Abstract:
A broadband cage antenna about a mast of a vessel is described, containing a first plurality of wires aligned in a substantially vertical orientation and arranged circumferentially around the mast; a second plurality of wires aligned in a substantially horizontal orientation and placed around the mast, a first wire of the second plurality of wires joining, near a top portion of the mast, all the first plurality of wires, and a second wire of the second plurality of wires joining, near a bottom portion of the mast, all the first plurality of wires; and an antenna feed coupled to the second wire of the second plurality of wires, wherein the first and second plurality of wires are electrically insulated from the mast, to form a broadband antenna having a VSWR response of less than 4 over a designated frequency range.

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     This invention (Navy Case No. 099086) is funded by the United States Department of the Navy. Licensing inquiries may be directed to the Office of Research and Technical Applications, Space and Naval Warfare Systems Center, San Diego, Code 2112, San Diego, Calif., 92152; voice 619-553-2778; email T2@spawar.navy.mil. 
    
    
     BACKGROUND 
     This disclosure relates to communication systems. More particularly, this disclosure relates to a broadband cage antenna system surrounding a ship&#39;s mast. 
     SUMMARY 
     The foregoing needs are met, to a great extent, by the present disclosure, wherein systems and methods are provided that in some embodiments provide for a broadband antenna composed of open wires disposed over a ship&#39;s mast. 
     In accordance with one aspect of the present disclosure, a broadband cage antenna about a mast of a vessel is provided, comprising: a first plurality of wires aligned in a substantially vertical orientation and arranged circumferentially around the mast; a second plurality of wires aligned in a substantially horizontal orientation and placed around the mast, a first wire of the second plurality of wires joining, near a top portion of the mast, all the first plurality of wires, and a second wire of the second plurality of wires joining, near a bottom portion of the mast, all the first plurality of wires; and an antenna feed coupled to the second wire of the second plurality of wires, wherein the first and second plurality of wires are electrically insulated from the mast, to form a broadband antenna having a VSWR response of less than 4 over a designated frequency range. 
     In accordance with another aspect of the present disclosure, a broadband cage antenna about a mast of a vessel is provided, comprising: a plurality of first conducting means for conducting electrons, aligned in a substantially vertical orientation and arranged circumferentially around the mast; a plurality of second conducting means for conducting electrons aligned in a substantially horizontal orientation and placed around the mast, a first conducting means of the plurality of second conducting means joining, near a top portion of the mast, all the plurality of first conducting means, and a second conducting means of the plurality of the second conducting means joining, near a bottom portion of the mast, all the plurality of the first conducting means; and a feeding means for receiving/transmitting electricity, coupled to the second conducting means of the plurality of the second conducting means, wherein the pluralities of the first and second conducting means are electrically insulated from the mast, to form a broadband antenna having a VSWR response below 4 over a designated frequency range. 
     In accordance with another aspect of the present disclosure, a method for A method for broadband coupling of electromagnetic waves using a cage of wires placed about a mast of a vessel, comprising: aligning a first plurality of wires in a substantially vertical orientation in a circumferential arrangement around the mast; aligning a second plurality of wires in a substantially horizontal orientation around the mast; joining a first wire of the second plurality of wires, near a top portion of the mast, to all the first plurality of wires; joining a second wire of the second plurality of wires, near a bottom portion of the mast, to all the first plurality of wires; and coupling an antenna feed to the second wire of the second plurality of wires, herein the first and second plurality of wires are electrically insulated from the mast, to form a broadband antenna having VSWR response of less than 4 over a designated frequency range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is an illustration of an exemplary cage antenna on a ship&#39;s mast. 
         FIG. 2  is a representation of an exemplary scale cage antenna on a scale ship&#39;s mast over a small ground plane. 
         FIGS. 3A-B  are VSWR and Smith chart response plots of the exemplary scale cage antenna system of  FIG. 2 . 
         FIG. 4  is a representation of another exemplary scale cage antenna on a scale ship&#39;s mast over a large ground plane. 
         FIGS. 5A-B  are VSWR and Smith chart response plots of the exemplary scale cage antenna system of  FIG. 4 . 
         FIGS. 6A-B  are VSWR and Smith chart response plots of another exemplary scale cage antenna system. 
         FIGS. 7A-B  are VSWR and Smith chart response plots of another exemplary scale cage antenna. 
