Abstract:
An antenna for a Radio-Frequency IDentification (RFID) system is disclosed that comprises a pair of resonant cavities. The antenna is realized by folding the ends of a ribbon of conductive material, such as metal foil, over the middle part of the ribbon. The antenna generates a higher voltage than prior-art antennas used in RFID systems, and it makes possible RFID systems with an improved range. In an alternative embodiment, the antenna comprises a reflector that enables the RFID system to better tolerate the presence of nearby metal objects.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The underlying concepts, but not necessarily the language, of the following cases are incorporated by reference: 
     (1) U.S. provisional application No. 61/207,467; and 
     (2) U.S. provisional application No. 61/273,814. 
     If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case. 
     This case claims benefit of the following provisional applications: 
     (1) U.S. provisional application No. 61/207,467; and 
     (2) U.S. provisional application No. 61/273,814. 
     This case is a Continuation-in-Part and claims priority of co-pending U.S. case No. 12/535,768 titled “Multiple-Resonator Antenna” and filed on Aug. 5, 2009. 
    
    
     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 antenna 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 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. 
     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. 
     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 higher 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 the design of passive-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 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. 
     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 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.” 
     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 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. 
     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 . 
     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. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention comprise a pair of resonant structures implemented as resonant cavities. Cavities are realized by interconnecting sheets of conductive material such as, for example, metal foil. Two cavities are combined to achieve an antenna structure that, when used as a receiving antenna, has a source impedance that is higher than prior-art antennas. For a given received signal strength, the higher source impedance yields a higher voltage at the antenna output port, resulting in a longer distance of operation for RFID tags based on the present invention. 
     An embodiment of the present invention comprises a ribbon of conductive material, such as metal foil, wherein the two ends of the ribbon are folded over the middle part of the ribbon. Between each folded end of the ribbon and the middle part of the ribbon there is a layer of supporting material that supports the ribbon and maintains the folded end of the ribbon at a fixed distance from the middle part of the ribbon. The volume of space between one end of the ribbon and the middle part of the ribbon, which is occupied by the supporting material, forms one electromagnetically-resonant cavity. The supporting material also acts as dielectric. 
     A load element is connected between the two folded ends of the ribbon to make an RFID tag. The folded ribbon is the tag&#39;s antenna; it has a higher impedance than prior-art antennas for RFID tags, with the result that a higher voltage is generated across the load element. 
     For situations where an RFID tag is used near a large metal object, embodiments of the present invention comprise an additional sheet of conductive material, referred to as a “reflector.” For embodiments implemented as a folded ribbon, the reflector sheet is placed parallel to the middle part of the ribbon, on the side opposite the folded ends. A layer of supporting material is between the reflector and the middle part of the ribbon and serves to maintain a fixed distance between them. The presence of the reflector reduces the disruption of tag performance caused by large metal objects in the vicinity of the tag. 
    
    
     
