Abstract:
Antennas for transmitting and receiving circularly polarized UHF SATCOM radio signals include a mast which has four circumferentially spaced apart element mounts that protrude radially from the mast, each having a mechanical coupling mechanism holding an electrically conductive tubular antenna element disposed radially from the mast for use and parallel to the mast to minimize the envelope size of the antenna when not in use. Replaceable elements in one version of the antenna have a threaded stud threadably receivable in a threaded socket on the element mount. Each element of a foldable version of the antenna has a tapered support peg which is insertably receivable in a tapered socket in a boss on the element mount and releasably held therewithin by a tensioning spring within the element. Optionally, a fifth conductive element is disposed longitudinally within the mast to transmit and receive linearly polarized radio signals.

Description:
BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The present invention relates to antennas to transmit and receive ultra-high frequency radio signals. More particularly, the invention relates to novel transportable X-WING type UHF SATCOM antennas which are attachable to a vehicle, ship or other support structure, and which are highly resistant to impact damage and readily repairable in the field. 
     B. Description of Background Art 
     Government agencies such as U.S. military services that utilize personnel operating in remote field locations have a need for instantaneous, reliable communication systems. Such systems are required for conveying data between personnel in field locations and fixed command and control sites. As a practical matter, communication systems which meet the various requirements for reliable communications of the type alluded to above generally utilize radio transceivers. Thus, the U.S. military services and other governmental agencies typically use for their communications between remote field location, and between remote field locations and command and control sites, small, readily transportable radio transceivers. Such transceivers, which are typically installed in vehicles or ships, usually operate at power levels of 200 watts or less. To achieve long distance communication capability, and to avoid line-of-sight signal transmission obstructions such as mountainous terrain, portable communication transceivers used for applications such as those described above often utilize a transponder located in an earth-orbiting satellite, and are hence used in communication systems referred to as Satellite Communication (SATCOM) systems. 
     Radio transceivers of the type described above must of course use an antenna to transmit and receive radio signals through space. Thus, transportable transceivers which are used to communicate over long distances and/or rugged terrain where line-of-sight communication is not feasible often utilize transmissions between an earth-orbiting satellite to provide the needed range and terrain obstruction avoidance. For such applications, small SATCOM antennas mountable to vehicles, ships or portable shelters and operable in ultrahigh frequency (UHF) radio bands are frequently used. 
     Vehicle mountable SATCOM antennas currently in use are required to have a reasonably high gain in UHF radio bands located generally between about 225 MHZ and 400 MHZ. Typical SATCOM antennas are constructed to utilize circularly polarized signals. Circular polarization is required for satellite communication because ionized particles in the upper part of the atmosphere known as the ionosphere rotate the plane of polarization of a linearly polarized radio signal, thus causing a polarization mismatch in linearly polarized antennas. One type of SATCOM antenna in common use has a “turnstile” type external appearance, or “form factor,” which includes a central straight, longitudinally disposed mast that has protruding radially outwards from the upper end of the mast four radiating elements which are spaced circumferentially apart at 90-degree intervals. The active part of each radiating element which is effective in transmitting or receiving radio frequency electromagnetic waves is an elongated straight electrical conductor, which may be in the form of a blade or rod. The conductors of one pair of diametrically opposed elements comprise an electric dipole antenna that is electrically connected to a first port of a hybrid antenna coupler network. The conductors of a second pair of elements oriented at 90-degrees to the first pair comprise a second electric dipole antenna, and are connected to a second port of the antenna coupler network, which is shifted in phase 90-degrees from the first port by circuitry in the coupler network. This arrangement results in the transmission of a circularly polarized signal. The arrangement also enables the conductors of the elements of the antenna to intercept and receive at relatively high gain radio signals of various polarizations, when the antenna is operated in a receive mode, with no transmitting signals applied to the radiating elements. 
     When viewed from above or below, the radiating elements of SATCOM antennas of the type described above, which typically consist of four conductive rods which extend perpendicularly outwards from the antenna mast, form an X-shaped pattern. Thus such antennas are commonly referred to as “X-WING” antennas. 
     Portable X-WING SATCOM antennas which are intended for use in field operations are typically mounted to the hood, roof or the fender of a vehicle, such as a HumVee. Therefore, the outwardly protruding radiating elements of such antennas are subject to impact damage from contact with low-hanging tree branches, for example. Such damage can degrade or completely destroy the functionality of X-WING antennas currently in use, and thus jeopardize the success of missions which require reliable communications implemented with the antenna. Accordingly, it would be desirable to provide an improved X-WING UHF SATCOM antenna which had superior impact resistance. For the same reason, it would be desirable to provide an improved impact resistant X-WING UHF SATCOM antenna which utilized field-replaceable radiating elements. Also, it would be desirable to provide an improved UHF SATCOM antenna which utilized radiating elements that could be readily replaced in the field if damaged. 
     An additional problem with prior art X-WING UHF SATCOM antennas is the large amount of container space which is required to store and ship such antennas to the field. As can be readily envisioned, the form factor of an X-WING antenna, which includes a longitudinally elongated cylindrical mast that has four straight rod-like radiating elements approximately equal in length to the length of the mast protruding radially outwards from the upper end of the mast, requires essentially a storage or shipping space having the shape of a rectangular block whose height is equal to the mast height, and whose base sides are equal to the length of the radiating elements. Accordingly, it would be desirable to provide an improved X-WING UHF SATCOM antenna which could be configured to a smaller space for shipping, and quickly and easily be re-configured to an operational configuration in the field. 
