Abstract:
A slotted antenna comprises a plurality of loop structures and interconnecting conductors that define a slot. The antennas can operate in a single band or over multiple bands. Flexible or inflatable substrates enable easy storage aboard an underwater craft and facilitate deployment and towing behind an underwater craft with minimal chances of detection.

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 
     This invention generally relates to antennas and more specifically to an antenna that covers a wide frequency band and that can be deployed from an underwater location, such as from a submarine. 
     (2) Description of the Prior Art 
     As known, communications between the outside the world and underwater craft, such as a submerged submarine, can be achieved through a floating cable antenna system. With the advent of satellite communications, such antenna systems enable a submarine to remain submerged while communicating with other facilities throughout the world by means of satellite communications in the UHF and other frequency bands. 
     More specifically, such underwater craft deploy an antenna to the surface to establish communications while the craft remains submerged. After communications are completed, the antenna is reeled back into a storage area. Consequently, the presence of the antenna at the sea surface is minimized to reduce the possibility of detection. Specifically, the antenna as a physical radar contact is detectable only during its presence on the surface. 
     As an example, U.S. Letters Patent No. 2,067,337 granted in 1933 to Polatzek discloses a flexible tube or hose for deploying an antenna from a submarine. The hose is inflated with air under compression to overcome any loading on an aerial wire in the hose. 
     U.S. Letters Patent No. 3,788,255 granted in 1974 to Tennyson discloses an expendable submarine receiving antenna. A buoyant capsule has an opening therethrough for release from an ejection chamber in a submarine. The capsule contains a coil of lead-in wire with electrical insulation suitable for use in seawater and having a length that extends between the submerged submarine and the surface. A free end of the wire extends freely through the opening in the capsule for withdrawal and severance of a selected length of the wire for connection at the free end to radio communication equipment aboard the submarine. 
     U.S. Letters Pat. No. 3,972,047 granted in 1976 to Lombardi for a floating cable antenna system discloses an antenna system in which a submerged submarine tows a buoy by means of an electromechanical cable. A cable reel stores the inflatable buoyant cable and has a pressure accumulator containing a medium under given pressure attached to one end of the buoyant cable. Slip rings provide a communication with the electromechanical cable radio communications. 
     U.S. Letters Pat. No. 5,132,696 granted in 1992 to Cobb discloses a pneumatic extendable antenna for a water deployable buoy. A whip antenna extends from a shortened configuration to a lengthened configuration. The antenna body comprises a plurality of hollow frusto-conical segments that slidably nest inside each other when the antenna is in its shortened or compact configuration. Filling the container with a pressurizing gas expands the segments relative to each other. A weighted ballast and electronic control circuit attached to one end of the antenna and an air filled stability bag disposed about the antenna near its weighted end orients the antenna in a vertical direction. 
     U.S. Letters Pat. No. 5,933,117 granted in 1999 to Gehard discloses a flexible ferrite loaded loop antenna assembly. A buoyant loop antenna is deployed along a cable with a core region that comprises a plurality of annular ferrite beads. The ferrite beads are aligned with the concave end of one bead against the convex end of another bead so the cable can flex while the beads maintain contact with each other thereby providing flexibility and resistance to crushing. The core region has a looped wire wrapped helically around it forming the loop antenna. The looped wire elements start and end at the same end of the core region forming a loop. The loop allows reception in an athwart (side to side) direction. The wire loop antenna can be combined with a straight wire antenna to provide reception in a fore and aft direction thereby to provide an omni-directional cable antenna assembly. 
     Each of these references discloses an antenna that is relatively large and therefore readily detectable at the surface by modern radar systems. With the exception of the Gerhard patent with its complicated ferrite beads that provides some flexibility, the antennas are rigid and not adapted for wrapping on a reel. Many of them require external gas in order to inflate and rise to the surface. Further, each of them tends to be an end fed antenna with the exception of the Gerhard patent that discloses multiple antenna elements to obtain an omni-directional range. The Gerhard patent additionally is directed to VLF/LF transmission bands that incorporate entirely different signal requirements than the typical transmission frequencies in the 200-400 MHz band. 