         FIGS. 8A-B  are VSWR and Smith chart response plots of another exemplary scale cage antenna system. 
     
    
    
     DETAILED DESCRIPTION 
     Antennas and their placement on a ship are an ongoing concern with large and small vessels. This concern is exacerbated with the need for deployment of multiple antennas in a limited space platform, particularly on the deck of a war-faring vessel. As such, the ship&#39;s mast has been used as the principal platform for placing antennas. Because the mast is elevated from the deck of the ship, it offers less interference for the antennas. However, most of the ship&#39;s existing antennas are already on the mast, rendering it difficult to attach any additional antennas without increased inter-antenna interference. Also, high frequency (HF) antennas are understood to be physically large, rendering them to be difficult to position on the mast without interference, either physically or electromagnetically. 
     As disclosed below, a proposed solution is to use a cage antenna that is situated about the mast. Specifically, a cage antenna is fitted to the mast as a wire frame. The openness of the cage antenna enables it to be placed near the mast without physically blocking other antennas on the mast, and it is understood not to electrically interfere with these other antennas because of its frequency differentiation. The wires are placed in a predominately longitudinal orientation, stretching from the base of the mast to an upper portion of the mast. In the context of this disclosure, the term “vertical” may be used as a proxy to the term “longitudinal” and is understood to connote a general direction rather than an absolute form. Also, it should be noted that the wires described herein, may be covered with insulation to provide a degree of protection and/or resilience to the elements. 
     In this exemplary configuration, the cage antenna and the mast can be designed to operate as coupled mutual impedances. By varying the distance and shape of the cage from the surface of the mast, the coupling can be adjusted to affect the resulting frequency response. Also, by varying the height of the cage, the frequency response can be changed. Therefore, the complex mast geometry with the cage antenna can form the equivalent of a “fat” monopole antenna, which is known to have broadband capabilities. Based on this understanding, measurements and experiments have been performed to evaluate the broadband performance of a cage antenna positioned around a ship&#39;s mast. 
     In the experiments shown below, a 1:48 scale model of the cage antenna is placed over a 1:48 scale model of a ship&#39;s mast and evaluated from scale frequencies of 96.0 MHz to 432.0 MHz. Given that a 1:48 scale is used, the scale frequencies can be converted to non-scale frequencies by simply dividing by 48. For example, a scale frequency of 350 MHz is equivalent to a non-scale frequency of 7.29 MHz. (i.e., 350/48=7.29). Of note in the following Figures is that under certain protocols, an antenna can be considered “acceptable” if it demonstrates a voltage standing wave ratio (VSWR) of less than 4 over a non-scale frequency range of 2-7 MHz. Accordingly, if a 1:48 scale antenna demonstrates a VSWR of less than 4 between 96 MHz to 336 MHz, it is understood that a non-scaled version of the antenna will be an acceptable broadband antenna. 
       FIG. 1  is a diagram illustrating a ship  10  with an exemplary cage antenna  12  positioned about the ship&#39;s mast  14 . The exemplary cage antenna  12  is composed of four vertically oriented wires  16  and two horizontally oriented rings, upper ring  18   a  and lower ring  18   b , to form upper and lower sections of the cage antenna  12 . The upper ring  18   a  is placed just below the top yard arm  13  of the mast  14  and runs through the closed loop formed by the mast  14 , the top yard arm  13 , and support members  17 . The lower ring  18   b  is placed at the base of the mast  14 . The vertically oriented wires  16  are connected to the two rings  18   a  and  18   b , to bound the vertical arms of the cage antenna  12 . On the lower ring  18   b , a transmitter/receiver is  19  is attached to the cage antenna  12  via an antenna feed assembly  15 . 
     It should be noted that while  FIG. 1  illustrates the vertically oriented wires  16  as being substantially parallel to the ship&#39;s mast  14 , the vertically oriented wires  16  may be bowed or shaped to provide the desired clearances and frequency responses, as needed, as also made apparent in the ensuing Figures. Also, the terms vertically oriented and horizontally oriented are understood to signify the general orientation of the wires and is not to be construed to provide an absolute, unyielding direction. Thus, these terms may be used with the term substantial to signify their overall orientation, without loss of generality. 
