Abstract:
An antenna including a feed tube with radial fins and circular plates at the ends of the tube and fins thereby forming a boundary for a plurality of resonant cavities. Curved plates, connected to the tube by switches of a switching system, partially encompass and subtend to the length of the tube. Interior to the tube, a transmission line from an end plate terminus conducts radio-frequency energy from the terminus to a hub and onto a switch of the switching system in which the switch is mechanically reactive to and actuated by a righting action of the curved plates when the curved plates encounter a sea state. When actuated, energy from the switch distributes to a proximate resonant cavity and curved plate to form a radiation pattern based on the difference in phase of the resonant cavity and curved plate.

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. 

   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The present invention relates to antennas and more particularly to radiators for low profile, towed antennas. 
   (2) Description of the Prior Art 
   Present submarine communications with battlegroups or shore sites utilize surface antennas for a variety of requirements including SATCOM, LOS, etc. The use of surface antennas typically interferes with the covert operation of the submarine. For example, data exchange or the receipt of commands is accomplished by using antennas within a mast, which must be extended whenever transmission or reception is required. For communications in coastal or littoral areas, raising a mast renders the submarine vulnerable to visual or radar detection. To mitigate such detection, buoyant cable antennas (BCA) are often used. However, current BCAs cannot be used effectively for transmission, due to their extremely low radiation efficiency. 
   Furthermore, antennas towed on the ocean surface are subjected to dynamic forces that act to cause the antenna to pitch, yaw and sometimes roll under varying sea states. These antenna movements can easily result in transmission and reception interruption, especially so with the use of directional antennas. As a result, the towing submarine must operate in a station keeping status or must constantly adjust course headings in order to obtain optimal antenna performance. 
   In Rivera et al. (U.S. Pat. No. 6,127,983), there is disclosed a wideband antenna capable of transmission and reception while the antenna is towed horizontally in the ocean behind the submarine or vessel. Specifically, the antenna of the cited reference is formed as a metal cylinder having a longitudinal slot with the longitudinal slot open at one end and closed at the other end. The cylindrical shape in a towing container provides a strong righting moment to the antenna with the result of efficient broadband coverage under varying sea states. 
   Also, by setting the terminations of the antenna, that is, the open end, the closed end, and the feedpoint (along with the antenna diameter and thickness, and slot length and width) an antenna having a good impedance match over a wide frequency band is produced. 
   As disclosed, the above antenna is clearly suitable for wideband transmission when being towed in the ocean; however, an alternative antenna is desirable to produce an increased effectiveness during operation and an increased range of use when compared to the above antenna as well as for other known buoyant antennas. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is a general purpose and primary object of the present invention to provide an antenna that can transmit a directionalized radiation pattern with minimal interruption when operating in varying sea states. 
   It is a further object of the present invention to provide an antenna in which the antenna construction is simple and economical. 
   It is a still further object of the present invention to provide an antenna with an increased antenna gain. 
   It is a still further object of the present invention to provide an antenna that operates efficiently over a wide band of frequencies. 
   It is a still further object of the present invention to provide an antenna in which the operation of the antenna is roll stable. 
   It is a still further object of the present invention to provide an antenna that emits a symmetrical radiation pattern in the fore/aft and athwart directions. 
   To attain the objects described there is provided a gravity-actuated antenna suitable for towing horizontally on the ocean surface in which the antenna includes a switching system that actuates the antenna when facing “up” toward the sky or ocean surface. The antenna comprises a cylindrical feed tube with three radially extending fins and disk plates secured to ends of the feed tube and the fins. A plurality of the curved plates spaced apart an extending plane of the fins and projecting from an end plate partially encompass and subtend to the length of the feed tube with each curved plate connected to the feed tube by the protecting structure of a gravity-actuated electrical switch. 
   The fins of the antenna are spaced evenly around the circumference of the feed tube. Each fin is sized to form a longitudinal radiation boundary of a resonant cavity and the end plates are sized to form an athwart radiation boundary of the resonant cavity with the exterior of the feed tube forming the base of the resonant cavity. The boundaried resonant cavity is shallow enough that the cavity is not shadowed by the radial fins and the end plates. Without a shadow condition restricting a wavelength generated in the resonant cavity during antenna actuation, a resultant symmetrical radiation pattern can be transmitted in conjunction with the actuation of a specified curved plate. 
