Abstract:
An antenna employs cylindrical Fresnel zone plate (CFZP) construction in combination with a reflective ground plate and a sectorial reflector to enhance antenna gain, while lowering assembly cost and improving antenna placement flexibility. By forming a surface of symmetry for the antenna, the ground plate allows the antenna to mimic the operation of a symmetrical CFZP antenna using only half the nominal number of Fresnel elements. Further, the sectorial reflector restricts radiated emissions over a desired sector angle, minimizing radiation in undesirable directions, such as toward mounting walls or other nearby surfaces that would cause unwanted signal reflections, such as might aggravate multipath signal phenomenon.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention generally relates to antennas for electromagnetic signal reception and transmission, and particularly relates to cylindrical Fresnel zone plate (CFZP) antennas.  
           [0002]    Antennas form integral elements in essentially all communication systems or devices. One notes that antennas run the gamut in terms of size, shape, and configuration, in dependence on intended use, cost considerations, and the involved signals of interest. Despite such physical variations, a common set of performance parameters generally apply to essentially all antenna types. Antenna gain and directionality are, for example, properties generally of some importance.  
           [0003]    CFZP antennas are a type of antenna exhibiting relatively good gain characteristics. In contrast to flat Fresnel zone plate antennas, which comprise a supporting disc with an array of concentric Fresnel rings, CFZPs use a cylinder to support a vertical array of metallic rings acting as Fresnel zones. Such antennas exhibit a generally good omni-directional horizontal signal, making them suitable for use in certain communication system applications.  
           [0004]    However, this omni-directionality is not always desirable, particular where there is a need to restrict signal radiation in particular directions, such as might be desired where reflective surfaces would otherwise contribute to multipath signal problems. Indoor wireless network installations represent such an environment.  
           [0005]    Further, typical implementations of CFZP antennas require some number of discrete Fresnel elements spaced apart in accordance with the signal frequencies and gain requirements at hand. These implementation requirements sometimes result in undesirably large and, consequently undesirably awkward, and possibly expensive, antennas.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    The present invention provides an apparatus for implementing an efficient cylindrical Fresnel zone plate (CFZP) antenna having good signal gain, low cost, compact package, and directional radiation. In an exemplary embodiment, the inventive antenna is constructed as a sectorial cylindrical Fresnel zone plate (S-CFZP) antenna. In such embodiments, the antenna comprises a ground plate, a dielectric support positioned perpendicular to the base, a plurality of Fresnel elements arrayed in vertically spaced apart fashion on an inner face of the dielectric support, a sectorial reflector positioned on an outer face of the support, and a feeder positioned on the base at the foci of the Fresnel elements.  
           [0007]    The dielectric support is generally cylindrically shaped, and might comprise a cylindrical sector or a complete right circular cylinder. Likewise, the Fresnel elements are generally cylindrically shaped flat hoops or rings, and might be complete or partial hoops. In some embodiments, both the dielectric support and the Fresnel elements form cylindrical sectors, though not necessarily at the same sector angles. In other embodiments, one or both the support and Fresnel elements form complete cylinders. Further, the arrangement of Fresnel elements on the inner face of the dielectric support varies between embodiments, and it is not necessary to maintain the same number of Fresnel elements, or to use uniform spacing between them. Implementation details regarding placement and spacing of the Fresnel elements may be varied as needed.  
           [0008]    Regardless of the number or spacing of the Fresnel elements, the ground plate acts as a reflective surface lying parallel to the Fresnel elements, which placement allows it to function as a surface of symmetry. With this surface of symmetry, the antenna operates as if an additional, symmetric plurality of Fresnel elements is implemented on a side opposite the ground plate. As such, the antenna offers the performance advantage of symmetric pluralities of Fresnel elements, but with only half the number Fresnel elements required for symmetry physically implemented. Attendant cost and size advantages flow from the use of the reflective ground plate.  