     In addition to these antennas, other antennas have been proposed that provide radiation patterns that are more advantageous particularly with respect to satellite communications, but not readily adapted for deployment from underwater craft. For example, U.S. Letters Pat. No. 2,622,196 granted in 1952 to Alford discloses an ultra-high frequency antenna that generates horizontally polarized waves. The antenna comprises a number of small loops shunted across a balanced transmission line arranged so that a large number of loops may be cophasally energized thereby attaining a large concentration of radial power in a plane in which the radiation is distributed in a substantially circular pattern. 
     U.S. Letters Pat. No. 2,812,562 granted in 1957 to Carter discloses loop antennas for television signals with a loop antenna array of a plurality of loops coupled together by a section of transmission line that has a quarter wavelength, or any odd multiple thereof, at the frequency of operation and having field patterns superimposed in phase quadrature relationship. The loop is preferably a single turn arrangement having a circumference in the order of one or a few wavelengths at the operating frequency and made of a large diameter conductor. 
     U.S. Letters Pat. No. 3,626,418 granted in 1971 to Barryman discloses a VHF antenna comprising a plurality of closed loop radiating elements that are parallel fed by a tapered pair feed line. Each loop comprises a single turn of conductive material whose dimensions are uniform over the entire loop so each loop is electrically uniform and continuous. The loops are fed in parallel by uniformly tapered feed lines comprising two congruent strips of conductive material that diverge at a small angle. A first loop of said plurality of loops has a circumference equal to one half length at the lowest desired frequency. A second loop has a circumference equal one-half wavelength at the highest desired frequency. The remaining loops are of intermediate size between the first and second loops. 
     U.S. Letters Pat. No. 3,999,185 granted in 1976 to Polgar, Jr. et al. discloses plural antennas on a common support with feed line isolation. This structure includes a tunable high power MF/HF transmitting antenna having a vertical access and shorting assembly driven along a vertical axis to tune the high power antenna. A plurality of additional antennas are disposed in a vertically stacked relationship above the high power antenna. A tunable ferrite isolator is disposed below a drive shaft and includes a conduit that enables the conduit and the service conductors to pass through the high power antenna with a minimum modification to the performance of the high power antenna. 
     Of all these antennas, it has been found that the loop antenna, such as disclosed in U.S. Letters Pat. No. 2,622,196, has the potential for providing a desired radiation pattern to a number of applications. However, this structure is a rigid structure that also is not readily adapted for undersea applications. Specifically, it is difficult to store such a rigid structure and to provide any structure that would allow an antenna to rise to the surface. 
     SUMMARY OF THE INVENTION 
     Therefore it is an object of this invention to provide an improved antenna structure that can be deployed from underwater craft, such as submarines. 
     Another object of this invention is to provide an antenna structure that can be deployed by providing a low radar signature. 
     Still another object of this invention is to provide an antenna for underwater craft, such as submarines, that is readily stowed with its cable without special housings or storage containers. 
     Yet another object of this invention is to provide an antenna structure for use as a deployable antenna from a submarine that maintains an impedance match over a wide frequency band. 
     Yet still another object of this invention is to provide a deployable antenna for use with underwater craft, such as submarines that provides elliptically polarized signals. 
     Still yet another object of this invention is to provide an antenna structure that operates as a solid sheet metal slotted antenna. 
     In accordance with one aspect of this invention, an antenna that is deployable from an underwater housing comprises a support and a slotted antenna structure. The support extends along an antenna axis, and the antenna is flexible about radii transverse to the axis. The slotted antenna structure is formed on the flexible support with an axis coincident with the antenna axis and with a plurality of antenna loops extending along the antenna axis such that each antenna element is substantially transverse to the antenna axis. 
     In accordance with still another aspect of this invention, a deployable antenna for use in underwater craft includes a flexible support that has compact and expanded states. The flexible support carries an elongated antenna structure to be in operating condition in the expanded state. A gas contained within the flexible support provides appropriate expansion of the flexible support from its compact to its extended state as the antenna rises to the surface. A retainer device maintains the flexible support in its compact structure. 