     The foregoing limitations of prior art X-WING UHF SATCOM antennas, and the foregoing improvement objectives, were motivating factors for the present invention. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to provide a portable X-WING type UHF SATCOM antenna which has protruding perpendicularly outwards from a mast radiating elements that are capable of withstanding substantially large impacts from objects without destroying the functionality of the antenna. 
     Another object of the invention is to provide an impact resistant X-WING type UHF SATCOM antenna which includes an elongated straight tubular mast that has protruding radially outwardly from the cylindrical wall surface of the mast near the upper transverse end wall of the mast radiating elements which are threadably and removably attached to the mast. 
     Another object of the invention is to provide a dual function X-WING UHF SATCOM antenna which has a hollow tubular mast that has disposed longitudinally therewithin an electrically conductive cylindrical tube which functions as a vertically polarized broadband monopole antenna, and cylindrical rod-shaped elements which protrude radially and perpendicularly outwards from the upper end of the mast tube and are spaced circumferentially apart at ninety degree intervals to function as a circularly polarized cross dipole antenna, the elements being threadably attached to the mast to enable field replaceability of the elements. 
     Another object of the invention is to provide an impact resistant X-WING UHF SATCOM antenna with foldable elements which has an elongated straight tubular mast that has protruding radially and perpendicularly outwards from the upper end of the mast straight cylindrically-shaped tubular conductive radiating elements which are pivotable downwardly to orientations parallel to the longitudinal axis of the mast for storage and shipment, and pivotable upwards to an operational orientation perpendicular to the mast, where a spring mechanism within each radiating element locks the element into a perpendicular operational orientation. 
     Another object of the invention is to provide a dual function impact resistant X-WING UHF SATCOM antenna with foldable radiating elements which has a hollow tubular mast made of an electrically non-conductive material such as fiberglass which has disposed longitudinally through the bore of the mast an elongated straight hollow electrically conductive cylinder which functions as a vertically polarized broadband monopole antenna, and which has protruding radially outwards from the upper end of the mast straight cylindrically-shaped tubular conductive radiating elements which are pivotable downwardly to orientations parallel to the longitudinal axis of the mast for storage and shipment, and pivotable upwards to an orientation perpendicular to the mast, where a spring mechanism within each radiating element locks the element into a perpendicular use orientation. 
     Various other objects and advantages of the present invention, and its most novel features, will become apparent to those skilled in the art by perusing the accompanying specification, drawings and claims. 
     It is to be understood that although the invention disclosed herein is fully capable of achieving the objects and providing the advantages described, the characteristics of the invention described herein are merely illustrative of the preferred embodiments. Accordingly, I do not intend that the scope of my exclusive rights and privileges in the invention be limited to details of the embodiments described. I do intend that equivalents, adaptations and modifications of the invention reasonably inferable from the description contained herein be included within the scope of the invention as defined by the appended claims. 
     SUMMARY OF THE INVENTION 
     Briefly stated, the present invention comprehends improved X-WING UHF SATCOM antennas which can withstand substantially powerful impacts without degrading the capability of the antennas to transmit and receive UHF radio signals. 
     A basic embodiment of an impact resistant X-WING UHF SATCOM antenna according to the present invention includes a thin, flat, hollow rectangular block-shaped mounting base which has a flat lower mounting surface for mounting the antenna to a support structure such as a hood, fender or roof of a vehicle, or to a structural component of a ship or shelter. The basic embodiment of an impact resistant X-WING UHF SATCOM antenna according to the present invention includes an elongated hollow circular cross-section cylindrically-shaped tubular mast which is mounted centrally to the upper surface of the mounting base and extends perpendicularly upwards therefrom. A circular cover cap fits over the upper entrance opening to a bore disposed longitudinally through the length of the mast. Four tubular radiating elements spaced circumferentially apart at ninety-degree intervals extend perpendicularly outward from the outer circumferential wall surface of the mast. The four elements are located near the upper end of the mast, with a transverse plane tangent to the upper surfaces of the elements approximately aligned with the upper transverse face of the mast cover cap. 
     According to the invention, each element includes an electrically conductive cylindrically shaped shell which has a cylindrical bore disposed through its length. The straight, electrically conductive shell of each radially disposed antenna element functions as the active component of each element, which is effective in transmitting and receiving electromagnetic waves at UHF radio frequencies. A first pair of diametrically opposed antenna elements comprises a first electric dipole antenna component. The second pair of diametrically opposed elements, spaced circumferentially apart at ninety degrees to the first pair of elements, functions as a second electric dipole antenna. As is known in the art, two such dipole antennas oriented perpendicularly to each other are effective in transmitting and receiving circularly polarized electromagnetic waves when driven by a radio frequency power source which feeds a sinusoidal signal of a first phase to one dipole element pair, and feeds a sinusoidal signal shifted in time phase by ninety degrees from the first signal to the second dipole pair. The required phase shifting is preferably accomplished by an electrical network known as a 3-db quarter wove coupler or 90-degree power divider/combiner which includes 90-degree phase shifter circuitry and is preferably implemented as a hybrid circuit module. 
     According to the present invention, an antenna coupler network of the type described above is located in a hollow interior space within the mounting base of the antenna. Coaxial cables connected to each conductor of each of the four elements located at the top of the mast are disposed through the hollow interior bore of the mast to terminals of the coupler network. In an example embodiment of the invention, the two coaxial cables consist of a first, 0-degree cable connected to the 0-degree port of a hybrid coupler network located in the base, and a second, 90-degree phase cable connected to the 90-degree port of the coupler network. 
