Abstract:
An antenna capable of being joined to an antenna feed and being positioned perpendicular to a ground plane includes a conductive cylinder having a longitudinal slot. The antenna feed is connected across the slot. A plurality of dielectric rods are provided parallel to the slot with rod being positioned much less than one wavelength of the maximum operating frequency away from adjacent rods. The rods each have a length of at least 25 times its mean diameter is made from a material having a dielectric constant greater than 30. The combination of the conductive cylinder and dielectric rods provides increased bandwidth. A kit for modifying existing antennas is further provided.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention is directed to a cylindrical antenna having a broader bandwidth and a method for making such an antenna. 
     (2) Description of the Prior Art 
     Slotted cylinder antennas have been proposed in submarine applications before. For example, in U.S. Pat. No. 6,127,983, Rivera and Josypenko disclose a horizontally mounted slotted cylinder antenna for use in a towed buoy. Though somewhat broadband in performance, it is not suitable for vertical mounting over a groundplane. Removed from floating at the ocean&#39;s surface, the antenna becomes resonant and has a narrow bandwidth. 
     Slotted cylinder antennas are popular antennas for use in line of sight communications systems, especially where the carrier frequency exceeds 300 MHz.  FIG. 1  provides a diagram of a prior art slotted cylinder antenna  10 . Antenna  10  includes a metallic cylinder  12  having slot  14  cut into the wall of the cylinder  12 . Cylinder  12  can be any thickness as long as skin effects are avoided. Slot  14  is parallel to an axis  16  of cylinder  12 . Axis  16  is perpendicular to a ground plane  18 . In the antenna shown, slot  14  extends the entire length of the cylinder  12 . The interior of the cylinder or cavity is typically filled with air, but another dielectric material can be used. 
       FIG. 1  shows an end-fed version of this antenna, but this antenna can also be center-fed. In the end-fed version, a transmission line having a first conductor  20  is provided through the ground plane  18  and connected across the slot  14  near one end of the slot  14 . A second conductor  22  is shown grounded to the ground plane  18 . Transmission line can be either a balanced line, such as a twisted pair, or an unbalanced line, such as a length of coaxial line (shown). In either case, the feeding transmission line  18  must have two conductors in order to connect across slot  14 . The optimal frequency of this antenna  10  is given by the length of the slot  14 . The size of the cavity and the slot width govern bandwidth. 
     The dimensions of the antenna  10  components are critical to operating frequencies. Metallic cylinder  12  is typically made of copper and has an inner radius a, a thickness d and a height h 1 . Cylinder  12  is raised above the ground plane  18  by a distance h 2  so that it is not in contact with the ground plane. Slot  14  has a width w. Slot  14  is cut so that it extends the entire length of cylinder  12 . Slot  14  is parallel to axis  16 . 
     In this embodiment, antenna  10  is fed by a coaxial feed arrangement that penetrates the ground plane  18  beneath the antenna  10 . Outer conductor  22  of the coaxial feed is connected to ground plane  18  and to the bottom of cylinder  12  on the right hand side of slot  14 . Center conductor  20  of the coaxial feed is connected to the bottom of cylinder  12  on the left hand side of slot  14 . The coaxial feed is designed to have a standard 50 Ohm characteristic impedance. 
       FIG. 2  shows a computed voltage standing wave ratio (VSWR) for this antenna. The VSWR is a figure of merit used in determining the impedance bandwidth of the antenna. Typically, this bandwidth is defined as the continuous range of frequencies for which VSWR&lt;3:1. The passband of the antenna is indicated at  26 . For the example shown in  FIG. 2 , resonant character of the antenna can be seen in the oscillatory nature of the VSWR curve, and modest bandwidth in each passband. 
     SUMMARY OF THE INVENTION 
     It is a first object of the present invention to provide a vertically deployable antenna. 
     Another object is to provide such an antenna with greater bandwidth. 
     Yet another object is to provide an ability to modify preexisting slotted cylindrical antennas in order to enhance the bandwidth. 
     Accordingly, there is provided an antenna capable of being joined to an antenna feed and being positioned perpendicular to a ground plane includes a conductive cylinder having a longitudinal slot. The antenna feed is connected across the slot. A plurality of dielectric rods are provided parallel to the slot with rod being positioned much less than one wavelength of the maximum operating frequency away from adjacent rods. The rods each have a length of at least 25 times its mean diameter is made from a material having a dielectric constant greater than 30. The combination of the conductive cylinder and dielectric rods provides increased bandwidth. A kit for modifying existing antennas is further provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is made to the accompanying drawings in which are shown an illustrative embodiment of the invention, wherein corresponding reference characters indicate corresponding parts, and wherein: 
         FIG. 1  is a perspective view of a prior art antenna; 
         FIG. 2  is a graph of VSWR versus frequency for the prior art slotted antenna of  FIG. 1 ; 
         FIG. 3  is a partially cut-away perspective view of one embodiment of an antenna; 
         FIG. 4  is a graph of VSWR versus frequency for the antenna shown in  FIG. 3 ; 
         FIG. 5  is a perspective view of another embodiment of an antenna; 
         FIG. 6  is a perspective view of a third embodiment of the antenna; and 
         FIG. 7  is a perspective view of a fourth embodiment of the antenna. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 3 , there is shown an embodiment of an antenna  30 . Antenna  30  includes a slotted cylinder  12  having a slot  14  formed longitudinally therein. Slot  14  is parallel with an axis  16 . Slotted cylinder  12  is perpendicular to ground plane  18 . Slotted cylinder is end fed by a two conductor feed including a first conductor  20  and a second conductor  22 . First conductor  20  is joined to slotted cylinder  12  at a first side of slot  14 . Second conductor  22  is joined to slotted cylinder  12  at a second side of slot  14  opposite the first side. An insulator  24  is provided between slotted cylinder  12  and ground plane  18 . A plurality of dielectric rods  32  are provided outside slotted cylinder  12 . Rods  32  are arranged radially and regularly spaced around slot  14  with the axis of each rod  32  being parallel to slot  14 . In the tested embodiment, ten rods  32  were utilized, but more or fewer rods  32  could be utilized. The number of rods is selected so that the spacing between the rods is much smaller than the shortest wavelength of operation; however, spacing between rods  32  is not critical as long as the spacing is much smaller than the shortest wavelength. Spacing between the rods was about 2.4% of the wavelength at the highest frequency in the tested embodiment. It is believed that this spacing could be as much as 5% of the wavelength at the highest frequency or as little as 1% while still maintaining this broadening effect. The spacing between the rods should also be at least the diameter of one of the rods. This avoids the rods acting as a solid cylinder of material. The rods should extend around the entire slotted cylinder  12  to interact with all of the near field energy produced by the slotted cylinder  12 . This means that the rods  32  should extend beyond the maximum and minimum vertical extents of cylinder  12 . A cylindrical rod holder  34  has a plurality of apertures  36  formed longitudinally therein. Apertures  36  are dimensioned and arranged to accommodate the dielectric rods  32 . Holder  34  is provided over rods  32  to maintain their orientations and spacings. 
     Slotted cylinder  12  is a regular hollow metallic right cylinder. This can be made from any highly conductive metal such as copper or the like in order to conduct electric current. The thickness of slotted cylinder is not critical; however, the length of the cylinder and the width of the slot relate to the design frequency of the antenna. Cylinder  12  is separated from ground plane  18  by an insulator  24  which can be an air gap or an insulating material. In a tested embodiment, slotted cylinder was 4 inches long with an outer diameter of 0.75 inches. The slot was 0.125 inches. Cylinder  12  was insulated from ground plane  18  using insulator  24  which was a 0.0625 inch layer of Rogers Duriod® which is a commercially available insulator. 