       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-cavity antenna with a load element in accordance with a first illustrative embodiment of the present invention. 
         FIG. 6  depicts a dual-cavity antenna with non-equal cavities in accordance with a second illustrative embodiment of the present invention. 
         FIG. 7  depicts a dual-cavity antenna with a reflector in accordance with a third illustrative embodiment of the present invention. 
         FIG. 8  depicts a dual-cavity antenna with a dielectric in accordance with a fourth illustrative embodiment of the present invention. 
         FIG. 9  depicts a dual-cavity antenna with multiple dielectrics and a reflector in accordance with a fifth illustrative embodiment of the present invention. 
         FIG. 10  depicts a dual-cavity antenna with delay elements in accordance with a sixth illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 5  depicts dual-cavity-antenna-with-load-element  500  in accordance with a first illustrative embodiment of the present invention. Dual-cavity-antenna-with-load-element  500  comprises: conductive ribbon  510 , load element  520 , and connection points  530 - 1  and  530 - 2  interrelated as shown. In particular, the two ends,  540 - 1  and  540 - 2 , of conductive ribbon  510 , are folded over the middle part  550  of conductive ribbon  510  and they are on the same side of the middle part  550  of conductive ribbon  510 . The two folded ends  540 - 1  and  540 - 2  do not touch one another. Connection points  530 - 1  and  530 - 2  are on the two folded ends,  540 - 1  and  540 - 2 , of conductive ribbon  510 . 
     Each of the two folded ends  540 - 1  and  540 - 2  forms a resonant cavity together with the middle part  550  of conductive ribbon  510 . The two cavities are electrically connected together via the shared middle part  550  of conductive ribbon  510 . Compared to prior-art folded-dipole antenna  300 , dual-cavity antenna with load element  500  has a higher impedance. In traditional radio systems, the higher impedance is not an advantage—indeed, in many traditional radio systems it is a disadvantage—but the higher impedance is advantageous in passive RFID tags. The use of a conductive ribbon to form two cavities, instead of using two resonant structures formed by a wire, is a salient difference between folded-dipole antenna  300  and dual-cavity antenna with load element  500 ; this difference gives the latter antenna the advantageous higher impedance. The other illustrative embodiment of the present invention set forth in this disclosure also provide the advantage of a higher impedance. 
     Although the two cavities formed by the two folded ends  540 - 1  and  540 - 2  are depicted in  FIG. 5  as equal to one another, 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 cavities are different. 
     Although connection points  540 - 1  and  540 - 2  are depicted in  FIG. 5  as being placed near the center of folded ends of ribbon  540 - 1  and  540 - 2 , respectively, 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. For example and without limitation, connection points  540 - 1  and  540 - 2  can be near corners of folded ends of ribbon  540 - 1  and  540 - 2 . 
     Although connection points  540 - 1  and  540 - 2  are depicted in  FIG. 5  as direct electrical connections such as are known in the art as “ohmic” connections, 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 realized differently. For example and without limitation, connection points  540 - 1  and  540 - 2  can comprise capacitors or inductors or more complex impedance-matching networks. 
     Although the portions of conductive ribbon  510  wherein the folds occur are depicted as semicircular 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 with folds having different shapes. For example, and without limitation,  FIG. 8  below depicts an alternative embodiment of the present invention that can be implemented by folding a conductive ribbon in a different manner. 
       FIG. 6  depicts dual-cavity-antenna-with-non-equal-cavities  600  in accordance with a second illustrative embodiment of the present invention wherein the two cavities are not equal. As with the first illustrative embodiment, this antenna comprises a conductive ribbon  610 , whose ends,  620  and  630 , are folded over the middle part  640  of the ribbon. However, folded end  630  is longer than folded end  620 , and folded end  630  is at a distance  650  from middle part of ribbon  640  that is less than the distance  660  between the shorter folded end of the ribbon  620  and the middle part of the ribbon  640 . 
     For the purpose of visual clarity,  FIG. 6  does not show connection points or a load element. 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 . It will be clear to those skilled in the art, after looking at  FIG. 5  and reading this disclosure, how to place connection points and how to attach a load element to dual-cavity antenna with non-equal cavities  600  in a manner similar to the manner shown in  FIG. 5  for dual-cavity antenna with load element  500 . Hereinafter, for the purpose of visual clarity, other figures that depict alternative embodiments of the present invention will also not explicitly show connection points or a load element. It will be understood that connection points and a load element are also present in all such embodiments, and it will be clear to those skilled in the art, after looking at  FIG. 5  and reading this disclosure, how to place connection points and how to attach a load element, in such embodiments, in a manner similar to the manner shown in  FIG. 5  for dual-cavity antenna with load element  500 . 
     Although, in  FIG. 6 , the two cavities differ from one another because the lengths of folded ends  620  and  630  are different, and because distances  650  and  660  are different, 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 cavities differ from one another in other ways. For example, and without limitation, the two cavities can differ by:
         i. having different lengths,   ii. having different widths,   iii. the two folded ends having different distances from the middle part of the ribbon,   iv. being made of different conductive materials,   v. having different shapes,   vi. comprising different dielectric materials,   vii. comprising different amounts of dielectric materials,   viii. comprising different combinations of multiple dielectric materials,   ix. having different corners,   x. having differently-finished edges, or   xi. a combination of i, ii, iii, iv, v, vi, vii, viii, ix, or x.       

     Although values for distance  650  and distance  660  are not explicitly specified in  FIG. 8 , it will be clear to those skilled in the art, after reading this disclosure, how to make and use embodiments of the present invention with specific values for distance  650  and distance  660 . For example, and without limitation, both distances might be less than: 
     (i) the length of the ribbon, and 
     (ii) the width of the ribbon. 
     Also, for example and without limitation, the values for distance  650  and distance  660  might within the range of 3 mm to 10 mm, inclusive; the length of the ribbon might be within the range of 200 mm and 300 mm, inclusive; and the width of the ribbon might be no less than 6 mm. 
       FIG. 7  depicts dual-cavity-antenna-with-reflector  700  in accordance with a third illustrative embodiment of the present invention. Dual-cavity-antenna-with-reflector  700  comprises conductive ribbon  710  and conductive reflector sheet  720 . Conductive ribbon  710  implements a dual-cavity antenna in accordance with the first illustrative embodiment or in accordance with the second illustrative embodiment set forth above. 
     Although  FIG. 7  shows conductive ribbon  710  as having the same shape as conductive ribbon  510  as depicted 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 a dual-cavity antenna with reflector in accordance with the present invention wherein conductive ribbon  710  has the same shape as conductive ribbon  610  as depicted in  FIG. 6 . Furthermore, it will also be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of a dual-cavity antenna with reflector in accordance with the present invention wherein conductive ribbon  710  is replaced by one of the alternative embodiments of a dual-cavity antenna according set forth in this disclosure. For example, and without limitation, one such embodiment of a dual-cavity antenna with reflector is depicted in  FIG. 9  below. 
     Although conductive reflector sheet  720  is depicted as a thin sheet, as might be implemented with metal foil, that extends slightly beyond the outline of conductive ribbon  710 , 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 conductive reflector sheet  720  is realized differently. For example and without limitation, conductive reflector sheet can be:
         i. much larger than conductive ribbon  710 ,   ii. a solid block of conductive material,   iii. part of a metal structure that also provides mechanical support,   iv. part of the housing of an RFID system, or   v. a combination of i, ii, iii, or iv.       