     According to the invention, the two coaxial cables disposed upwards through the bore of the mast are electrically connected to a circular disk-shaped printed circuit board (PCB) which is mounted coaxially in the mast bore, near the upper transverse end of the mast. The PCB board also has depending downwardly therefrom interconnected lengths of coaxial cable which produce 180-degree phase shifted signals for driving opposed conductors of each of the two pairs of dipole elements. The PCB board thus has four antenna element connection terminals. Each of the PCB board element connection terminals is electrically connected to a separate one of four electrically conductive antenna element mounts which are attached to the outer circumferential wall surface of the outer, fiberglass shell of the mast. 
     According to the invention, there are four antenna element mounts spaced circumferentially apart at 90-degree intervals 
     Each antenna element mount has a thin, arcuately curved rectangular plan view base plate which has an inner concave surface that has the same radius of curvature as that of the outer surface of the mast housing, so that the base plate can fit conformally to the mast housing. 
     Each antenna element mount also has protruding radially outwardly from the outer convex surface of the base plate a cylindrically-shaped, circular cross-section boss which has disposed through its length a threaded bore which extends through the base plate. 
     The four antenna element mounts are attached to the outer surface of the antenna mast housing near the upper transverse end of the housing, with the upper transverse edge of each rectangular antenna element mount base plate aligned with the upper transverse annular end wall of the mast. Each antenna element mount is securely fastened to the antenna mast housing, as by a pair of screws inserted into a pair of holes located on opposite sides of the base plate and tightened into a pair of aligned holes through the mast housing. Each antenna element mount is made of an electrically conductive material, such as aluminum or stainless steel, and the internal threaded bore of each mount is electrically conductively connected to a separate one of the four output terminals of the PCB, by a screw disposed perpendicularly downwards through a conductive metal bushing which contacts the upper surface of a strip conductor on the PCB. The screw depends perpendicularly downwards through a hanger flange bracket which protrudes perpendicularly inwards from the upper edge of the antenna element mount into a notch formed in the upper transverse annular end wall of the mast housing. 
     According to the invention each of the four antenna elements is removably attachable to a separate one of the internally threaded antenna element mounts by an externally threaded stud which extends perpendicularly outwards from the center of the inner transverse end face of the element. The stud protruding from each antenna element is electrically conductively connected to the straight electrically conductive tubular shell of the element. Preferably, the inner annular ring-shaped peripheral end face of each antenna element which encircles the protruding threaded stud has adhesively adhered thereto a lock washer which is slipped onto the stud and pressed against the annular end face. 
     With the foregoing construction, a complete impact resistant UHF SATCOM antenna with field replaceable elements according to the present invention can be stored and shipped in a rectangular carton which has a length equal to the height of the antenna mast plus the height of coaxial connectors which protrude perpendicularly downwards from the lower surface of the antenna base plate. The cross-section dimension of the storage and shipping carton need not be any larger than the cross-section of the rectangular antenna base plate. This efficient packing method for storage and shipment is made possible because each of the four loose antenna elements has a length less than that of the antenna mast, and can thus be laid alongside the mast and secured thereto with layers of bubble wrap or other protective shipping filler material. When the antenna arrives at a use location, is can be rapidly assembled by threading the four antenna elements into the bores of the four antenna element mounts, applying a layer of LOCTITE or similar adhesive to the face of the lock washer and tightening the threaded element stud into the element mount bore by hand, or by a wrench which engages a pair of parallel flats formed in diametrically opposed wall surface of each element housing, which extend a short distance longitudinally outwards from the inner annular end face of the antenna element. 
     An antenna according to the present invention and described above is inherently very rugged and impact resistant. However, if any element of the antenna suffers catastrophic damage in the field, it can be readily replaced by unscrewing the element from its mount, using an open-end wrench or pliers, and quickly and easily replaced with an undamaged element. 
     In a foldable modification of the basic embodiment of an impact resistant UHF SATCOM X-WING antenna described above, each of the four elements is attached to the upper end of the antenna mast by a separate tensioned socket joint. This construction enables the elements to be pulled outwardly from a socket joint and folded downwardly and parallel to the longitudinal axis of the antenna mast to minimize the antenna profile for storage and shipment when not in operation, and orbited upwardly and inserted into sockets which lock the elements into radially perpendicular orientations from the antenna mast when the antenna is used to transmit and receive signals. 
     The four antenna element mounts for the foldable antenna according to the present invention are similar in construction and mounting locations to those of the replaceable element antenna described above, with the following modifications. 
     The antenna element support boss of each antenna element of the foldable antenna according to the invention has disposed radially inwardly from the outer longitudinally disposed circular face thereof a tapered, smooth wall blind bore instead of the helically threaded, uniform internal diameter bore disposed through the length of the support bosses used in the replaceable element antenna. The smooth-wall, blind bore in the antenna element support boss of each antenna mount has a tapered, frusto-conic shape, terminated in an inner end wall disposed transversely to the longitudinal axis of the bore and constitutes a socket for supporting an antenna element. The inner end wall is of smaller diameter than the outer entrance opening of the bore. The foldable antenna support boss also has cut into the lower side of the outer longitudinally disposed annular wall thereof a vertically disposed, rectangularly-shaped slot which penetrates the inner cylindrical wall surface of the bore and extends downwardly through a flat formed in the lower side of the outer cylindrical wall surface of the boss, and radially inwardly about half the radial length of the boss. 
     According to the invention, each of the four antenna elements of the foldable antenna has protruding longitudinally from an inner end thereof a tapered, frusto-conically shaped support peg which is of the proper size and shape to be insertably received in an interference fit in the tapered socket bore of the element mount boss. The support peg is an integral part of a circular cross-section body which has a cylindrically-shaped plug portion that is fitted into an open inner end of a tubular antenna element housing, and secured to the housing. The plug portion of the body has an inner cylindrical part which has at one end thereof a transverse face that is aligned with the inner transverse end wall of the element housing. The frusto-conic support peg has a base diameter which is smaller than the outer diameter of the plug, and is coaxially centrally located on the face of the plug, thus forming a flat transversely disposed annular ring-shaped end face on the plug. 