     In accordance with another aspect of this invention, an antenna is provided that operates with the characteristics of a solid slot antenna. The antenna comprises a plurality of conductive loops spaced along and substantially transverse to an antenna axis. The loops are oriented in a substantially parallel relationship. Each loop has first and second spaced, facing end portions. A first conductor interconnects the first end portions, and a second conductor interconnects the second end portions. The first and second conductors are spaced and form a slot path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The appended claims particularly point out and distinctly claim the subject matter of this invention. The various objects, advantages and novel features of this invention will be more fully apparent from a reading of the following detailed description in conjunction with the accompanying drawings in which like reference numerals refer to like parts, and in which: 
         FIG. 1  is a perspective view of an antenna structure that is useful in this invention; 
         FIGS. 2A through 2E  depict the radiation pattern for the antenna in  FIG. 1  over defined frequency band; 
         FIG. 3  is a perspective view of a portion of the antenna constructed in accordance with this invention with a support in an expanded state; 
         FIG. 4  is a perspective view of the antenna in  FIG. 3  with its support in a compact state; 
         FIG. 5  depicts the deployment of the antenna from a submarine; 
         FIG. 6  depicts the storage position of the antenna on a drum in a submarine; 
         FIGS. 7 through 13  depict alternative embodiments of an antenna such as shown in  FIG. 1 ; and 
         FIG. 14  depicts still another embodiment of an antenna structure that is useful in accordance with this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In  FIG. 1  a basic slotted antenna structure  20  extends along an antenna axis  21  and comprises a series of metal loops  22  that lie in parallel planes transverse to the antenna axis  21 . Each metal loop  22  is an open loop with counterfacing first and second ends  23  and  24  on each loop define an opening or slot. First and second spaced parallel conductors  25  and  26  interconnect the first and second ends  23  and  24 , respectively. That is, the first conductor  25  attaches to all of the first ends  23  of each loop while the second conductor  26  attaches to all of the second ends  24  of each loop. In this particular embodiment, the conductors  25  and  26  are parallel and spaced to define the slot further. Ends of the parallel conductors  25  and  26  provide a means for attaching an RF communication system. 
     It is desirable that the number and spacing of the open loops  22  produce an antenna structure that emulates a slotted-cylindrical antenna made from sheet metal. The metrics for determining the usefulness of such an antenna include an analysis of the radiation, propagation and impedance properties in the slot region. 
     A useful radiation property is free spaced directivity that measures how the radiated energy is spatially concentrated around the antenna. Directivity for such an antenna structure can be approximated by: 
             D   =       3   ⁢     n   2         2   ⁢     {     n   +     6   ⁢       ∑     m   =   1       n   -   1       ⁢           ⁢       (     n   -   m     )     ⁡     [         cos   ⁡     (   mks   )           (   mks   )     3       -       sin   ⁡     (   mks   )           (   mks   )     2         ]             }                 (   1   )             
 
Where n and s are the number of loops and spacing between the loops respectively, k is a free space-wave number that is 2π/λ and m is a summation index; basically the loop number, so that D is found by summing from loop  1  (m=1) to loom m=n−1 (n being the total number of loops). It has been observed that the plot with one thin loop provides an antenna directivity, D, with a value of 3/2. As the number of loops are increased without bound while constrained over a finite axial length,  1 , the directivity increases but asymptotically approaches the directivity of the slotted sheet metal radiator of corresponding length. Consequently Equation (1) can be recast as Equation (2) where Si (X) is the sine integral defined as: 
             D   =         (   kL   )     2       2   ⁡     [       cos   ⁡     (   kL   )       +       sin   ⁡     (   kL   )       kL     +     kLSi   ⁡     (   kL   )       -   2     ]                 (   2   )             
 
     This equation assumes that the ratio of the antenna perimeter, (p) to the wavelength (λ) is small. In a submarine application this perimeter-wavelength ratio is desirable since it yields a slender antenna that minimizes the potential for radar detection. Moreover, as will be described, this condition permits the antenna to have a toroidal pattern in which the pattern null is on the antenna axis. 
     A model with ten loops yields a directivity that is 8% above the final level value given by Equation (2). Doubling the number of loops yielded the directivity that was 4% above the final value. Thus, a given antenna length will have a combination of loop number, n, and spacing, s, such that the resulting directivity is approximately the limiting value described by Equation (2). 
