Patent Publication Number: US-2007120761-A1

Title: High performance compact half wave antenna

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to antennas and, more particularly, to half wave antennas.  
      2. Background of the Invention  
      In response to consumer demand, new mobile communication devices continue to be developed which are dimensionally smaller than previous models. For example, some communication devices that are now being developed are significantly shorter and thinner than models that they will replace. Such devices can be easily carried in one&#39;s pocket, making their use convenient.  
      Unfortunately, making a mobile communication device dimensionally smaller creates challenges for the RF engineer. In particular, as antennas for the devices become smaller, engineers are forced to operate the antennas in quarter-wave mode. With all other parameters being equal, the specific absorption rate (SAR) of an antenna in quarter-wave mode is higher than the SAR of an antenna operating in half-wave mode.  
     SUMMARY OF THE INVENTION  
      The present invention relates to an antenna that includes an outer helical radiator having a first diameter and an inner helical radiator having a second diameter that is smaller than the first diameter. The inner helical radiator can be positioned at least in part interior to the outer helical radiator. The outer helical radiator and the inner helical radiator can be substantially coaxially aligned. For example, the inner helical radiator can be attached to a dielectric member such that the dielectric member maintains the position of the inner helical radiator interior to the outer helical radiator.  
      The antenna also can include a whip to which the inner helical radiator is attached. The inner helical radiator can be movable between a first position wherein at least a portion of the inner helical radiator is positioned interior to the outer helical radiator, and a second position wherein the inner helical radiator is positioned exterior to the outer helical radiator. Further, the antenna can include a dielectric member that insulates the inner helical radiator from the outer helical radiator when the inner helical radiator is in the first position.  
      The outer helical radiator can be communicatively linked to a signal source. In addition, the inner helical radiator can be communicatively linked to the signal source when the inner helical radiator is in the second position.  
      The present invention also relates to a method for tuning performance characteristics of an antenna. The method can include positioning at least a portion of an inner helical radiator interior to an outer helical radiator having a first diameter. The inner helical radiator can have a second diameter smaller than the first diameter. The method also can include coaxially aligning the outer helical radiator and the inner helical radiator. The inner helical radiator can be insulated from the outer helical radiator with a dielectric member.  
      The inner helical radiator can be electrically connected to a whip such that the inner helical radiator is movable between a first position wherein at least a portion of the inner helical radiator is positioned interior to the outer helical radiator, and a second position wherein the inner helical radiator is positioned exterior to the outer helical radiator.  
      The outer helical radiator can be communicatively linked to a signal source. The inner helical radiator can remain unconnected to the signal source when the inner helical radiator is in the first position, and the inner helical radiator can be communicatively linked to the signal source when the inner helical radiator is in the second position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Preferred embodiments of the present invention will be described below in more detail, with reference to the accompanying drawings, in which:  
       FIG. 1  depicts a side view of an antenna that is useful for understanding the present invention.  
       FIG. 2  depicts a top view of the antenna of  FIG. 1 .  
       FIG. 3  depicts a side view of another arrangement of the antenna that is useful for understanding the present invention.  
       FIG. 4  depicts the extendable antenna of  FIG. 3  in an extended position.  
       FIG. 5  depicts a top view of the antenna of  FIG. 3 .  
       FIG. 6  is a flowchart which is useful for understanding the present invention. 
    
    
     DETAILED DESCRIPTION  
      While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the description in conjunction with the drawings. As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting but rather to provide an understandable description of the invention.  
      The present invention relates to a high performance compact half-wave antenna. More particularly, the antenna of the present invention achieves a low specific absorption ratio (SAR) and a voltage standing wave ratio (VSWR) that is at least very near the ideal value of 1. Both of these performance specifications are achieved even though the antenna is small enough to be compactly integrated into today&#39;s very small mobile communication devices.  
