Abstract:
The invention provides methods and apparatuses for a helical antenna assembly that are constructed by placing a metallic tape strip diagonally onto non-metallic tape. The tape assembly is then rolled on a dielectric core. The metallic tape strip is coupled to an electrical connector and a center conductor that is located through the center of the dielectric core. The tape assembly may include one or two tabs that are bent over the ends the dielectric core to prevent the tape assembly from separating from the dielectric core. The tabs may be pinned by eyelets that are affixed to the center conductor. The pitch of the conductive portion of the tape assembly is determined to provide desired electrical characteristics when the tape assembly is wrapped around the dielectric core. The conductive portion of the tape assembly may be trimmed to obtain desired electrical characteristics.

Description:
This application claims priority to provisional U.S. Application Ser. No. 60/650,249 (“Small Broadband Helical Antenna”), filed Feb. 4, 2005. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to small broadband antennas, and more particularly helical antennas that may be used with wireless microphones. 
     BACKGROUND OF THE INVENTION 
     Wireless applications are becoming even more prevalent with the growing utilization of untethered computers, wireless telephones, and other wireless devices. However, in order to effectively support wireless applications, a RF signal is typically transmitted or received between wireless devices through a radio antenna. Radio antennas are typically bulky and incur a cost that may adversely increase the price of a wireless device. A “rubber ducky” antenna is an example of a radio antenna that is popularly used in wireless applications. A “rubber ducky” antenna is often constructed by wrapping wire around a core insulator and covered by protective material. Consequently, a “rubber ducky” antenna is often bulky, obstructive, and costly. Moreover, the electrical characteristics of a “rubber ducky” antenna may be insufficient. For example, the operating frequency bandwidth tends to be narrow, while many wireless applications may require broadband operation. Additionally signal loss due to the proximity of a user&#39;s hand may be excessive. 
     The approaches of the prior art, as described heretofore, provide antenna assemblies having construction attributes, electrical characteristics and associated costs that are often lacking for wireless applications. Thus, there is a real need in the market place to provide a radio antenna, e.g., a helical antenna, that is low cost, small, easy to assemble, and broadband with low sensitivity to hand proximity. 
     BRIEF SUMMARY OF THE INVENTION 
     Aspects of the invention provide solutions to at least one of the issues mentioned above, thereby enabling one to construct a radio antenna with conductive material that is affixed on tape. The tape is secured to a base material. 
     With one aspect of the invention, a helical antenna assembly is constructed by placing a metallic tape strip diagonally onto a rectangular piece of non-metallic tape. The tape assembly is then rolled on a dielectric core. The metallic tape strip is then coupled to an electrical connector. 
     With another aspect of the invention, a center conductor is inserted through the center of the dielectric core. The center conductor is electrically coupled to an electrical connector. The tape assembly includes one or two tabs that bend over the ends the dielectric core to prevent the tape assembly from separating from the dielectric core. The tabs may be further pinned by eyelets. 
     With another aspect of the invention, the pitch of the conductive portion of the tape assembly is determined to provide desired electrical characteristics when the tape assembly is wrapped around the dielectric core. 
     With another aspect of the invention, the conductive portion of the tape assembly is trimmed in length to obtain desired electrical characteristics, including the center operating frequency. Parasitic effects of surrounding components may be compensated when tuning the antenna assembly. 
     With another aspect of the invention, a helical antenna is formed by determining a length of a conductive portion to obtain desired characteristics of the helical antenna, laminating the conductive portion to a base portion to form a tape assembly in which the conductive portion is diagonally placed on the base portion, wrapping the tape assembly around a dielectric core, and electrically coupling an electrical connector to the conductive portion. 
     With another aspect of the invention, a helical antenna assembly includes a dielectric core, a tape assembly that is wrapped around the dielectric core where the tape assembly further includes a base portion and a conductive portion, and an electrical connector that is coupled to the conductive portion of the tape assembly. The conductive portion is diagonally placed on the base portion with a determined pitch and has a length and a width in order to obtain desired electrical characteristics. 