     With respect to the propagation constant, the feed region of an antenna comprised of a parallel wire line has electrical properties that are similar to a solid cylindrical slotted antenna. More specifically a complex number γ(=α+jβ) typically has a small value in the attenuation constant, α, and an increase in the phase constant, β, in the band of interest. Below the band of interest, i.e., below 225 MHz in a typical submarine application, α, increases and β decreases. An intersection at the cutoff frequency below which wave propagation in the slot region is evanescent and the antenna behaves as a lossy transmission line. The values at cutoff, α c  and β c , are related to a normalized cutoff wave number (k c a e =p/λ c ) by 
               α   c     =       β   c     ≈           5   ⁢   π       2   ⁡     [     5   -     (     p   /     λ   c       )       ]         ⁡     [       1   +     16   ⁢       (     p   /     λ   c       )     4           1   +     10   ⁢       (     p   /     λ   c       )     4           ]                   (   3   )             
 
where p is the mean perimeter of the antenna cross section. It has been found that with a cutoff frequency of 220 MHz, an antenna can be constructed with a mean perimeter of 18.2 inches to yield α c =β c =1.10 m −1 . Lower values of k c α e , may be obtained by increasing the perimeter, p, or decreasing the slot width. Analysis of both an antenna structure as shown in  FIG. 1 and a  solid structure demonstrate that the propagation constants are very similar with a slight displacement of the α-β, intersection points. Given this, an antenna with twenty loops  22  is sufficient to simulate a solid radiator in the frequency range of 200-400 MHz.
 
     With respect to impedance, it has been found that the feed point impedance at any arbitrary location along the parallel conductors  25  and  26  in  FIG. 1  may be computed approximately by 
               Z   in     =       Z   0     ⁡     [         coth   ⁡     (     γ   ⁢           ⁢     l   1       )       ⁢     coth   ⁡     (     γ   ⁢           ⁢     l   2       )             coth   ⁡     (     γ   ⁢           ⁢     l   1       )       +     coth   ⁡     (     γ   ⁢           ⁢     l   2       )           ]               (   4   )             
 
where γ=α+jβ, 1 1  and 1 2  are the distances from each end to the feed point, respectively, and Z 0  is the characteristic impedance of the slot region. When the feed point is positioned at the center such that 1 1 =1 2 , Equation (4) reduces to 
               Z   in     =         Z   0     2     ⁢     coth   ⁡     (     γ   ⁢           ⁢   l     )                 (   5   )             
 
where  1  is now the half-length of the antenna. This analysis indicates that the feed point resistances have reactances of a twenty-loop antenna structure  20  and the solid antennas are essentially similar with the values of Z 0  and γ roughly equal.
 
       FIGS. 2A through 2B  depict the radiation patterns from a twenty-loop antenna structure according to the foregoing designs at 200 MHz, 250 MHz, 300 MHz, 350 MZ and 400 MHz respectively. From this it can be seen the antenna is directional over the entire band. 
     An antenna structure, such as the antenna structure  20  in  FIG. 1 , will have appropriate characteristics for towing at the water surface for high frequency communications in the 200 MHz through 400 MHz bandwidth.  FIG. 3  depicts one embodiment of such a structure. Specifically, it includes a collapsible support  31  that extends along the antenna axis  21 .  FIG. 3  depicts the antenna in an expanded state in which the support, formed of an insulating material such as Mylar, has the antenna elements deposited thereon. More specifically,  FIG. 3  depicts a plurality of loops  32  that extend between first ends, such as the first end  33  and second ends, such as the second end  34 . An axially extending conductive path  35  on the substrate  30  spans the first ends  33 ; a conductive path  36 , the second ends  34 . The substrate  31  is formed of a flexible material. In a compact form the substrate  31  assumes a pleated or similar configuration carrying the deposited elements of the antenna structure including the loops  32  and the conductors  35  and  36  into the pleated or compacted configuration.  FIG. 4  depicts a retaining sleeve  38  that can receive the compact antenna and retain it in position. 
