Abstract:
Various resonant modes of a multiresonant antenna structure share at least portions of the structure volume. The basic antenna element has a ground plane and a pair of spaced-apart conductors electrically connected to the ground plane. Additional elements are coupled to the basic element, such as by stacking, nesting or juxtaposition in an array. In this way, individual antenna structures share common elements and volumes, thereby increasing the ratio of relative bandwidth to volume.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to co-pending application Serial No. 09/901,134, entitled “Multimode Grounded Multifinger Patch Antenna” by Gregory Poilasne et al., owned by the assignee of this application and incorporated herein by reference. 
     This application also relates to co-pending application Serial No. 09/781,779, entitled “Spiral Sheet Antenna Structure and Method” by Eli Yablonovitch et al., owned by the assignee of this application and incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of wireless communications, and particularly to the design of an antenna. 
     2. Background 
     Small antennas are required for portable wireless communications. With classical antenna structures, a certain physical volume is required to produce a resonant antenna structure at a particular radio frequency and with a particular bandwidth. A fairly large volume is required if a large bandwidth is desired. Accordingly, the present invention addresses the needs of small compact antenna with wide bandwidth. 
     SUMMARY OF THE INVENTION 
     The present invention provides a multiresonant antenna structure in which the various resonant modes share at least portions of the structure volume. The frequencies of the resonant modes are placed close enough to achieve the desired overall bandwidth. Various embodiments are disclosed. The basic antenna element comprises a ground plane; a first conductor extending longitudinally parallel to the ground plane having a first end electrically connected to the ground plane and a second end; a second conductor extending longitudinally parallel to the ground plane having a first end electrically connected to the ground plane and a second end spaced apart from the second end of the first conductor; and an antenna feed coupled to the first conductor. Additional elements are coupled to the basic element, such as by stacking, nesting or juxtaposition in an array. In this way, individual antenna structures share common elements and volumes, thereby increasing the ratio of relative bandwidth to volume. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 conceptually illustrates the antenna designs of the present invention. 
     FIG. 2 illustrates the increased overall bandwidth achieved with a multiresonant antenna design. 
     FIG. 3 is an equivalent circuit for a radiating structure. 
     FIG. 4 is an equivalent circuit for a multiresonant antenna structure. 
     FIG. 5 is a perspective view of a basic radiating structure. 
     FIG. 6 is a perspective view of an alternative basic radiating structure. 
     FIG. 7 is a top plan view of one embodiment of a multiresonant antenna structure. 
     FIG. 8 is a perspective view of the antenna structure of FIG.  7 . 
     FIG. 9 a  is a perspective view of another embodiment of a multiresonant antenna structure. 
     FIG. 9 b  is a perspective view of a further embodiment of a multiresonant antenna structure. 
     FIG. 10 is a perspective view of still another embodiment of a multiresonant antenna structure. 
     FIG. 11 is a perspective view of yet another embodiment of a multiresonant antenna structure. 
     FIG. 12 is a perspective view of another embodiment of a multiresonant antenna structure. 
     FIG. 13 is a perspective view of another embodiment of a multiresonant antenna structure. 
     FIG. 14 is a perspective view of another embodiment of a multiresonant antenna structure. 
     FIGS. 15 a-b  are top plan and side views, respectively, of another embodiment of a multiresonant antenna structure. 
     FIG. 16 diagrammatically illustrates a multiresonant antenna structure with parasitic elements. 
     FIG. 17 is a Smith chart illustrating a non-optimized multiresonant antenna. 
     FIG. 18 is a Smith chart illustrating an optimized multiresonant antenna. 
     FIG. 19 is a side view of one of the elements of the antenna structure of FIG.  16 . 
     FIG. 20 illustrates optimization of the coupling of the elements of the antenna structure of FIG.  16 . 
     FIG. 21 illustrates optimization of the feed point of a driven element of the antenna structure of FIG.  16 . 
     FIG. 22 illustrates an antenna structure with a two-dimensional array of radiating elements. 