     According to the invention, each element of the foldable antenna includes a tensioning mechanism to maintain a radially inwardly directed force on the element which retains the element support peg aligned within the element mount socket bore. In a preferred embodiment, the tensioning mechanism includes longitudinally disposed within the bore of each element, a longitudinally elongated helical tension spring which has an end portion that fits into a blind cylindrical bore which extends inwardly into an outer transverse end wall of the outer end of the plug, which is located within the bore of the element. The spring is longitudinally movably located within a cylindrical guide tube which is mounted in the blind cylindrical bore in the outer face of the plug and extends into the tubular element housing for an appreciable fraction of the length of the spring. The outer transverse end of the spring is capped by a cylindrically-shaped stop sleeve, which has attached thereto a flexible wire cable which extends longitudinally inwardly through the central bore of the spring, and out through a cable hole which is disposed longitudinally through the center of the transverse end wall of the spring holder bore in the plug, and outwardly through the center of the outer transverse end face of the frusto-conic peg. 
     The inner end portion of the tensioning cable extends through a cable bore that extends through the center of the inner transverse end face of the frusto-conic socket bore in the element mount boss, and into the interior of the antenna mast. The inner end of the tensioning cable is secured against radially outward movement by an inner stop bushing which is fastened to the inner end of the cable, and retained in a cup-shaped blind bore which extends into the inner longitudinally disposed end wall of the mounting base. 
     With the foregoing construction, each antenna element may be re-configured from an operational use position, in which the antenna element support peg is secured with the socket bore of an element mount by tension in the spring within the element, to a downwardly oriented stowed position by grasping the element and pulling it radially outwardly from the antenna mount sufficiently far for the support peg to be withdrawn from the support peg socket bore in the antenna element mount. Thus freed, the element may be folded downwardly towards an orientation parallel to the mast, with the tension cable sliding into the slot in the lower wall of the element mount boss. In this position, the tensioning spring forces the flat inner transverse end face of the antenna element support peg into compressive contact with the lower surface of a flat formed in the lower side of the element support boss, thus retaining the element in a folded position. 
     To re-configure an antenna element from a stowed orientation to an operational orientation, each antenna element is grasped and pulled downwardly to unseat the flat end of the antenna element support peg from the bottom flat of the element mount, and swung upwardly in an arc until the peg is aligned with an element mount socket bore, whereupon pulling tension on the element is released, thus enabling tension in the element spring to pull the peg into the element mount socket bore and thus secure the element in a radially outwardly disposed operational orientation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an upper perspective view of a replaceable element, impact resistant antenna according to the present invention. 
         FIG. 2  is a front element view of the antenna of  FIG. 1 . 
         FIG. 3  is an upper plan view of the antenna of  FIG. 1 . 
         FIG. 4  is a lower plan view of the antenna of  FIG. 1 . 
         FIG. 5  is a schematic diagram of the antenna of  FIG. 1 . 
         FIG. 7  is a fragmentary view of the antenna structure of  FIG. 6 , on an enlarged scale. 
         FIG. 8  is a fragmentary view of the antenna structure of  FIG. 6 . 
         FIG. 8A  is a fragmentary lower plan view of the structure of  FIG. 8 . 
         FIG. 8B  is a transverse sectional view of the antenna structure of  FIG. 8 , taken in the direction  8 B- 8 B. 
         FIG. 9  is an upper plan view of the antenna of  FIG. 1 , with a top cover of the antenna removed. 
         FIG. 10  is a fragmentary perspective view of the antenna of  FIG. 1 , showing how elements thereof are attached and removed. 
         FIG. 11  is an elevation view of an element of  FIG. 10 , on an enlarged scale. 
         FIG. 12  is a fragmentary lower plan view of the antenna of  FIG. 1 , showing a lower cover plate removed from the base of the antenna. 
         FIG. 13  is a lower perspective view of a foldable element, impact resistant antenna according to the present invention. 
         FIG. 14  is a front elevation view of the antenna of  FIG. 13 . 
         FIG. 15  is a perspective view of the antenna of  FIG. 13 , showing a first step in re-configuring one of four elements from an operational use position, to a compact storage and shipment configuration, and showing a fourth and final step in re-configuring the element from a stowed configuration to a use configuration. 
         FIG. 16  is a view similar to that of  FIG. 15 , showing a second step in folding the element down and a third step in folding the element up. 
         FIG. 17  is a view similar to that of  FIG. 16 , showing a third step in folding the element down, and second step in folding the element up. 
         FIG. 18  is a view similar to that of  FIG. 17 , showing a fourth and final step in folding the element down, and a first step in folding the element up. 
         FIG. 19  is a perspective view showing all four elements folded down for storage. 
         FIG. 20  is a side elevation view of one of the four identical elements of the antenna of  FIG. 13 . 
         FIG. 21  is a medial vertical sectional view of the element of  FIG. 20  showing the element attached to an antenna element mount of the antenna of  FIG. 1 . 
         FIG. 22  is an outer end elevation view of the element of  FIG. 20 . 
         FIG. 23  is an inner end elevation view of the element of  FIG. 20 . 
         FIG. 24  is an upper plan view of the element of  FIG. 20 . 
         FIG. 25  is a lower plan view of the element of  FIG. 20 . 
         FIG. 26  is a fragmentary view of the antenna of  FIG. 15 . 
