Patent Publication Number: US-9407010-B1

Title: Slotted antenna with anisotropic covering

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 slotted antenna having enhanced broadband characteristics. 
     (2) Description of the Prior Art 
     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 . 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  18  is provided through the cylinder  12  and connected across the slot  14  near one end of the slot  14 . Transmission line  18  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 is given by the length of the slot. The size of the cavity and the slot width govern bandwidth. 
     SUMMARY OF THE INVENTION 
     It is a first object of the present invention to provide a compact antenna capable of transmitting and receiving. 
     Another object is to provide such an antenna having broader band characteristics than heretofore known. 
     Yet another object is to provide enhancements to an existing slotted antenna. 
     Accordingly, there is provided an antenna that includes a tubular, conductive radiator having a longitudinal slot formed therein from a first end of the conductive radiator to a second end of the conductive radiator. An antenna feed can be joined to the conductive radiator adjacent to and across the slot. An anisotropic plate is positioned a uniform distance from the conductive radiator, centered above the slot. The plate extends beyond the length of the radiator and is electrically insulated therefrom. An anisotropic tube surrounds the plate and radiator. The anisotropic tube is electrically insulated from the plate and radiator. In use, this antenna gives enhanced bandwidth over ordinary slotted antennas. This can also be applied to preexisting slotted antennas for enhanced bandwidth. 
    
    
     
       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 cut away view of one embodiment of the antenna; 
         FIG. 3  is a graph of VSWR versus frequency for a prior art slotted antenna; 
         FIG. 4  is a graph of VSWR versus frequency for an antenna according to the current invention; and 
         FIG. 5  is a perspective view of another embodiment of an antenna in accordance with the current invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  provides a cut-away view of a slotted antenna  20  having a feed  18  joined to a slotted cylinder inner radiator  12 . Radiator  12 , slot  14  and feed  18  are substantially the same as shown in  FIG. 1 . Hidden portions of these objects are shown with dashed lines. Antenna  20  further includes an anisotropic cylinder portion  22  positioned apart from radiator  12  above slot  14 . Portion  22  is coaxial with radiator  12  and extends angularly 60 degrees on either side of slot  14 . Total angular extent of cylinder portion  22  is about 120 degrees. Testing determined that this was sufficient to capture emissions from radiator  12 ; however, this could have a smaller overlap. An anisotropic outer housing  24  is positioned outside of and surrounding cylinder portion  22  and radiator  12 . Outer housing  24  is spaced apart from radiator  12  and portion  22  and coaxial with radiator  12  and portion  22 . Outer housing  24  and portion  22  extend beyond the ends of radiator  12  by about 10% of the length of radiator  12  in order to utilize the electromagnetic fields that fringe around the ends of radiator  12 . This was experimentally determined and could be optimized to a smaller or larger extension to completely enclose the field generated by radiator  12 . 
     The interior regions of radiator  12 , portion  22  and outer housing  24  can be filled with a dielectric material having a low dielectric constant. This can be a gas such as air or a solid such as syntactic foam. A liquid having this property could also be used. 
     Cylinder portion  22  and outer housing  24  are made from an anisotropic dielectric material. The dielectric tensor of both the cylinder portion  22  and outer housing  24  are engineered to impact the electric field that is generated in radiator  12 . Due to the way in which this antenna is fed by the coaxial line, this electric field will be in the circumferential direction relative to the axis of the antenna. In the preferred embodiment, the material has a uniaxial dielectric tensor |∈| in cylindrical coordinates in which ∈ ρρ =∈ zz =1 and ∈ φφ ˜10. (Off-diagonal elements in the tensor are negligibly close to zero.) Note that this tensor assumes a cylindrical coordinate system with the z axis coincident with the axis of the antenna. In another embodiment the tensor may be expressed in rectangular coordinates and be biaxial such that ∈ xx =∈ yy &gt;1 and ∈ zz ˜1. 
     The anisotropic dielectric material can be any such material known in the art having these characteristics. In one embodiment this material is a rectangular mesh having conductors oriented circumferentially and longitudinally printed on a dielectric backing. This material could also be a semiconductor material or a nanostructured material having these characteristics. 
     The presence of the two anisotropic layers, portion  22  and housing  24 , increases the electrical length of the slot  14  above its first resonance and keeps the equivalent magnetic current density relatively stable, leading to improved impedance bandwidth. 
     Measured plots of the voltage standing wave ratio (VSWR) for a prototype are shown in  FIG. 3  and  FIG. 4 .  FIG. 3  shows the VSWR for an antenna  10  without anisotropic portion  22  and cylinder  24 .  FIG. 4  is the VSWR plot for antenna  20  shown in  FIG. 4 . The VSWR of an antenna is typically used to define its bandwidth; normally a value less than 2:1 is considered the maximum VSWR in the passband. In  FIG. 3 , the valley indicated at  26  indicates a narrow passband at 1.25 GHz. The antenna has relatively high impedance in the frequencies above this frequency. The data shown in  FIG. 4  indicates a first resonance in the valley shown at  28  centered about 1.125 GHz. A second passband is given by the broad region  30  above first resonance that runs from approximately 2.18 to 4.48 GHz. This is roughly one octave of bandwidth. This is more than is expected for a slotted cylinder antenna of this size. 
       FIG. 5  shows an alternate embodiment of slotted antenna utilizing a slot  34  in a rectangular inner radiator  32 . Feed  36  is joined across slot  34 . As before, inner radiator  32  is made from a metallic material having sufficient thickness to prevent skin effects. An anisotropic portion  38  is positioned adjacent to inner radiator  32  at slot  34 . Portion  38  should be less than one wavelength of the operating frequency away from slot  34 . As before, portion  38  should not be in electrical conduction with inner radiator  32 . Portion  38  can be flat in conformance with the outer surface of inner radiator  32 . For optimal performance, portion  38  should extend a sufficient distance on either side of slot  34  to capture electromagnetic radiation from slot  34 . An outer anisotropic housing  40  is positioned outside of inner radiator  32  and portion  38 . Outer housing  40  should be separated from inner radiator  32  by less than one wavelength at the operating frequency of the antenna. Portion  38  and outer housing  40  extend beyond the ends of inner radiator  32  sufficiently far to capture electromagnetic radiation emanating from the ends of inner radiator  32  in order to maximize efficiency. 
     Both portion  38  and outer housing  40  are made from an anisotropic dielectric material. This material must have a much greater resistivity in the direction across the face of portion  38  and housing  40 , perpendicular to slot  34 . This corresponds to the cylindrical coordinates given with reference to  FIG. 2 . Like the cylindrical embodiment of  FIG. 2 , rectangular embodiment of  FIG. 5  utilizes portion  38  and outer housing  40  to give enhanced bandwidth; however, it is believed that this embodiment will have a less effective pattern than that of the cylindrical embodiment. Other geometries can be used of this antenna depending on the space available and the required beam pattern of the antenna. 
     This antenna has a greatly improved bandwidth, meaning that it can be used to support multiple communications services where several separate antennas might have been needed before. Given that the slotted cylinder antenna is one that is sometimes used in cellular communications towers as well as in digital television broadcast towers, a broadband version of this antenna might have significant applications in use on communications towers in order to host multiple services on a single antenna. 
     This antenna can be made by modifying existing slotted antennas by retrofitting these antennas with anisotropic portions and anisotropic outer radiators. This will improve the bandwidth of the existing antenna and allow greater flexibility. 
     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. 
     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.