           [0009]    Further operating advantages derive from using the sectorial reflector. Positioned on the outer face of the dielectric support over a desired cylindrical sector angle, the reflector serves at least a twofold purpose. First, the reflector enhances antenna gain by reflecting electromagnetic signals from or to the feeder through the portions or bands of the dielectric support not covered by the Fresnel elements. Second, the reflector blocks backward radiation through the portion of the dielectric support covered by the reflector. Thus, the otherwise omni-directional horizontal radiating pattern of the antenna is restricted to a desired sector, or, more appropriately, is blocked over a desired sector angle, by use of the sectorial reflector.  
           [0010]    Use of the inventive antenna structure is not limited to a particular application, or even to a range of applications. However, it is expected that the present invention will be applied to antenna structures for use in wireless LAN communications, broadcast satellite reception, mobile communication, and various other wireless networking and communication applications. For example, the ability to restrict or otherwise reduce radiated energy in a given sector with the inventive antenna structure facilitates its use in wireless LAN applications, where it may be undesirable to radiate energy toward a mounting wall or other surface on which the antenna is positioned, because radiation in those directions generally produces reflective waves that exacerbate multi-path, propagation within the indoor environment.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a diagram of a conventional CFZP antenna implemented as a full cylinder.  
         [0012]    [0012]FIG. 2A is a diagram of a CFZP antenna implemented as a partial cylinder.  
         [0013]    [0013]FIG. 2B is a diagram illustrating a surface of symmetry as used to modify CFZP antenna structures according to some embodiments of the present invention.  
         [0014]    [0014]FIG. 3 is a diagram illustrating the electromagnetic image principle employed by exemplary embodiments of the present invention.  
         [0015]    [0015]FIG. 4. is a diagram of an exemplary embodiment of a sectorial CFZP (S-CFZP) according to the present invention.  
         [0016]    [0016]FIG. 5 is a diagram of another exemplary embodiment of a S-CFZP antenna.  
         [0017]    [0017]FIG. 6 is a diagram of another exemplary embodiment of a S-CFZP antenna.  
         [0018]    [0018]FIG. 7 is a diagram illustrating a variation of the antenna of FIG. 6.  
         [0019]    [0019]FIG. 8 is a diagram illustrating another variation of the antenna of FIG. 5.  
         [0020]    FIGS.  9 A- 9 D are diagrams illustrating a few of the variations possible for the ground plate used in exemplary S-CFZP antennas.  
         [0021]    [0021]FIG. 10 illustrates an exemplary segmented variation of an S-CFZP antenna. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    [0022]FIG. 1 illustrates a conventional Cylindrical Fresnel Zone Plate (CFZP) antenna  10 . Such antennas utilize symmetric Fresnel zones  12  and  14  that are disposed in upper and lower vertical arrays on the inner face of a cylindrical support  18 , where the radii of the Fresnel zones  12  and  14  are the focal length of the antenna  10 . Traditionally, antennas of this type are either complete or half cylinders and provide omni-directional horizontal radiation pattern centered at the feeder  16 . When placed inside a building, such as in wireless LAN communications, the omni-direction radiation of the antenna  10  exacerbates multipath signal propagation because of, among other things, potentially strong signal reflections from reflective surfaces nearby the antenna  10 .  
         [0023]    [0023]FIGS. 2A and 2B illustrate exemplary embodiments of an antenna  20  according to the present invention. In FIG. 2A, the antenna  20  comprises a feeder  22 , a dielectric support  24 , upper Fresnel elements  26 , e.g.,  26 - 1 ,  26 - 2 , and so on, symmetric lower Fresnel elements  28 , e.g.,  28 - 1 ,  28 - 2 , and so on, and a sectorial reflector  30 . Use of the reflector  30  enhances directional radiation from the inner face of the support  24 , i.e., the support surface facing the feeder  22 , and blocks outward radiation from the antenna  20  over the portion of the support&#39;s outside face that is covered by the reflector  30 . In this manner, the antenna  20  can be mounted to a reflective surface, such as a wall, without it strongly radiating into the wall and thereby causing unwanted signal reflections.  
