Patent Publication Number: US-6219902-B1

Title: Method for manufacturing a protectively coated helically wound antenna

Description:
The present invention generally concerns a method for manufacturing a protectively coated helically wound antenna, such as those typically used in portable communication devices. More specifically, the present invention concerns a method for manufacturing a helically wound antenna which forms the protective coating using injection molding, but avoids deformation of the helical winding during molding by precise and repeatable control of the molding point relative to the winding. 
     BACKGROUND OF THE INVENTION 
     Portable communicators, such as cell phones, frequently utilize antennas including a helical winding. Helical windings permit a relatively long effective antenna length with a small physical antenna length. This is convenient in cell phones and other portable communicators since small physical size is beneficial and since a certain antenna length is necessary to achieve particular broadcast and reception frequencies. Accordingly, antennas are frequently formed, in whole or part, from a helical conductor. Small size also dictates that the wire used to form the helical conductor be thin. This requires the helical conductor to be encased in a protective material, since cell phone antennas are often subjected to forces which would permanently deform delicate helical windings. 
     The typical helical windings are formed from a thin and delicate conductive wire. Thin wires help preserve the desired small size and low weight which is desirable in portable communicators. Thin conductive wires also facilitate the low power transmission and reception functions of portable communicators. 
     The coating of such thin helical conductors with protective material has proved difficult. Injection molding is an efficient and widely used coating technique, but often deforms delicate helical antenna conductors. The helical winding is placed in a mold, typically while it is mounted on a core, and thermoplastic material is injected into the mold. Significant forces are applied to the helical winding during the injection, and deform the winding by changing its pitch, i.e., the spacing between windings, and causing the pitch to be nonuniform. This changes the electrical characteristics of the antenna in a manner which may vary from one antenna to the next during manufacturing. Compensation for these variances is often achieved through additional processing, such as testing and trimming to tune the antenna to a desired frequency. Even still, a significant percentage of manufactured antennas may be unsuitable for use. Obviously, this increases both the cost and difficulty of manufacturing. In addition, performance tolerances must be generous enough to accommodate the variances experienced in those antennas which are still suitable for use. 
     It is known to wind the helical structure around supports prior to injection to attempt to avoid deformation. Exemplary techniques are disclosed in Bumsted, U.S. Pat. No. 5,648,788, Jul. 15, 1997, and in Valimaa et al., U.S. Pat. No. 5,341,149, Aug. 23, 1994. In the first technique, a relatively complex molding process is disclosed, where a sliding bar locks a coil onto a special handle assembly for molding. The mold includes mold pads for holding the coils in place during molding. This leaves portions of the coils exposed, requiring additional processing. 
     Valimaa also recognizes the potential for thin helical windings to deform during injection molding, and discloses a threaded support core, used for molding of helical coils. The core is completely molded into the coil and therefore cannot control the point of injection relative to the beginning of the winding. Neither Bumsted or Valimaa recognizes or addresses the need to control this point to avoid deformation in the first few windings. 
     In sum, there is a need for an improved and efficient method of manufacturing a protectively coated helically wound antenna which addresses shortcomings of prior techniques. In addition, there is a need for an improved and efficient method for manufacturing such an antenna which produces a repeatable consistent helical structure, avoids deformation throughout the winding, and avoids significant post-processing trimming and tuning. 
     SUMMARY OF THE INVENTION 
     The present method is an improved, efficient and repeatable method for manufacturing a protectively coated helical conductor antenna. The helical conductor is threaded onto a temporary, removable support, such as a bolt including an asymmetrically shaped head and a shank having threads to hold the helical conductor. After being threaded on the shank, the entire winding and the associated portion of the bolt shank are placed within an injection mold cavity. The mold also preferably includes a recess outside of the injection cavity for accommodating the bolt head, with the recess being shaped to permit a specific orientation for the bolt. The shape of the bolt head and the recess define a repeatable orientation for the bolt and helical conductor. As a result, the point of injection relative to the helical winding may be exactly repeated time and time again. This allows selection of an injection point relative to the helical winding that produces minimal deformation, and the obtainment of consistent results thereby reducing variation and necessary manufacturing tolerances 
     A preferred application of the present method is manufacture of an antenna having an elongated conductor, with a helical conductor attached at an end of the elongated conductor. The helical conductor and elongated conductor are joined by an electrode. A shank end of the bolt mates with the electrode and presses against the beginning of the helical winding. The shaped head of the bolt orients the beginning of the helical winding, which is locked between the electrode and shank, adjacent the thermoplastic injection point. Pressing of the bolt at this point prevents deformation of the helical winding at the point where the force created by thermoplastic injection is most powerful, while the shank threads oppose deformation of the helical winding throughout its remainder. 
     After injection, the mold is opened, and the bolt and antenna are removed. Because the head was kept outside of the injection cavity, the bolt is conveniently removed by unthreading. An opening left where the bolt was removed may be capped. A core may be inserted in the space occupied by the shank during the injection molding. Alternatively or additionally, center fill molding may be used to finish the open end. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will be apparent to those skilled in the art with reference to the detailed description and the drawings, of which: 
     FIG. 1A shows a bolt and helical conductor assembly used in the method of the present invention; 
     FIG. 1B shows a preferred electrode for mounting a helical conductor in the method of the invention; 
     FIG. 1C shows a top view of the bolt of FIG. 1A; 
     FIG. 2 shows a mold used in the method of the invention; and 
     FIGS. 3A-3B illustrate preferred finishing steps in the method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Broadly stated, the present method provides for repeatable and precise control of an injection point for injecting thermoplastic material around a delicate helical conductor in a repeatable and precisely controlled fashion. Helical windings molded according to the method of the invention are uniform and avoid the need for tuning and trimming that is inherent in many conventional techniques used to mold helical windings. 