         FIG. 27  is a annular lower plan view of the structure of  FIG. 21 . 
         FIG. 28  is a transverse sectional view of the antenna structure of  FIG. 26 , taken in the direction  28 - 28 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1-12  illustrate a basic embodiment of an X-WING UHF SATCOM antenna according to the present invention, which has replaceable elements. 
       FIGS. 13-28  illustrate a modification of the antenna shown in  FIGS. 1-12 , which has foldable elements. 
     Referring first to  FIGS. 1 and 2 , an impact resistant X-WING UHF SATCOM antenna  50  with replaceable elements may be seen to include a mast  51  having an elongated, straight vertically disposed hollow cylindrical circular cross-section housing  52 . The lower end of housing  52  fits into a short circular cross-section, ring-shape support boss  53  that protrudes upwardly from the center of the upper surface  54  of a rectangular upper cover plate  55  of a thin rectangular box-shaped base housing  56 . 
     As shown in  FIG. 6 , housing  52  of antenna mast  51  has the shape of an elongated, thin wall circular cross-section cylindrical shell. The housing  52  is made of a durable electrically non-conductive material, such as fiberglass. As shown in  FIGS. 3 and 6 , housing  52  has disposed through its length a circular cross-section bore  57 , which extends from the lower transverse annular end wall  58  to the upper transverse end wall  59  of the mast housing. 
     As shown in  FIGS. 2 ,  4  and  6 , base housing  56  has generally the shape of a thin rectangular cross-section box which has disposed through the upper rectangular plate cover  55  thereof a centrally located circular aperture  60  which communicates with a rectangular block-shaped, hollow interior space  61  of the base housing. Base housing also has a lower rectangular base plate  56 -B. Aperture  60  has a smaller diameter than that of ring-shaped antenna mast support boss  53 , which coaxially overlies the aperture, thus forming a flat annular ring-shaped shoulder ledge  62 , which supports the lower transverse end wall of mast housing  52 . The mast housing  52  is secured to mast support boss  53  by an adhesive bond. 
     Referring to  FIGS. 1 ,  3  and  6 , it may be seen that the upper opening of bore  57  through mast housing  52  is closed by a circular top cover  63 . 
     As shown in  FIGS. 1 and 3 , antenna  50  includes four identical straight cylindrically shaped, circular cross-section tubular elements  64 - 1 ,  64 - 2 ,  64 - 3 ,  64 - 4 . Each element has a length less than the height of mast  51  above upper surface  54  of base housing cover plate  55 , and a diameter less than that of the mast housing  52 . As shown in  FIGS. 3 and 6 , the four antenna elements  64  protrude radially in a horizontal plane perpendicularly outwards from the outer circumferential wall surface  65  of mast housing  52 . 
     As shown in  FIG. 11 , each antenna element  64  includes an elongated, straight hollow circular cross-section cylindrically shaped tubular housing  68  which has longitudinally disposed through its length a bore  69 . The antenna element housing  68  has an outer transverse end  70  that is covered by a circular element end cap  71  that has the same diameter as the outer diameter of element housing. 
     The tubular housing  68  of each antenna element  64  is made of aluminum or another such electrically conducting material and functions as the active component of the element in receiving and transmitting radio frequency electromagnetic waves. 
     As may be seen best by referring to  FIG. 3 , the four antenna elements  64 - 1 ,  64 - 2 ,  64 - 3 ,  64 - 4  are spaced circumferentially apart at 90-degree intervals. As will be described in detail below, each of the two pairs of diametrically opposed elements, e.g.,  64 - 1 / 64 - 3 ,  64 - 2 / 64 - 4  is electrically configured as a dipole antenna  66 - 1 ,  66 - 2 , respectively. The two dipole antennas are perpendicular to one another, thus forming a crossed dipole or X-WING antenna configuration. As is well known by those skilled in the art, the crossed dipole or X-WING configuration of antenna elements  64  is suitable for transmitting and receiving circularly polarized radio waves. 
     As shown in  FIG. 11 , the tubular housing  68  of each antenna element  64  has located at an inner transverse end wall  72  thereof an element adapter  73  for removably attaching the element to antenna masts. Each element adapter  73  is made of an electrically conductive material such as aluminum, and has generally the shape of cylindrical body  74  which has a same outer diameter as that of antenna element tubular housing  68 , and a reduced diameter plug section  75  which extends outwardly from an outer transverse face  76  of the body. The plug section  75 , which preferably has a knurled outer cylindrical surface, fits into the inner entrance opening  77  of the bore  69  in element housing  68 , and is secured to the element housing by a press-fitted adhesive bond. 
     As shown in  FIG. 11 , element adapter structure  73  includes a straight threaded stud  78  which is coaxially aligned with the longitudinal axis of element housing  68 . Stud  78  has a smaller diameter than that of adapter body  73 , and extends perpendicularly from an inner transverse end face  79  of body  74 . Preferably, stud  78  is received within central aperture  80  of an annular ring-shaped lock washer  81 , which preferably is adhesively adhered to the inner transverse end face  79  of adapter body  74 . Also, as shown in  FIG. 11 , body  73  preferably has formed in the outer cylindrical wall surface  82  thereof a pair of diametrically opposed, parallel, longitudinally disposed wrench flats  83 - 1 ,  83 - 2  to facilitate torquing element  64  about its longitudinal axis. 
     FIGS.  1  and  7 - 10  illustrate how antenna elements  64 - 1 ,  64 - 2 ,  64 - 3 ,  64 - 4  are removably attached to antenna mounts  94 - 1 ,  94 - 2 ,  94 - 3 ,  94 - 4 . 