   The feed tube encompasses a first transmission line from a feedpoint terminus at one end plate to a cylindrical feed hub within the feed tube. The transmission line is capable of conducting radio-frequency energy from the terminus to the hub and onto an individual electrical switch when the switch is gravity-actuated as a result of a righting motion of the curved plates. Energy from the hub via the switch and onto a specified curved plate and further onto the resonant cavity results in a current distribution across the curved plate and the resonant cavity such that a difference in phase between both results in the radiation pattern beamed from the antenna. Based on the sizing of the components of the antenna, the resultant radiation pattern can be transmitted from a fore and aft direction in relation to the antenna as well as at an athwart direction and at a direction perpendicular to the axis of the feed tube. 
   By decreasing the diameter of the transmission line from the feedpoint terminus to the hub, the transmission line performs an impedance transformation over its length. The impedance transformation of the transmission line among varying diameters presents a variable load (Ω) at the feedpoint terminus thereby allowing the antenna to emit over a range of frequencies. 
   A second transmission line with a diameter equal to the smallest diameter of the first transmission line and electrically connectable to the hub, continues from the hub onto a second terminus at the other end plate. The second transmission line and the second terminus behave as a reactive impedance to match the impedance at the connection of a pin of the switch and the hub. By matching the impedance, an optimum amount of radio-frequency energy can be transferred onto the actuated switch and curved plate with a result in increased gain of the antenna. 
   The above and other features of the invention, including various and novel details of construction and combinations of parts will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular devices embodying the invention are shown by way of illustration only and not as the limitations of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention and many of the attendant advantages thereto will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
       FIG. 1  is a perspective view of the gravity-actuated antenna of the present invention showing the physical configuration of the antenna; 
       FIG. 2  is an alternate perspective view of the antenna of the present invention with the view taken from reference line  22  of  FIG. 1 ; 
       FIG. 3  is a cross-sectional view of the antenna of the present invention with a curved plate of the antenna removed for a clarified view of the electrical transmission structure of the antenna with the view taken from reference line  3 — 3  of  FIG. 2 ; 
       FIG. 4  is an end view of the antenna of the present invention with a curved plate, the feed tube and the radial fins of the antenna removed and with the view inverted for a clarified view of the electrical switch configuration of the antenna with the view taken from reference line  4 — 4  of  FIG. 2 ; 
       FIG. 5  is a cross-sectional view of the conductive relationship of the feed hub to the electrical switches of the antenna of the present invention with the view taken from reference line  5 — 5  of  FIG. 4 ; 
       FIG. 6  is a three-dimensional view of a radiation pattern formed by the antenna of the present invention; 
       FIG. 7  is a cross-sectional view of a first variant of the electrical switch of the antenna of the present invention; and 
       FIG. 8  is a cross-sectional view of a second variant of the electrical switch of the antenna of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings wherein like numerals refer to like elements throughout the several views, one sees that  FIG. 1  depicts the gravity-actuated submarine antenna  10  of the present invention. The antenna  10  is preferably cast with a rigid thickness from aluminum with brass electrically conductive components attached. Other commonly acquired materials or methods known to those skilled in the art may be used in forming the antenna  10 . Such a variant in antenna formation would be molding the antenna  10  from plastic and plating the antenna with a conductive material. Another non-exclusive variant in antenna formation would be molding the antenna  10  from conductive material. 
   The simplified structure of the antenna  10  generally comprises a cylindrical feed tube  12  with radially extending fins  14  and disk plates  16 ,  18  secured to ends of the feed tube  12  and the fins  14 . A plurality of curved metal plates  20  spaced apart from the fins  14  and projecting from the end plate  16  partially encompass the length of the feed tube  12  with each curved plate  20  connected to the feed tube  12  by a flange  21  and the protective structure of an electrical switch  22 . 
   Each curved plate  20  of the antenna  10  projects at a distance (A) of λ/3 from the end plate  16 , wherein λ is the wavelength corresponding to the center design frequency. The center design frequency is the geometric mean frequency between the frequencies provided to the antenna  10 . Each curved plate  20  subtends to the feed tube  12  at an angle in the range of 45° to 90°, with the high end of the range preferred for broadened antenna bandwidth. 
   The radial fins  14  of the antenna  10  are spaced at 120° from each other around the circumference of the feed tube  12 . Each radial fin  14  is sized to form a longitudinal radiation boundary of a resonant cavity  23  (a volume shown) with the dimensions of each radial fin  14  at λ/22 in width (B) and 2×λ/5 in length (C). The end plates  16 ,  18  are sized to form an athwart radiation boundary of the resonant cavity  23  with the diameter of each of the end plates  16 ,  18  sized to be λ/8. An exterior of the feed tube  12  forms the base of the resonant cavity  23 . 