     The antenna feed assembly  15  may be located anywhere between the transmitter/receiver  19  and the cage antenna  12 . The antenna feed assembly  15  may comprise several devices. For example, a current probe may be used in the antenna feed assembly  15  to excite/sample the cage antenna  12 . Also, the antenna feed assembly  15  may be matched to the cage antenna  12  using a passive or active matching network. Further, an antenna tuner (e.g., frequency tuner—not shown) may be incorporated in the antenna feed assembly  15 . Alternatively, a direct excitation of the cage antenna  12  may be utilized. Accordingly, it is apparent that multiple forms of antenna feed assemblies may be used, whether in combination with the devices described or with other devices/capabilities, as is known in the art. Therefore, modifications to the antenna feed assembly  15  and attendant devices may be made without departing from the spirit and scope of this disclosure. 
     It is noted that the vertically oriented wires  16  and horizontally oriented rings  18   a  and  18   b  are electrically insulated from the mast and the inherent ground plane formed by the ship&#39;s deck and ocean. Also, it is noted that while the exemplary embodiments described herein use only four vertically oriented wires  16 , more or less wires may be used according to design preference. Out of simple convenience and for experimental expediency, only four vertically oriented wires  16  were utilized. However, in some instances, it may be desirable to use more or even less wires, depending on design objectives. 
     Using the exemplary cage antenna  12  shown in  FIG. 1 , a series of empirical measurements were performed, varying the length of the vertically oriented wires  16  (and ensuing separation distance from the mast  14 ) to test the performance of the overall antenna system. These performance tests were compared to baseline values and also to each other to see which cage configuration provided the best VSWR/Smith chart response. This was then matched with corresponding photos to visualize the physical shape. 
     An Anritsu S312D SiteMaster analyzer was used as the principal measuring device. The analyzer was used to measure the VSWR of the input which was transferred to a computer for Smith chart generation. In the experimental setup, a scale model of the mast  14  was secured to the ground plane with electrically conducting tape. With the scale model in place, the analyzer was calibrated and measurements were taken from 96-432 MHz. Photos were taken of the setup so that the VSWR results could be compared to the physical model. 
       FIG. 2  is a representation of an exemplary 1:48 scale cage antenna on a 1:48 scale ship&#39;s mast, and is instructive in showing that the vertically oriented wires  16  may be curved. The outward curvature arises from the length of the vertically oriented wires  16  being longer than the actual height of the cage antenna. In consideration of the 1:48 scale, the vertically oriented wires  16  are equivalent to approximately 70 feet in straight length for a non-scaled antenna. The cage antenna system of  FIG. 2  is shown as being tested inside a building. Consequently, the ground plane surface  22  of the cage antenna system is understood to be limited in size as compared to the ground available in an outdoor testing range. 
       FIGS. 3A-B  illustrate the VSWR and Smith chart response plots for the cage antenna system of  FIG. 2 . As noted earlier, if the VSWR can be constrained to be below 4 from 98-336 MHz, then is can be considered an “acceptable” broadband antenna. In this example, the VSWR plot of  FIG. 3A  shows that the cage antenna system demonstrates an unacceptable VSWR level (above 4) at the lower end of the plot and near the 336 MHz area of the plot.  FIG. 3B  reinforces this observation by showing the real and imaginary values of the input impedance of the cage antenna system cycling away from the center of the Smith chart through the different frequencies. The starting frequency of 98 MHz is illustrated by reference number  36  and the stopping frequency of 432 MHz is illustrated by reference number  38 . 
     Given the poor performance of FIG.  2 &#39;s cage antenna system, the lengths of the vertically oriented wires  16  were changed, as shown in the following Figures. 
       FIG. 4  is a representation of another exemplary scale cage antenna on a scale ship&#39;s mast over a large ground plane. FIG.  4 &#39;s cage antenna system principally differs from FIG.  2 &#39;s cage antenna system in that the vertically oriented wires  16  are approximately 100 feet in length (non-scale) and that testing was performed outdoors over a large ground plane  42 . With longer vertically oriented wires  16 , the cage antenna is more displaced from the ship&#39;s mast, forming more a spherical shape than that of FIG.  2 &#39;s prolate shape. 