     As shown in  FIG. 1 , the four antenna mounts  94  are fastened to the outer cylindrical wall surface  95  of antenna mast housing  52  at circumferentially spaced apart intervals of 90 degrees, adjacent to the upper transverse end wall  59  of the mast housing. 
     As shown in  FIGS. 7 and 9 , each antenna element mount  94  has a thin uniform thickness, arcuately curved base plate  96  which has an inner longitudinally disposed arcuately curved surface  97  that has the same radius of curvature as outer cylindrical wall surface  95  of mast housing  52 , so that the inner surface of the base plate can fit conformally to the outer surface of the mast housing. Each base plate  96  has protruding perpendicularly inwards of the inner surface  97  thereof a hanger bracket plate  98  which has a flat horizontally disposed upper surface  99  which is coextensive with the upper edge surface  100  of the base plate. As shown in  FIGS. 7 and 9 , hanger bracket plate  98  has a generally rectangular plan view shape, which has a width less than the circumferential arc length of the base plate. 
     As shown in  FIG. 9 , each hanger bracket plate  98  fits downwardly into a separate one of four rectangular notches  99 - 1 ,  99 - 2 ,  99 - 3 ,  99 - 4  which extend downwardly into upper transverse end wall  59  of antenna mast housing  52  at circumferentially spaced apart intervals of ninety degrees. As shown in  FIGS. 9 and 10 , each base plate  96  has a square outline shape, and is secured to mast housing  52  by a pair of circumferentially spaced apart screws  100 ,  101  which are inserted through holes  102 ,  103  located next to outer longitudinally disposed edges  104 ,  105  of the base plate, and through a pair of aligned hoes  106 ,  107  through mast housing  52  into bore  57  of the mast housing, where the nuts  108 ,  109  are tightened onto the threaded shanks of the screws. 
     As shown in  FIG. 10 , each antenna element mount  94  has protruding perpendicularly outwards from outer surface  110  of base plate  96  a circular cross-section, cylindrically-shaped boss  111 . Each boss  111  has disposed through its length a threaded bore  112  which is aligned with a through-bore  113  disposed radially through mast housing  52 . As shown in  FIG. 10 , the foregoing construction enables each antenna element  64  to be readily attached to and removed from an antenna element mount  94  by grasping the tubular housing  68  of the element and twisting it about its longitudinal axis to thus screw in the threaded stud  78  protruding inwardly from the inner end of the element into or out of the threaded bore  112  in the element mount boss  111 . The element  64  may be further tightened or loosened by engaging flats  83  of the element within the jaws of an open-end wrench or pliers. 
       FIGS. 5 through 9  illustrate other components of antenna  50  which are connected with elements  64  included in mounts  94  to comprise an X-WING UHF SATCOM antenna which is operable to transmit and receive circularly polarized UHF radio waves through elements  64 . 
     As shown in  FIGS. 7-9 , each rectangularly-shaped hanger bracket plate  98  of each antenna element mount  94  has a straight inner edge  114  which is perpendicular to the longitudinal axis of antenna element housing  51 , and which is located radially inwards of the inner circumferential surface  115  of antenna mast housing  52 . 
     As shown in  FIGS. 7-9 , each of the hanger bracket plates  98  has depending perpendicularly downwards from the lower surface  117  thereof the shank of a screw  118  which is disposed through a hole  119  near the inner edge  114 , through a tubular conductive spacer bushing  120 , through a hole  121  through a printed circuit board (PCB)  122  and through a nut  123  tightened onto the shank of the screw against the lower surface  124  of the PCB to thus support the PCB. 
     As shown in  FIGS. 5 and 9 , PCB  122  has affixed to the upper surface  125  thereof strip-line conductors  126  which connect at outer radial ends thereof to conductive spacer bushings  120 , and at inner radial ends thereof to conductive metal eyelet pairs  127 ,  128 . 
     As shown in  FIGS. 5 ,  6 , and  7  there are four eyelet pairs  127 - 1 ,  127 - 2 ,  127 - 3  and  127 - 4 . Two sets of eyelet pairs  127 ,  128 - 1  are connected to the center and outer conductors  129 ,  130 - 1  of a pair of coaxial cables  131 ,  132 , which are disposed perpendicularly downwards form the PCB  122  through antenna mast housing  57  to a hybrid antenna coupler  135  in base housing  56 . 
     As shown in  FIGS. 6 and 12 , the coaxial cables  131 ,  132  are electrically connected at lower ends thereof located within the hollow interior space  61  of base housing  56  of antenna  50  to a 0-degree port  135 - 2  and a 90-degree port  135 - 3  of hybrid antenna coupler  135  located in the interior space of the housing. Hybrid antenna coupler  135  is a reciprocal device, which has a first interface port  135 - 1 , which in a transmit mode receives a modulated UHF radio signal input to a “high-angle”, i.e., circular polarization mode coaxial N-connector  136  through a high angle mode, i.e., circular polarization mode, coaxial cable  136 - 1 . Connector  136  is mounted in a hole that penetrates base plate  56 -B of housing  56 . 
     Hybrid antenna coupler  135  functions in a transmit mode as a power divider, splitting the power input to first interface port  135 - 1  into a first, 0-degree signal at 0-degree port  135 - 2  which has one-half the input power level, i.e., is attenuated by 3-db. Similarly, a second, 90-degree antenna signal shifted in phase by 90-degrees from the 0-degree signal and also attenuated by 3-db, is output at hybrid terminal  135 - 3 . The two signals, separated in phase by 90 degrees, when input to 90-degree displaced dipole pairs comprised of elements  64 - 1 / 64 - 3 ,  64 - 2 / 64 - 4 , cause the antenna to launch right-hand circularly polarized (RHCP) electromagnetic waves axially from the elements, i.e., along the longitudinal axis of antenna housing  52 . 