     With another aspect of the invention, a double-helical antenna assembly includes a dielectric core, a tape assembly that is wrapped around the dielectric core where the tape assembly further includes a base portion and a conductive portion, and an electrical connector that is coupled to a center feed-point of the conductive portion. The conductive portion includes two diagonal conductive sections that join at the center feed-point with a determined pitch. Each diagonal conductive portion has a length and a width to obtain desired electrical characteristics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1C  show components of a broadband helical antenna in accordance with an embodiment of the invention; 
         FIGS. 2A and 2B  show a tape assembly and illustrates a procedure for wrapping the tape assembly around dielectric material to form an antenna assembly in accordance with an embodiment of the invention; 
         FIGS. 3A-3C  show a helical antenna assembly in accordance with an embodiment of the invention; 
         FIG. 4  shows components of a helical antenna assembly and a resulting assembled antenna assembly in accordance with an embodiment of the invention; 
         FIG. 5  shows a microphone assembly that includes a helical antenna assembly in accordance with an embodiment of the invention; 
         FIG. 6  shows tape assemblies for different frequency operating ranges in accordance with an embodiment of the invention; and 
         FIGS. 7A and 7B  show a double helical antenna assembly in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows components of a broadband helical antenna in accordance with an embodiment of the invention. Tape assembly  101  comprises base portion  104  and conductive portion  103  (which comprises copper tape in the embodiment shown). In the embodiment, base portion  104  is constructed from a vinyl core material that is laminated with copper tape  103  with electro tin plating. (In the embodiment shown, 3M™ number 9471 adhesive with an approximate thickness of 2.0 mils is used for laminating the copper tape  103  with base portion  104 . Copper tape  103  may be electroplated on base portion  104  and laser trimmed or mechanically trimmed to provide the desired width and length dimensions. Also, as will be discussed, copper tape  103  may be subsequently cut at line  151 , in which the excessive length of copper tape is removed, in order to adjust and tune the helical antenna assembly. The frequency characteristics are determined by a number of parameters that include length (L)  153 , width (W)  155 , and pitch (θ)  156  of copper tape  103 . In the exemplary embodiment shown in  FIG. 1 , tape assembly  101  is approximately 10 cm long and 14 mm wide with conductive portion  103  having width  155  of approximately 7 mm and corresponding to a frequency operating range of 578-650 MHz. 
     As shown in  FIG. 1 , tape assembly  101  includes tab  111  on which copper tape  103  is extended to be electrically coupled to other components of the antenna assembly as will be discussed. Copper tape  103  forms hole  105  on tab  111  to support the electrical coupling. 
     Tape assembly  101  comprises tab  111 , although other embodiments of the invention may support more than one tab (e.g., tabs  211   a  and  211   b  as shown in  FIG. 2 .) 
     As will be discussed, tape assembly  101  is wrapped around dielectric core  107  (corresponding to top view  107   a  and side view  107   b ). Center conductor  109  (corresponding to top view  109   a  and side view  109   b ) is located at essentially the center of dielectric core  107  and extends through the entire length of dielectric core  107 . The length of center conductor  109  is typically longer than the length of dielectric core  107  so that the ends of center conductor  109  extend beyond dielectric core  107  for mechanical and electrical coupling. As will be discussed, an eyelet flange and a SMA connector may be attached to the ends of center conductor  109 . In the embodiment, the length of dielectric core  107  is approximately 14 mm (to match the width of tape assembly  101 ) and the diameter of dielectric core  107  is approximately 0.680 to 0.684 inches. 
     In an embodiment of the invention, dielectric core  107  is formed from Texin® 285 urethane thermoplastic elastomer (manufactured by Bayer MaterialScience). Texin® 285 possesses fairly constant consistent dielectric properties with a dielectric constant between 5.6 and 6.5 and a good electrical strength of approximately 445 Kv/in. 
       FIG. 2  shows tape assembly  201  and illustrates a procedure for wrapping tape assembly  201  around dielectric material  207  to form an antenna assembly in accordance with an embodiment of the invention. Tape assembly  201  (corresponding to top view  201   a  and side view  201   b ) comprises conductive portion  203  and base portion  204 . 