   The boundaried resonant cavity  23  is shallow enough that the cavity is not shadowed by the radial fins  14  nor the end plates  16 ,  18 . Without a shadow condition restricting a wavelength generated in the resonant cavity  23  during actuation of the antenna  10 , a resultant symmetrical radiation pattern  24  can be transmitted in conjunction with the actuation of a specified curved plate  20 . As discussed below for  FIG. 6 , the resultant radiation pattern  24  can be transmitted from a fore and aft direction as well as at an athwart direction and at a direction perpendicular to the axis of the feed tube  12 . 
   The end plate  16  further includes a stub terminus  25  to the feed tube  12  through a central portion of the end plate  16  and as shown in  FIG. 2 , the end plate  18  includes a feedpoint terminus  26  to the feed tube  12  through a central portion of the end plate  18 . The terminus  26  and the terminus  25  are respectfully at the ends of the coaxial transmission lines  30  and  32  shown in FIG.  3 . 
   As shown in the cross-sectional view of  FIG. 3 , the feed tube  12  encompasses and protects the transmission line  30  with the transmission line  30  continuing from the terminus  26  to a cylindrical feed hub  34 . The diameter of the feed tube  12  is sized to contain the transmission lines  30  and  32  without impacting the impedance seen at the hub  34  such that the diameter of the feed tube is slightly larger than the hub  34 . 
   The transmission line  30  is capable of conducting radio-frequency energy from the terminus  26  to the hub  34  and onto an individual electrical switch  22  when the switch  22  is actuated by the electrical connection of the hub  34  to the switch  22  (the connection of conducting wire  36  within the switch  22  is shown in  FIG. 5 , FIG.  7  and FIG.  8 ). Energy from the switch  22  and onto a specified curved plate  20  and outward to the resonant cavity  23  results in the radiation pattern  24  of the antenna  10 . 
   By decreasing the diameter of the transmission line  30  in a stepwise or tapered manner, the transmission line  30  performs an impedance transformation over its length. The impedance transformation of the transmission line  30  among varying diameters presents a variable load (Ω) at the terminus  26  thereby allowing the antenna  10  to emit over a range of frequencies. Because the switch  22  and the curved plate  20  would each have a unique impedance based on their structure and size, the degree of tapering of the transmission line  30  (or lack thereof) also depends on the dimensions of the switch  22  and the curved plate  20 . 
   As further shown in  FIG. 3 , the second transmission line  32  has a diameter equal to the smallest diameter of the transmission line  30 . The second transmission line  32  is electrically connectable to the hub  34  and continues from the hub  34  onto the terminus  25  such that the transmission line  32  the terminus  25  behave as a short-circuit electrically in parallel with the connection of a pin  38  of the switch  22  and the hub  34 . The length and the diameter of the transmission line  32  determines the amount of reactive impedance of the transmission  32  to match the impedance at the connection of the pin  38  and the hub  34 . By matching the impedance, an optimum and undistorted amount of radio-frequency energy can be transferred onto the actuated switch  22  and curved plate  20  with a result in increased gain of the antenna  10 . 
   As shown in  FIG. 4 , the antenna  10  preferably includes three switches  22  positioned equidistant along the circumference of the feed tube  12  with the attached curved plates  20  also positioned equidistant. Since three curved plates  20  are attached, the chord width (D) of the curved plate  20  can be maximized to enhance a angular range of a righting or “facing up” action that mechanically actuates the switch  22 . By maintaining the righting action of the actuated switch  22  over a widened range, the operation of the antenna  10  thereby becomes roll-stable during towing. Additionally, the maximum chord width (D) of the curved plate  20  permits a greater bandwidth to be emitted from the antenna  10 . Because the attachment point of the switch  22  to the curved plate  20  also affects the impedance bandwidth of the antenna  10 , the preferred attachment point  42  is λ/6 from the open edge  44 . 
   A cross-sectional view of the electrical switch  22  of the antenna  10  used for the actuation described below is shown in  FIG. 5 ; however, other suitable variations of the switch  22  are described for FIG.  7  and FIG.  8 . As stated above, the dimensions of the switch  22 , specifically its supporting structure, can affect the impedance seen at the terminus  26 . As such, the desired diameter (E) of the switch  22  is λ/45 and the desired height (F) of the switch  22  is λ/ 22 . The conical taper  50  of the switch  22  preferably has an angle of 45° and occupies 25% of the switch height (F). While the dimensions of the supporting structure of the switch  22  are preferred for a center design frequency over which the antenna  10  maintains a good impedance match, other supporting structures for the switch  22  such as a cylinder without a taper may be used with compensating changes in the diameter (E) and the height (F). 