     In a preferred embodiment of this invention the substrate  31  forms a sealed compartment that contains a small amount of gas, such that in its compact form the antenna structure  30  has some buoyancy even as it is transferred into the ocean at depth. As the buoyant antenna structure  30  rises to the surface, it expands. As will be apparent, the gas pressure in the expanded state exceeds the pressure that would lead to substrate failure. 
       FIG. 5 , that is not to scale, depicts a submarine  40  trailing the antenna structure  30  at the surface  41  of the water while the submarine remains in an undersea portion  42 . A cable  43  tethers the antenna structure  30  to the submarine  40 . 
       FIG. 6  depicts the antenna structure  30  and cable  43  in a stored position. In this specific example, a drum  44  rotates on an axis  45  to wrap the cable  43 . When the antenna structure  30  reaches the drum, it will have been partially compressed by the water pressure at the exterior of the submarine  40 . Then the retaining sleeve  38  can be placed in position to keep the antenna structure in the compact form shown in FIG.  4 . One of the advantages of this invention will now become apparent. Specifically, the spacing of the loops  32  shown in  FIG. 3  allows the antenna to have flexibility. 
     Thus, an antenna structure, such as the antenna structure  20  in  FIG. 1 , will have appropriate characteristics for an antenna to be towed at the water surface for high-frequency communications in the 200 MHz through 400 MHz bandwidth.  FIG. 3  depicts on one embodiment of such a structure. More specifically, using metallic loops instead of a sheet metal surface as normally used in a slotted antenna provides spaces between the adjacent loops that serve as gaps. The gaps allow the antenna to bend with a certain radius. This feature allows the antenna to be stored on a reel or drum, such as the antenna  30  on the drum  44  in FIG.  6 . Similarly, if the antenna conductors are embedded in an elastomer capable of stretching with applied gas pressure, the antenna could be made to inflate. This would allow alternate inflation and deflation would provide the expanded and compacted states of  FIGS. 3 and 4  directly. This again is useful for stowage purposes. 
     The physical attributes of this antenna structure also facilitate its construction. For example, the antenna might be blow molded in a manner similar to that used for liquid containers. After molding, the exterior structure could be plated with a thin layer of metal to form the antenna. Thus, in a pattern such as the pattern shown in  FIG. 3  or corresponding to any other patterns as more specifically described later. The interior of the support  31  shown in  FIG. 3  can also be filled with a syntatic foam formulation to provide strength while maintaining light weight. Alternatively, the metallic structure can be imbedded into a rubbery material. If an antenna is made with an elastomeric material such as polyurethane, it can be fabricated as a bladder with air voids with the flexible conducting members comprising the antenna inserted between the bladder walls. In this arrangement, if the antenna assembly is deployed from the submerged ship toward the ocean surface, the decrease in hydrostatic pressure allows the antenna to assume its form for operation. 
     As known, the major advantage of a submarine is its stealth. Floating a transmitting antenna on the surface can provide a radar signature. It has been found, however, that an antenna constructed in accordance with this invention exhibits a significantly decreased radar signature over a corresponding slotted solid antenna. 
     When a solid slotted antenna is at the surface, a degree of capacitive coupling between the antenna structure and surrounding seawater ground plane can vary effective gain. In such environments gain is a function of angular rotation. An antenna constructed in accordance of this invention minimizes the effect of function because only a small portion of the antenna surface couples to the seawater at any given instant of time. 
     The combination of the foregoing attributes provides advantages over conventional submarine antennas. When an antenna according to this invention is deployed on the surface, reflections due to wave effects or sea clutter may be much larger in any radar image of the area. This has the potential of providing an antenna that is undetectable by radar. It is also expected that the cooling effect of the seawater wash over the antenna will tend to make any infrared signature indistinguishable. 
     The basic antenna structure shown in  FIG. 1  can be modified to provide a number of different antenna embodiments that may be used as substitutes for the structure of  FIG. 1  in an inflatable or flexible antenna or in a rigid antenna. For example,  FIG. 7  depicts an antenna structure  50  that acts as a hybrid slot-dipole antenna extending along an antenna axis  51 . The antenna structure  50  includes first loops  52  that are similar in a configuration as loops  22  in FIG.  1 . Each loop  52  has a first end  53  and a second end  54 . Space parallel conductors  55  and  56  interconnect with the first and second ends of the loops  52  respectively. 