     FIGS. 23 a - 23   d  illustrate alternative antenna structures with two-dimensional arrays of radiating elements. 
     FIG. 24 illustrates a physical embodiment of a radiating element for the antenna structures of FIGS. 22-23. 
     FIGS. 25 a  and  25   b  illustrate alternative physical embodiments of radiating elements for the antenna structures of FIGS. 22-23. 
     FIG. 26 illustrates a parasitic antenna element having a spiral configuration. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods and devices are omitted so as to not obscure the description of the present invention with unnecessary detail. 
     The volume to bandwidth ratio is one of the most important constraints in modern antenna design. One approach to increasing this ratio is to re-use the volume for different orthogonal modes. Some designs, such as the Grounded Multifinger Patch disclosed in patent application Serial No. 09/801,134, use this approach, even though the designs do not optimize the volume to bandwidth ratio. In the previously mentioned patent application, two modes are generated using the same physical structure, although the modes do not use exactly the same volume. The current repartition of the two modes is different, but both modes nevertheless use a common portion of the available volume. This concept of utilizing the physical volume of the antenna for a plurality of antenna modes is illustrated generally in FIG. 1. V is the physical volume of the antenna, which has two radiating modes. The physical volume associated with the first mode is designated V1, whereas that associated with the second mode is designated V2. It can be seen that a portion of the physical volume, designated V12, is common to both of the modes. 
     We will express the concept of volume reuse and its frequency dependence with what we refer to as a “K law” . The common general K law is defined by the following: 
     
       
         Δf/f=K•V/λ 3    
       
     
     Δf/f is the normalized frequency bandwidth. λ is the wavelength. The term V represents the volume that will enclose the antenna. This volume so far has been a metric and no discussion has been made on the real definition of this volume and the relation to the K factor. 
     In order to have a better understanding of the K law, different K factors are defined: 
     K modal  is defined by the mode volume V i  and the corresponding mode bandwidth: 
     
       
         Δf i /f i =K modal •V i /λ 3   i    
       
     
     where i is the mode index. K modal  is thus a constant related to the volume occupied by one electromagnetic mode. 
     K effective  is defined by the union of the mode volumes V 1 ∪V 2 ∪ . . . V i  and the cumulative bandwidth. It can be thought of as a cumulative K; 
     
       
         Σ i   Δf   i   /f   i   =K   effective ·( V   1   ∪V   2   ∪ . . . V   i )/λ c   3    
       
     
     where λ c  is the wavelength of the central frequency. K effective  is a constant related to the minimum volume occupied by the different excited modes taking into account the fact that the modes share a part of the volume. The different frequencies f i  must be very close in order to have nearly overlapping bandwidths. 
     K physical  or K observed  is defined by the structural volume V of the antenna and the overall antenna bandwidth: 
     
       
         Δf/f=K physical •V/λ 3    
       
     
     K physical  or K observed  is the most important K factor since it takes into account the real physical parameters and the usable bandwidth. K physical  is also referred to as K observed  it is the only K factor that can be calculated experimentally. In order to have the modes confined within the physical volume of the antenna, K physical  must be lower than K effective . However these K factor are often nearly equal. The best and ideal case is obtained when K physical  is approximately equal to K effective  and is also approximately equal to the smallest K modal . It should be noted that confining the modes inside the antenna is important in order to have a well-isolated antenna. 
     One of the conclusions from the above calculations is that it is important to have the modes share as much volume as possible in order to have the different modes enclosed in the smallest volume possible. 
     For a plurality of radiating modes i, FIG. 2 shows the observed return loss of a multiresonant structure. Different successive resonances occur at the frequencies f 1 , f 2 , f i , . . . f n . These peaks correspond to the different electromagnetic modes excited inside the structure. FIG. 2 illustrates the relationship between the physical or observed K and the bandwidth over f 1  to f n . 
