incident electromagnetic signal. Because the two resonant structures are connected in series, the output voltage is the algebraic sum of the resonant voltages. 
     The output voltage as a function of time is the electrical signal, s T , that the antenna generates in response to the incident electromagnetic signal. The two resonant voltages as functions of time are the two signals, s 1  for the first resonant structure and s 2  for the second resonant structure, generated across each structure in response to the incident electromagnetic signal. These signals should be understood to be sinusoidal at a given frequency, the same for all of them. Accordingly, each signal is characterized by an amplitude and a phase. It is well known in the art how to relate a sinusoidal signal to its amplitude and phase; in particular, the amplitude of a signal, s, is max[|s|], where s can be s T , s 1 , or s 2 . 
     In general, there is a phase difference between s 1  and s 2 . The phase difference depends on the spatial characteristics of the incident electromagnetic signal. In particular, the phase difference can be analyzed for the specific case when the incident electromagnetic signal is a polarized plane wave. In such a case, the resulting phase shift can be measured as a function of the direction of arrival and the polarization of the plane wave. Because plane waves are a complete set within the vector space of electromagnetic signals (equivalent to spherical harmonics) this is a complete characterization of the antenna at the frequency of s T , s 1 , and s 2 . 
     The subset of all possible directions of arrival and polarizations for which the amplitude of s T  is larger than the amplitude of s 1  is denoted by A 1 ; i.e., A 1  is the subset for which max[|s T |]&gt;max[|s 1 ]. The subset of all possible directions of arrival and polarizations for which the amplitude of s T  is larger than the amplitude of s 2  is denoted by A 2 ; i.e., A 2  is the subset for which max[|s T |]&gt;max[|s 2 |]. The intersection of these two subsets, A 1 ∩A 2 , corresponds to the subset of all possible incident plane waves for which the phase shift is sufficiently small that the amplitude of s T  is larger than the amplitude of either s 1  or s 2  individually. In such cases, the antenna, when used in an RFID tag, provides improved performance compared to an antenna that comprises only one or the other of the two resonant structures. 
     It is well known in the art how to measure the size of the A 1 ∩A 2  subset. In particular, directions of arrival correspond to points on the surface of a sphere and, therefore, a set of directions can be measured in units of steradians. Polarization states can also be represented as points on a sphere (for example, on the Poincaré sphere) and, therefore, a set of polarizations can also be measured. It is a characteristic of the first illustrative embodiment of  FIG. 5  that the A 1 ∩A 2  subset comprises the majority (i.e., more than one-half) of all possible directions of arrival and polarizations when signals s T , s 1 , and s 2  have a frequency within the resonant bands of both resonant structures. 
     In the first illustrative embodiment, the two resonant structures have resonant bands that overlap over a common portion (hereinafter “common band”). For example, the two resonant structures can be identical in shape and, therefore, have the same resonant band. For electromagnetic signals at frequencies within the common band, the antenna of  FIG. 5  provides a larger voltage, compared to an antenna that comprises only one or the other of the two resonant structures, for a majority of all possible directions of arrival and polarizations. Therefore, an RFID tag based on the antenna of  FIG. 5  has, in most cases, an improved range. 
     Although  FIG. 5  shows the conductive sheets as solid sheets of electrically conductive material, 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 conductive sheets are not solid. For example, and without limitation, each of the conductive sheets can:
         i. be a grid of wires, or a mesh, or   ii. be perforated with holes arranged at random or in a regular pattern, or   iii. be a printed circuit board with one or more interconnection layers,   iv. comprise notches or jagged edges,   v. have an uneven or rough surface with bumps or lumps, or   vi. comprise electronic components, such as, for example, resistors, capacitors or integrated circuits,   vii. comprise mechanical fasteners such as, for example, screws, nuts, or rivets, or   viii. comprise solder joints, welds or other electrical or mechanical joints, or   ix. be an array of parallel wires substantially parallel to the prevailing direction of electrical currents within the sheet.   x. be a combination of i, ii, iii, iv, v, vi, vii, viii, or ix.       