       FIG. 8  depicts dual-cavity-antenna-with-dielectric  800  in accordance with a fourth illustrative embodiment of the present invention. Dual-cavity-antenna-with-dielectric  800  comprises: conductive sheets  810 - 1 ,  810 - 2 , and  810 - 3 , electrical connections  820 - 1  and  820 - 2 , and dielectric material  830 , interrelated as shown. 
     Electrical connections  820 - 1  and  820 - 2  perform the same functions as the curved portions of conductive ribbon  510  in the first illustrative embodiment of the present invention. Conductive sheet  810 - 1  performs the same function as middle part of ribbon  550  in the first illustrative embodiment of the present invention. Conductive sheets  810 - 2  and  810 - 3  performs the same functions as folded ends of ribbon  540 - 1  and  540 - 2  in the first illustrative embodiment of the present invention. In particular, conductive sheets  810 - 2  and  810 - 3  form two resonant cavities, respectively, together with conductive sheet  810 - 1 . 
     Although the combination of conductive sheets  810 - 1 ,  810 - 2 , and  810 - 3 , and electrical connections  820 - 1  and  820 - 2  can be realized by folding a ribbon of conductive material similar to conductive ribbon  510  with sharp bends around dielectric material  830 , 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 realized in a different manner. For example and without limitation, electrical connections  820 - 1  and  820 - 2  can be realized as:
         i. single wires or multiple wires,   ii. portions of sheet material bent in different shapes,   iii. single or multiple connections at single or multiple points along the edges of the interconnected sheets,   iv. separate sheets of conductor formed by a stamping process and press fitted together as desired   v. solder joints, screws, pins, or other electrically conductive fasteners,   vi. plated-through via holes,   vii. a combination of i, ii, iii, iv, v, or vi
 
Furthermore, the electrical connections can extend over larger or smaller sections of one or more edges of the conductive sheets.
       

     Although conductive sheets and conductive ribbons are depicted in the figures of this disclosure as solid sheets of electrically conductive material such as, for example, metal foil, 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 and conductive ribbons are realized differently. For example, and without limitation, a conductive sheets or a conductive ribbon can:
         i. be a grid of wires, or a mesh,   ii. be made of any conductive materials such as metals (e.g., copper, aluminum) or, for example, conductive ink, or conductive paint,   iii. be perforated with holes arranged at random or in a regular pattern,   iv. be a printed circuit board with one or more interconnection layers,   v. comprise notches or jagged edges,   vi. have an uneven or rough surface with bumps or lumps,   vii. comprise electronic components, such as, for example, resistors, capacitors or integrated circuits,   viii. comprise mechanical fasteners such as, for example, screws, nuts, or rivets,   ix. comprise solder joints, welds or other electrical or mechanical joints,   x. be an array of parallel wires substantially parallel to the prevailing direction of electrical currents within the sheet or ribbon.   xi. be a combination of i, ii, iii, iv, v, vi, vii, viii, ix, or x.       