       FIG. 1  depicts a side view an antenna  100  which is useful for understanding the present invention.  FIG. 2  depicts a top view of the antenna  100 . Making reference both to  FIG. 1  and to  FIG. 2 , the antenna can include an outer helical radiator  102  that comprises a helically wound electrical conductor  104 . The outer helical radiator  102  can be communicatively linked to a signal source and/or a signal receiver  124 . For instance, the outer helical radiator  102  can be communicatively linked to the source/receiver  124  via a conductive member  106  and a contact  108 . The contact  108  can provide electrical continuity with one or more circuit traces  126  on a printed circuit board  110  to which the source/receiver  124  is communicatively linked. The conductive member  106  can be T-shaped, as shown, although the invention is not limited in this regard. Indeed, the conductive member  106  can be any shape suitable for providing an electrical connection to the outer helical radiator  102 .  
      The antenna  100  also can include an inner helical radiator  112  that comprises a helically wound electrical conductor  114 . A diameter  116  of the inner helical radiator  112  can be smaller than a diameter  118  of the outer helical radiator  102 . This configuration facilitates positioning at least a portion of the inner helical radiator  112  interior to the outer helical radiator  102  without the inner helical radiator  112  directly contacting the outer helical radiator  102 .  
      The outer helical radiator  102  and the inner helical radiator  112  can be substantially helically shaped, as shown in  FIG. 1 . However, the invention is not so limited. Indeed, at least one of the helical radiators  102 ,  112  can have a modified helical shape in which the diameter  116 ,  118  progressively changes along a length of the radiator  102 ,  112 . Still, other modifications to the helix shape can be made, and such modifications are within the scope of the present invention.  
      A dielectric member  120  can be provided to position the inner helical radiator  112  within outer helical radiator  102 , while minimizing flow of direct current between the outer helical radiator  102  and the inner helical radiator  112 . Moreover, the inner helical radiator  112  can be dielectrically isolated from any other components of the antenna  100 .  
      The dielectric member  120  can be any shape suitable for disposing the inner helical radiator  112  within the outer helical radiator  102 . For example, the dielectric member  120  can be T-shaped, as shown, or I-shaped. In another arrangement, the dielectric member  120  can be disposed in a region  122  defined between the inner helical radiator  112  and the outer helical radiator  102 . In yet another arrangement, one or more of the radiators  102 ,  112  can be encapsulated in the dielectric member  120 . Still, there are a myriad of other dielectric structures that can be used for positioning the inner helical radiator  112  and the invention is not limited in this regard.  
      In operation, the inner helical radiator  112  can electromagnetically couple to the outer helical radiator  102 . This coupling can increase the level of signal current in the outer helical radiator  102 , thus enabling the outer helical radiator to achieve lower transmission impedance than would otherwise be obtainable without use of the inner helical radiator  112 . For example, using known impedance matching methods, the antenna  100  can operate as a half wave antenna having a transmission impedance of 50 ohms with a VSWR of 2:1, while also achieving a very low SAR. In comparison to quarter wave antenna operation, operation as a half wave antenna can have the desirable effect of maximizing transmission currents in the antenna, while minimizing transmission currents in circuit traces  126  on the printed circuit board  110 .  
       FIGS. 3-5  depict another arrangement of the antenna  100  that is useful for understanding the present invention. In addition to the inner helical radiator  112 , the antenna  100  can comprise a whip  330  to which the inner helical radiator  112  is attached. The whip  330  can be moveable between a first position wherein at least a portion of the inner helical radiator  112  is positioned interior to the outer helical radiator  102 , as shown in  FIG. 3 , and a second position wherein the inner helical radiator  112  is positioned exterior to the outer helical radiator  102 , as shown in  FIG. 4 .  
      The inner helical radiator  112  can be attached to the whip  330  via an electrically conductive connector  332 . Accordingly, the whip  330 , connector  332  and the inner helical radiator  112  can form a radiating member  334 . In another arrangement, the inner helical radiator  112  and the whip  330  can be formed from a single conductor  336 , in which case the connector  332  would not be required as a component of the radiating member  334 .  