         [0024]    The thickness of the support  24  determines the distance separating the Fresnel elements  26  and  28  from the reflective surface  30 . Ideally, this thickness is configured as λ m /4, where λ m  represents the wavelength of a frequency of interest within the dielectric material. With the dielectric thickness set appropriately, radiated signals reflecting from the facing surfaces of the Fresnel elements  26  and  28 , and those signals reflecting from the reflector  30 , which must pass through the dielectric  24  twice, constructively interfere to enhance antenna gain. Thus, the reflector  30  aids antenna gain, as well as directly blocking unwanted rearward antenna emissions.  
         [0025]    [0025]FIG. 2B illustrates a further refinement of the antenna of FIG. 2A with the introduction of ground plate  34 , which enables antenna  20  to eliminate the Fresnel elements  28  below the ground plate  34  by employing the “image principle” known by those skilled in the art of electromagnetic theory. Here, the ground plate  34  serves as a reasonable approximation of a perfectly conductive, infinite ground plane provided that it sized large enough relative to the dimensions of the Fresnel elements  26  and made of suitable conductive material, such as zinc, brass, aluminum, steel, etc.  
         [0026]    With the ground plate  34  positioned parallel to the Fresnel elements  26  as shown, the antenna  20  mimics the symmetrical Fresnel element configuration shown in FIG. 2A, but with only the upper Fresnel elements  26  physically implemented. That is, with the ground plate  34  operating as a reflective surface for the antenna  20 , one need only implement one half of the symmetrical pluralities of Fresnel elements  26  and  28  otherwise required for symmetric operation.  
         [0027]    [0027]FIG. 3 illustrates such antenna operation in more detail, and demonstrates use of the image principle as a basis for analyzing the field behavior of the antenna  20 . Only one Fresnel element  26 −x (x=1, 2, 3, etc.) is shown in simplified form relative to the ground plate  34 , along with the corresponding Fresnel element  28 −x, which is not physically present but rather is depicted as the “mirror image” of element  26 −x. Thus, where Fresnel element  26 −x occupies a position at height “h” above the ground plate  34 , the image element  28 −x is assumed to occupy a mirror position at height h below the ground plate  34 .  
         [0028]    From the perspective of a receiver R, the resultant field from antenna  20  depends on the direct wave from Fresnel element  26 −x in combination with the reflected wave from the ground plate  34 . Using the image principle, the reflected wave may be assumed to radiate from the mirror image element  28 −x. Thus, one obtains the field at the receiver R by analyzing the problem based on the assumption that Fresnel element  28 −x is physically present, and is driven by a current relevant to that driving Fresnel element  26 −x. The resultant field pattern of antenna  20  if implemented with symmetric sets of Fresnel elements  26  and  28  but without the ground plate  34 , is essentially the same when antenna  20  is implemented using just one set of Fresnel elements in combination with the ground plate  34 .  
         [0029]    Of course, those skilled in the art will recognize that use of the ground plate  34  may change the antenna impedance characteristics as compared to the free-space characteristics of Fresnel elements  26 . As is well understood, such changes alter, for example, the required applied voltage for a given antenna power.  
         [0030]    [0030]FIG. 4 illustrates an exemplary embodiment of the antenna  20  that takes advantage of the image principle. Here, the antenna  20  comprises one set of Fresnel elements (set 26), the ground plate  34 , and the reflector  30 . In such a configuration, the antenna  20  is relatively compact, i.e., only the upper set of symmetric Fresnel elements  26  is implemented, and directional by virtue of the reflector  30 . One notes that the support  24 , the Fresnel elements  26 , and the reflector  30  are implemented here with the same cylindrical sector angle “TOP” defined by line segments “TO” and “OP.” Further, note the vertically spaced arrangement of the set of Fresnel elements  26  on the inner face of the support  24 . The height from the ground plate  34  to the edge of each Fresnel element  26 , i.e., individual elements  26 - 1 ,  26 - 2 , and so on, relative to the feeder  22  is given by the equation,  
                 r   n     =         nF                 λ     +       (       n                 λ     2     )     2           ,           (   1   )                               
 
         [0031]    where n equals the number of the particular edge of Fresnel elements  26  up to the Nth edge, F is the focal length of the antenna  20 , and λ is the free-space wavelength of the electromagnetic signal of interest. Thus, Equation (1) may be used to set the relative spacing of the Fresnel elements  26 . Additionally, where there are a total of I elements, the width (edge-to-edge) of the ith Fresnel element  26  is given as,  
           W   i   =r   2i+1   −r   2i ,   (2)  
         [0032]    where i=0, 1, 2, . . . , I, and W i  the width (edge-to-edge) of the ith Fresnel element  26 .  
         [0033]    With the above configuration, antenna  20  forms a S-CFZP antenna structure having a sectorial reflector  30  positioned a wavelength-dependent distance behind the Fresnel element  26 , and providing antenna gain and directionality control. While the reflector  30  is generally implemented as a partial cylindrical section (i.e., sector angle is less than 360 degrees), one or both the support  24  and the Fresnel elements  26  may be implemented as full or partial cylinders in any combination.  
         [0034]    When configured as a transmitting antenna, the feeder  22  functions as a radiating element, thereby serving as a radiating signal source for the antenna  20 . The Fresnel elements  26  direct the electromagnetic energy such that it is radiated outward from the antenna  20 . By positioning the reflector  30  behind the support  24 , radiated energy is greatly reduced behind the antenna  20 . Obviously, varying the size and position of the reflector  30  varies the areas relative to the antenna  20  at which radiated energy is controlled.  
         [0035]    One of the many advantages in being able to define one or more areas of reduced radiation is that the antenna  20  may be mounted on a wall or other reflective surface, without significant electromagnetic energy radiating backwards toward the mounting surface. This reduction in backward-radiated energy reduces the amount of reflected energy from mounting surfaces, thereby reducing multi-path propagation associated with the desired signals radiating from the antenna  20 . As noted earlier, radiation from the Fresnel elements  26  constructively interferes with the radiation from the reflector  30 , yielding a higher gain than is generally available with conventional dipole and monopole antennas.  
         [0036]    In general, the antenna  20  is subject to much variation in terms of its physical implementation. FIG. 5 illustrates several of these variations, where the placement of the Fresnel elements  26  is opposite that shown in FIG. 4, and where the monopole configuration of feeder  22  is replaced with a “microstrip” patch antenna configuration positioned at the foci of the Fresnel elements  26 . As such, the microstrip patch antenna  22  can be mounted or otherwise fixed to the ground plate  34 , but is not necessarily fixed to the ground plate  34 . Of course, the feeder  22  is not limited to monopole or microstrip patch antenna configurations, and may be implemented using a variety of other antenna feeder configurations, including various dipole configurations.  
         [0037]    In this particular configuration, the ground plate  34  comprises a circular disc, which may be solid or laminate in structure and preferably includes one or more conductive, planar layers, and which has a radius R substantially equal to the radius of curvature of the support  24 . As such, the feeder  22  is positioned at the center of the ground plate  34 . Of course, the feeder  22  may not be positioned at the geometric center of the ground plate  34  depending upon the shape of ground plate used.  
         [0038]    [0038]FIG. 6 illustrates further exemplary variations on the antenna  20 . Here, the support  24  is implemented as a complete right circular cylinder, and the Fresnel elements  26  form complete cylindrical hoops facing the feeder  22  and are positioned on the inner cylindrical surface of the support  24 . The reflector  30 , however, retains its implementation as a partial cylinder, and covers the outer face of the support  24  over a desired sector angle. Again, outward radiation from the antenna  20  is substantially blocked by the reflector  30  over this desired sector angle, while the reflector&#39;s inward reflections toward the feeder  22  tend to boost antenna gain.  
         [0039]    As was noted earlier, the support  24  and the Fresnel elements  26  may be implemented at essentially any sector angle between 0 degrees and 360 degrees, in any combination of sector angles between the support  24  and the Fresnel elements  26 . That is, one or both the support  24  and Fresnel elements  26  may comprise a complete cylinder or a portion thereof, in any combination. FIG. 7 illustrates one such variation, and deviates from the antenna  20  shown in FIG. 6 with its implementation of a full cylindrical support  24  and sectorized Fresnel elements  26 , i.e., partial cylindrical sections. While the sector angle of the Fresnel elements  26  is shown equal to the sector angle of the reflector  30 , it should be understood that the two sector angles do not have to be equal. Indeed, the sector angle of the Fresnel elements  26  may be greater than or less than the reflector sector angle.  
         [0040]    [0040]FIG. 8 illustrates yet another exemplary embodiment of the antenna  20  and, in converse relation to FIG. 7, illustrates the Fresnel elements  26  as comprising complete cylindrical hoops, while the support  24  comprises a cylindrical section. The sector angle of the support  24  is shown equal to that of the reflector  30 , but it should be understood that the two sector angles do not need to be equal; the support&#39;s sector angle may be more or less than that of the reflector  30 . While not shown, the forward portion of each Fresnel element  26 , i.e., the portion of the loop diametrically opposite the support  24 , might be supported by a dielectric rod or other structural element that may be supported by the ground plate  34 .  
         [0041]    As regards the ground plate  34 , one notes that FIG. 8 illustrates a rectangular plate rather than the circular configurations shown in the other embodiments. In practice, variations on the extent and shape of the ground plate  34  are tolerated without significant changes in antenna performance. Of interest beyond the rectangular shape of ground plate  34  in this embodiment, one notes that ground plate  34  here is formed as a conductive wire mesh. In general, wire mesh can be used to form the ground plate  34  in essentially any shape, e.g., circle, rectangle, general polygon, or in some non-uniform shape. Of course, the same versatility in ground plate shape is available where the ground plate  34  is implemented as one or more planar layers of conductive material.  
         [0042]    [0042]FIGS. 9A through 9D illustrate such shape-based variations on ground plate configurations, but it should be noted that such illustrations are not meant as an exhaustive catalog of all possible variations. Use of the ground plate  34  is in generally beneficial because it allows the antenna  20  to mimic symmetrical pluralities of Fresnel elements  26  and  28  without the need for physically implementing both sets; however, the specific size and shape of it are not overly significant, and it may be altered to suit usage considerations and practical convenience.  
         [0043]    [0043]FIG. 10 illustrates implementation flexibility beyond ground plate shape and construction. FIG. 10 is an exemplary, segmented version of the antenna  20  wherein the Fresnel elements  26 , the dielectric support  24 , and the reflector  30  are segmented. Of course, variations on this segmenting approach include embodiments where, for example, the Fresnel elements  26  are segmented but the support  24  and reflector  30  remain curvilinear. In any case, ease of transportability and assembly/disassembly may be gained through segmenting portions of the antenna  20 . For example, with the segmenting shown, the antenna  20  may be disassembled into a number of relatively small parts, thereby facilitating convenient transportation and storage.  
         [0044]    Where the Fresnel elements  26  are implemented as a series of joined segments, the number of segments is chosen such that the segmented ring approximates an overall curved shape. Thus, by selecting an appropriate number of segments, the Fresnel elements  26  may be formed as a ring or partial ring that substantially conforms to the curvature desired for the dielectric support  24  on which they are mounted.  
         [0045]    From the implementation variety illustrated by the included drawings, those skilled in the art will recognize that the inventive antenna  20  is subject to much variation. However, its underlying characteristics of directionality and relatively high gain are consistent across its range of implementations. As such, it should be appreciated that the foregoing information is exemplary only, and should not be construed as limiting the range of applications and the variations suitable for antenna  20 . Indeed, the scope of the present invention is limited only by the scope of the following claims, and their reasonable equivalents.