     Referring now to the drawings and particularly to FIG. 1A, shown is a bolt and helical conductor  18  winding preliminary assembly which is of critical importance to the method of the invention. While the method of the invention is applicable to molding of any helical conductor  18  structure antenna to an appropriate electrode, the method is illustrated with respect to a preferred dual conductor cell phone antenna. Specifically, the antenna includes an elongated conductor  10  having an insulating protective sleeve  12  around it, from which the elongated conductor  10  extends out of either end for electrical contact with remaining parts of the antenna and/or phone circuits to which the antenna is applied. At one end, the elongated conductor  10  is attached, preferably through crimping, to an electrode  14 . The electrode  14  is preferably crimped to the elongated conductor  10  on both sides of its cup  16 , which is used to accommodate a helical conductor  18  having a predetermined desirable length and pitch. It is important that the elongated conductor  10  not protrude from the end of the cup  16  which is on the side of the helical conductor  18 . The end of the elongated conductor  10  may exist anywhere within the cup  16 , but may not extend out the other end therefrom. 
     The cup  16  includes a concave recess on the side facing the helical conductor  18  to accommodate an end of the helical conductor which lays flat within the cup  16 . The cup  16  is crimped to the end of the helical conductor  18  which is accommodated therein. FIG. 1B shows a preferred cup electrode  16  including a rim extension  17  that is conveniently crimpable over the end of the helical conductor. After this connection of the elongated conductor  10  to the electrode  14  and the helical conductor  18 , a temporary bolt  20  is threaded into the helical conductor  18 . The bolt  20  includes threads which are numbered and pitched to accommodate the predetermined desirable number and pitch of threads on the helical conductor  18 . A tapered end  24  of the bolt  20  includes multiple generally convex surfaces  26  and  28  which serve to hold the bottom of the helical conductor  18  firmly against the electrode cup  16  when the bolt is threaded completely into the helical conductor  18 . An opposite end of the bolt includes a shaped head  30 . According to the invention, at least a portion of the shaped head must be asymmetrical so that it uniquely fits into an appropriately accommodating injection mold. 
     A preferred asymmetrical shape is illustrated in FIGS. 1A and 1C wherein an extension  32  extends from a single side of the bolt head  30 . As is best seen from the top view of FIG. 1C, the head  30  including the extension  32 , includes an asymmetrical shape about an elongate axis  34  of the bolt  20 . A preferred shape for the head  30  includes respective matching straight  36 , angled  38 , and curved  40  surfaces which serve to uniquely orient the head  30  and the entire bolt  20  along the bolt&#39;s elongate axis  34  when placed in a mold shaped according to the principles of the present invention. 
     Referring now to FIG. 2, the next step of the method of the invention is illustrated with respect to a preferred mold  42  which accommodates two antennas for molding. Half of the mold is shown in FIG. 2, with the other half being of corresponding shape. The mold  42  accommodates a portion of the bolt including the helical winding  18  in an injection molding cavity  44  which is fed by an injection molding point  46 . An end portion of the mold  42  extends to include a recess for uniquely orienting the asymmetrical bolt head  30 . Alternatively, a separate structure which cooperates with the mold  42  may be used for this purpose. In FIG. 2, portion  43  of the mold includes portions to uniquely orient the bolt head  30 . Thus, as will be appreciated by artisans, the bolt temporarily threaded into the helical conductor  18  will uniquely and repeatedly orient the helical conductor  18  within the injection mold  42 . This unique orientation preferably orients the injection point to be at the lowest part of the helical conductor  18  that contacts the cup  16 , thus placing the first winding which is not held between surfaces of the end of the bolt  20  and the cup  16  furthest away from the injection point  46 . It has been found that a preferred angle between the elongate axis  34  of the bolt be 35° to the angle of injection of thermoplastic material into the cavity  44 . This minimizes deformation of the helical conductor  18 . 
     Once the subassembly including the helical conductor  18  is placed within the mold, the mold is sealed and thermoplastic material is injected into the cavity  44  via conventional techniques. It is preferred that the thermoplastic material include a lubricant. 
     After molding around the helical conductor  18  has been completed, the mold is opened and the structure is removed. The bolt  20  is unthreaded from the now coated helical conductor  18 . From here, an additional step to finish the thermal plastic coating of the helical conductor  18  is conducted. This may comprise simply an additional molding step in which material is injected to the inside, but it is preferred that a core plug  48  is inserted prior to the finish mold, as seen in FIG.  3 A. This nonconductive plug  48  will be permanently molded into the helical conductor  18  structure, and protects the helical conductor  18  as center filling molds a cap  50 , seen in FIG. 3B, which bonds to the remaining thermoplastic material  52 . The removed bolt  20  is suitable for additional moldings. 
     Artisans will appreciate the repeatable nature of control the molding point according to the invention. Artisans will also appreciate that many antenna structures incorporating a helical conductor may be produced according to the method of the invention. These and many other variations will be apparent to artisans within the scope of the invention. 
     While various embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims. 
     Various features of the invention are set forth in the appended claims.