     Although the conductive sheets in  FIG. 5  are depicted as rectangular in shape, 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 conductive sheets have other shapes. It is well known in the art how to make resonant structures with a variety of alternative shapes. Such resonant structures can be combined in a manner equivalent to how the first resonant structure and the second resonant structure are combined in  FIG. 5  to achieve alternative embodiments of the present invention. 
     Although electrical connections  520 - 1  and  520 - 2  are depicted in  FIG. 5  as sections of conductive sheet material bent in semicircular shapes, 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 electrical connections between sheets are realized differently. For example and without limitation, electrical connections  520 - 1  and  520 - 2  can be:
         i. single wires or multiple wires, or   ii. portions of sheet material bend in different shapes, or   iii. single or multiple connections at single or multiple points along the edges of the interconnected sheets, or   iv. solder joints, screws, pins, or other electrically conductive fasteners, or   v. plated-through via holes, or   vi. a combination of i, ii, iii, iv, or v.
 
Furthermore, the electrical connections can extend over larger or smaller sections of one or more edges of the conductive sheets.
       

     Although the first resonant structure and the second resonant structure are connected in  FIG. 5  by virtue of sharing 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 the two resonant structures are electrically connected in other ways. For example and without limitation, the two resonant structures can be connected through:
         i. single wires or multiple wires, or   ii. portions of sheet material, or   iii. solder joints, screws, pins, or other electrically conductive fasteners, or   iv. plated-through via holes, or   v. a combination of i, ii, iii, or iv.       

     Although connection points  540 - 1  and  540 - 2  are depicted in  FIG. 5  as being placed near the center of the sheet edge on which they occur, and load element  530  goes through hole  550  near the center of 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 the connection points are in different places, and load element  530  is connected between sheets  510 - 2  and  510 - 3  in other ways. For example and without limitation, connection points  540 - 1  and  540 - 2  can be near corners of sheets  510 - 2  and  510 - 3 , and load element  530  can pass around the side of sheet  510 - 1 , or through a notch in the edge of sheet  510 - 1 . 
       FIG. 6  depicts dual-resonator antenna with dielectric  600  in accordance with a second illustrative embodiment of the present invention. Dual-resonator antenna with dielectric  600  comprises: conductive sheets  610 - 1 ,  610 - 2 , and  610 - 3 , electrical connections  620 - 1  and  620 - 2 , and dielectric material  660 , interrelated as shown. 
     For the purpose of clarity,  FIG. 6  does not show connection points, a load element or a hole in sheet  610 - 1 . Such elements in the second illustrative embodiment are identical to the corresponding elements in the first illustrative embodiment and should be understood to be present even though they are not depicted in  FIG. 6 . The salient elements of the second illustrative embodiment that differ from the corresponding elements of the first illustrative embodiment are:
         i. dielectric material  660  occupies part of the volumes of the two resonant structures, and   ii. electrical connections  620 - 1  and  620 - 2  are realized as sections of conductive sheet material bent at right angles.       