     Hybrid antenna coupler  135 , which, as stated above, is a reciprocal device is also effective in a receive mode of operation of combining 90-degree phase shifted signals induced in dipole pairs  64 - 1 / 64 - 3 ,  64 - 2 / 64 - 4  by a circularly polarized signal received and input to antenna ports  135 - 2 ,  135 - 3 , to a single-phase output signal at interface port  135 - 1  of the hybrid network. 
     As shown in  FIGS. 1 and 6 , antenna  50  optionally and preferably includes additional components which enable the antenna to transmit and receive linearly polarized radio waves. Thus, as shown in  FIGS. 5 and 6 , antenna  50  preferably includes a cylindrical cup-shaped eclectically conductive shell  137  which is contained coaxially within the bore  57  of antenna housing mast  52 . The shell  137  has a cylindrical body  138  which is terminated at the lower end thereof by a circular disk-shaped electrically conductive base  139 . Base  139  of shell  137  is electrically conductively connected to an antenna port  140  of a band pass filter  141 , which is connected through a “low-angle” coaxial cable  142  to a low-angle interface port coaxial connector  143 , as shown in  FIG. 5 . Connector  143  is mounted in a hole that penetrates base plate  56 B of housing  56 . A pair of coiled coaxial inductors  144 ,  145  are connected in series with the two hybrid antenna coupler ports  135 - 2 ,  135 - 4  to provide electrical isolation between operation of the low-angle antenna conductor  137 , which is effective in transmitting and receiving signals which are linearly polarized in a direction parallel to the longitudinal axis of antenna housing  52 , i.e., vertically polarized signals, and circularly polarized signals transmitted and received by radially disposed elements  64 . 
       FIGS. 13-28  illustrate a modification  150  of the replaceable antenna element  50  shown in  FIGS. 1-12  and described above. Modified antenna  150  is substantially similar in electrical function to replaceable element antenna  50 . However, antenna  150  utilizes radially disposed elements which are attached to the mast of antenna by a novel construction which enables the elements to be folded downward to a small profile for storage and shipment configuration, and foldable upward to a radially disposed operational configuration. 
     Referring now to  FIG. 13 , it may be seen that an impact resistant X-WING UHF SATCOM antenna  150  with foldable elements has an external appearance and construction which are substantially similar to that of replaceable element antenna  50  described above. Thus foldable element antenna  150  has a base housing  156 , an elongated tubular mast  151  which extends perpendicularly upwards from the center of an upper cover plate  155  of a base housing  156 , and four elements  164  which protrude radially outwards from the outer cylindrical wall surface  165  of mast housing  152  at 90-degree circumferential intervals. 
     As shown in  FIGS. 20-25 , each antenna element  164  includes an elongated, straight hollow circular cross-section cylindrically-shaped tubular electrically conductive housing  168  which has disposed through its length a bore  169 . Each antenna element housing  168  has an outer transverse end  170  that is covered by a circular element end cap  171  that has the same diameter as the outer diameter of the element housing. 
     As shown in  FIGS. 17 and 21 , each of the four antenna elements  164  of foldable antenna  150  has located at an inner transverse end  172  thereof an element adapter  173  for attaching the element to a separate one of four antenna mounts  194  attached to the outer cylindrical wall surface  195  of antenna mast housing  152  at circumferentially spaced apart intervals of 90 degrees, adjacent to the upper transverse end wall  159  of the mast housing. Each element adapter  173  is made of an electrically conductive material such as aluminum, and has generally the shape of a circular cross-section body  174  which has an outer cylindrical plug section  175  that preferably has a knurled surface, fits into the inner entrance opening  177  of the bore  169  in element housing  168 , and is secured to the housing in electrically conductive contact therewith by an adhesive bond. 
     As shown in  FIGS. 17 and 21 , each element adapter  173  includes a frusto-conically shaped, tapered antenna element support peg  178  which extends perpendicularly from an inner transverse end face  179  of plug section  175  of body  174 . Support peg  178  has a smaller base diameter than the diameter of body  174 , and is coaxially aligned with the plug section of the body and tubular element housing  168  into which the plug section  175  of the body fits. A transversely disposed flat annular ring-shaped flange surface  178 A is formed in the transverse end face of plug section  175  of body  174 . 
     Referring still to  FIGS. 17 and 21 , it may be seen that the support peg  178  of each antenna element  164  is receivable in a socket bore  212  of separate antenna element mount  194 . Thus, as shown in the figures, each antenna element mount  194  has a thin uniform thickness, arcuately curved base plate  196  which has an inner longitudinally disposed arcuately curved surface  197  that has the same radius of curvature as outer cylindrical wall surface  195  of mast housing  152 , so that the inner surface of the base plate can fit conformally to the outer surface of the mast housing. Each base plate  196  has protruding perpendicularly inwards of the inner surface  197  thereof a hanger bracket plate  198  which has a flat horizontally disposed upper surface  199  which is coextensive with the upper edge surface  200  of the base plate. 
     As shown in  FIG. 29 , each hanger bracket plate  198  fits downwardly into a separate one of four rectangular notches  199 - 1 ,  199 - 2 ,  199 - 3 ,  199 - 4 , which extend downwardly into upper transverse end wall  159  of antenna mast housing  152  at circumferentially spaced apart intervals of 90 degrees. 