     At other positions along the axis, the antenna  50  compresses loops  57  with an essentially reverse s-shape in the perspective of FIG.  7 . Each such loop  57  terminates at a first free end  60  and a spaced second free end that is not visible in the perspective of FIG.  7 . Each free end  60  is a portion of a lower loop element  61 , and each loop  57  also includes an upper element  62 . The loops  57  are split at the center to connect to portions  55 A and  56 A of the parallel conductors. 
     More specifically, the parallel conductors  55  and  56  meander by shifting radially from the outer position shown at their connection to loops like the loops  52  to the substantially axial position of portions  55 A and  56 A. Portions like the portion  55 A drive each element  61 ; portions like the portion  55 A drive each element  62 . This radial meandering of the conductors  55  and  56  produces a structure that constitutes an array of dipoles. The vector addition of the fields radiated from the composite structure produces a beam with maximum lobes tilted 45° from broadside. 
       FIG. 8  depicts an antenna structure  70  extending along an axis  71  with loops  72  like the loops  22  in FIG.  1 . Each loop  72  has a first end  73  and a second end  74 . Spaced parallel conductors  75  and  76  attach to the first and second ends  73  and  74  respectively. The conductors  75  and  76  meander radially from the outer position shown at loops  72  to a substantially center or axial position along the axis  71  where they connect to a second set of loops  77 . Each loop  77  has a closed outer loop  80  element. Additional elements from the mid-point of opposite sides such as elements  81  and  82 , extend radially; that is they extend horizontally in the perspective of  FIG. 8 , to connect to the center portions  75 A and  76 A. The parallel conductors  75  and  76  therefore define a slot that meanders radially in the orientation of FIG.  8 . Varying the pattern of the meander such as the number of undulations of the slot controls either impedance or pattern. 
       FIG. 9  depicts a circumferentially or helically meandering slot. Specifically, an antenna  90  extends along an antenna axis  91  with plurality of circular loops  92  that have end portions  93  and  94 . Conductors  95  and  96  interconnect with the ends  93  and  94  respectively. In this particular embodiment the location of the ends varies circularly along the axis. Consequently, a slot  97  defined by the conductors  95  and  96  meanders circumferentially or helically relative to the axis  91 . Like the antenna shown in  FIG. 8 , controlling the pitch of the meander provides impedance and pattern control. 
       FIG. 10  depicts another embodiment in the form of an antenna  100  that extends along an axis  101 . This antenna  100  comprises loops  102  characterized by having an opening at spaced ends  103  and  104 . In this embodiment alternate loops are designated by references  102 A and  102 B. Spaced parallel conductors  105  and  106  interconnect the ends  103  and  104  respectively to form a slot. In this particular embodiment alternate loops, such as loops  102 B, incorporate a PIN switch  107  opposite the slot. When all the PIN switches  107  are closed, the antenna  100  acts as a slot antenna such as shown in FIG.  1 . When the PIN switches  107  are open, the antenna acts as a hybrid dipole-slot antenna and the beam tilts 45°. Alternate switching mechanisms can be substituted and the position of the switches can be altered for different phasing effects. 
       FIG. 11  depicts another embodiment of an antenna  110  that extends along an antenna axis  111 . The antenna  110  comprises a plurality of spaced loops  112  having open ends  113  and  114 . In this embodiment conductors  115  and  116 , that form the slot, rotate about themselves to connect to the ends  113 , and  114  in an alternating fashion. For example, the conductor  115  connects to each first end  113  of loops  112 A and  112 B, while the conductor  116  connects to each first end  113  of loops  112 C and  112 D. In  FIG. 11  the twists are shown continuously. The twists can also be separated to produce an antenna with a twisted slot and a straight slot over axially displaced portions. Twisting the slot as shown in  FIG. 11  results in a phase shift along the slot that can generate radiation patterns having different shapes. 