     Although dielectric material  660  is shown in  FIG. 6  as occupying most of the volume between sheet  610 - 1  and sheets  610 - 2  and  610 - 3 , 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 only a portion of the volume is occupied by dielectric material, or dielectric material extends beyond the volume between the conductive sheets. Furthermore, 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 more than one type of dielectric material is used, or wherein the two resonant structures comprise dielectric materials that are different or have different shapes. 
     Many different dielectric materials are known in the art for making resonant structures. For example, and without limitation, dielectric material  660  can be acetate, ABS (Acrylonitrile Butadiene Styrene) of various densities, polyphenylsulphone, polyethersulfone, polysulfone, PETG (Polyethylene Terephthalate Glycol), polycarbonate, teflon, polystyrene, or polyethylene. 
       FIG. 7  depicts multiple-resonator antenna  700  in accordance with a third illustrative embodiment of the present invention. Multiple-resonator antenna  700  comprises: conductive sheets  710 - 1 ,  710 - 2 ,  710 - 3 ,  710 - 4 , and  710 - 5 , and electrical connections  720 - 1 ,  720 - 2 ,  720 - 3 , and  720 - 4 , interrelated as shown. 
     For the purpose of clarity,  FIG. 7  does not show connection points or a load element. Such elements are present in the third illustrative embodiment and are shown in  FIG. 8 . 
     The salient feature of the third illustrative embodiment is that the antenna comprises four resonant structures. In particular, conductive sheets  710 - 1  and  710 - 2 , together with electrical connection  720 - 1 , form a first resonant structure similar to resonant structure  450 . Conductive sheets  710 - 1  and  710 - 3 , together with electrical connection  720 - 2 , form a second resonant structure similar to resonant structure  450 . Conductive sheets  710 - 4  and  710 - 3 , together with electrical connection  720 - 3 , form a third resonant structure similar to resonant structure  450 . Conductive sheets  710 - 4  and  710 - 5 , together with electrical connection  720 - 4 , form a fourth resonant structure similar to resonant structure  450 . 
     The first and second resonant structures share conductive sheet  710 - 1 ; because conductive sheet  710 - 1  is shared between the two resonant structures, it provides an electrical connection between the two structures whereby the two resonant structures are connected in series. The second and third resonant structures share conductive sheet  710 - 3 ; because conductive sheet  710 - 3  is shared between the two resonant structures, it provides an electrical connection between the two structures whereby the two resonant structures are connected in series. The third and fourth resonant structures share conductive sheet  710 - 4 ; because conductive sheet  710 - 4  is shared between the two resonant structures, it provides an electrical connection between the two structures whereby the two resonant structures are connected in series. 
     Overall, the four resonant structures are connected in series and, as a result, the four signals produced by the four structures, s 1 , s 2 , s 3 , and s 4 , are added together to produce an overall signal, s T , that can be applied to a load element, as shown in  FIG. 8 . 
     It will be clear to those skilled in the art, after reading this disclosure, how the comments presented for the first illustrative embodiment can be extended to the third illustrative embodiment. In particular, for each resonant structure, i, wherein i=1, 2, 3, 4, (for the first, second, third and fourth resonant structure, respectively) there is a subset, A i , of all possible directions of arrival and polarizations for which max[|s T |]&gt;max[|s i |]. The intersection of these four subsets, A 1 ∩A 2 ∩A 3 ∩A 4 , corresponds to the subset of all possible incident plane waves for which the phase shift is sufficiently small that the amplitude of s T  is larger than the amplitude of s 1 , s 2 , s 3 , or s 4  individually. In such cases, the antenna, when used in an RFID tag, provides improved performance compared to an antenna that comprises only one of the four resonant structures. 
     In the third illustrative embodiment, the four resonant structures have resonant bands that overlap over a common band. For electromagnetic signals at frequencies within the common band, the antenna of  FIG. 7  provides a larger voltage, compared to an antenna that comprises only one resonant structures, for a majority of all possible directions of arrival and polarizations. 
       FIG. 8  depicts multiple-resonator antenna with load element  800  in accordance with the third illustrative embodiment of the present invention. Multiple-resonator antenna with load element  800  comprises: multiple-resonator antenna  700 , load element  830  and connection points  840 - 1  and  840 - 2 , interrelated as shown. 
     Load element  830  is connected to connection point  840 - 1  on conductive sheet  710 - 2  and to connection point  840 - 2  on conductive sheet  710 - 5 , such that the voltage applied to load element  830  is the voltage resulting from the four resonant structures connected in series. 
     Although load element  830  is depicted as positioned around the edges of the conductive sheets, 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 load  830  is positioned differently. Furthermore, although the first, second and third illustrative embodiments comprise two or four resonant structures, 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 a different number of resonant structures, including, without limitation, three resonant structures or more than four resonant structures. 
     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.