     For a particular radiating mode with a resonant frequency at f 1 , we can consider the equivalent simplified circuit L 1 C 1  shown in FIG.  3 . By neglecting the resistance in the equivalent circuit, the bandwidth of the antenna is simply a function of the radiation resistance. The circuit of FIG. 3 can be repeated to produce an equivalent circuit for a plurality of resonant frequencies. 
     FIG. 4 illustrates a multiresonant antenna represented by a plurality of LC circuits. At the frequency f 1  only the circuit L 1 C 1  is resonating. Physically, one part of the antenna structure resonates at each frequency within the covered spectrum. Again, neglecting real resistance of the structure, the bandwidth of each mode is a function of the radiation resistance. 
     As discussed above, in order to optimize the K factor, the antenna volume must be reused for the different resonant modes. One example of a multimode antenna utilizes a capacitively loaded microstrip type of antenna as the basic radiating structure. Modifications of this basic structure will be subsequently described. In all of the described examples, the element of the multimode antenna structures have closely spaced resonant frequencies. 
     FIG. 5 illustrates a single-mode capacitively loaded microstrip antenna. If we assume that the structure in FIG. 5 can be modeled as a L 1 C 1  circuit, then C 1  corresponds to fringing capacitance across gap g. Inductance L 1  is mainly contributed by the loop designated by the numeral  2 . Another configuration of a capacitively loaded microstrip antenna is illustrated in FIG.  6 . The capacitance in this case is a facing capacitance at the overlap designated by the numeral  3 . 
     A top plan view of a tri-mode antenna structure is shown in FIG.  7 . This structure comprises three sections corresponding to three different frequencies. The feed is placed in area  7 , which is similar to the feed arrangement used for the antennas of FIG.  5  and FIG.  6 . This structure has three sets of fingers,  4 / 5 ,  8 / 9 , and  10 / 11 , configured similarly to the antenna of Fig ire  5 . The different inductances are defined by the lengths of fingers  4 ,  5 ,  8 ,  9 ,  10  and  11 . The different capacitances are defined by the gaps  6 ,  12  and  14 . 
     FIG. 8 is a perspective view of the antenna structure shown in FIG.  7 . In this configuration, there is a separate capacitance and inductance for each of the frequencies. The different L i  and C i  are set in order to have closely spaced frequencies fi. The slots S 1  and S 2  isolate the different parts of the antenna and therefore separate the frequencies of the antenna. This case shows that it is possible to partially reuse the volume of the antenna structure since the area  7  associated with the feed is common to all of the modes. However, some portions of the volume are dedicated to only one of the frequencies. 
     Another solution for the reuse of the structure volume is depicted in FIGS. 9 a  and  9   b . FIG. 9 a  is a variation of the basic structure shown in FIG. 5, whereas FIG. 9 b  is a variation of the basic structure shown in FIG.  6 . In each case, slits  15  are placed near the sides of the antenna, along its length. The slits create a resonant structure at one frequency, but are electromagnetically transparent at a second characteristic frequency of the structure. The spacing of the resonant frequencies of the structure is mainly controlled by the dimensions  16 ,  17 ,  18  and  19 . In both FIGS. 9 a  and  9   b   5 , two different antennas can be visualized—one by removing the material in the slits  15 , which resonates at a first frequency, and the other by filling in the slits, which resonates at a second frequency. These two antennas in one clearly share the same volute. 
     An embodiment of a multifrequency antenna structure composed of overlapping structures is shown in FIG. 10. A plate  20  connected to another plate  21  is placed over a structure S like that shown in FIG.  6 . The underlying structure S defines a capacitance C 1  and an inductance L 1  and is resonant at a frequency f 1 . The plate  20  is placed at a distance  23  from one edge. The plate  21  is placed at a distance  22  from the underlying structure, which defines a second capacitance C 2 . A second frequency f 2  is characterized by the inductance L 2  of loop  24  and the capacitance C 2  associated with gap  22  (the size of which is exaggerated in the figure). By optimizing C 1 , C 2 , L 1  and L 2  it is possible to achieve a set of two close frequencies that will indeed increase the K factor while reusing the same volume. In this case the volume V 1  is included within the volume V 2 . It should be noted that f 2  is not necessarily lower than f 1 . 