   In the operation of the antenna  10 , the feedpoint terminus  26  of the transmission line  30  is connected to a energized feed source (not shown) at a portion of the UHF spectrum from 240-270 MHz. The transmission line  30  allows the radio-frequency energy to be conducted via the hub  34  and onto an electrical switch  22 . The conductive function of the switch  22  is actuated by gravity whenever the attached curved plate  20  is righted or faces “upwards” as a result of wave action buoying the curved plate  20 . The attached curved plate  20  is typically able to be righted at an angle greater than 17° relative to a horizontal plane. 
   When the curved plate  20  is righted and the switch  22  inclines, a metal sphere  60  rolls to contact the conducting wire  36 , conductive to the structure of the switch  22 , with a wire  64  in contact with the pin  38 . Energy from the hub  34  via the pin  38  continues to the curved plate  20 . The energy to the curved plate  20  results in a sinusoidal current distribution flowing along and across a surface  66  of the curved plate  20 . The direction and intensity of the current distribution varies with the frequency of the antenna  10 . 
   When energized, the switch  22  also emits a sinusoidal wave that sets up a current distribution on a surface  67 ,  68  of the fins  14  and a surface  69  of the feed tube  12  in the resonant cavity  23 . The differences in phase from the various radiating surfaces  66 ,  67 ,  68  and  69  contributes to the generally hemispherical radiation or beam pattern  24 , shown in FIG.  6 . 
   In  FIG. 6 , the radiation pattern  24  is depicted as a mathematical surface known as a horn cyclide (a variant of a toroid) with a null  72  from the center the horn cyclide to the lower point  73  of a surface  74 . The horn-cyclide shaped radiation pattern  24  is advantageous because when the antenna  10  is placed on the ocean surface, the radiation pattern  24  in the air space above the ocean surface (shown by the area  76  above the plane defined by the “x” and “y” coordinates) has a minimal null area. As such, the radiation pattern  24  in the air space permits full directionalized transmission allowing the towing submarine to communicate when is the antenna  10  is subject to conditions of pitch, yaw, and varying degrees of roll since the antenna  10  will be righted to the plane defined by the “x” and “y” coordinates and coincident to the ocean surface. 
   Since the emitting area of the radiation pattern  24  is symmetrical, problems associated with asymmetrical radiation patterns are avoided. The symmetrical radiation pattern  24  of the antenna  10  allows the submarine or ship to operate the antenna for optimal antenna performance without station keeping or adjusting course headings. 
   An additional feature of the present invention is that the structural ratio (identified by the wavelength dimensioning above) of the various components of the antenna  10  allows the radiation pattern  24  to remain symmetrical while maintaining the compactness of the antenna  10 . The compactness of the antenna  10  is naturally advantageous for many reasons including detection minimalization and reduced drag. In defining the compactness feature, the outer physical boundary of the antenna  10  is based on the size and placement of the end plates  16 ,  18  and the curved plates  20 . For example, each curved plate  20  of the antenna  10  projects at a distance (A) of λ/3 from the end plate  16  with the diameter of the end plates  16 ,  18  sized to be λ/8, therefore any remaining structure of the antenna  10  would be within a circumferential boundary created by the above dimensions. Also, the radial fins  14  of the antenna  10  are 2 times λ/5 in length (C) therefore any remaining structure of the antenna  10  would be within a longitudinal boundary created by the dimension of the radial fins  14 . 
   While the metal sphere  60  shown in  FIG. 5  is used in the actuation of the switch  22  described above, other variations of electrical contact within the switch  22  may be used. In a first variant of the switch  22  shown in  FIG. 7 , the sphere  60  of the switch  22  is substituted with a metal plunger  80 . The use of the plunger  80  may be preferred in some circumstances since the shape as well as the size of the plunger  80  can affect the angle of gravity-actuation. 
   In a second variation of the switch  22  shown in  FIG. 8 , the plunger  80  or sphere  60  is substituted with a gravity-actuated magnet  90 . When the curved plate  20  is righted and the switch  22  inclines, the magnet  90  slides to close the normally open contacts of the reed switch  96 . This allows the reed switch  96  to be conductive to the structure of the switch  22  by the conducting wires  38  and  64 . The magnetic material for the switch  22  must have a substantial mass to perform a switch but the material also must have a stable magnetic field. In order not to affect the magnetic field or impedance properties of the antenna  10 , the switch  22  may be lined with magnetic shielding foil material  98 . 
   Thus by the present invention its objects and advantages are realized and although preferred embodiments have been disclosed and described in detail herein, its scope should be determined by that of the appended claims.