     Tape assembly  201  includes tabs  211   a  and  211   b  which form holes  205   a  and  205   b , respectively. Hole  205   a  is formed through conductive portion  203 , an electrical connector may be electrically coupled to conductive portion  203  near hole  205   a  by soldering an electrical connector (e.g., SMA connector  315  as shown in  FIG. 3 ) to a center conductor (not shown) which protrudes through hole  205   a . An eyelet flange (not shown) may be fastened to the other end of the center conductor through hole  205   b.    
     Tape assembly  201  (shown as side view  201   b ) is wrapped around dielectric core  207 . (An adhesive may be applied to tape assembly  201  to prevent tape assembly  201  from detaching from dielectric core  207 .) In the embodiment, dielectric core  207  is wrapped from right to left in order to show indicia (not shown) that may be on tape assembly  201 . The indicia may be used for identification purposes of the antenna assembly. However, tape assembly  201  may be wrapped from left to right without significantly altering the electrical characteristics of the antenna assembly. 
     After tape assembly  201  is wrapped around dielectric core  207 , tabs  211   a  and  211   b  are bent to be flush with the ends of dielectric core  207 . In the exemplary embodiment shown in  FIG. 2 , notches are formed between each tab  211   a  and  211   b  and the main portion of tape assembly  201  to facilitate the bending of tabs  211   a  and  211   b.    
     In the embodiment, the pitch of conductive portion  203  is selected so that conductive portion  203  does not overlap when tape assembly  201  is wrapped around dielectric core  207 . 
       FIG. 3  shows helical antenna assembly  321  (corresponding to side view  321   a , bottom view  321   b , and top view  321   c ) in accordance with an embodiment of the invention. Side view  321   a  illustrates conductive portion  303  wrapped around dielectric core (not labeled). Center conductor  309  goes through the center of the dielectric core. The core pin of SMA connector  315  (corresponding to side view  315   a  and bottom view  315   b ) is soldered to conductive extension  311  (which is an extension of conductive portion  303 ) and center conductor  309 . A ground for helical antenna assembly  321  is established by the conductivity properties of the microphone enclosure. Flange  313  (corresponding to top view  313   b  and side view  313   a ) is fastened to the other end (opposite of SMA connector  315 ) of center conductor  309 . Flange  313  may be machined as part of center conductor  309  or may be formed by fastening an eyelet on center conductor  309 . Also, an eyelet may be fastened on the connector end to maintain the positioning of conductive extension  311  before assembling SMA conductor  315 . 
     Antenna assembly  321  utilizes one tab (corresponding to conductive extension  311 ). However, other embodiments of the invention may use more than one tab (e.g., tabs  211   a  and  211   b  as shown in  FIG. 2 . Using two tabs helps to prevent the copper tape from un-rolling in high humidity and moister environments. In the associated embodiments, the tabs are bent across the top and bottom of the dielectric core and pinned with the eyelet that is used to connect the antenna to the RF connector. A tab may be lengthened to ensure that the metal end of the tape assembly is covered after being wrapped. 
       FIG. 4  shows components of a helical antenna assembly and a resulting assembled antenna assembly  421  in accordance with an embodiment of the invention. Antenna assembly  421  includes tape assembly  401 , dielectric core  407 , and SMA connector  415 .  FIG. 4  illustrates the position of eyelet  413  in relation to dielectric core  407 . As with the embodiments shown in  FIGS. 2 and 3 , dielectric core  407  has a hole drilled through the center to accommodate a center conductor (not visible). 
       FIG. 5  shows microphone assembly  500  that includes helical antenna assembly  527  in accordance with an embodiment of the invention. (Microphone assembly  500  includes acoustical transducers (not shown) and a microphone cover (not shown) located at the left side of  FIG. 5 .) Helical antenna assembly  527  connects to electronic circuitry that converts an audio signal into an electrical signal that is transmitted through helical antenna assembly  527 . Helical antenna assembly  527  is positioned by housing  531  and covered by antenna cover  529 . 
     In the embodiment shown in  FIG. 5 , antenna cover  529  comprises Santoprene® 103-50 thermoplastic rubber that is manufactured by Advanced Elastomer Systems. Santoprene® 103-50 exhibits a dielectric constant of approximately 2.3 with a dielectric strength of approximately 498 Kv/inch. 