     FIG. 11 illustrates an extension of the structure shown FIG. 10 in which several plates  20 - 21 ,  29 - 30 ,  31  and  32  have been superposed on an underlying structure S to create a plurality of loops  25 ,  26 ,  27 ,  28 . Each of these loops is associated with a different resonant frequency. This concept can be extended to an arbitrary number of stacked loops. 
     FIG. 12 illustrates an antenna having a first structure  34  of the type shown in FIG. 5 included within a second such structure  33 . The feeding point could be coupled to the end of either plate  35  or plate  36  or along any of the open edges. Here, the volume of one antenna is completely included in the volume of the other. 
     FIG. 13 illustrates another embodiment in which a plurality of structures share common parts and volumes. In this case, the loops associated with the characteristic inductance  3  of the structures are numbered  37  and  38 . This concept can be extended to more than two frequencies. The dimensions of the structures may be adjusted to achieve the desired capacitance values as previously described. It should be noted that the selected dimensions may give rise parasitic frequencies and that these may be used in adjusting the overall antenna characteristics. 
     Another approach to making a multiresonant antenna is illustrated in FIG.  14 . Here, multiple antennas are combined in such a way that the coupling is low. The basic antenna element is the same as shown in FIG. 6. A set of such elements Fp1, Fp2, . . . Fpi are stacked upon one another. One part of each Fpi is also a part of Fpi+ 1  and Fpi− 1 . The common parts will help to define the related capacitances C i . The entire structure may have a common feeding point at Fp1 or separate feeding points may be located at Fp2. . . Fpi. 
     It is interesting to note that the width of the antenna structure does not have a critical influence on either the resonant frequency or the bandwidth. There is an optimum width for which the bandwidth of the basic element is at a maximum. Beyond this, the bandwidth does not increase as the width is increased. 
     The limited effect of the antenna width on bandwidth allows consideration of the structure shown in FIGS. 15 a-b , which nests the individual antenna elements in both the vertical and horizontal directions. This allows more freedom in organizing the capacitive and inductive loading. This arrangement provides for the total inclusion of the inner antenna elements within the overall antenna volume, each element sharing a common ground. At different frequencies, only one element is resonating. 
     FIG. 16 illustrates an antenna structure comprising an array of elements, each of the general type shown in FIG. 6, having a driven element  40  and adjacent parasitic elements  41 - 43 . Impedance matching of this structure is illustrated by the Smith chart shown in FIG.  17 . The large outer loop  50  corresponds to the main driven element  40 , whereas the smaller loops  51 - 53  correspond to the parasitic elements. This is a representation of a non-optimized structure. Various adjustments can be made to the antenna elements to influence the positions of the loops on the Smith chart. The smaller loops may be gathered in the same area in order to obtain a constant impedance within the overall frequency range. 
     In the case of a typical 50 ohm connection, an optimized structure will have all of the loops gathered approximately in the center of the Smith chart as shown in FIG.  18 . In order to gather the loops in the center of the Smith chart (or wherever it is desired to place them), the dimensions of the individual antenna elements are adjusted, keeping in mind that each loop corresponds to one element. 
     FIG. 19 illustrates a single element, such as  41 , of the antenna structure shown in FIG.  16 . By reducing the dimension  1 , the corresponding loop rotates clockwise on the Smith chart. By adjusting the length of the parasitic elements, all of the different loops can be gathered. Then, if necessary, the group of loops can be rotated back in the counter-clockwise direction on he Smith chart by reducing the length of the main driven element. 
     In order to optimize the bandwidth of the antenna structure, the main loop must have a large enough diameter. With reference to FIG. 20, the diameter of the main loop is controller by the amount of coupling between each element and its neighbor, which is determined by the distance d1 between the adjacent elements. The amount of coupling is also controlled by the with of the elements. The narrower the elements are, the closer the elements can be in order to keep the same loop diameter. The ultimate size reduction is obtained when each element comprises a single wire. Furthermore, the elements can also be placed closer together by making the gap  45  smaller. 
     Finally, the main loop may be centered on the Smith chart by adjusting the location of the antenna feed on the main driven element. Referring to FIG. 21, impedance matching of the antenna structure is optimized by adjusting the dimension 1f. By increasing 1f, the diameter f the main loop is increased. In this way, the small loops can be centered at the desired location on the Smith chart. 
     FIG. 22 illustrates a polarized multi-resonant antenna structure in which polarization diversity is achieved through the use of two interleaved arrays of antenna elements. In the case illustrated, the two arrays are arranged orthogonally to provide orthogonal polarization. The two arrays may be interconnected in various ways or they may be totally separated. It is easiest to have the arrays make contact where they cross, otherwise the manufacturing is more difficult. However it is not necessary that the arrays contact one another, and, in some cases, isolating the array elements from each other can be used for adjusting the impedance matching characteristics of the antenna. In any case, it is always possible to match the antenna by adjusting the various dimensions of the array elements as discussed earlier. 
     The use of one- or two-dimensional arrays of antenna elements allows the antenna structure to be co-located on a circuit board with other electronic components. The individual array elements can be placed between components mounted on the board. The electronic behavior of the components may be slightly affected by the presence of the radiating elements, but this can be determined through EMC studies and appropriate corrective measures, such as shielding of sensitive components, may be implemented. However, the electronic components will generally not perturb the electromagnetic field and will therefore not change the characteristics of the antenna. 
     The two-dimensional array shown in FIG. 22 can be extrapolated to other array designs as illustrated in FIGS. 23 a-d . The elements of the array can be arranged in various configurations to achieve spatial and/or polarization diversity. Other configurations in addition to those shown in FIGS. 23 a-d  are possible. In each case, the elements of the array may be interconnected in various ways or may be electrically isolated from one another. In addition, the individual elements may or may not be shorted to ground. All of these design parameters, including those previously discussed, permit the design of an antenna structure having the desired electromagnetic characteristics. 
     The design of an antenna structure must, of course, take into account manufacturing considerations, the objective being to achieve an antenna with both high efficiency and a low manufacturing cost. In achieving this objective, the problem of loss maybe a big issue. The electric field inside the capacitive part of the antenna is very high. Therefore, no material should be in between the two metallic layers. 
     A first solution, as illustrated in FIG. 24, utilizes an antenna element consisting of two wires  60 ,  61  connected to a ground. The distance between the two wires is very important for frequency tuning. Therefore, it is important to have a spacer that maintains the two wires at a fixed distance. In order to minimize the loss contributed by the presence of the spacer, the spacer should not intrude into the space between the wires. FIG. 24 shows a simple solution configured like a conventional surface mounted resistor. The wires are secured within a plastic hollow cylinder  62  and the protruding wires are then soldered to the ground. 
     A second solution, as illustrated in FIGS. 25 a-b , utilizes an antenna element constructed as a printed circuit. Each element is printed on a very thin, low-loss dielectric substrate in order to achieve good efficiency. The printed circuit element is then placed vertically on the ground. FIG. 25 a  shows a simple two-arm element. FIG. 25 b  shows a similar two-arm element with the ground printed on the substrate. 
     The parasitic elements of the antenna array need not be limited to the basic two-wire design shown in FIGS. 5 and 6 and in the later described structures based on these elements. Referring to FIG. 26, the parasitic elements may instead have a spiral configuration. The resonant frequency of the spiral element will be a function of the number of turns. It should be noted that when such a spiral element is coupled to a driven element having the configuration shown in FIG. 5 or FIG. 6, the capacitive coupling is reduced since the driven element acts as a dipole, whereas the spiral element acts as a quadrupole. 
     It will be recognized that the above-described invention may be embodied in other specific forms without departing from the spirit or essential characteristics of the disclosure. Thus, it is understood that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.