     In the embodiment shown, the dielectric rods  32  are arranged parallel to and equidistantly from slot  14  at a fixed radius. This radius should be approximately the same as the shortest operating wavelength of the antenna. Rods should be at least 10% longer than cylinder  12  and slot  14  in order to influence the electromagnetic radiation extending from cylinder  12 . All of the rods  32  have an identical length. Rods  32  are made from a material with a high dielectric constant relative to free space. Testing found that a dielectric constant of approximately 30 was acceptable. Dielectric materials with a lower dielectric constant are unacceptable because the high impedance of the specified rods provides a contrast with the impedance of the surrounding space. Since impedance varies as the square root of the reciprocal of dielectric constant, the rods must have a fairly high dielectric constant of around 30 to get a proper contrast in impedances of greater than 5:1. The rods must also be long in comparison to their mean diameter. In the preferred case, the rods are at least 25 times longer than their diameter. The plurality of rods  32  can be rods having a circular cross-section. Rods  32  having other cross-sections are possible. In these embodiments the length and the mean diameter of the cross-section is used to give the proper aspect ratio. In a tested embodiment, rods  32  were 7 inches long and had a diameter of 0.25 inches. Rods  32  were made from a barium-titanate and epoxy resin material. 
     Cylindrical rod holder  34  must be made from a material having a dielectric constant lower than that of rods  32  by a factor of at least 1:10 in order to preserve the contrast between rods  32  and surrounding space. In the tested embodiment, rod holder  34  was made from polycarbonate and had a dielectric constant of approximately 2.4. Holder  34  was 7 inches tall with a 3.5 inch outer diameter and a 2.5 inch inner diameter. 0.25 inch longitudinal channels were drilled in holder  34  to accommodate rods  32 . 
     The tested VSWR of antenna  30  is shown in  FIG. 4 . As before, bandwidth is indicated as the region where VSWR is around 3:1. This region is indicated as  36 . When compared with the prior art antenna of  FIG. 1 , the embodiment shown in  FIG. 3  provides an increase in bandwidth  36  over that indicated in the prior art plot provided as  FIG. 2 . 
       FIG. 5  shows an alternate embodiment  30 ′ of the current antenna. This antenna  30 ′ features slotted cylinder  12  having slot  14 . As before, slotted cylinder  12  is fed by a two conductor feed including first conductor  20  and second conductor  22  positioned on either side of slot  14 . Slotted cylinder  12  is insulated from ground plane  18  by insulating material  24 . Dielectric rods  32  are positioned equidistantly from slot  14  and parallel to slot  14 . Rods  32  are separated from one another by the same angle. Rods  32  extend perpendicular to ground plane  18 . Antenna  30 ′ of  FIG. 5  differs from antenna  30  of  FIG. 3  by omission of cylindrical rod holder  34 . It is suggested that this configuration renders antenna  30 ′ lighter while making it less durable. 
       FIG. 6  shows yet another alternate embodiment of the current antenna. In this embodiment, antenna  30 ″ utilizes retaining brackets  38  in place of rod holder  34 . Similar components of this antenna  30 ″ are numbered as before. Retaining brackets  38  are positioned at various lengths along rods  32 . Retaining brackets  38  are circular with apertures having a radius and spacing to accommodate rods  32 . The number and positioning of brackets  38  is dictated by the need for structural support of rods  32 . Brackets  38  can be fixed to rods by means known in the art. Brackets can be made from any material having a dielectric constant near that of the operating environment so as to avoid influencing the antenna. 
       FIG. 7  shows another embodiment of antenna  30 ′″ that utilizes arbitrary positioning of rods  32  about cylinder  12  and slot  14 . Rods  32  are generally spaced apart by between 1 and 5% of the wavelength of the highest operating frequency. Rod spacing should not be limited to circular and regular positioning. 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. For example, specific measurements are provided for components of the antenna; however, these measurements can be scaled to give different pass bands making the antenna applicable to operating frequencies other than those disclosed. Furthermore, the rods and retaining cylinder or brackets can be formed by other means known in the art such as by additive manufacturing. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description only. It is not intended to be exhaustive, nor to limit the invention to the precise form disclosed; and obviously, many modification and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.