      A dielectric member  338  that slideably engages the whip  330  can dielectrically insulate the whip  330  from a conductive member  306 . Accordingly, the inner helical radiator  112  can electromagnetically couple to the outer helical radiator  102  to increase the level of signal current in the outer helical radiator  102 , as previously described, while the inner helical radiator  112  and the outer helical radiator  102  are not electrically connected.  
      Referring to  FIG. 3 , a stop member  340  and stop bracket  342  can be provided to stop movement of the radiating member  334  at a selected retracted position. For instance, it may be desirable for the inner helical radiator  112  to be positioned at a specific location interior to the outer helical radiator  102  to optimize performance of the antenna  100  when the radiating member  334  is retracted. In one arrangement, the stop member  342  can be dielectrically insulated from any circuit traces  126  on the printed circuit board  110 .  
      In another arrangement, the stop member can provide an electrical connection between the radiating member  334  and one or more circuit traces  126  on the printed circuit board  110 . For example, the outer helical radiator  102  and the radiating member  334  each can be electrically connected to different portions of a circuit. Such an arrangement can provide greater flexibility in the application of impedance control devices that optimize performance of the antenna  100  when the antenna  100  is in the retracted position.  
      Referring to  FIG. 4 , the radiating member  334  can be extended such that the inner helical radiator  112  extends outward of the outer helical radiator  102 . The stop member  340  can interface with the conductive member  306  to stop movement of the radiating member  334  at an appropriate position. In one arrangement, the stop member  340  can form an electrically conductive connection with the conductive member  306  so that the radiating member  334  and the outer helical radiator  102  form a single radiator  344 . In another arrangement, a switch, such as one known to the skilled artisan, can be used to form the electrically conductive connection between the stop member  340  and the outer helical radiator  102 .  
      In yet another arrangement, the bushing  340  can be formed from a dielectric material. In this arrangement, the radiating member  334  can be extended to decouple the inner helical radiator  112  from the outer helical radiator  102 . For example, this could be used to capacitively couple energy from the conductive member  306  to the whip  330 . Such an arrangement could allow a half-wave antenna to be formed in the extended mode, with the whip  330  and inner helical radiator  112  being the primary radiators.  
       FIG. 6  is a flowchart that presents a method  600  which is useful for understanding the present invention. Beginning at step  610 , at least a portion of an inner helical radiator can be positioned interior to an outer helical radiator having a first diameter. The inner helical radiator can have a second diameter smaller than the first diameter. At step  620 , the inner helical radiator can be coaxially aligned with the outer radiator.  
      In one arrangement, the inner helical radiator can be fixed into a static position and insulated from the outer helical radiator with a dielectric member. In another arrangement, the inner helical radiator can be electrically connected to a whip such that the inner helical radiator is moveable between a first position wherein at least a portion of the inner helical radiator is positioned interior to the outer helical radiator, and a second position wherein the inner helical radiator is positioned exterior to the outer helical radiator.  
      At step  630 , the outer helical radiator can be communicatively linked to a signal source. In the arrangement in which the inner helical radiator is moveable, the inner helical radiator can remain unconnected to the signal source when the inner helical radiator is in the first position, and the inner helical radiator can be communicatively linked to the signal source when the inner helical radiator is in the second position.  
      The terms “a” and “an,” as used herein, are defined as one or more than one. The term “plurality”, as used herein, is defined as two or more than two. The term “another”, as used herein, is defined as at least a second or more. The terms “including” and/or “having”, as used herein, are defined as comprising (i.e., open language). The term “coupled”, as used herein, is defined as connected, although not necessarily through a conductive path, and not necessarily mechanically, e.g. linked through an electromagnetic field. The term electrically connected, as used herein, is defined as being connected via a continuously electrically conductive path (i.e. a path that, relative to the devices being connected, has low DC resistance). The term communicatively linked, as used herein, is defined as being linked via a signal path. The signal path can be a direct electrical connection having low DC resistance, but is not limited in this regard. For instance, a signal path also can comprise series components, such as capacitors, that impede or block DC current while propagating RF signals.  
      This invention can be embodied in other forms without departing from the spirit or essential attributes thereof. Accordingly, reference should be made to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.