     As shown in  FIGS. 19-25 , each base plate  196  has a square outline shape, and is secured to mast housing  152  by a pair of circumferentially spaced apart screws  200 ,  201  which are inserted through holes  202 ,  203  located next to outer longitudinal edges  204 ,  205  of the base plate, and through a pair of aligned holes  206 ,  207  through mast housing  152  into bore  157  of the mast housing, where nuts  208 ,  209  are tightened onto the threaded shanks of the screws. 
     As shown in  FIG. 19 , each antenna element mount  194  has protruding outwards from outer surface  210  of base plate  196  a circular cross-section cylindrically-shaped boss  211 . Each boss  211  has disposed through its length a frusto-conically tapered smooth-wall blind socket bore  212 . Socket bore  212  terminates in an inner circular disk-shaped end wall  213 , which is disposed transversely to the longitudinal axis of the bore, and parallel to longitudinal axis of antenna mast  152 . Also, socket bore  212  of antenna element mount  194  is of the proper size and shape to receive in an interference fit the support peg  178  of an antenna element  168 . According to the invention, each element  168  of the foldable antenna  150  includes a tensioning mechanism to maintain a radially inwardly directed force on a support peg  178  inserted into a socket bore  212  to retain the peg in a the socket bore, as will now be described. 
     A may be seen best by referring to  FIGS. 21-25 , a tension mechanism  215  for releasably exerting a radially inwardly directed force on antenna element support peg  178  to thus hold the peg in element mount socket bore  212  and thereby maintain antenna element  164  in a radially disposed orientation relative to antenna mast  152  includes a longitudinally elongated helical tension spring  216  located coaxially within the bore  169  disposed longitudinally through element housing  168 . Spring  216  has at one end thereof a longitudinally disposed portion which fits coaxially within the bore  217  of an elongated cylindrically-shaped guide tube  218 . Guide tube  218  fits coaxially within a blind coaxial bore  219  which extends longitudinally into the center of an inner transverse end wan  220  of cylindrical body  174  of element adapter  173  located within bore  169  of element housing  168 . 
     Spring  216  is longitudinally movable within bore  217  of guide tube  218 , and has abutting an outer transverse end thereof a cylindrically-shaped stop sleeve  221 . Stop sleeve  221  has extending longitudinally from the center of the inner transverse face  222  thereof an elongated flexible wire tensioning cable  224 . Tensioning cable  224  extends longitudinally inwardly through spring  216  along the center line of the spring, and through a small diameter wire bore  225  which extends longitudinally inwardly through the inner face  226  of guide tube bore  217 , and out from the outer transverse end face  227  of antenna element support peg  178 . 
     The inner end portion of tensioning cable  224  extends through a cable bore  228  that extends through the center of the inner transverse end wall  213  of socket bore  212  in the element mount boss  211 , and into the bore  157  through the antenna mast housing  152 . The inner end of tensioning cable  224  is secured against radially outward movement by an inner stop bushing  229  which is fastened to the inner end of the cable and retained in a cup-shaped blind bore  230  which extends into the inner longitudinally disposed end wall  231  of the element support boss  211 . 
     As shown in  FIGS. 19 ,  25  and  27 , each foldable antenna element support boss  211  has cut into the lower side of the outer longitudinally disposed annular wall  232  thereof a vertically disposed, rectangularly-shaped slot  233  which penetrates the inner cylindrical wall surface  234  of the socket bore  212  in the boss and extends downwardly through a flat  235  formed in the lower side  236  of the outer cylindrical wall surface  237  of the boss, and radially inwardly about half the radial length of the boss. 
     With the foregoing construction, elements  164  of foldable element antenna  150  may be re-configured from an operational use position as shown in  FIGS. 13 and 15 , in which each element support peg  178  is secured with the socket bore  212  of an antenna element mount boss  211  by tension spring  216  within the element, to a compact, folded configuration as shown in  19 . 
     As shown in  FIGS. 16-18 , re-configuration of antenna  150  from an operational to a folded configuration is accomplished by first grasping in turn each element  164  and pulling the element radially outwards from element mount  194  against tension afforded by spring  216 , sufficiently far for the element support peg  178  protruding from the inner end of the element to be withdrawn from the socket bore  212  in the antenna element mount boss  216 . Thus freed, the element  164  may be folded downwardly towards parallel alignment with antenna mast housing  152 , with the tensioning cable  224  sliding into the slot  233  in the lower wall  232  of the element mount boss  211 . When pulling force exerted on the element  164  is then released, tension in spring  216  draws the flat outer face  227  of element support peg  178  into compressive contact with the flat  235  in the lower surface of antenna element mount boss  211 , thus retaining the element in a downwardly oriented, folded position. 
     Re-configuration of antenna  150  from a folded configuration to an operational configuration is accomplished as shown in the sequence of  FIGS. 19 ,  18   17 ,  16  and  15 . As shown in that sequence of figures, re-configuration to an operational configuration is accomplished by in turn grasping each individual antenna element  164  and pulling the element downwards against tension of spring  216  sufficiently far to unseat the flat end face  227  of antenna element support peg  178  from the flat  235  at the bottom of an antenna element support boss  211 . The element  168  is then orbited upwardly in an arc until the antenna element support peg  178  is longitudinally aligned with an element mount socket bore  212 . Pulling tension in the element  164  is then released, thus enabling tension in element spring  216  to pull element support peg  178  into element mount socket bore  212  and thus secure the element in a radially outwardly disposed operational orientation. 
     Other than differences in antenna elements  164  and mounts  194  described above, the structure and functions of foldable element modification  150  of replaceable element antenna  50  described previously and antenna  50  are identical. Thus, as shown in  FIGS. 6  and  9 , foldable element antenna  150  preferably also includes the construction shown in  FIG. 6  to enable the foldable element antenna to function in a linearly polarized mode as well as a circularly polarized mode.