     Although dielectric material  830  is shown in  FIG. 8  as occupying most of the volume between sheet  810 - 1  and sheets  810 - 2  and  810 - 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 none of the volume or only a portion of the volume is occupied by dielectric material, or dielectric material extends beyond the volume between the conductive sheets. It will also be clear to those skilled in the art, after reading this disclosure, how to make and use variants of the illustrative embodiments set forth in this disclosure wherein part or all of the volume of space within one or both of the cavities comprises one or more dielectric materials. 
     Many different dielectric materials are known in the art for making resonant structures. For example, and without limitation, dielectric material  830  can be acetate, ABS (Acrylonitrile Butadiene Styrene) of various densities, polyphenylsulphone, polyethersulfone, polysulfone, PETG (Polyethylene Terephthalate Glycol), polycarbonate, teflon, polystyrene, difunctional epoxy resin (FR4), epoxy glass, or polyethylene. 
     Although values for the dimensions of conductive sheets  810 - 1 ,  810 - 2 , and  810 - 3  and for the distances between them are not explicitly specified in  FIG. 8 , it will be clear to those skilled in the art, after reading this disclosure, how to make and use embodiments of the present invention with specific values for such dimensions and distances. For example, and without limitation, sheets  810 - 1 ,  810 - 2 , and  810 - 3  might be arranged such that the distance between the plane of sheet  810 - 1  and the plane of sheet  810 - 2  is less than: 
     (i) the square root of the area of sheet  810 - 1 , 
     (ii) the square root of the area of sheet  810 - 2 , and 
     (iii) the square root of the area of sheet  810 - 3 . 
     Also, sheets  810 - 1 ,  810 - 2 , and  810 - 3  might be arranged such that the distance between the plane of sheet  810 - 1  and the plane of sheet  810 - 3  is less than: 
     (i) the square root of the area of sheet  810 - 1 , 
     (ii) the square root of the area of sheet  810 - 2 , and 
     (iii) the square root of the area of sheet  810 - 3 . 
       FIG. 9  depicts dual-cavity-antenna-with-multiple-dielectrics-and-reflector  900  in accordance with a fifth illustrative embodiment of the present invention. Dual-cavity-antenna-with-multiple-dielectrics-and-reflector  900  comprises: conductive sheets  810 - 1 ,  810 - 2 , and  810 - 3 , electrical connections  820 - 1  and  820 - 2 , conductive reflector sheet  720 , and dielectric materials  930 - 1 ,  930 - 2 , and  930 - 3 , interrelated as shown. 
     Conductive sheets  810 - 1 ,  810 - 2 , and  810 - 3 , electrical connections  820 - 1  and  820 - 2  are identical to conductive sheets  810 - 1 ,  810 - 2 , and  810 - 3 , electrical connections  820 - 1  and  820 - 2  in  FIG. 8 , respectively. Conductive reflector sheet  720  is identical to conductive sheet  720  in  FIG. 7  and it provides the same advantage as in the illustrative embodiment depicted in  FIG. 7 . 
     In this fifth illustrative embodiment of the present invention, the volume of space inside the two cavities is occupied by two layers of different dielectric materials,  930 - 1  and  930 - 2 . The volume of space between conductive reflector  720  and conductive sheet  810 - 1  is occupied by dielectric material  930 - 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 the volumes of space described in this paragraph are occupied by one or more dielectric materials arranged in one or more layers or in other geometric arrangements. 
       FIG. 10  depicts dual-cavity-antenna-with-delay-elements  1000  in accordance with a sixth illustrative embodiment of the present invention. Dual-cavity-antenna-with-delay-elements  1000  comprises: conductive sheets  810 - 1 ,  810 - 2 , and  810 - 3 , electrical connections  820 - 1  and  820 - 2 , dielectric material  830 , load element  520 , and delay elements  1010 - 1  and  1010 - 2 , interrelated as shown. 
     Conductive sheets  810 - 1 ,  810 - 2 , and  810 - 3 , electrical connections  820 - 1  and  820 - 2  and dielectric material  830  are identical to conductive sheets  810 - 1 ,  810 - 2 , and  810 - 3 , electrical connections  820 - 1  and  820 - 2  and dielectric material  830  in  FIG. 8 , respectively. Load element  520  is identical to load element  520  in  FIG. 5 . 
     The salient difference between this illustrative embodiment and the previous illustrative embodiments is the way in which load element  520  is connected to conductive sheets  810 - 2  and  810 - 3 . It is well known in the art how to make a delay element using a so-called “serpentine” structure, sometimes also referred-to as a “meandering” structure. Such a structure is depicted in  FIG. 10  as implementing delay elements  1010 - 1  and  1010 - 2 , and can be regarded as having an electrical behavior similar to an inductor or similar to a delay line. By connecting load element  520  through one or two such delay elements, it is possible to reduce the length of one or both resonant cavities without an increase in the resonant frequency. This is advantageous because, in the absence of such delay elements, a reduction in the size of a resonant cavity, if other cavity parameters are kept unchanged, is generally accompanied by an increase in the cavity&#39;s resonant frequency. In an alternative embodiment of the present invention, one or both of delay elements  1010 - 1  and  1010 - 2  can be serpentine ribbon structures with electric-field couplings to conductive sheets  810 - 2  or  810 - 3 , respectively. 
     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 in accordance with an embodiment of the present invention. 
     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.