       FIG. 6  shows tape assemblies for different frequency operating ranges in accordance with an embodiment of the invention. Tape assemblies  601   a ,  601   b ,  601   c ,  601   d , and  601   e  correspond to frequency ranges of 518-578 MHz, 578-638 MHz, 638-689 MHz, 740-814 MHz, and 798-862 MHz, respectively. Conductive portions  603   a - 603   e  are trimmed to obtain the desired electrical characteristics when exposed to anticipated parasitic effects. In order to identify characteristics of an antenna assembly, indicia may be laser cut, stamped, or printed on the tape assembly. When the tape assembly is rolled on the dielectric core, the indicia are visible to provide easy identification during and after the construction of the antenna assembly. 
     Each tape assembly  601   a - 601   e  uses the same pitch. However, the length of the conductive portions is adjusted to provide the desired electrical characteristics. An approximate length is determined without the parasitic effects of the antenna cover and microphone case. For example, the shape and material of the antenna cover and microphone case will affect the electrical characteristics. However, the parasitic effects are not typically large and may be compensated by trimming the conductive portion (e.g., the laminated copper tape) of the tape assembly. 
       FIGS. 1-6  illustrate exemplary embodiments of the invention that support a wireless microphone (which functionally operates as a handheld transmitter). However, embodiments of the invention may support other wireless applications in which radio frequency signals are generated. Experimental data suggests that the embodiments shown in  FIGS. 1-6  are low cost, small, and easy to assemble. 
     An antenna assembly (e.g., antenna assembly  527 ) has broadband frequency characteristics with a bandwidth greater than 10% with center frequencies greater than 500 MHz. The embodiments exhibit low sensitivity to hand placement or hand proximity. 
     The embodiments shown in  FIGS. 1-6  enable one to easily adjust the center frequency of operation. For example, the length of conductive portion  103  (which comprises copper tape) may be shortened by cutting conductive portion  103  along line  151  as shown in  FIG. 1 . The antenna assembly is typically tuned to compensate for parasitic effects (e.g., the effects of antenna case  529  as shown in  FIG. 5 ) by tuning conductive portion  103 . Moreover, the embodiments that are shown in  FIGS. 1-6  exhibit repeatable results. 
     The embodiment shown in  FIGS. 1-6  have exhibited VSWR values of 1.2:1 within the operating frequency range whether the microphone is positioned in a stand or held by a user. The embodiments typically exhibit VSWR values of less than 3:1 for the entire frequency range. 
     In the embodiments shown in  FIGS. 1-6 , the pitch of the conductive portion (e.g., conductive portions  603   a - 603   d  as shown in  FIG. 6 ) is essentially the same. In order to obtain the desired frequency range, the conductive portion is trimmed to the necessary length. However, other embodiments of the invention may tune the frequency characteristics by adjusting other parameters, e.g., the dielectric constant of the dielectric core or the width of the conductive portion. Moreover, the wider the conductive portion, the lower the Q of the antenna assembly, thus resulting in a wider frequency bandwidth of operation. (However, increasing the width of the conductive portion reduces the maximum length of the conductive portion for a given diameter of the dielectric core in order to avoid overlapping the conductive portion.) 
     While the embodiments shown in  FIGS. 1-6  illustrate exemplary embodiments of wireless microphones, other embodiments of the invention may support other wireless applications that require a wireless device for either receiving or transmitting a RF signal. 
     While the embodiments shown in  FIGS. 1-6  illustrate exemplary embodiments of a helical antenna, other embodiments of the invention support other types of antennas.  FIG. 7  shows a double-helical (ram&#39;s horn) antenna assembly in accordance with an embodiment of the invention. Tape assembly  701  comprises copper tape  703  forming a “vee” shape with a center feed-point  751   a . Tape assembly  701  is wrapped around a dielectric core to form antenna assembly  721 . RF energy is provided to antenna assembly  721  through SMA connector  715 , which is soldered to center feed-point  751   b.    
     While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims.