Abstract:
A cast core process is used to fabricate a very small, precision wound helical antenna having readily repeatable configuration parameters for use in a high GHz multi-element (e.g., phased array) antenna. A dielectric core member is formed by shaping a solid mandrel having a precision helical groove. After a mold is formed around the mandrel and cured, the mandrel is extracted, so that it may be used to make additional identical molds. A dielectric mixture is injected into the mold&#39;s cavity, and cured. The mold is then removed, and antenna wire is tightly wound and bonded into the dielectric core&#39;s helical groove. The antenna wire-wrapped core is then mechanically and electrically attached to a baseplate laminate structure, that includes a tuning circuit, so that the antenna may be physically mounted to a support member and connected to an associated transmit—receive module.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to the manufacture and assembly of helical antennas for very high frequency applications (e.g., several tens of GHz), and is particularly directed to a cast core-based fabrication of a very small precision wound helical antenna having readily repeatable configuration parameters for use in a phased array antenna. 
     BACKGROUND OF THE INVENTION 
     Continuing improvements in circuit manufacturing technologies in developing smaller sized components for achieving higher operational frequencies (smaller wavelengths) has been accompanied by a need to reduce the dimensions of both signal processing and interface circuitry support hardware and their associated radio frequency antenna structures. In such reduced size, high frequency communication systems, helically wound antennas, such as those supported by low loss foam cores, are particularly attractive, as their radiation characteristics and relatively narrow physical configurations readily lend themselves to implementing physically compact phased array architectures that provide for electronically controlled shaping and pointing of an antenna&#39;s directivity pattern. 
     However, as operational frequencies have reached into the multidigit GHz range, achieving dimensional tolerances in large numbers of like components has become a major challenge to system designers and manufacturers. For example, in a relatively large number element phased array antenna operating at frequency in a range of 15-35 GHz, and containing several hundred to a thousand or more antenna elements, each antenna element may have on the order of twenty turns helically wound within a length of only several inches and a diameter of less than a quarter of an inch. 
     While conventional fabrication techniques, such as those which employ crossed-slat templates, diagrammatically illustrated in FIG. 1 at  11  and  12  to form a winding  14 , may be sufficient to form helical windings for relatively large sized applications (since relatively small variations in dimensions or shape may not significantly degrade the electrical characteristics of the overall antenna), they are inadequate for very small sized elements (multi-GHz applications), where minute parametric variations are reflected as substantial percentage of the dimensions of each element. As a consequence, unless each element is effectively identically configured to conform with a given specification, there is no assurance that the antenna will perform as intended. This lack of predictability is essentially fatal to the successful manufacture and deployment of a high numbered multi-element antenna structure, especially one that may have up to a thousand elements. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the drawbacks of using conventional helical antenna fabrication techniques for high frequency designs are effectively obviated by a precision cast core-based manufacturing process to construct any number of very small helically wound antenna elements, each of which has readily repeatable configuration parameters. Pursuant to the invention, a dielectric core member upon which the helical antenna is wound is formed by shaping a solid mandrel to conform with the intended contour of the core member, and such that the eventually realized core member provides the intended characteristic impedance of the antenna. In addition, a precision helical groove is machined in the surface of the mandrel, to a shape and depth that provide for precision seating of the antenna wire that is wound around the dielectric core member. 
     After machining the mandrel to its intended shape, a silicone mold is formed around the mandrel and cured. The mandrel is then extracted, leaving the silicone mold with a shaped cavity having an embossed helical ridge that replicates the shape of the groove in the mandrel. The mandrel may now be repeatedly used to make additional dielectric cores of the identical shape and dimensions. A dielectric core epoxy—glass bead mixture is then injected into the silicone mold&#39;s cavity, and cured. 
     The silicone mold is then removed, and a length of antenna wire that is slightly longer than the length of helical groove is tightly wound in the dielectric core&#39;s helical groove, leaving wire extensions that project from the base and distal ends of the core. The antenna wire is then adhesively secured in the core groove at selected locations, thereby realizing a dielectric core-supported helical winding that is dimensionally stable, conforming exactly with the precision helical groove machined in the outer surface of the original mandrel. 
     The antenna wire-wrapped core is then mechanically and electrically attached to a baseplate laminate structure, so that the antenna may be physically mounted to a support member and connected to an associated transmit—receive module. The baseplate laminate structure includes a microstrip tuning circuit connected between the feed end of the helical antenna wire and the center pin of a standard self-mating connector, which provides a direct low loss connection to the transmit—receive module. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 diagrammatically illustrates the conventional use of a pair of crossed-slat templates for forming a relatively large sized, low frequency helical antenna; 
     FIG. 2 is a diagrammatic perspective side of the configuration of a precision, cast core-wound helical antenna in accordance with the present invention; 
     FIGS. 2A and 2B are respective side and end views of a baseplate of the helical antenna of FIG. 2; 
     FIGS. 3 and 4 are respective end and side views of a cup structure base plate in which the helical antenna of FIG. 2 is inserted; 
     FIG. 5 shows a microstrip-configured tuning circuit for the helical antenna of FIG. 2; and 
     FIGS. 6-11 diagrammatically illustrate the precision, cast core-wound helical antenna element of FIG. 2 at respective stages of its manufacture. 
    
    
     DETAILED DESCRIPTION 
     A precision helical antenna manufactured in accordance with the cast core-based fabrication scheme of the present invention is diagrammatically shown in the side view of FIG. 2 as comprising an integrated arrangement of a cup structure  20 , a baseplate  22 , dielectric core member (or simply core)  30 , a multiturn conductive helix  40 , a tuning circuit  50  for the antenna, and a (self-mating) connector  60 . 
     The cup structure  20 , which is shown in greater detail in the end view of FIG.  3  and the side view of FIG. 4, provides mechanical support for each of the baseplate  22 , dielectric core member  30  and helical winding  40 , and antenna tuning circuit  50 . The cup structure  20  and the baseplate  22  are made of conductive material, such as aluminum plated with thin nickel and gold layers. In cross-section the cup structure  20  has a generally inverted cylindrical ‘T’ shape, defined by a generally hollow, centrally located cylindrical cup  21 , which projects from a generally flat circular plate member  23 . A plurality of mounting holes  24  are formed through the plate member  23  and are sized to receive fasteners such as screws and the like for affixing the baseplate to the cup structure. A blind or self-mate connection through-hole  25  is formed in plate member  23  at the bottom of the cup  21  for providing attachment of the antenna tuning circuit  50  to an associated transmit—receive module. 
     As shown in FIG. 5, the tuning circuit  50  is formed of a generally circular laminate structure comprised of a thin dielectric substrate  51 , such as a 0.01 inch thick Rogers 6002 substrate, upon a first side  52  of which a layer of (half-ounce) rolled metallic (e.g., copper) foil  53  is formed. The metallic foil layer is selectively etched to form a microstrip tuning element  54 , such as a quarter-wave transformer that extends from a feed port  55  to a connection port  56 . 
     An attachment layer, such as a layer of 0.002 inch thick (acrylic) adhesive film, having a protective backing layer, is attached to the unplated side of the dielectric substrate  51 . Mounting holes are drilled through the tuning circuit laminate  50  to be aligned with the through holes  28  in the baseplate. A 0.020 inch diameter hole  69  is drilled through the tuning circuit laminate  50 , to provide access to the feed port  55  for the multiturn conductive helix  40 . The tuning circuit laminate  50  is attached to the top of the baseplate  22  by means of the adhesive layer  55 , after the protective backing layer is peeled off the adhesive film, and the tuning circuit  50  is aligned with the baseplate  22 , by pressing the two components together. 
     The feed port  55  of the tuning circuit  50  is connectable to a conductive pin  61  of a standard (GPO shroud) self-mating connector  60 , so as to facilitate a direct low loss connection to the transmit—receive module. The connection may be effected by a conductive ribbon (e.g., a 0.002 inch thick by 0.010 inch wide by 0.020 inch long gold ribbon) that extends from the end of the pin  61  to the microstrip tuning element  54 . The tuning circuit  50  may be attached to the GPO connector  60  by any conventional means, such as solder, thermo-compression, welding, and the like. Once the ribbon connector is attached, a portion of the input side of the tuning circuit, such as a length of 0.020 inch of the etched tuning circuit trace, as a non-limiting example, is pre-tinned for attachment to the antenna&#39;s helical wire  40 . 
     The dielectric support member  30 , upon which the multiturn conductor  40  is helically wound and supported, comprises a generally cylindrically shaped elongated dielectric core member or rod  30 , having a feed or base end  31 , that is glued to the baseplate  22 , using a suitable epoxy adhesive, such as Hysol 9320 epoxy. While the major length portion  33  of the dielectric rod  30  has a constant diameter cylindrical shape, the distal end  35  of the rod  30  terminates with a slight taper, as shown at  37  in the side view of FIG.  2 . Extending along a helical path formed in the outer surface  34  of the dielectric core member  30 , including both the major length portion  33  and the tapered portion  37  of the rod, is a precision formed groove  32 , that serves as support path or track for the conductive winding  40  of the antenna element. 
     As described briefly above, the dielectric core member  30  is formed by a cast process diagrammatically illustrated in FIGS. 6-11, to realize a very small precision wound helical antenna, that has readily repeatable configuration parameters. For this purpose, as shown in FIG. 6, a solid mandrel  71 , such as a cylindrical aluminum rod, is shaped (e.g., by machining) to conform with the intended elongated, partially tapered cylindrically elongated shape of the dielectric core member  30  shown in FIG.  2 . As described above, the dimensions of the mandrel are based upon the dielectric properties of the material used in casting the dielectric core member  30 , so that the eventually realized core provides the requisite characteristic impedance and resonant frequency for the helical antenna. 
     In addition to shaping the mandrel to provide the taper at its distal end, a precision helical groove  73  is formed (e.g., machined) in the outer surface  75  of the mandrel, as shown in FIG.  7 . The shape and depth of the helical groove  73  are defined in accordance with the cross-sectional characteristics of the wire to be used for the helical winding  40  (e.g, #31 AWG wire, as a non-limiting example). To provide for precision seating of the wire within the helical groove  73 , the groove may be formed in the mandrel surface  75  up to a depth of half the diameter of the antenna wire. 
     Next, as shown in FIG. 8, a cast or mold  81  (such as, but not limited to a silicone mold) is formed around the mandrel  71 . After the mold  81  has cured, the mandrel  71  is extracted, thereby leaving the mold with a shaped cavity  83  having an embossed helical ridge  85  that replicates the shape of the groove  73  in the mandrel  71 , as shown in FIG.  9 . The mandrel  71  is now available for use in making another mold. 
     Next, as shown in FIG. 10, a dielectric core epoxy—glass bead mixture  91 , such as Emerssen and Cummings Eccospheres Grade SI, Shell Epon 828 epoxy, and SmoothOn Senite  19  hardener, is injected into the mold cavity  81 , and allowed to cure. As a non-limiting example, this glass epoxy mixture of may be cured at a temperature on the order of 65° C. for 90 minutes. After the dielectric core mixture  91  has cured—to realize the dielectric core member  30  containing the helical groove  32  that replicates the helical ridge  85  of the mold cavity  83 —the mold  81  is removed. 
     As shown in FIG. 11, antenna wire  40  is then tightly wound in the dielectric core&#39;s helical groove  32 , leaving extra lengths of wire  93  and  95  projecting from the base end the distal end of the dielectric core member  30 . The antenna wire is then tacked in place at selected locations within the core groove  32 . For example, the antenna wire may be tacked at top, middle and bottom locations  94 ,  96  and  98 , by a suitable curable adhesive, such as Hysol 9320 epoxy, and cured for 60 minutes, at 80° C., as a non-limiting example, thereby securely bonding the antenna wire to the helical groove and thereby retaining the antenna wire  40  around the dielectric core member  30  in a helical shape that conforms exactly with the precision helical groove  32  around the core. 
     The wire-wrapped dielectric core member  30  may be readily attached to the baseplate  22  by means of a suitable adhesive, such as Hysol 9320 epoxy, referenced above. When attaching the base end of the dielectric core member  30  to the baseplate  22 , the core member may be rotated at an angle on the order of 45° counter clockwise, as viewed from the tapered, distal end of the dielectric core member. The epoxy adhesive may be cured at a temperature on the order of 80° C. for 60 minutes. 
     The extension length  93  of the antenna wire  40  at the base end of the dielectric core member  30  is pulled taut over the connection port  56  of the tuning circuit  50 , so that it is flush with the tuning circuit substrate  51 , and overlaps the connection port  56  by a prescribed distance (e.g., 0.020 inches). It is then cut and soldered in place. The extension length of wire  95  at the distal end  35  of the core member is trimmed, so that it terminates with the distal end of the helical groove  32 . 
     The completed helical antenna, shown in FIG. 2, is readily mountable to an associated antenna system support structure, such as a phased array mounting plate, by means of suitable fasteners inserted through mounting holes  24  in the plate member  23 , and associated holes in the mounting plate, and with the GPO self-mating connector  60  extending through an associated aperture in the mounting plate for connection to an antenna interface port of the transmit receive module. As pointed out above, because the cast core process of the invention ensures repeatability of the dimensional parameters of each helical winding and its supporting dielectric core member, it is particularly useful in constructing a high frequency phased array antenna (e.g., one operating in a frequency range of 15-35 GHz), containing several hundred to a thousand or more helical antenna elements, each of which may have a pitch on the order of less than an eighth of an inch. 
     While we have shown and described an embodiment in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as are known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.