       FIGS. 5A-B  are VSWR and Smith chart response plots of the exemplary scale cage antenna of  FIG. 4 . From  FIG. 5A , it is evident that an unacceptable VSWR level occurs near 200 MHz, rendering this antenna non-suitable for use. FIG.  5 B&#39;s Smith chart details the real and imaginary input impedance track with starting frequency indicated by reference number  52  and stopping frequency indicated by reference number  54 . 
       FIGS. 6A-B  are VSWR and Smith chart response plots of another exemplary scale cage antenna with wires  16  of approximately 88 feet (non-scaled) in length. It is noted in  FIG. 6A , that there is an unacceptable VSWR level occurring at approximately 360 MHz. However, this value is above the 336 MHz threshold (non-scaled frequency of 7 MHz) discussed above. And the VSWR values from 98 MHz to 336 MHz are below 4. Therefore, this cage antenna configuration fits within the requirements for an acceptable antenna. This is further evidenced in  FIG. 6B , where the starting frequency point is indicated by reference number  62  and the stopping frequency point is indicated by reference number  64 ; and a smooth initial circle is traced about the center of the Smith chart, for the lower frequencies. In summary, the antenna of  FIG. 6 , using 88 feet long vertically oriented wires  16 , is demonstrated as a suitable antenna for operation between 2-7 MHz. 
       FIGS. 7A-B  are VSWR and Smith chart response plots of an exemplary scale cage antenna using wires  16  of approximately of 76 feet (non-scaled). Here we see in  FIG. 7A  that the VSWR is below 4 for most of the desired frequency range. However, at around 310 MHz an unacceptable VSWR level is detected. Based on this result, the length of 76 feet (non-scaled) does not provide a better antenna than of  FIGS. 6A-B .  FIG. 7B  shows the attendant Smith chart behavior. 
       FIGS. 8A-B  are VSWR and Smith chart response plots of the exemplary scale cage antenna of  FIG. 2  placed over a large ground plane in an outdoor testing range. We see in  FIG. 8A  that the VSWR is markedly lower over the whole frequency range, though not sufficient for antenna performance acceptance. Consequently, it is understood that the size of the ground plane can affect the measured results. Given that the ocean is considered to have ground plane-like characteristics, this test scenario is understood to be more representative of actual conditions.  FIG. 8B  is the corresponding Smith chart showing the start frequency point  82  and the stop frequency point  84 . 
     Based on the above results, it has been shown that a cage antenna having only four vertically oriented wires  16  can be successfully operated with a low VSWR between the non-scale frequencies of 2-7 MHz. Of course, as one of ordinary skill may be aware, other frequencies can be successfully operated in by appropriately adjusting the length of the vertically oriented wires  16 , or in some instances the height of the cage antenna structure. As shown in the above embodiments, a reasonable and repeatable trial-and-error procedure was used to obtain the desired cage antenna, by simply increasing or decreasing the length of the vertically oriented wires  16 , for a fixed cage antenna height on a fixed ship&#39;s mast  14 . Thus, it is within the scope of this disclosure to implement other modifications, such as contouring the vertically oriented wires  16  in a more acute angle, for example, versus a smooth curve, to enable more degrees of freedom around the ship&#39;s mast  14 . Or, to have the vertically oriented wires  16  form a gentle screw-thread like curve as they rise from the base of the mast  14 . Also, fewer wires may be used, for example three vertically oriented wires  16 , which would provide more spacing for accommodation of mast protrusions, other antennas, and such. 
     For reference purposes, the above cage antenna examples were tested using a pre-configured ship&#39;s mast  14  having a defined yardarm span and profile. The ship&#39;s mast  14  being at 1:48 scale, the equivalent non-scale dimensions of the ship&#39;s mast  14  would be: 
     Lower yardarm span of approximately 70 feet. 
     Upper yardarm span of approximately 60 feet. 
     Stub mast (portion above the upper yardarm) of approximately 28 feet. 
     Base of approximately 11 feet in diameter and approximately 4 feet in diameter at the top. 
     Using the above non-scaled dimensions, the cage antenna&#39;s overall height would be approximately 66 feet, from the bottom ring  18   b  to the top ring  18   a . The diameter of the rings  18   a ,  18   b  would be approximately 11 feet. Thus, based on the type of ship&#39;s mast  14  being used, the cage antenna&#39;s parameters may be accordingly altered, without departing from the spirit and scope of this disclosure. 
     What has been described above includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments. It will, therefore, be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated 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.