     The antennas shown in  FIGS. 1 and 7  through  11  each operate over a wide band. In some applications it may be desirable that the antenna operate over two wide bands that have widely separated center frequencies.  FIG. 12  depicts a dual-band embodiment of an antenna  120  that extends along an axis  121 . The antenna comprises a plurality of loops  122 . In this particular embodiment, however, each loop  122  includes a bottom portion  123  and vertical portions  124  and  125  that extend to upper horizontal elements  126  and  127  respectively. The upper elements  126  and  127  terminate at facing and spaced first and second ends  128  and  129 . Slot conductors  130  and  131  interconnect the loops at the ends corresponding to the upper first ends  128  and  129 , respectively. This portion of each loop defines the lower of the operating frequency band. Horizontal elements  131  and  132  extend toward each other from vertical portions  124  and  125  to terminate at ends  133  and  134  respectively. Parallel conductors  135  and  136  interconnect elements ends  133  and  134  respectively. This defines a second loop orientation including the bottom leg  123 , the horizontal legs  131  and  132  and the vertical legs  124  and  125  intermediate bottom leg  123 , the horizontal legs  131  and  132 . The operating characteristics of this second lobe define the upper operating frequency. 
     Still referring to  FIG. 12 , the lower frequency band connections are made to the conductors  130  and  131 ; the upper frequency connections, to the conductors  135  and  136 . It has been found that the lower frequency slot produced by the conductors  131  and  132  is capacitively coupled to the edges of the higher frequency antenna. The degree of coupling can be adjusted by spacing for optimal performance. For example, such an antenna might be constructed to cover both the UHF and L frequency bands. Similar mechanical arrangements might stack or more slots to allow the structure to operate with three or more widely separated bands. 
       FIG. 13  depicts an alternative embodiment of an antenna  140  that is capable of shifting the radiation pattern between end-fire and broadside lobes. Like the other antennas, the antenna  140  lies along an antenna axis  141  and comprises a plurality of loops  142 . Each of the loops has first and second spaced ends  143  and  144 . A pair of spaced conductors  145  and  146  interconnect the first ends  143  and second ends  144  respectively to define a slot. This embodiment includes two feed points. The first feed point comprises a feed  147 ; the second, a feed  148 . Adjusting the phase difference of the signal applied to the two feeds has the effect of either shifting the radiation pattern to two end-fire lobes (with a 180° phase shift) or a broad-side mode with a 0° phase shift. If the signal is applied with a 270° phase shift, one end fire lobe may be radiated. 
       FIG. 14  depicts still another embodiment of a slot antenna that is adapted for operating against a ground plane. This embodiment includes an antenna  150  that lies along an axis  151  and comprises a plurality of semi-circular loops  152 . Each loop has a first end  153  and a second end  154 . A conductive element  155  interconnects each of the first ends  153 . Another conductive element  156  interconnects each of the second ends  154 . The second conductor  156  and the second ends  154  connect to a ground plane  157 . A center feed  160  attaches to drive the conductor  155 . Such an antenna could be particularly useful in a car or on a surface ship. In addition, the antenna structure in  150  could have further modifications. For example, the structure could include spaced shorting pins at  161  and  162  as shown in phantom. Shorting pins could be placed at other arbitrary points to define different slot regions such as the slot region  158 . Such shorting pin placements would-control pattern or impedance characteristics. 
     In summary, there has been disclosed a basic antenna structure of spaced loops that define a slot. The basic configuration is shown with a number of modifications, including square and circular loops, straight and meandering paths, loops that comprise multiple loop portions. Loop shapes and slot paths other than these specifically disclosed can be substituted. These antennas provide performance corresponding to a solid cylindrical slotted antenna. In addition, the configuration enables each antenna structure to be constructed on a flexible substrate such that the portions of the loop opposite from the slots can be bent toward each other thereby to provide a structure that is flexible. Moreover, as the antenna can be take the form of a structure such as shown in any of the  FIGS. 1 and 7  through  12  and supported by a compressible material or can have a form that is deposited on a collapsible support such as shown on  FIGS. 3 and 4 . 
     This invention has been disclosed in terms of certain embodiments. It will be apparent that many modifications can be made to the disclosed apparatus without departing from the invention. Therefore, it is the intent of the appended claims to cover all such variations and modifications as come within the true spirit and scope of this invention.